Read more: Instant Expert: Extreme weather
The thin layer of gas making up the Earth鈥檚 amazing atmosphere is prone to moods of spectacular beauty 鈥� and stunning violence. To understand what makes the weather go wild, we must start by looking at the intricate workings of the forces that set the atmosphere in motion and thus drive our planet鈥檚 weather.
Instability and precipitation
Instability occurs when air is less dense than its surroundings. It is greatest when there is cold, dense air aloft, and warm, moist air at low levels, since moist air is less dense than dry air. In an unstable atmosphere, air that starts moving upwards will keep moving upwards.
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As air moves upwards, it expands further. Expansion causes cooling, since as the molecules of air move farther apart, the amount of kinetic energy in a given volume falls. At some point it becomes too cool for water vapour in the rising air to remain in the gas phase. When air reaches this point, called the dew-point temperature, water vapour begins condensing out of the air, forming clouds and precipitation 鈥� rain, hail or snow. Thus, two ingredients are needed to generate precipitation: sufficient water vapour in the air, and a mechanism to lift air so that it cools to its dew-point temperature.
The three main ways air gets lifted to cause cooling and condensation are:
- 1) solar heating of the ground, causing thermals to develop
- 2) air masses of different densities meeting and creating 鈥渇ronts鈥� that push air upwards, and
- 3) air being forced upwards by mountains in its way.
While air temperatures fall with altitude in the lower atmosphere, or troposphere, at an altitude of around 11 kilometres the air suddenly begins to warm again. This 鈥渢emperature inversion鈥� marks the bottom of the stratosphere. It is caused by the heating of air as ozone absorbs ultraviolet light. No clouds form in the stratosphere, since air from the troposphere cannot rise above the inversion. This puts a lid on instability. If there was no inversion, we would get more extreme weather.
Driver of the weather
Earth鈥檚 atmosphere is heated unequally at the poles and equator. This occurs because of simple geometry. We live on a sphere orbiting the sun, and sunlight falls from directly overhead on the equator, but at a sharply slanted angle near the north and south poles. The polar regions thus receive less sunlight for a given area than the equator. This difference is the fundamental driver of weather on the planet. Heat naturally moves from hotter to colder areas, so the atmosphere and oceans transport heat from the equator to the poles. A planet without temperature differences would be a planet without weather, where the wind never blows. But on Earth the wind always blows, and sometimes it blows very hard.
Wind belts and forces
If Earth did not rotate, global wind patterns would be very simple. Hot air would rise at the equator, then spread out horizontally towards the poles once it reached the top of the atmosphere. At the poles it would cool, sinking as it became more dense, then flow along the surface back to the equator. Surface winds would thus flow only from north to south in the northern hemisphere, and south to north in the southern hemisphere.
On a rotating sphere, the surface 鈥� and the air above it 鈥� moves fastest at the equator and not at all at the poles. Thus, Earth鈥檚 rotation deflects winds to the right in the northern hemisphere, and left in the southern. This deflection is called the Coriolis effect.
The rotation of the Earth creates a Coriolis effect strong enough to produce three interlocking bands of surface winds in each hemisphere: the equatorial trade winds, the mid-latitude westerlies and the polar easterlies (see diagram). If Earth spun faster, there would be more of these wind belts. Jupiter鈥檚 very fast rotation rate 鈥� its days last just 10 hours 鈥� gives that planet many more bands of winds than Earth. At high altitudes, fast west-to-east bands of wind called jet streams develop above the slower-moving surface winds. While this general pattern of wind belts predominates, because we do not live on a uniform sphere but on one with oceans, mountains, forests and deserts, actual wind patterns are far more complicated and variable.
鈥淚f Earth spun faster, there would be more of these wind belts, as can be seen on Jupiter鈥�
Temperature and pressure
Air is warmed in three main ways: radiation, conduction and convection. The sun radiates photons that are absorbed by air molecules, making the molecules move faster 鈥� that is, get warmer. It has the same effect on the ground, whose molecules then conduct heat energy to the thin layer of air in contact with it.
As the molecules in this parcel of warm air zing around more rapidly, its volume increases. Since this makes the air parcel less dense than the surrounding air, it becomes more buoyant and thus rises. Cooler, heavier air flows into the space it has vacated, where it in turn becomes heated and rises, continuing the cycle. This vertical movement of heat is called convection, and the rising parcels of air are known as thermals.
In this way, temperature differences cause variations in density and pressure that drive winds both vertically and horizontally as the air flows to try to equalise the pressure.
