My Science School

          Hurricanes are very powerful, spiralling storms that produce winds of up to 300km/h (185mph). A combination of wind and torrential rain causes widespread flooding of the land and damage to buildings. Neteorologists call hurricanes tropical cyclones, due to the nature of their movement and the areas in which they form. They are also known variously as typhoons and willy-willies.

          A hurricane is a type of tropical cyclone, which is a generic term for a low pressure system that generally forms in the tropics. The cyclone is accompanied by thunderstorms and, in the Northern Hemisphere, a counterclockwise circulation of winds near the earth's surface. Tropical cyclones are classified as follows:

Tropical Depression

          An organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of 38 mph (33 kt) or less. Sustained winds are a 1-minute average wind measured at about 33 ft (10 meters) above the surface. While 1 knot = 1 nautical mile per hour or 1.15 statute miles per hour and is abbreviated as "kt".

Tropical Storm

          An organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of 39-73 mph (34-63 kt)

Hurricane

          An intense tropical weather system of strong thunderstorms with a well-defined surface circulation and maximum sustained winds of 74 mph (64 kt) or higher

          Hurricanes are categorized according to the strength of their winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest wind speeds, while a Category 5 hurricane has the strongest. These are relative terms, because lower category storms can sometimes inflict greater damage than higher category storms, depending on where they strike and the particular hazards they bring. In fact, tropical storms can also produce significant damage and loss of life, mainly due to flooding.

Hurricane Names

          When the winds from these storms reach 39 mph (34 kts), the cyclones are given names. Years ago, an international committee developed names for Atlantic cyclones. In 1979 a six year rotating list of Atlantic storm names was adopted — alternating between male and female hurricane names. Storm names are used to facilitate geographic referencing, for warning services, for legal issues, and to reduce confusion when two or more tropical cyclones occur at the same time. Through a vote of the World Meteorological Organization Region IV Subcommittee, Atlantic cyclone names are retired usually when hurricanes result in substantial damage or death or for other special circumstances.

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          An anemometer is an instrument used to measure the speed of the wind, which is a common weather station instrument or to calculate any form of current gas. The term is derived from the Greek word anemos meaning wind which was first explained by Leon Battista Alberti who was an Italian artist and an architect in the year 1450. He used a mechanical anemometer which was placed perpendicular to the wind direction such that the wind velocity as indicated by the angle of inclination of the disc. Some early versions had a ball attached to a swinging arm that travelled up a curved scale according to the strength of the wind. Most anemometers consist of three or more cups mounted on arms that spin around a pole. Inside the pole, a mechanism records the number of rotations in a certain period of time. The speed is usually given in kilometres or miles per hour, although marine anemometers may give the speed in knots.

Types Of Anemometers

          The basic classification of anemometers depends on the measurement of the velocity of wind and measurement of pressure of the wind. The 5 major types of anemometer are:

Cup anemometer: This consists of four hemispherical cups that are mounted on one end of the horizontal arms at equal angles on a vertical shaft. The positioning of the cups is such that the air passes through them in the horizontal direction making the cups rotate such that it is proportional to the speed of the wind. By calculating the turns of the cups over a period of time, the average speed of the wind is found. These are used by researchers, meteorologists, and educational institutions for research and commercial activities.

Hotwire anemometer: This consists of thin wire which is heated up to a certain temperature slightly more than atmospheric temperature. The wire cools as the air flows past the wire. To calculate the velocity, the relation between the resistance of the wire and the velocity of the wind must be obtained as most metal’s resistance depends on the temperature of the metal. These find applications in HVAC (heating, ventilating and air conditioning) businesses measuring the airflow through the building ducts.

Windmill anemometer: This is a mechanical device similar to the regular windmill used to calculate the velocity of the wind. It consists of the axis of rotation running parallel to the direction of the wind. It also has an aerovane as the direction of the wind is not constant helping the axis change the direction. The aerovane consists of a propeller and a tail to obtain precise wind speed and direction measurements.

