Climate and weather: the system that moves heat

Weather is the state of the atmosphere now or over days. Climate is the statistical pattern of weather over decades. A cold week says nothing about climate, in the same way a single coin landing heads says nothing about the coin.

The distinction matters because the two are predictable in opposite ways. Individual weather becomes unpredictable beyond roughly two weeks because small errors grow, yet long-run climate averages are constrained by energy balance and are more tractable.

Sunlight strikes the equator nearly head on and the poles at a shallow angle, spreading the same energy over more surface. That imbalance means the tropics gain more heat than they lose and the poles do the reverse, and the atmosphere and oceans continually move heat poleward to compensate.

Rising warm air at the equator, sinking dry air around thirty degrees latitude, and the deflection produced by the earth's rotation create the major circulation cells and the prevailing winds. The world's great deserts sit where air descends, warming and drying as it does.

• Warm air holds more water vapour than cold air, which is why cooling produces condensation.
• Clouds form when rising air expands, cools and reaches its dew point.
• Fronts form where air masses meet: a warm front lifts gently and brings prolonged light rain, a cold front lifts steeply and brings brief intense rain.
• Ocean currents carry enormous heat, which is why western Europe is far milder than places at the same latitude in Canada.

The earth absorbs short-wave solar radiation and re-emits energy as long-wave infrared. Certain gases, chiefly water vapour, carbon dioxide, methane and nitrous oxide, absorb infrared and re-radiate it in all directions, including back down. Without this the surface would average roughly thirty degrees colder and be uninhabitable.

The concern is not the effect itself but its intensification. Burning fossil fuels transfers carbon from long-term geological storage into the atmosphere far faster than natural processes remove it, and measurements show atmospheric concentrations well above the range of the past several hundred thousand years.

Feedbacks decide how much warming a given forcing produces. Ice reflects sunlight, so melting ice exposes darker ocean that absorbs more heat and drives more melting: a positive feedback. Warmer air holds more water vapour, itself a greenhouse gas, which is another.

Some feedbacks are stabilising, such as increased radiation to space as the surface warms, and cloud responses that can push either way and remain the largest source of uncertainty in projections. Understanding climate arguments means paying attention to feedbacks rather than to direct effects alone.