WIND Wind is movement of air caused by differences in air pressure. The greater the difference, the faster the wind moves. Air flows from areas of high pressure to areas of low pressure. Differences in air pressure are caused by the uneven heating of Earth (because Earth is curved). Ultimate energy source for most wind is the Sun. Anemometer used to measure wind velocity. Wind vane is used to measure wind direction.

Air Pressure Highest at sea level and decreases with increasing altitude. Why don’t we feel the tons of air pressing on us? Why don’t objects collapse under the weight of the air above them? Air pressure is measured using a barometer. As air pressure increases the mercury rises in the barometer. The Standard atmospheric pressure at sea level is 29.92 inches.

Barometers Aneroid barometer Mercury barometer

Factors affecting wind Air Pressure differences Coriolis effect Friction

Pressure Belts Uneven heating produces pressure belts which occur every 30° latitude. As warm air rises at the equator and moves towards the poles it cools. As it cools some of the air sinks around 30° north and south of the equator. At the poles, cold air sinks and moves towards the equator. Around 60° north and south, the air begins to heat up and rise.

How does coriolis affect wind/atmospheric circulation? In the northern hemisphere, wind is deflected to the right (clockwise). In the southern hemisphere, wind is deflected to the left (counterclockwise).

If the earth was non-rotating there would be no coriolis effect so the atmosphere would circulate in one large cell in each hemisphere. Hot surface air at the equator would rise and flow toward the poles. Cold air at the poles would sink and flow toward the equator.

GLOBAL WINDS Because the earth is rotating, coriolis effect causes global circulation to be divided into three cells. Trade winds: blow between 30° north/south and the equator. They curve to the west. Westerlies: blow between 30° and 60° north and south. These winds curve to the east. Polar Easterlies: blow between the poles and 60° north and south. These winds curve to the west.

Rotating Earth Model

ISOBARS

ISOBARS Isobars are lines on a map connecting places of equal air pressure. Closely spaced isobars indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure gradient and light winds.

Jet Streams They are narrow belts of high speed winds that blow in the upper troposphere and lower stratosphere. (between 7 km and 16 km above Earth’s surface) Jet stream flow parallel to isobars. They blow from west to east at average speeds between 97 and 185 km/hr. They follow boundaries between hot and cold air. Airplanes travelling in the same direction as the jet stream go faster than those travelling in the opposite direction of a jet stream.

Cyclone: low pressure centers (pressure decreases towards the center); winds blow counter clockwise Anticyclone: high pressure centers (pressure increases towards center); winds blow clockwise