Laser Doppler anemometer: A beam of light is used which is further split into two beams. The velocity is determined by calculating the amount of light that has been reflected off by the moving air particles when one of the two beams is made to enter the anemometer. These find applications in high-tech jet engines and also in river hydrology.

Sonic anemometer: Sound waves are used to obtain the speed of the wind. These waves are made to pass through the transducer. These find applications in aircraft and scientific wind turbines.

Uses Of Anemometer

Other than measuring the speed of the wind, the anemometer is used for the following purposes:

• To measure the wind pressure


• To measure the flow of the wind


• To measure the direction of the wind


• It is used by the drone users or RC plane users to check the weather conditions before testing their devices


• Also used by long range shooters and pilots


• Used by skydivers to evaluate wind velocity before they leap into the abyss


• Used in aerodynamics to measure the airspeed

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           Kites use the strength of the wind to  keep them in the air. Held by one or more strings, the kite deflects the force of the wind downwards. The wind produces a reaction force that acts in the opposite direction of the pull to the string, supporting the kite in the air. Different designs of kites are suitable for use in different wind strengths.

            Most kites have three main components: the kite body (which comes in many different shapes and sizes), the bridle (or harness), and the control line (or tether).  The kite body is made up of a framework and outer covering.  The framework is usually made from a lightweight material like wood or plastic.  Paper, fabric, or plastic is then stretched over the framework, turning it into a sort of wing.  The bridle and the control line help the kite flyer control the kite.  In flight, the kite is connected to the kite flyer by the control line, which is connected to the kite by the bridle.  The kite pivots and dives about the point where the bridle connects to the control line.

            The four forces of flight (i.e. Lift, Weight, Drag, and Thrust) affect kites in the same way they affect airplanes, and anything else that flies.  Lift is the upward force that pushes a kite into the air.  Lift is generated by differences in air pressure, which are created by air in motion over the body of the kite.  Kites are shaped and angled so that the air moving over the top moves faster than the air moving over the bottom. Daniel Bernoulli, an 18th century Swiss mathematician, discovered that the pressure of a fluid (like air) decreases as the fluid speeds up.  Since the speed of the air above the kite is greater than the speed of air below, the pressure above is less than the pressure below and the kite is pushed into the air and — Tada — lift!  Weight is the downward force generated by the gravitational attraction of the Earth on the kite.  The force of weight pulls the kite toward the center of the Earth.  Thrust is the forward force that propels a kite in the direction of motion.  An airplane generates thrust with its engines, but a kite must rely on tension from the string and moving air created by the wind or the forward motion of the kite flyer to generate thrust.  Drag is the backward force that acts opposite to the direction of motion.  Drag is caused by the difference in air pressure between the front and back of the kite and the friction of the air moving over the surface of the kite.  To launch a kite into the air the force of lift must be greater than the force of weight.  To keep a kite flying steady the four forces must be in balance.  Lift must be equal to weight and thrust must be equal to drag.

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          Damage to buildings during a storm obviously varies according to the construction and location of the building, but damage generally occurs above Force 9 or 10. Chimney pots, roofing tiles and slates are the parts of buildings most at risk from storm damage.

          As wind passes over and around a building, two things happen.  First, positive wind pressure applies to building components on the side(s) of the building that face the incoming wind (the “windward” direction).  The windward wind load is essentially the force of the blowing wind pressing on the building.  Second, negative wind pressure applies to building components on the side(s) of the building that face away from the incoming wind (the “leeward” direction).  The negative wind pressure is also known as “suction.”  The suction force will be applied to vertical surfaces such as walls, and also horizontal or sloped surfaces such as roofs.  The suction force can be thought of as acting like a vacuum that pulls on a building and its components.

          During an event with moderately strong winds, building materials such as asphalt shingles or vinyl siding may displace away from the building.  Sometimes, windows break and the broken shards end up outside of the building.  In such cases, a common misconception held by many people in the construction industry is that wind must have gotten under or behind the surface of the material to “blow” it out.  However, acknowledging and understanding the concept of wind suction can explain how those materials were damaged.  Wind does not only apply a direct blowing force on buildings, but it also induces a suction force.  Depending on various factors, the suction force can be significant enough to cause damage to individual building components or the structure of the building itself.

