Wind physics

1. Wind physics How are winds created Global wind changes Seasonal wind changes

2. How are winds created • The wind is the flow of gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. • The wind happens due to difference in air pressure between two neighboring areas . • What causes the difference in air pressure ?

3. Different solar energy absorption rates at different climate zones

4. How does the air flow ? • The air doesn’t flow in straight line from high pressure area to low pressure area , it actually influenced by the earth circulation around it self which generates a coriolis force which makes circulates making vortices . • Except for air pressure disturbances that accur when low air pressure and high air pressure are close such as thunder storms .

5. Wind flow pattern

6. Wind strength • The closer the high and low pressure areas are together, the stronger the "pressure gradient", and the stronger the winds. • The higher difference in pressure the stronger the wind. • . On weather maps, lines of constant pressure are drawn (as in the example, above) which are called "isobars". These isobars are usually labeled with their pressure value in millibars (mb). The closer these lines are together, the stronger the wind. 

7. Wind speed • The curvature of the isobars is also important to the wind speed. Given the same pressure gradient (isobar spacing), if the isobars are curved anticyclonically (around the high pressure in the above example) the wind will be stronger. If the isobars are curved cyclonically (around the low pressure in the example above) the wind will be weaker

8. Wind speed • Near the surface of the Earth, friction from the ground slows the wind down. During the day, when convective mixing is stirring up the lower atmosphere, this effect is minimized. At night, however, when convective mixing has stopped, the surface wind can slow considerably, or even stop altogether . • Convective mixing is the vertical movement of air .

9. Fastest ever recorded • SPEED The highest wind speed ever recorded on the surface of the Earth was 231 mph on April 12 1934, atop Mt. Washington, New Hampshire.

10. Global wind patterns • Polar Easterlies: At about the latitude of Norway and northward (60-90 degrees) the Polar easterliese blow irregularly from the east and north. • Westerlies: At about the latitude of Western Europe and the United States (30-60 degrees) the Westerliese blow from the west, tending somewhat toward the north. This causes most weather in the United States to move from west to east.

11. Global wind patterns • Trade Winds: South of about 30 degrees the northern or northeast trade winds blow mostly from the northeast toward the equator. These were the sailor's favorite winds since the weather was warm and the winds usually blew steadily in an advantageous direction. Columbus used these to sail to the Caribbean

12. Global wind patterns • Southern hemisphere: In the southern hemisphere the belts are reversed. The southeast trade winds blow from the southeast toward the equator. The southern equivalent of the horse latitudes (or Variables of Cancer) is called the Variables of Capricorn. The southern westerlies start somewhat south of South Africa. They tend to be stronger than the northern westerlies because they are mostly over water (roaring forties). The southern polar easterlies are mostly over Antarctica.

13. Global wind patternsThe six major wind belts .

14. Wind patterns with time • Seasonal shifts: All of the belts move north during the northern summer and south during the northern winter. Because global heating and cooling lags behind the position of the sun they reach their northernmost latitude at or after the end of the northern summer. This brought the trade winds within reach of the Spain and Portugal and determined the sailing time of the Spanish treasure fleet. The northernmost position of the wind belts corresponds to the Atlantic hurricane season.

15. Wind patterns with time • Land and sea breezes: Land gains and loses heat more rapidly than water. During the day the land warms more rapidly than the water. The air above land warms, becomes thinner and rises drawing cooler air landward from the sea. At night the process reverses and cool heavy air from the land flows out to sea. These land and sea breezes are important along the coast.

16. Wind patterns with time • Monsoon: The annual equivalent of the daily land and sea breezes is the yearly monsoon. During summer the continents heat more rapidly than the oceans. Air over the continents warms, thins and rises drawing cooler moist ocean air landward, producing a wet season. During winter the process reverses and cold, dry heavy air flows outward from the continents, producing a dry season. The monsoon is most striking in south Asia because of the size of the Eurasian landmass and because the Himalayas tend to bottle up the air above the continent. Approximations of the Indian monsoon exist in other places but they are poorly developed.

17. Coriolis force and Haldley cell • The Coriolis force : In the northern hemisphere the Coriolis effect causes wind and water currents to bend to the right (clockwise). Cold heavy air flows south from the north pole and is bent west, forming the polar easterlies. Warm air rises at the equator drawing air from the north which bends to the west, contributing to the trade winds. The Corioliseffect bends the westerlies and trade winds slightly clockwise in the northern hemisphere.

18. Coriolis force and Haldley cell • Hot air rises at the doldrums. As it rises it cools producing thunderstorms. The dry air flows north at a high altitude and descends at the horse latitudes and flows back to the equator with the trade winds. This is called the Hadley cell. There is also a Ferrel cell over the westerlies and a polar cell over the pole. There are other complexities, not all of which are properly understood.