Temperature
Temperature measures how hot or cold the air is — physically, the average kinetic energy of air molecules. It's measured with thermometers and reported in °C (SI) or °F.
/ Learn to observe
Understand the sky above you.
Short, honest explainers for every observer. Each topic starts simple and links onward to full references — full multi-level guides are being written on this foundation.
Weather basics
Humidity
Humidity is the amount of water vapour in the air. Relative humidity compares current vapour to the maximum the air can hold at that temperature — 100% means saturation, fog or rain.
Air pressure
Atmospheric pressure is the weight of the air column above you, measured in hectopascals (hPa). Falling pressure often signals approaching storms; rising pressure usually means settling weather.
Wind
Wind is air moving from high-pressure areas toward low-pressure areas. The bigger the pressure difference over a distance, the stronger the wind.
Clouds & rain
Clouds
Clouds form when moist air rises, cools, and its water vapour condenses onto tiny particles. The main families are cirrus (high, wispy), cumulus (puffy) and stratus (layered).
How rain forms
Rain begins when cloud droplets collide and grow heavy enough to fall. In cold clouds, ice crystals grow instead and melt on the way down — the Bergeron process.
Rain gauges
A rain gauge is a simple cylinder that collects rainfall over a known area so it can be measured in millimetres. One millimetre of rain equals one litre of water per square metre.
Storms
Thunderstorms
Thunderstorms are cumulonimbus clouds powered by strong convection — warm moist air rising fast. They produce lightning, thunder, heavy rain, gusty winds and sometimes hail.
Lightning
Lightning is a giant electrical discharge between cloud and ground (or within a cloud), heating air to ~30,000 °C. The rapid expansion of that air is the thunder you hear.
Hail
Hail forms when strong thunderstorm updrafts carry raindrops upward into freezing air, adding layer upon layer of ice until the stone is too heavy to stay aloft.
Monsoon
A monsoon is a seasonal reversal of winds bringing a wet and a dry season, driven by land heating faster than the ocean. South Asia's summer monsoon delivers most of the region's annual rain.
Safety first: never approach tornadoes, lightning, floodwater or storm damage to make an observation. Report from a safe place, and follow your national weather service for official warnings.
