1. Unit 3-4: Air Pressure

2. The Study of Air Pressure
What is it?
Air pressure is the weight of the air in the atmosphere pressing down.
Pressure in general is force applied over an area.
Why is it important?
Air pressure affects winds and precedes weather changes.

3. How to Measure Air Pressure
Barometer
Tool for measuring air pressure.
Two different types of barometer:
Aneroid –
Uses a thin metal can with a pen attached to a long arm.
As the air pressure around the can changes, the pen moves up or down.
Works in a similar way to the metal thermometer.

5. How to Measure Air Pressure
Mercury Barometer
A thin column of glass houses mercury.
Mercury is loose in a pan at the bottom of the glass column.
As air pressure changes, the level of the mercury column inside the glass will rise or fall.
At sea level, the column of mercury will be 76 cm tall.

7. Units of Air Pressure
There are two different units of air pressure used.
The first is given in inches of mercury
Standard pressure is inches of mercury
The second is in millibars
Standard pressure is mb

8. Isobars Isobars are a way of showing air pressure on a weather map.
They are similar to isotherms in that they connect all the points of equal pressure.
Useful in determining the distribution of air pressures over areas and the changes in pressure over time.
The pressure is printed on each isobar.
The L indicates a localized low pressure area
An H would indicate a localized high pressure area.

9. Why air pressure changes
If you were to observe a barometer for several day, you would see the pressure change. Why? There are several reasons:
Temperature affects air pressure.
The warmer air gets, the less dense it is and the more space it takes up.
This increase in air temperature results in lower air pressure at the ground.
This is also the principle that allows hot air balloons to work.

10. Why air pressure changes
The second reason is that the level of water vapor in the air changes from day to day.
The amount of water vapor in the air is known as humidity.
Water vapor is less dense than the different gases that make up air,
So the more water vapor there is in the air, the lower the pressure.

11. Why air pressure changes
Warm humid air is lighter than cool dry air.
Temperature and humidity are the two biggest factors in air pressure changes.
Meteorologists observe that falling barometer readings will indicate warmer weather and more humid air.

12. Why air pressure changes
Meteorologists also observe that rising barometer readings will indicate cooler temperatures, and less humid air.
The changes in air pressure can indicate weather patterns.
Falling barometer readings may indicate precipitation, such as rain or snow.

13. High and Low Pressure
On a weather map, isobars close around each other. Either the pressure is going up or going down as the circles get smaller.
The highest pressure reading is referred to as a pressure high, and the lowest is referred to as a pressure low.
Pressure gradient is the rate of how quickly the pressure changes over an area. Indicated by how close the isobars are to each other.
Pressure gradient = (Pressure 2 – Pressure 1) Distance between

15. Winds
What is wind?
Wind is the movement of air from an area of high pressure to an area of low pressure.
This is an attempt to maintain equilibrium.
As the air pressure over land decreases in summer since the air is getting warmer, the high pressure air over the ocean will move towards land.

16. Plotting Winds
Meteorologists can plot the path and strength of wind by looking at an isobar weather map.
The winds will move from the high pressure areas to the low pressure areas across the isobars.
The winds will be stronger in the areas where the isobars are close together.

17. Plotting Winds
At the Earth’s surface, the winds do not travel in a straight line across the isobars.
They will cross the isobars and turn due to the Coriolis force.
The Coriolis force is an effect felt by all objects moving towards or away from the equator due to the Earth’s rotation.

18. Plotting Winds
In the northern hemisphere, winds will turn to the right as they cross isobars.
In the southern hemisphere, winds will turn left as they cross the isobars.
Remember, winds travel from high pressure areas to low pressure areas.

19. Characteristics of Wind
Wind direction is based on the direction it comes from.
A weather vane is a tool that shows direction of wind, it point to the direction the wind is coming from.
Speed of wind varies with the difference in pressure and friction.

20. Characteristics of Wind
The greater the difference between the pressure levels, the faster the wind.
As the wind rises away from the surface, friction decreases and speed rises.
As altitude reaches about 1000m, the winds run along the isobars, instead of across them.
Wind speed is measured with an anemometer.

21. Characteristics of Wind
Winds are rarely steady at the surface.
They often blow in sudden bursts called gusts.
Gusts are mainly due to rough surfaces on land.
Winds are usually measured in miles per hour or in knots.
One knot is about 1.15 miles per hour.