Download Air Pressure Differences and Breezes

Air Pressure and Wind Notes
Air Pressure is the weight of the gases in the atmosphere pushing on the surface
of the Earth.
At sea level, standard air pressure is 14.7 lb/in2 or 1013.2 mb or 29.92 in Hg.
Air Pressure is measured with a Barometer.
Two types: Aneroid and Mercury
Why Does Air Pressure Change?
1. Elevation
2. Humidity
3. Temperature
1. Elevation – As elevation increases, the pressure decreases since there is less air pushing on
you from above
- The greatest change in air pressure occurs in the lower atmosphere (troposphere)
2. Humidity – As humidity increases, pressure decreases because water molecules are less
massive than air.
3. Temperature – As temperature increases, pressure decreases because the molecules are
further apart.
How is air pressure shown on a weather map?
 Air pressure can be analyzed on a weather map using isobars – lines that connect points
of equal air pressure.
 The pressure difference between each isobar always increases or decreases by the same
increment.
 On the map pictured, each line differs in pressure by 4 millibars.
 Isobars on a map are used to determine the location of pressure system and act as a way
to forecast weather.
Drawing & Reading Isobars
 Every point along the top isobar, the pressure is 996mb
 Every point along the bottom isobar, the pressure is 1000mb
 Points above the 1000 mb isobar have a lower pressure
 Points below that isobar have a higher pressure.
A Pressure Gradient is a change in pressure between two
places.
This creates a force that makes air move from areas of high
pressure to areas of low pressure (aka WIND)
Wind speed and direction can be determined by calculating the pressure gradient.
- [Change in air pressure] ÷ [distance]
Units: mb/mile
- The closer the isobars are together, the greater the gradient, the faster the winds.
Air Pressure Differences and Breezes
Differences in the heating of land and water creates different pressure systems.
- Land heats up and cools faster than water.
- On a local level, this difference in heating creates Breezes.
During Day = Sea Breeze
During Night = Land Breeze
Air Pressure and Wind Notes
How does air move around areas of High pressure compared to Low pressure?
View from side
VERTICAL MOTION
View from above
SURFACE WIND MOTION
Characteristics of High and Low Pressure Systems
High Pressure




Cool temperatures
Rising barometer
Fair weather
Air moves out from center - clockwise
Low Pressure




Warm temperatures
Falling barometer
Poor weather
Air moves toward centercounterclockwise
NOTES: CORIOLIS EFFECT & GLOBAL WIND PATTERNS
Measuring Wind Speed and Wind Direction
Anemometer - used to measure wind speed
Wind Vane - measures wind direction.
Wind and water appear to travel in curved paths because of the CORIOLIS EFFECT.
Coriolis Effect – is caused by the rotation of the Earth.

Wind and water are deflected to the right in the Northern Hemisphere and to the left in
the Southern Hemisphere.
Try it!
Draw a straight line in the blank space below beginning at the bottom of the page towards the
top. Do it again but this time twist your paper to the left while drawing. What happens?
GLOBAL PRESSURE BELTS –are caused by the uneven heating of the Earth which results in
high and low pressure areas.
GLOBAL WINDS are the result of air moving from high pressure belts to low pressure belts.
-Air always moves from high to low pressure.
-Remember, winds are named for the direction they are coming from.