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          During high winds, some bridges may be closed for safety reasons. The structure of the bridges is rarely in doubt, although there have been cases of bridges collapsing in strong winds. The chief concern is for the safety of the vehicles that cross the bridge, particularly high-sided lorries and trucks. Those bridges in especially high positions are most prone to closure.

           When the wind reaches speeds of 65 miles an hour, the bridge closes to traffic.

          A category one hurricane starts at 75 miles per hour.

         The Mackinac Bridge Authority says there was a gust up to 72 miles per hour on Sunday.

          It caused both a camper and a boat on a trailer to tip over on the bridge.

         Typically the bridge would be closed at that wind speed, but that storm front only took a couple of minutes to go from calm winds to hurricane gusts.

         “We’re expected to get some pretty bad storms this week so everybody should take their time driving across the bridge," said Bob Sweeney from the Mackinac Bridge Authority. "If you’re driving across the bridge during a high wind event, even if it’s only 20 miles per hours, or the winds only 20 miles per hour, drive slow, drive 20 miles per hour and you’ll safely get across the bridge.”

         At 35 miles per hour, the bridge authority starts escorting high profile vehicles like trucks or trailers.

        At 55 miles per hour, it is closed to those high profile vehicles.

        To keep an eye on the wind, the bridge authority watches weather reports online, and they have their own wind meter on the bridge.

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          The Beaufort scale was devised for use by sailors. By observing the wind's effect on the ship's rigging and the waves, sailors would know how much sail should be carried or stowed in order for the ship to sail efficiently and safely. The 12 levels of wind strength have since been adapted for use on land.

          Beaufort scale, in full Beaufort wind force scale, scale devised in 1805 by Commander (later Admiral and Knight Commander of the Bath) Francis Beaufort of the British navy for observing and classifying wind force at sea. Originally based on the effect of the wind on a full-rigged man-of-war, in 1838 it became mandatory for log entries in all ships in the Royal Navy. Altered to include observations of the state of the sea and phenomena on land as criteria, it was adopted in 1874 by the International Meteorological Committee for international use in weather telegraphy.

          The Beaufort scale as originally drawn up was calibrated to Beaufort’s assessment of the various effects of the wind on a full-rigged man-of-war. Somewhat arbitrarily, he identified 13 states of wind force on his vessel and ranked them 0 to 12. The scale, however, made no reference to the speed of the wind, and various attempts, particularly during the 20th century, have been made to correlate the two. An attempt made in 1912 by the International Commission for Weather Telegraphers was interrupted by World War I. In 1921 G.C. Simpson was asked to formulate equivalents, which were accepted in 1926 by the Committee. In June 1939 the International Meteorological Committee adopted a table of values referring to an anemometer at a height of 6 metres (20 feet). This was not immediately adopted by the official weather services of the United States and Great Britain, which used the earlier scale referring to an anemometer at an elevation of 11 metres (36 feet). The Beaufort force numbers 13 to 17 were added by the U.S. Weather Bureau in 1955.

          The scale is now rarely used by professional meteorologists, having been largely replaced by more objective methods of determining wind speeds—such as using anemometers, tracking wind echoes with Doppler radar, and monitoring the deflection of rising weather balloons and radiosondes from their points of release. Nevertheless, it is still useful in estimating the wind characteristics over a large area, and it may be used to estimate the wind where there are no wind instruments. The Beaufort scale also can be used to measure and describe the effects of different wind velocities on objects on land or at sea.

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          The strength of the wind varies between gentle breezes and destructive storms. Knowing the strength of the wind and its effect is important for the safety of people and property, particularly for those at sea. In 1805, Sir Francis Beaufort devised a scale by which the strength of the wind could be determined by observing its effect on the environment. This is known as the Beaufort scale.

          Wind has both speed and direction. Anemometers measure wind speed and wind vanes measure wind direction.

          A typical wind vane has a pointer in front and fins in back. When the wind is blowing, the wind vane points into the wind. For example, in a north wind, the wind vane points northward.

