Air Pressure and Wind

Goal: Explain the formation of wind based on differences in air pressure

What is Air Pressure?

• Reminder: Air pressure is thickest near Earth’s surface and becomes thinner as we move up towards the exosphere

• Air pressure is the pressure you feel from surrounding atmospheric gases.

• To be technical it is the pressure exerted by the weight of the air above.

• At sea level, the average weight of the air is 1 kilogram per square centimeter

Why don’t we feel air pressure?

• If air pressure is force that pushes on you, then why don’t we collapse as a result of all of that pressure?!

• Air pressure is exerted in all directions ( down, up, and sideways)

• This means that the air pressure pushing down on an object perfectly balances the air pressure that is pushing up on an object

Measuring Air Pressure

• Air pressure is measured in millibars. (Just like most temperature is measured in degrees)

• Sea level pressure is 1013.2 millibars ( just like body temperature is 97 degrees,

• Standard Air pressure is typically described as high or low

• Air pressure is measured using a barometer (just like temperature is measured using a thermometer)

Mercury Barometer • A tool used to

measure air pressure

• When air pressure increases, the mercury in the tube will rise.

• So when air pressure decreases, the mercury in the tube will sink.

WIND

• Goal: Explain the relationship between air density and wind

Wind and Air Pressure

• We recently learned about air pressure.

• In order to understand what wind is, we need to understand how wind is created.

How does wind form?

• Wind is created when there are horizontal differences in air pressure! High→Low

• Air will always flow from areas of higher pressure to lower pressure and that creates ...WIND

• Example: If you open an air tight container, you feel a ‘pull’ because the air is rushing from high pressure (outside of the container) to low pressure (inside of the container).

Unequal heating = Wind

• Basically, wind is nature’s way of balancing out the differences in air pressure

• Larger scale explanation: The unequal heating of Earth’s surface creates pressure differences, which will cause wind.

• Therefore, Solar radiation is the ultimate energy source for most wind.

Measuring Wind

• Wind is measured by the directions from which it flows and its speed

• Wind direction-

• Speed-

• pg. 545

• Anemometer- a tool used to measure wind

• Goal: Explain interacting air masses control wind patterns

Pressure differences control wind speed

• Reminder: wind is created from differences in air pressure.

• Large difference in air pressure = high wind speed (ex: 1013mb. to 1025mb)

• Small difference in air pressure = low wind speed (ex: 1013.2mb to 1014.2mb)

Mapping air pressure and wind

• On a weather map, scientists use barometric readings to predict wind speed. How do you think this works?

• Isobars- are lines on a map that connect places of equal air pressure (iso=equal, bar=pressure)

• The spacing of the isobars will indicate wind speed this is called the pressure gradient

Pressure gradient determines wind

• Pressure gradient= a term used to describe changes in air pressure on a weather map (based on isobar spacing)

– Closely spaced isobars indicate a steep pressure gradient and high winds.

– Widely spaced isobars indicate a weak pressure gradient and light winds

Steep = fast speed! Weak (gentle)= slow speed

Wide space Close space

• Use this to remember:

• Closely spaced = large difference = high wind speed (example= riding a bike on a steep hill)

• Widely spaced = small difference = low wind speed (example: riding a bike on a gentle hill)

PRESSURE CENTERS

Pressure centers

• Winds will blow around areas of high pressure and low pressure.

• How fast the wind moves depends on the amount of pressure in the center of the area.

• Classification =low pressure centers and high pressure centers

Highs and Lows

• A center of low pressure is called a cyclone = BAD

• A center of high pressure is called an anticyclone = GOOD

Cyclone= Low Pressure Center

In a cyclone (L) = the air pressure will decrease from the outer isobars towards the center.

Pressure is lowest in the center of a cyclone

(because warm air is rising away)

Anticyclone= High Pressure Center

In an anticyclone (H)= the air pressure will increase from the outer isobars towards the center.

Pressure is highest in the center of an anticyclone- (cold air sinks)

Global vs Local Winds

Global Winds

• Reminder- Earth’s surface is not heated equally. More radiation is absorbed at the poles than the equator.

• Therefore: The atmosphere balances these differences by acting as a giant heat transfer system.

• Warm air rises up to the poles and cool air sinks down toward the equator

Coriolis Explained: Northern Hemisphere vs. Southern Hemisphere

Objects are deflected to your

right right

Objects are deflected to your

left

R L

R L

What controls the movement of global winds?

• Coriolis Effect- This effect describes how Earth’s rotation impacts all moving objects- including wind. Winds are deflected to the right of their path of motion in the N. Hemisphere (opposite in S. Hemisphere)

• Watch this video –> Coriolis Effect

• The coriolis effect only changes wind direction, but not the speed

Global Winds

• Trade Winds

• Westerlies

• Polar Easterlies

• Polar Front

Tradewinds • Trade Winds- 2 belts of winds that blow almost

constantly from the east.

• They originate between subtropical highs and the equator-Warm air that rise towards the poles

West vs. East • Westerlies -make up the dominant west to

east motion of our atmosphere

• Easterlies- are winds that blow from the polar high towards the subpolar low

Local Winds

Local Wind Formation

• Local winds are caused by:

– Topographic effects

– Differences in land vs. water

Land vs. Sea Breezes

• Sea Breeze=In coastal areas during the warm summer months, the land surface is heated more intensely during the daylight hours than the ocean

• As a result, the air above the land surface heats, rises, and expands. The cool air above water rushes into to take its

• At night the reverse takes place= Land Breeze

Sea Breeze

Land Breeze

Valley vs. Mountain Breezes

• During the day, the sun heats up valley air rapidly.

• Convection causes the warm air to rise, causing a valley breeze.

• At night, the process is reversed. Mountain air cools rapidly at night and "falls" downslope, causing a mountain breeze.

Valley Breeze Mountain Breeze