ATMOSPHERIC PRESSURE AND WINDS

ATMOSPHERIC PRESSURE AND WINDS

Air Pressure

Air exerts pressure on the earth's surface. The weight of air on a unit area of the earth is called Air Pressure.

Atmospheric Pressure

It refers to the force per unit area exerted by the weight of the air above that surface.

Pressure is expressed in millibars and measure with a mercury barometer.

• Atmospheric Pressure is a very important factor for changes that happen in the weather.

• Contrasts in temperature causes changes in air density, which are responsible for variations in pressure.

• The variations cause horizontal movement of air called winds.

Measurement of Pressure

• Pressure is normally measured in Millibars.

• The variations in pressure are shown on maps by means of Isobars.

• These are lines joining the same barometric pressure.

• There are two types of Barometers:

• Fortin's Barometer and Aneroid Barometer.

Pressure Gradient

Pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases more rapidly.

In other words, the rate of change of atmospheric pressure between two points on the earth's surface is called pressure gradient.

Multiple Choice

The weight of air on a unit area of the earth is called as _________.

• Pressure Gradient

• Air Pressure

Multiple Choice

Pressure is expressed in ___________.

• Millibars

• Degrees

Multiple Choice

Pressure is measured with a _________.

• Thermometer

• Mercury barometer

Multiple Choice

Horizontal movements of air is called as ______.

• Winds

• Air Currents

Open Ended

Name the two types of barometers used to measure atmospheric pressure.

Multiple Choice

the rate of change of atmospheric pressure between two points on the earth’s surface is called ________

• Pressure gradient.

• Air Pressure

Factors affecting Atmospheric Pressure 

ALTITUDE

• The atmospheric pressure decreases with altitude.

• The decrease in pressure is about 1 cm of mercury for every 110 m ascent.

• This is because at higher altitudes the air is thinner or less dense than the air at the sea level.

• The fewer number of gas molecules at higher altitudes means fewer molecules collision and decrease in air pressure.

Open Ended

Why atmospheric pressure decreases with altitude?

Open Ended

Why the density of air is less at higher altitudes?

Open Ended

Why do mountain climbers have to use bottled oxygen?

TEMPERATURE

• Atmospheric pressure decreases with increase in temperature.

• This is because when the temperature rises, air expands. The molecules of air move far apart and hence less pressure is exerted.

• If the temperature decreases the air gets compressed and the space between molecules decreases and hence more pressure is exerted. 

Open Ended

Why does atmospheric pressure decrease with increase in temperature?

Open Ended

What happens to the air molecules as the temperature rises?

Open Ended

What happens to the air molecules as the temperature falls?

WATER VAPOUR 

• Molecular weight of water is less than the average molecular weight of air. 

• When water evaporates and enters the atmosphere as gas the water vapour molecule takes the place of other gas molecules in the air.  

Open Ended

Why is the volume of wet air less than the dry air?

ROTATION OF THE EARTH 

• Temperature and rotation of the earth together contribute to the formation of world pressure belts.