A cup anemometer is a common tool to measure wind speed. The cups catch the wind and produce pressure difference inside and outside the cup. The pressure difference, along with the force of the wind, causes the cups to rotate. Electric switches measure the speed of the rotation, which is proportional to the wind speed.

          At wind speeds below about 3 mph, the cup anemometer is prone to error because friction keeps the cups from turning. At wind speeds above 100 mph, cup anemometers often blow away or give unreliable measurements. In freezing rain, the anemometer can literally freeze up and stop turning.

          Propellers also can measure wind speed. The propeller blades rotate at a rate proportional to the wind speed.

          A windsock often is used at airports. A windsock is a cone-shaped bag with an opening at both ends. When it is limp, winds are light; when it is stretched out, winds are strong. Pilots can quickly determine the wind direction and speed along a runway just by observing the shape and direction of a windsock.

          Sonic anemometers use sound waves humans cannot hear to measure wind speed and direction. The instrument determines the wind velocity by measuring the time between when the instrument sends a sonic pulse and when it is received.

          An anemometer looks like a weather vane, but instead of measuring which direction the wind is blowing with pointers, it has four cups so that it can more accurately measure wind speed. Each cup is attached to the end of a horizontal arm, each of which is mounted on a central axis, like spokes on a wheel. When wind pushes into the cups, they rotate the axis. The faster the wind, the faster the cups spin the axis.

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          Windmills usually face into the prevailing wind, but they can also be adjusted should the wind direction change. Some types of windmills can be completely rotated according to the wind direction; in others, the angle of the sails can be adjusted to receive the maximum amount of wind power. Some wooden sails have spring shutters that open and close according to the wind strength. If the wind gusts, the shutters open up, if it drops, they close. In this way, a constant wind force is maintained on the windmill sails.

          Up until recently, people still only had visual impressions of what a windmill is, often associating it with the past and particularly before the industrial revolution. Today, things have come full circle, if you will and there is now a growing demand for large, technologically advanced windmills across the world. The term wind energy or wind power describe the process through which wind turbines convert the kinetic energy in the wind into electrical energy by the use of generator.

         What this introductory guide seeks to do is describe the apparatus in layman’s terms and also outline how they work and what they were intended for originally and the purposes for which they are used today. We begin with a brief definition of what a windmill is.

          It was originally a structure with sails, much like that on pre-industrial ships, and was originally used to produce flour from corn. In order to do this, the wind would have to prompt the sails to turn. They were also originally built by master craftsmen.

         A dictionary definition explains it thus; it is a machine which is propelled by the wind from a horizontal shaft which extended onto sails. Windmills still used today, mainly in parts of the world which have traditionally relied on them, are powered by electricity or water.

       The dictionary expounds this definition further by relating it to a human physical exercise technique which replicates the symbolism and movement of the original windmill. It is also famously symbolic in Cervantes’ classic of Don Quixote. This definition reminds readers that the original mill was also used to pump water and generate power.

       In modern terms, the advanced windmill operates with just three blades mainly to generate sustainable sources of electricity and energy. Today, these windmills are also referred to as wind turbines.

      Winds are produced due to uneven heating of the atmosphere by the sun, the rotation of the earth and the irregularities of the earth’s surface. Wind flow patterns differ from place to place and are modified by bodies of water, vegetation, and differences in terrain. This next section explains briefly but accurately how windmills work. Sourcing more extensive information, readers will learn that understanding technical processes initiated in wind turbines will be easy to follow because the manner in which windmills work follows a simple process. Here we continue to rely on layman’s terms.

       A number of different options were tried when modern wind turbines were first built. Today, the universal mechanizing principle is to operate the turbine by using just three blades placed around a rotor which is connected to a shaft. Note that numbers of variations have been tried, two blades and even one blade. But, three blades work the best.

       As its name states, the windmill’s only source of energy is derived from the wind. The wind turns the blades which spins a shaft, in turn, prompt a generator to produce electricity. These blades are connected to a generator, sometimes through a gearbox and sometimes directly. In both the cases, the generator converts the mechanical energy into electrical energy. Interestingly, most modern turbines turn in a clockwise direction. Depending on wind speed, most modern turbines can operate at speeds from as little as four meters per second to as much as 15 mps.

