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# Air Pressure and Wind PowerPoint PPT Presentation

Air Pressure and Wind. Air Pressure. Air Pressure – the weight of the gases in the atmosphere pushing on the surface of the Earth . Air Pressure. At sea level, standard air pressure is 14.7 lb/in 2 or 1013.2 mb or 29.92 in of Hg. The Weight Of The Atmosphere On Our Shoulders .

Air Pressure and Wind

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### Air Pressure

• Air Pressure – the weight of the gases in the atmosphere pushing on the surface of the Earth.

### Air Pressure

• At sea level, standard air pressure is 14.7 lb/in2 or 1013.2 mbor 29.92 in of Hg

### The Weight Of The Atmosphere On Our Shoulders

• At sea level, the weight of the column of air above a person is about 9,800 newtons (2,200 pounds)!

• This is equal to the weight of a small car.

• Why aren’t we crushed by this pressure?

### How does air pressure change with altitude?

• Atmospheric pressure is a measurement of the force due to air molecules at a given altitude.

• The pressure of the atmosphere decreases as altitude increases.

This plastic bottle was sealed at 14,000 feet on top of Mauna Kea, and was crushed by the increase in atmospheric pressure (at 9,000 feet and 1,000 feet) as it was brought down towards sea level.

### Why does air pressure change with altitude?

• Remember, air pressure is the weight of the atmosphere pushing down from above.

• The higher you go in the atmosphere the smaller the column of air that is pushing down from above.

### How is air pressure measured?

Air pressure is measured with a Barometer

There are Two Types of Barometers:

Mercury Barometer

Aneroid Barometer

### How Does Air Pressure Change With Altitude?

• In the following activity you will investigate how air pressure changes with altitude. You will also determine the air pressure at various altitudes above sea level.

### Record & Average Sea-Level Pressures

• Air pressure can be analyzed on a weather map using Isobars – lines that connect points of equal air pressure.

• Each isobar differs in pressure by 4 millibars.

### Why Does Air Pressure Change?

• So far we know the air pressure decreases with altitude because there are less air molecules pushing down from above.

• It also changes with temperature & humidity.

### Characteristics of High and Low Pressures

• Warm temperatures

• Falling barometer

• Poor weather

• Air moves toward center- counterclockwise

• Cool temperatures

• Rising barometer

• Fair weather

• Air moves out from center - clockwise

Light Winds

Strong Winds

• Pressure Gradient – a change in pressure between two places. This creates a force that makes wind move from areas of high pressure to areas of low pressure

### What causes wind & pressure differences?

Differential heating of land and water –

• Differences in the heating of land and water creates different pressure systems.

• Land heats up and cools off faster than water.

• Land temperatures change quickly while sea temperatures change slowly.

Sea Breeze

### On a local level, this difference in heating creates breezes.

Land Breeze

Sea/land breeze animation

Valley Breeze

Mountain Breeze

### Wind and water appear to travel in curved paths because of the CORIOLIS EFFECT.

• Caused by the rotation of the Earth.

• Wind and water are deflected to the right in the Northern Hemisphere

• to the left in the Southern Hemisphere

[Coriolis Effect animation]

The rotation of the Earth also causes the rotation that is seen with tornadoes and hurricanes

[Hurricane Katrina]

### How do we measure wind speed and wind direction?

Anemometer- measures wind speed

Wind Vane – measures wind direction

### Global Pressure Belts are caused by the uneven heating of the Earth which results in High and Low Pressure areas

90º N

Polar High

Subpolar Low

60º N

Subtropical High (Horse Latitudes)

30º N

Equatorial Low (Doldrums or ITCZ)

Subtropical High (Horse Latitudes)

30º S

Subpolar Low

60º S

Polar High

90º S

### The Global Winds are the result of air moving from high pressure belts to low pressure belts.

• Air always moves from high to low Pressure.

• Winds are named for the direction they are coming from.

90º N

Polar Northeasterlies

60 º

Prevailing Southwesterlies

30 º

0 º