What is weather?
Weather is the condition of the atmosphere at a certain time and place. Is it hot or cold? Sunny or cloudy? Calm or windy? Weather affects what we wear, how we travel, which crops grow, and even our moods.
All weather happens in the lowest layer of the atmosphere, the troposphere, which reaches about 10–15 km up.
The Sun drives weather
The Sun heats Earth unevenly. Land heats faster than water, and the equator gets more direct sunlight than the poles. This uneven heating makes air move, causing wind, clouds and storms. Water evaporating and condensing adds more energy to the system.
Elements of weather
| Element | What it is | Tool | Unit |
|---|---|---|---|
| Temperature | How hot or cold the air is | Thermometer | °C |
| Air pressure | Weight of air pressing down | Barometer | kilopascals (kPa) |
| Humidity | Amount of water vapour in the air | Hygrometer | % |
| Wind speed | How fast air moves | Anemometer | km/h |
| Wind direction | Where wind comes from | Wind vane | N, S, E, W |
| Precipitation | Water falling from clouds | Rain gauge | mm |
| Cloud cover | How much of the sky is covered | Observation | Clear, partly cloudy, overcast |
A west wind blows from the west.
Air pressure
Air has weight. At sea level, air pressure is about 101 kPa.
| Pressure system | Air movement | Usual weather |
|---|---|---|
| High pressure (H) | Air sinks and spreads out | Clear, calm, dry |
| Low pressure (L) | Air rises, cools and forms clouds | Cloudy, rainy, stormy |
A falling barometer often means stormy weather is coming.
Humidity
Humidity is how much water vapour is in the air. Warm air can hold more water vapour than cold air.
- Relative humidity compares how much vapour is in the air to the most it could hold.
- At 100% relative humidity, the air is saturated, and water condenses into clouds, fog or dew.
- The dew point is the temperature at which this happens.
On humid summer days, sweat doesn’t evaporate easily, so we feel hotter. Canada uses the humidex to describe how hot it feels.
Precipitation
| Type | How it forms |
|---|---|
| Rain | Water droplets grow heavy and fall |
| Snow | Ice crystals form in cold clouds and fall without melting |
| Sleet (ice pellets) | Snow melts, then refreezes before hitting the ground |
| Freezing rain | Rain freezes on contact with cold surfaces; causes ice storms |
| Hail | Raindrops are carried up and down in thunderstorms, adding icy layers |
The 1998 ice storm in Ontario, Quebec and the Maritimes coated the region in ice for days, leaving millions without power.
Air masses
An air mass is a huge body of air with similar temperature and moisture.
| Air mass | Formed over | Brings |
|---|---|---|
| Continental arctic | The Arctic | Bitterly cold, dry air |
| Continental polar | Northern Canada | Cold, dry air |
| Maritime polar | North Pacific or Atlantic | Cool, damp air |
| Maritime tropical | Gulf of Mexico | Warm, humid air |
Fronts
A front is where two air masses meet. Most weather changes happen at fronts.
| Front | What happens | Weather |
|---|---|---|
| Cold front | Cold air pushes under warm air, forcing it up fast | Thunderstorms, heavy rain, then cooler and clearer |
| Warm front | Warm air slides up over cold air | Long periods of light rain or snow, then warmer |
| Stationary front | Neither air mass moves much | Days of cloudy, wet weather |
On weather maps, cold fronts are blue lines with triangles; warm fronts are red lines with half-circles.
Canada’s weather
Canada’s weather usually moves from west to east, carried by the westerlies and the jet stream, a fast river of air high in the atmosphere. When the jet stream dips south, Arctic air can plunge into southern Canada.
Canada also experiences chinooks: warm, dry winds that rush down the eastern slopes of the Rockies and can raise temperatures in Calgary by 20 °C or more in a few hours.
Weather forecasting
Meteorologists use satellites, radar, weather balloons, ground stations and computer models to predict weather. Forecasts for the next few days are quite accurate, but beyond about 10 days they become much less reliable because the atmosphere is so complex.
Common mistakes
- Mixing up weather and climate.
- Thinking clouds are water vapour. They’re tiny water droplets or ice crystals.
- Thinking wind direction means where wind is going. It’s where it comes from.
Try it yourself
- Keep a weather journal for two weeks: temperature, clouds, wind and precipitation.
- Make a simple rain gauge from a straight-sided jar and a ruler.
- Look at today’s weather map. Find a high, a low and a front. Predict tomorrow’s weather.
Quick recap
- Weather is the state of the atmosphere at a time and place.
- The Sun’s uneven heating drives weather.
- Temperature, pressure, humidity, wind and precipitation describe weather.
- Air masses and fronts bring weather changes.
- Meteorologists use many tools to forecast weather.