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 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
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
• 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 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
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 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
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.