Air Pressure and Wind

Air Pressure and Wind

• Changes in air pressure cause air to move in order to try to equalize the pressure

• The moving air causes wind• When air circulates inward from all directions

toward low pressure it forces air to rise in the sky to make room for incoming air

• This rising air is what causes clouds and storms

Air circulates outward from high pressure then air moves down (low pressure moving in) and we often have clear skies.

Three forces affect winds

• Air pressure• Friction• Coriolis Effect

Air Pressure

– Bigger the difference in air pressure the stronger the wind blows

– Remember air pressure is lower closer to the surface of the earth because the air contains more molecules due to the pull of earths gravity

– Changes in vertical air pressure much greater than changes in horizontal air pressure

– Horizontal pressure changes may only be 5% but this is enough to make winds blow up to hundreds of miles

Friction

• When air is blowing and bumps into something its path must be diverted and this causes turbulence

• The effects of friction on air extend about 2/3 of a mile up into the atmosphere

• Air above the friction layer is called the free atmosphere or the air aloft

• How does the landscape of the land influence wind? More wind in the city or the country?

• Turbulence when flying especially over mountains

Coriolis Effect

• Change in direction of the wind due to earths rotation

• Wind in the northern hemisphere curves to the right and in the southern hemisphere to the left

• This causes winds to circulate clockwise around high pressure and counter clockwise around low pressure in the northern hemisphere and the opposite in the southern hemisphere

• Why our weather generally moves from west to east

The Westerlies

• The winds that typically blow from west to east across the middle latitudes in the northern hemisphere

• Normally the winds would blow from the poles to the equator because the upper air would travel toward the poles

• The Coriolis effect causes the wind to bend at the middle latitudes

The trade winds

• At about 30 degrees north and south of the equator the air pressure combined with the earths rotation causes the winds to circulate clockwise around high pressure north of the equator and counter clockwise south of the equator

• So the wind blows from east to west in both hemispheres

• These winds are called the trade winds• Reliable, steady winds that sailors used to

navigate

The Doldrums

• Area near the equator where air pressure is constant and the Coriolis effect is minimal

• Very still with little wind• Ships could go nowhere

The Horse Latitudes

• Area just north of the equator where there was no wind

• Sailors could not go anywhere and would fear starvation

• Horses were common cargo and were either thrown over board or eaten

• Came to be known as the horse latitudes• Just about at 30 degrees north

Coastal breezes

• Air over water is at different temp than air over land

• During the day air over land is warmer and at night air over water is warmer

• This causes breezes to blow out to water during the day and in from water during the evening

Valley and Mountain breezes

• Air in valleys heat up more rapidly during the day causing winds to blow up the slopes of mountain

• At night mountains cool more intensely and the denser air sinks back down the slope as a mountain breeze

•

Chinooks

• Occur in the Rockies• Westerly winds of the winter and spring encounter a north

south mountain range they push the air up the westerly slopes often causing condensation such as rain or snow.

• As air reaches the mountain top air is drained of its moisture and slides down the eastern slope warming and becoming dense

• This creates very strong winds called Chinooks• When the reach the cold and snowy Great Plains they can

cause shocking rises in temperatures. • Within minutes they temp can climb more than 40 degrees• Sometimes called snow eaters

Haboobs

• Whirlwinds or dust devils• Hot rising winds begin spinning in local turbulence• Occur with violent thunderstorms over sandy deserts• Winds derive their name form the Arabic word habb

for a wind causes by a downdraft• Causes tons of sand and dust to become airborne• Creates dense dark, cloud that extends for miles and

can completely engulf a desert town or city• Most common in the Africa Sudan and the

southwestern desert of the US