       Quite a number of green energy advocates and NGO’s describe the wind-generator process more succinctly by correlating it closely with the environmental sustainability initiatives.

       Once the turbine’s blades turns a shaft located inside of a box placed on top of the turbine, gearbox mode is propelled and more speed rotation is given off. A transformer within the turbine then converts electricity into a voltage suitable for distribution to a national grid.

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          Land breezes occur at night, as the land cools down more quickly than the sea. The cold air sinking over the land pushes out to the low-pressure area over the sea. Land breezes tend to be lighter than sea breezes, as the difference in temperature between the sea and the land during the night is only slight. Land and sea breezes help make a coastal climate very different from that inland.

          Land breeze, a local wind system characterized by a flow from land to water late at night. Land breezes alternate with sea breezes along coastlines adjacent to large bodies of water. Both are induced by differences that occur between the heating or cooling of the water surface and the adjacent land surface. The land breeze is typically shallower than the sea breeze since the cooling of the atmosphere over land is confined to a shallower layer at night than the heating of the air during the day. Since the surface flow of the land breeze terminates over water, a region of low-level air convergence is produced. Locally, such convergence often induces the upward movement of air, fostering the development of clouds. Therefore, it is not uncommon to see clouds lying off the coast at night, which are later dissipated by the daytime sea breeze.

          The air over the ocean is now warmer than the air over the land. The land loses heat quickly after the sun goes down and the air above it cools too.  This can be compared to a blacktop road. During the day, the blacktop road heats up and becomes very hot to walk on. At night, however, the blacktop has given up the added heat and is cool to the touch. The ocean, however, is able to hold onto this heat after the sun sets and not lose it as easily. This causes the low surface pressure to shift to over the ocean during the night and the high surface pressure to move over the land. This causes a small temperature gradient between the ocean surface and the nearby land at night and the wind will blow from the land to the ocean creating the land breeze.

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          On a hot and sunny day, coastal areas will experience sea breezes. The land and the sea heat up and cool down at different rates, producing moving currents of air. The land heats more quickly than the sea, producing an area of low pressure, into which the cooler sea air moves. This breeze may move in a completely different direction from the prevailing wind and can blow up to 30km (18 miles) inland.

           A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes are more localized than prevailing winds. Because land absorbs solar radiation far more quickly than water, a sea breeze is a common occurrence along coasts after sunrise. By contrast, a land breeze or offshore breeze is the reverse effect: dry land also cools more quickly than water and, after sunset, a sea breeze dissipates and the wind instead flows from the land towards the sea. Sea breezes and land breezes are both important factors in coastal regions' prevailing winds. The term offshore wind may refer to any wind over open water.

          Wind farms are often situated near a coast to take advantage of the normal daily fluctuations of wind speed resulting from sea or land breezes. While many onshore wind farms and offshore wind farms do not rely on these winds, a near shore wind farm is a type of offshore wind farm located on shallow coastal waters to take advantage of both sea and land breezes. (For practical reasons, other offshore wind farms are situated further out to sea and rely on prevailing winds rather than sea breezes.)

Cause

          The sea has a greater heat capacity than land, so the surface of the sea warms up more slowly than the land's. As the temperature of the surface of the land rises, the land heats the air above it by convection. The warming air expands and becomes less dense, decreasing the pressure over the land near the coast. The air above the sea has a relatively higher pressure, causing air near the coast to flow towards the lower pressure over land. The strength of the sea breeze is directly proportional to the temperature difference between the land and the sea. If a strong offshore wind is present (that is, a wind greater than 8 knots (15 km/h)) and opposing the direction of a possible sea breeze, the sea breeze is not likely to develop.

