The Atmosphere

Weather & Meteorology Fundamentals

Course 1 · Chapter 2 · The Atmosphere

Everything in this course happens in a thin shell of gas around a rock. If the Earth were the size of an apple, the part of the atmosphere in which nearly all weather occurs would be thinner than the apple's skin (check the comparison, which is a common illustration and depends on how the layer is defined). Before looking at clouds, wind and storms, it helps to know what the air is made of, how it is arranged in layers and why it is the lowest layer that matters. Figures are given from general knowledge and flagged where they should be checked.

What the Air Is Made Of

Gas	Share of dry air	Note
Nitrogen	About 78 per cent	Mostly inert in everyday weather
Oxygen	About 21 per cent	What animals breathe
Argon	About 0.9 per cent	An inert gas
Carbon dioxide	About 0.04 per cent	A small share, but important for heat balance (check the current value)
Water vapour	Roughly 0 to 4 per cent	Highly variable; the fuel of clouds and rain

The bulk gases make little difference to day-to-day weather. The interesting actors are the smallest and most variable: water vapour above all, and the tiny particles of dust, salt and smoke around which cloud droplets form (Chapter 3).

Layers

The atmosphere is divided into layers according to how its temperature changes with height. The boundaries are approximate and vary with latitude and season.

Troposphere

From the ground to roughly 8 to 18 kilometres, higher at the equator than at the poles (check). Temperature falls with height. It holds most of the air and nearly all the water vapour, and it is where weather happens.

Stratosphere

Above the tropopause, up to about 50 kilometres. Temperature rises with height because the ozone layer absorbs ultraviolet light (check). Airliners fly in its lower edge, and storm tops flatten against its base.

Mesosphere

Up to about 85 kilometres, where temperature falls again. Meteors burn up here.

Thermosphere and beyond

Very thin air that is heated by the sun's most energetic radiation. Auroras occur here, and satellites orbit within it or beyond. The edge of space is conventionally placed around 100 kilometres (check).

The ceiling of weather The tropopause, the boundary between the troposphere and the stratosphere, acts like a lid. A towering thundercloud rises until it reaches this layer, where the air stops cooling with height, and then spreads out sideways in the familiar anvil shape. You can see the lid, on a summer day, in the flat top of a big storm cloud.

Pressure and Height

Air has weight, and the pressure at any point is the weight of the column of air above it. At sea level the standard value is about 1013 hectopascals (the same as millibars, check). Pressure falls quickly with height at first and more slowly later, roughly halving for every 5 to 6 kilometres climbed (check). Half of the atmosphere's mass therefore lies below about that height, and on the summit of Mount Everest the pressure is about a third of the sea-level value (check).

Why pressure is the weather's best clue Pressure at a place changes as masses of air of different density move over it. Falling pressure usually means an approaching system and more unsettled weather, and rising pressure means settling conditions (check the rule, which is a rough guide only). That is why barometers have long hung in hallways and on ships, and why pressure charts are the working map of the forecaster.

Temperature with Height

In the troposphere, air cools as it rises, by an average of about 6.5 degrees Celsius per kilometre in the standard atmosphere (check). That is why mountains are cold and snow lies on their tops even in summer. The rate varies with moisture and conditions, and the way in which it varies decides whether rising air continues to rise or sinks back, which is the central question in Chapter 3.

Measuring the Air: A Short History

The barometer. The Italian physicist Evangelista Torricelli made the first mercury barometer in the 1640s (check). It showed that air has weight.
A French experiment. In 1648 a barometer was carried up a mountain in central France, the Puy de Dôme, at the request of Blaise Pascal, and the mercury fell as the party climbed, confirming that pressure drops with height (check the details and who carried out the climb).
A Scottish observatory. An observatory on the summit of Ben Nevis, Britain's highest mountain, recorded the weather every hour for two decades from the 1880s and produced a valuable record of conditions at altitude (check the dates).
Balloons and radiosondes. Instrumented balloons, released twice a day around the world, now send back temperature, pressure, humidity and wind from the whole depth of the troposphere (Chapter 7).

Check the details The heights of the layers, the sea-level pressure and the lapse rate are all averages from a standard model, and real values differ from day to day and place to place. Confirm figures against an official source before using them in calculations.

Exercises

Exercise 1

Name the four lowest layers of the atmosphere in order, and say how temperature changes with height in each.

→ Solution

Exercise 2

Using a lapse rate of 6.5 degrees per kilometre, estimate the temperature at the top of a 1,300-metre mountain if the valley is at 12 degrees, and say why a real figure might differ.

→ Solution

Exercise 3

Explain why nearly all weather occurs in the troposphere, and what the tropopause does to a growing thundercloud.

→ Solution

Quick Reference

Air: about 78 per cent nitrogen, 21 per cent oxygen, plus argon, carbon dioxide and variable water vapour
Layers: troposphere, stratosphere, mesosphere, thermosphere
Troposphere: about 8 to 18 km deep; temperature falls with height; holds the weather
Tropopause: the lid that flattens storm tops into anvils
Pressure: about 1013 hPa at sea level; roughly halves every 5 to 6 km
Lapse rate: about 6.5 degrees C per km on average
Check: layer heights, standard values, historical dates

Next chapter: Heat, Moisture and Clouds, which covers how the sun's energy and water vapour make clouds.