Weather: Atmospheric Pressure

There is a layer of gases around the Earth. This is our atmosphere. 

Did you know?

The word atmosphere comes from two Greek words. Atmos means vapour and sphaira means ball.

Scientists who study our atmosphere describe it with five layers.

Did you know?

Earth’s atmosphere actually includes the Moon!

Atmospheric Pressure

Our atmosphere plays an important role in weather and wind patterns. It may seem invisible and weightless. But the gases that make up the atmosphere have both mass and weight. 

Misconception Alert

Mass and weight do not mean the same thing. Mass is a measure of the number of atoms in an object. Weight is a measure of gravity acting on it. 

Atmospheric pressure is the downward force caused by the molecules of air in the atmosphere. Imagine a one square metre column of air that reaches from sea level to the top of the atmosphere. This weighs about 1 000 kilograms! The exact weight of an air column depends on the number and types of molecules it contains.

Air is made of gas molecules. The more gas molecules there are in an area, the more dense the air is. More molecules also mean the air has more weight and pressure. Fewer molecules mean the air has lower density, weight and pressure. 

Like all molecules, molecules of gases in the atmosphere are always moving. At higher temperatures, the molecules move quickly and spread out. This means that there are fewer molecules in an area. Fewer molecules result in lower air pressure. The opposite is true at lower temperatures. The molecules stay closer together and air pressure is higher. 

Air pressure is measured with a tool called a barometer.

High and Low Pressure Systems

Atmospheric pressure changes from day to day and from place to place. You might hear a weather forecaster talk about a pressure system or pressure area. 

A low pressure system is an area of low pressure surrounded by higher pressure. Air with low density and high temperature is surrounded by air with high air density and low temperature. This causes the air to form an inward spiral. You can see this shape in the picture below.

Usually, low pressure systems have warm, humid air. They often create clouds and sometimes precipitation. This happens mostly when they run into high pressure systems. 

A high pressure system is an area of high pressure surrounded by lower pressure. This means there is cold, dense air surrounded by warmer, less dense air. This causes the air to form a spiral outward. High pressure systems usually cause weather with cool temperatures, dry air and few clouds. 

On a weather map, low pressure areas are labelled with a capital L. High pressure areas are labelled with a capital H.

Weather Fronts

Weather fronts are the border areas between air of different densities and temperatures. 

Cold fronts happen where a mass of colder air is moving towards a mass of warmer air. The colder, denser air quickly pushes the warmer, less dense air up into the atmosphere. The air then cools and condenses. This often causes short-lived showers and thunderstorms.

Warm fronts are where a mass of warmer air is moving towards a mass of colder air. The warm air slowly moves over the cold air and up into the atmosphere. There, it condenses and causes precipitation. Precipitation caused by warm fronts lasts much longer than the precipitation caused by cold fronts. 

On a weather map, the symbol for a cold front is a blue line with blue triangles. The triangles point in the direction the front is travelling.

The symbol for a warm front is a red line with red half-circles. The half-circles point in the direction the front is travelling.

Sometimes you will see a purple line with both triangles and half-circles. This shows the location of an occluded front. An occluded front is where three air masses come in contact with each other. The shapes point in the direction of the colder air.