Predicting the sky
Will it rain on the class trip? Should farmers harvest today? Will a snowstorm close the highways? Weather forecasting helps answer these questions. Meteorologists combine observations, science and powerful computers to predict what the atmosphere will do.
A short history
| Era | How people forecast |
|---|---|
| Ancient times | Watching the sky, animals and seasons; weather sayings |
| 1600s | Invention of the thermometer and barometer |
| 1840s | The telegraph allowed weather reports to be shared quickly |
| 1871 | Canada’s Meteorological Service was founded |
| 1950s | First computer weather forecasts |
| 1960 | First weather satellite, TIROS-1 |
| Today | Supercomputers, satellites, radar and millions of observations daily |
Step 1: Observing
Forecasts start with measuring what the atmosphere is doing right now, all over the world.
| Tool | What it measures |
|---|---|
| Weather stations | Temperature, pressure, humidity, wind, precipitation at ground level |
| Weather balloons | Conditions up to about 30 km high; launched twice a day from about 900 sites worldwide, including about 30 in Canada |
| Weather satellites | Clouds, storms, sea temperatures, snow cover, from space |
| Doppler radar | Where rain or snow is falling, how heavy it is, and how winds inside storms are moving |
| Ocean buoys and ships | Conditions over the oceans |
| Aircraft | Temperature and wind at flight levels |
Satellites
- Geostationary satellites stay over one spot, about 36 000 km up, and send pictures every few minutes.
- Polar-orbiting satellites circle closer to Earth, passing over the poles and scanning the whole planet twice a day.
Doppler radar
Radar sends out radio waves that bounce off raindrops, snowflakes and hail. Doppler radar also detects whether precipitation is moving toward or away from the radar, which reveals rotation in storms that might form tornadoes.
Step 2: Computer models
All those observations are fed into computer models. These programs divide the atmosphere into a 3-D grid of boxes and use physics equations to calculate how temperature, pressure, moisture and wind in each box will change.
Running a global model takes trillions of calculations, so meteorologists use supercomputers. Environment and Climate Change Canada runs its models on supercomputers in Dorval, Quebec.
Meteorologists often run the same model many times with slightly different starting conditions. This is called an ensemble forecast. If most runs agree, forecasters are more confident.
Step 3: The forecaster
Computers don’t do everything. Meteorologists:
- compare different models
- use their knowledge of local geography, like mountains and lakes
- issue watches and warnings for severe weather
- explain the forecast to the public
Reading a forecast
Probability of precipitation
A probability of precipitation (POP) of 40% means there’s a 4-in-10 chance that measurable rain or snow will fall at a given location during the forecast period.
Weather maps
| Symbol | Meaning |
|---|---|
| H | High pressure: usually fair weather |
| L | Low pressure: usually clouds and precipitation |
| Blue line with triangles | Cold front |
| Red line with half-circles | Warm front |
| Isobars | Lines of equal pressure; close together means strong winds |
How accurate are forecasts?
Forecasts have improved enormously. A five-day forecast today is about as accurate as a one-day forecast was in 1980.
| Forecast length | Accuracy |
|---|---|
| 1–3 days | Very good |
| 5 days | Good |
| 7 days | Fair |
| Beyond 10 days | Only general trends |
Why can’t we forecast further?
The atmosphere is chaotic: tiny differences in starting conditions grow into big differences over time. Meteorologist Edward Lorenz called this the butterfly effect: in theory, a butterfly flapping its wings could change whether a storm forms weeks later. Since we can never measure everything perfectly, forecasts lose accuracy after about two weeks.
Weather sayings: true or false?
| Saying | Science |
|---|---|
| "Red sky at night, sailor’s delight." | Often true in mid-latitudes: a red sunset can mean clear skies to the west, where weather comes from |
| "Ring around the Moon means rain soon." | Often true: a halo comes from high cirrostratus clouds that often come before a warm front |
| "Groundhogs predict spring." | No scientific basis |
Common mistakes
- Thinking a 30% chance of rain means it will rain for 30% of the day.
- Thinking forecasts are guesses. They’re based on physics and data.
- Expecting exact forecasts weeks ahead.
Try it yourself
- Record the forecast for tomorrow every day for two weeks, then record what actually happened. Calculate how often the forecast was right.
- Make your own forecast using a barometer reading, the clouds and the wind direction.
- Look at a satellite loop online and predict where a storm will move next.
Quick recap
- Forecasting starts with observations from stations, balloons, satellites and radar.
- Computer models use physics to predict how the atmosphere will change.
- Meteorologists interpret models and issue forecasts and warnings.
- Forecasts are very accurate for a few days but less so beyond a week.
- The atmosphere is chaotic, limiting long-range forecasts.