Weather is the state of the atmosphere at a particular place and time: temperature, air pressure, humidity, cloud, wind, and precipitation. Climate is the pattern those conditions follow over many years. A cold week does not change a climate, and a mild winter is not proof of one.
Why weather happens at all
Sunlight arrives unevenly. It strikes the tropics almost head on and the poles at a low angle, so the tropics absorb far more energy per square metre. Land heats and cools faster than water. Warm air expands, becomes less dense, and rises, which lowers the pressure beneath it. Cooler, denser air flows in to take its place, and that flow is wind. Almost every weather event traces back to this uneven heating and the movement it drives.
Air masses
An air mass is a large body of air with fairly uniform temperature and moisture, taking those properties from the surface it formed over. Air that sits over a warm ocean becomes warm and moist. Air that settles over a cold continental interior becomes cold and dry. Weather turns interesting where two different air masses meet.
Fronts
A front is the boundary between two air masses. The two common kinds behave differently enough that you can often tell which one passed by what the sky did.
| Front | What is happening | Typical weather |
|---|---|---|
| Cold front | Cold dense air pushes under warm air and forces it up steeply | Tall cumulonimbus cloud, short heavy rain or thunderstorms, then a sharp temperature drop and clearing skies |
| Warm front | Warm air slides up and over cooler air along a gentle slope | High cloud thickening to low grey cloud, long steady rain or drizzle, then milder and often humid air |
| Stationary front | Neither air mass advances | Cloud and precipitation that linger in one area for days |
Reading the clouds
- Cirrus. Thin and wispy, very high, made of ice crystals. Often the first hint that a warm front is on the way.
- Cumulus. Puffy with flat bases, the fair weather cloud, formed by rising bubbles of warm air.
- Stratus. Flat grey layers covering the whole sky, usually with drizzle.
- Cumulonimbus. A cumulus grown tall enough to make a storm, often with an anvil shaped top spreading out at high altitude.
Instruments
| Instrument | Measures | Units usually reported |
|---|---|---|
| Thermometer | Air temperature | Degrees Celsius or Fahrenheit |
| Barometer | Air pressure | Millibars or inches of mercury |
| Anemometer | Wind speed | Kilometres or miles per hour |
| Wind vane | Wind direction, named for where the wind comes from | Compass direction |
| Hygrometer | Humidity | Percent relative humidity |
| Rain gauge | Precipitation | Millimetres or inches |
Falling pressure on a barometer generally means unsettled weather is approaching, because low pressure goes with rising air, cloud, and precipitation. Rising pressure generally means clearing. Forecasters watch the trend more closely than the reading itself.
Check yourself
What is the difference between weather and climate?
Weather is what the atmosphere is doing right now in one place. Climate is the long-term pattern of weather for a region, measured over decades.
A north wind is blowing. Which way is the air moving?
From the north, toward the south. Winds are named for the direction they come from, which is what a wind vane points into.
The sky fills with high wispy cirrus, then lowers and greys over a day. What is likely coming?
A warm front. That slow thickening from high thin cloud down to a low grey layer, followed by steady rain, is the classic warm front sequence.
Why does a barometer help predict weather?
Because pressure tracks whether air is rising or sinking. Falling pressure signals rising air, which cools, forms cloud, and often brings precipitation. Rising pressure signals sinking air and clearer skies.
See also
Sources
- Air masses, frontal types, cloud families and standard meteorological instruments as taught in United States upper elementary earth science. Tables on this page are our own.