What the Atmosphere Really Is

An ocean of air, surprisingly thin

We live at the bottom of an ocean. Not water this time, but air: a blanket of gas held to the Earth by gravity, stretching from the grass in your yard up to the edge of space.

It feels endless when you look up at a big prairie sky, but it is remarkably thin. If the Earth were the size of a basketball, the part of the atmosphere where weather happens would be about as thick as two sheets of paper wrapped around it. Everything we talk about on this site, from a gentle July evening to a January blizzard, plays out inside that paper-thin skin.

There is no sharp edge where the air stops. It simply gets thinner and thinner the higher you go, until there is so little of it that it is effectively space. Most of the air is packed close to the ground, squeezed down by the weight of all the air above it, a bit like the bottom blankets in a tall pile being pressed flatter than the top ones.

What is air made of?

Dry air is a very steady mixture. Nearly all of it is just two gases:

• Nitrogen: about 78%
• Oxygen: about 21%
• Argon: just under 1%
• Carbon dioxide and other trace gases: tiny amounts, measured in parts per million

On top of that steady mix sits the most important ingredient for weather: water vapour. It is invisible, and its amount changes constantly. In a cold, dry Arctic air mass there is almost none; in a muggy summer air mass it can reach a few percent.

Why does such a small ingredient matter so much? Because water is the only common substance that changes between gas, liquid and solid at everyday temperatures. When vapour turns into cloud droplets, rain or snow, it releases heat that powers storms. When it evaporates, it soaks heat up. No water vapour, no clouds, no rain, no snow, and no thunderstorms.

The layers of the atmosphere

Scientists divide the atmosphere into layers based on one simple thing: whether the temperature falls or rises as you climb. Each time that pattern flips, a new layer begins.

Troposphere: where we live

From the ground up to roughly 10 to 12 kilometres over southern Alberta. It is deeper over the tropics and shallower over the poles, and it rises and sinks a little with the seasons and passing weather systems. Here the air generally gets colder as you climb, by about 6.5°C for every kilometre on average. That is why there can be snow on the peaks west of us while it is mild in town.

Stratosphere: calm and layered

Above the troposphere, up to about 50 km. Here the temperature stops falling and then rises with height, because the ozone layer absorbs ultraviolet sunlight and warms the air. Warm air sitting on top of colder air is very stable, like a lid. The boundary between the two layers is called the tropopause, and the tops of the biggest summer thunderstorms flatten out against it into their famous anvil shape.

Mesosphere and thermosphere: the high frontier

In the mesosphere, from about 50 to 85 km, temperatures fall again to the coldest in the whole atmosphere. It is where most meteors burn up as shooting stars. Above that is the thermosphere, where the few remaining gas molecules are heated intensely by the sun. The northern lights glow at these heights. The air there is so thin it would not feel hot to you at all.

Why weather lives in the bottom layer

Three reasons make the troposphere the weather layer:

• It holds most of the air. Roughly three-quarters or more of the atmosphere’s mass sits in the troposphere.
• It holds nearly all the water vapour. Moisture comes from oceans, lakes, soil and plants at the surface, and it stays mostly in the lower few kilometres.
• It is heated from below. Sunshine passes through the air and warms the ground first, and the ground then warms the air touching it. Warm air at the bottom and cold air above is a restless arrangement. Warm pockets rise, cool pockets sink, and the air is constantly stirring itself, building clouds as it goes.

Think of a pot of soup on the stove. Heat applied to the bottom makes the soup churn. The troposphere is the churning soup. The stratosphere, warm on top, is more like a still layer of oil floating above it.

Air has weight

Air seems weightless, but it is not. A cubic metre of air near the ground has a mass of roughly a kilogram. Stack up all the air above you, kilometres of it, and it adds up fast. The air pressing down on each square metre at sea level weighs about as much as a mid-sized truck: roughly 10 tonnes.

We do not get crushed because that pressure pushes equally in every direction, including from inside our bodies. But we feel it change: ears popping on the drive up to the mountains, or a sealed bag of chips puffing up when you take it to a higher elevation.

That weight of air is what we measure as air pressure. It is the push of the whole column of air above you, and it shrinks as you go up because there is less air left above you. At about 5.5 km up, half of the atmosphere is already below you.