Effects

          A sea-breeze front is a weather front created by a sea breeze, also known as a convergence zone. The cold air from the sea meets the warmer air from the land and creates a boundary like a shallow cold front. When powerful this front creates cumulus clouds, and if the air is humid and unstable, the front can sometimes trigger thunderstorms. If the flow aloft is aligned with the direction of the sea breeze, places experiencing the sea breeze frontal passage will have benign, or fair, weather for the remainder of the day. At the front warm air continues to flow upward and cold air continually moves in to replace it and so the front moves progressively inland. Its speed depends on whether it is assisted or hampered by the prevailing wind, and the strength of the thermal contrast between land and sea. At night, the sea breeze usually changes to a land breeze, due to a reversal of the same mechanisms.

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          The wind can make the air temperature feel colder than it actually is. A thin layer of warm air normally surrounds your body, creating an insulating "blanket" of air. If the wind is strong, this warm air gets blown away, making you feel a lot colder. This is known as the wind-chill factor. In a breeze blowing at 9km/h (5.6mph), an air temperature of 0°C (32°F) will feel like —3°C (27°F). If the breeze increases to around 15km/h (9.3mph), the wind-chill factor will make it feel like —10°C (14°F).

Meteorologists call this phenomenon the wind chill factor.

   Wind chill is what the air temperature feels like on our exposed skin due to wind. It's always lower than the actual air temperature.

   For example, even though the thermometer may indicate it's 35° F (1.6° C) outside, a 25-mile-per-hour wind will make it feel like it's only 8° F (-13.3° C)!

   The opposite effect can occur at temperatures above 50° F (10° C). At higher temperatures, humidity on the skin can make the air temperature feel hotter than the actual temperature. Meteorologists call this effect the heat index.

   It's important to note that wind chill is a prediction of what experts believe humans will perceive the temperature to be because of the wind. No matter how fast the wind blows, the air temperature is what it is and can be measured by a thermometer.

     The wind chill factor, on the other hand, is calculated using various formulas. There is no one formula that all scientists agree on. Most meteorologists in the United States use a standard formula accepted by the National Weather Service.

    What causes the wind chill effect? It's a result of the fact that the human body loses heat through a scientific process called convection.

     During convection, heated air molecules rise into the air and are replaced by cooler air molecules. How quickly your body loses heat by convection depends on air flow around your body.

     Your warm body usually loses heat slowly. When it's windy, though, the wind carries the warm air molecules away from your body more quickly, making you feel colder than the actual air temperature around you.

     The faster the wind blows, the faster your body loses heat by convection. As the air temperature around you falls, the effect of wind is magnified, making the wind chill effect greater the colder it gets.

     If you've ever blown on a hot bowl of soup to cool it down before eating, you've created your own wind chill effect on your soup!

     Even though the air temperature stays the same, the presence of wind makes us feel like it's colder outside than it actually is. The wind chill effect isn't all mental, though.

     Since wind chill speeds up heat loss by convection, our bodies experience heat loss and react as if the temperature were as low as it feels…even if the actual air temperature is much higher than the wind chill factor.

     Wind chill factors are calculated under the assumption that a person is properly dressed and dry. If you're not wearing suitable outdoor clothing, if your clothes are wet, or if you've been outside for an extended period of time, the wind chill factor will be magnified even further.

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          Prevailing Winds are those that blow constantly in certain parts of the world. They are produced by hot air moving north and south from the Equator and by cold air moving away from the poles. The prevailing winds are the Polar Easterlies, found in the extreme north and south; the Westerlies, blowing between 30° and 60° north and south of the Equator; and the Trade winds, which blow north-east and south-east, either side of the Equator.

          Since the atmosphere is fixed to the earth by gravity and rotates with the earth, there would be no circulation if some force did not upset the atmosphere's equilibrium.  The heating of the earth's surface by the sun is the force responsible for creating the circulation that does exist.

          Because of the curvature of the earth, the most direct rays of the sun strike the earth in the vicinity of the equator resulting in the greatest concentration of heat, the largest possible amount of radiation, and the maximum heating of the atmosphere in this area of the earth.  At the same time, the sun's rays strike the earth at the poles at a very oblique angle, resulting in a much lower concentration of heat and much less radiation so that there is, in fact, very little heating of the atmosphere over the poles and consequently very cold temperatures.

