Unit 3-4: Air Pressure. The Study of Air Pressure What is it? Air pressure is the weight of the air in the atmosphere pressing down. Pressure in general

Unit 3-4:Air Pressure

Unit 3-4:Air Pressure

The Study of Air PressureThe 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.

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.

How to Measure Air Pressure


BarometerTool 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.

BarometerTool 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.



Mercury BarometerA 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.

Mercury BarometerA 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.

Units of Air PressureUnits of Air Pressure

There are two different units of air pressure used.

The first is given in inches of mercuryStandard pressure is 29.92 inches of

mercuryThe second is in millibars

Standard pressure is 1013.2 mb

There are two different units of air pressure used.

The first is given in inches of mercuryStandard pressure is 29.92 inches of

mercuryThe second is in millibars

Standard pressure is 1013.2 mb

IsobarsIsobars

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.

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.

Why air pressure changesWhy 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.

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.


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.

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.


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.

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.


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.

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.

High and Low PressureHigh 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

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

WindsWinds

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.

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.

Plotting WindsPlotting 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.

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.


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.

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.


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.

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.

Characteristics of WindCharacteristics 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.

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.


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.

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.


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.

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.

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Unit 3-4: Air Pressure. The Study of Air Pressure What is it? Air pressure is the weight of the air in the atmosphere pressing down. Pressure in general