Météorologie: L’atmosphère

Lisibilité
6.56

Quels sont les liens avec mon programme d'études?

Renseigne-toi sur les couches de l’atmosphère, la pression atmosphérique, les systèmes dépressionnaires et les fronts.

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.

Layers of the atmosphere
Layers of the atmosphere including some of the things you can find there (Let’s Talk Science using an image by Bigmouse108 via iStockphoto).
LAYER HEIGHT DESCRIPTION
Troposphere 0 - 14 km
This is the part of our atmosphere that we experience every day. The troposphere is almost 80% of the total mass of the atmosphere. Most of Earth's weather occurs in the troposphere. All life as we know it exists in the troposphere.
Stratosphere 14 - 50 km This is a very stable layer. The air in the stratosphere is very dry. Because of this, there are few clouds here. The ozone layer is in the stratosphere. The ozone layer is very important. It helps protect us from ultraviolet radiation (UV) from the Sun.
Mesosphere 50 - 85 km Mesosphere comes from the Greek word meso, meaning middle. Most meteors burn up in the mesosphere. Reddish lightning called sprites can sometimes be seen above thunderstorms.These are in the mesosphere.
Thermosphere 85 - 600 km The Northern Lights (aurora borealis) and the Southern Lights (aurora australis) are sometimes visible in the thermosphere. This happens during solar flares.
Exosphere 600 - 1000 km The exosphere is the furthest from the surface of the Earth. It marks the edge of space. Only hydrogen and helium molecules are found here.

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.

Force of an air column
A one metre-squared column of air has a mass of 1 000 kg (Let’s Talk Science using an image by Bigmouse108 via iStockphoto).

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.

Barometer
A barometer (Source: connect11 via iStockphoto).

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.

Low pressure area over Iceland
Low pressure system over Iceland (Source: NASA [public domain] via Wikimedia Commons).

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.

High and low pressure systems on a map of Canada
Weather map of Canada showing high and low pressure systems (Source: YULIYA SHAVYRA via iStockphoto).

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. 

Diagram of cold front and warm front
Movement of air masses in a cold front (left) and warm front (right) (Let’s Talk Science using an image by VectorMine via iStockphoto).

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.

Cold front symbol
Weather map symbol for a cold front (Source: Public domain image via Wikimedia Commons).

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.

Warm front symbol
Weather map symbol for a warm front (Source: Public domain image via Wikimedia Commons).

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.

Références

Met Office. (n. d.). High and low pressure.

Rutledge, K. et al. (2011). Atmospheric pressure. National Geographic.

University Corporation for Atmospheric Research. (n. d.). The Highs and Lows of Air Pressure.

University Corporation for Atmospheric Research. (n. d.). Weather Fronts.