          Cold air, being more dense, sinks and hot air, being less dense, rises.  Consequently, the rising warm air at the equator becomes even less dense as it rises and its pressure decreases.  An area of low pressure, therefore, exists over the equator.

          Warm air rises until it reaches a certain height at which it starts to spill over into surrounding areas.  At the poles, the cold dense air sinks.  Air from the upper levels of the atmosphere flows in on top of it increasing the weight and creating an area of high pressure at the poles.

          The air that rises at the equator does not flow directly to the poles. Due to the rotation of the earth, there is a buildup of air at about 30° north latitude. (The same phenomenon occurs in the Southern Hemisphere).   Some of the air sinks, causing a belt of high-pressure at this latitude.

          The sinking air reaches the surface and flows north and south.  The air that flows south completes one cell of the earth's circulation pattern.  The air that flows north becomes part of another cell of circulation between 30° and 60° north latitude.  At the same time, the sinking air at the North Pole flows south and collides with the air moving north from the 30° high pressure area.  The colliding air is forced upward and an area of low pressure is created near 60° north.  The third cell circulation pattern is created between the North Pole and 60° north.

          Because of the rotation of the earth and the coriolis force, air is deflected to the right in the Northern Hemisphere.  As a result, the movement of air in the polar cell circulation produces the polar easterlies.   In the circulation cell that exists between 60° and 30° north, the movement of air produces the prevailing westerlies.  In the tropic circulation cell, the northeast trade winds are produced.  These are the so-called permanent wind systems of the each. 

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          The moving Air that produces the winds tries to take the most direct route possible between the different areas of pressure. However, it is deflected by the rotating movement of the Earth. This is known as the Coriolis effect. In the Northern Hemisphere, the winds are deflected to the right of the direction in which they are headed; in the Southern Hemisphere, they are deflected to the left.

          The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around  Earth. The Coriolis effect is responsible for many large-scale weather patterns.

          The key to the Coriolis effect lies in Earth’s rotation. Specifically, Earth rotates faster at the Equator than it does at the poles. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour.

          Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up.

          Now let’s pretend you’re standing at the North Pole. When you throw the ball to your friend, it will again to appear to land to the right of him. But this time, it’s because he’s moving faster than you are and has moved ahead of the ball.

          Everywhere you play global-scale "catch" in the Northern Hemisphere, the ball will deflect to the right.

          This apparent deflection is the Coriolis effect. Fluids traveling across large areas, such as air currents, are like the path of the ball. They appear to bend to the right in the Northern Hemisphere. The Coriolis effect behaves the opposite way in the Southern Hemisphere, where currents appear to bend to the left.

          The impact of the Coriolis effect is dependent on velocity—the velocity of Earth and the velocity of the object or fluid being deflected by the Coriolis effect. The impact of the Coriolis effect is most significant with high speeds or long distances. 

Weather Patterns

          The development of weather patterns, such as cyclones and trade winds, are examples of the impact of the Coriolis effect.

          Cyclones are low-pressure systems that suck air into their center, or “eye.” In the Northern Hemisphere, fluids from high-pressure systems pass low-pressure systems to their right. As air masses are pulled into cyclones from all directions, they are deflected, and the storm system—a hurricane—seems to rotate counter-clockwise.

          In the Southern Hemisphere, currents are deflected to the left. As a result, storm systems seem to rotate clockwise.

          Outside storm systems, the impact of the Coriolis effect helps define regular wind patterns around the globe. 

          As warm air rises near the Equator, for instance, it flows toward the poles. In the Northern Hemisphere, these warm air currents are deflected to the right (east) as they move northward. The currents descend back toward the ground at about 30° north latitude. As the current descends, it gradually moves from the northeast to the southwest, back toward the Equator. The consistently circulating patterns of these air masses are known as trade winds.

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          The wind is created by differences in air pressure and temperature —winds blow from areas of high pressure to those of low pressure. Rising warm air creates a low-pressure area, and the gap created is filled by high pressure produced by cooler air. The greater the difference in pressure, the stronger the wind.

