Weather
The state of the atmosphere now, and why it changes.
Weather is the condition of the atmosphere at a place and time — temperature, pressure, humidity, wind, cloud, precipitation. Climate is the long-run pattern of weather for a region, conventionally averaged over thirty years. A cold week says nothing about climate; a shifted thirty-year average does.
Pressure systems
Low pressure means air is rising. Rising air cools, water vapour condenses, and cloud and rain follow. High pressure means air is sinking, which warms it and evaporates cloud, giving settled conditions. Almost all day-to-day weather follows from that one distinction.
Fronts
| Front | What happens | Weather |
|---|---|---|
| Warm | Warm air rises gently over cold | Long spell of steady rain, then milder |
| Cold | Cold air undercuts warm, forcing it up steeply | Short heavy downpours, then clear and cooler |
| Occluded | Cold front catches the warm one | Mixed, often prolonged cloud |
Measuring it
Thermometer for temperature, barometer for pressure, anemometer for wind speed, wind vane for direction, hygrometer for humidity, rain gauge for precipitation. Readings are only comparable if the instruments are sited consistently — a thermometer in direct sun reads the sun, not the air.