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 Weak (gentle)= slow speed Steep = fast 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 L R R L Objects are deflected to your left
What controls the movement of global winds? Coriolis Effect- This effect describes how Earth s rotation impacts all moving objectsincluding 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