          Bob Dylan says, “You don’t need to be a weatherman to know which way the wind blows”. I say to Bob, “But, it doesn’t hurt!”

          Wind is a rather elusive meteorological variable, especially since we can’t really see it, like we can clouds or precipitation. Wind, during a storm, is something we expect. Wind can be an unpleasant nuisance though, especially on a bluebird day, to cyclists, sailors, paragliders, climbers, etc.

          The atmosphere is constantly adjusting itself, trying to balance the changes in temperature and humidity from one part of the planet to the other. This leads to different areas of high and low pressure that encircle the globe, and the bigger the difference in temperature, and/or humidity, from one area to another, the bigger the difference in pressure, and the faster the wind blows.

          That’s what gets it started in motion, always moving from high pressure towards lower pressure. Friction at the surface, mountains, buildings, etc. can slow the wind down and alter its direction. In the upper levels of the atmosphere, the wind starts moving from high to low, but it gets re-routed, and turned to the right in the northern hemisphere, because the earth is rotating. This is known as the Coriolis Effect.

          When we observe stronger winds, it means that there is a big difference in pressure across the region, or sometimes across the entire country. A big low-pressure center over the mid-western U.S. and a big area of high-pressure along the West Coast, for instance, could result in strong winds in-between, over the Rockies.

          That difference in pressure from Point-A to Point-B is known as a pressure-gradient. A strong pressure-gradient equals strong winds. You can track that each day by looking at a surface weather map, and look for big highs and big lows, and lots of pressure contour lines in-between, as well.

          The other thing that can cause strong winds at the surface is when the jet stream is directly overhead.

Example: The air pressure is higher in an inflated balloon than outside it. If a hole is made in the balloon, the air streams out, creating a wind that blows from the greater pressure in the direction of the lower. The wind settles when the pressure is the same inside the balloon as outside.In the atmosphere the pressure at the earth's surface reflects the weight of air above it, which in turn is determined mostly by its temperature, and as people generally know from everyday life, hot air is lighter than cold. This fits with the fact that depressions (low pressure systems) usually bring warm air.

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          A cold front is followed by an area of cold air. Thick, dark clouds, heavy rain and sometimes violent storms arrive immediately. If seen from the side, a cold front looks much steeper than a warm front. Cold air pushes beneath the warm air and rising water vapour condenses into clouds and then rain. Showers of rain will often follow as the front passes over.

          The cold front is depicted on a weather map as a blue line with triangles or as simply a blue line (Figure 9.28). A cold front, as discussed in the previous section, is the leading edge of colder air brought southward by winds around an area of low pressure. These fronts are most common during the active weather times of fall, winter, and spring.

          Winds ahead of the cold front are southwesterly in the warm sector of the mid-latitude cyclone. After the cold front passes a point, winds turn to the west, northwest, or north. Since the cold air is very dense it is very effective at displacing the warm air ahead of it. The dense cold runs under the warm air lifting it. The lifting of warm moist air usually causes cloudiness at the least. If the air is moist and unstable enough, rain and thunderstorms can accompany the passage of the front. Air pressure usually falls as a cold front approaches, rising rapidly after passage as the dense cold air moves in. The dew point falls indicating the change to a dry air mass. Usually there is little local observational evidence of a cold front approaching.

          A cold weather front is defined as the changeover region where a cold air mass is replacing a warmer air mass. Cold weather fronts usually move from northwest to southeast. The air behind a cold front is colder and drier than the air in front. When a cold front passes through, temperatures can drop more than 15 degrees within an hour.

          On a weather forecast map, a cold front is represented by a solid line with blue triangles along the front pointing towards the warmer air and in the direction of movement.

          There is usually an obvious temperature change from one side of a cold front to the other. It has been known that temperatures east of a cold front could be approximately 55 degrees Fahrenheit while a short distance behind the cold front, the temperature can go down to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

          Again, there is typically a noticeable temperature change from one side of the warm front to the other, much the same as a cold front.

          If colder air is replacing warmer air, it is a cold front, if warmer air is replacing cold air, then it is a warm front.

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