Winnipeg and Vancouver sit at almost the same latitude, about 49° to 50° N. Yet in January, Winnipeggers face an average of about −16 °C while Vancouverites see about +4 °C, and Vancouver gets more than twice as much precipitation in a year. The Sun shines on both cities at the same angle, so something else must be going on. Working out what that something is, using six factors you can remember with the word LOWERN, is the core skill of this lesson.
Weather versus climate
Weather is the condition of the atmosphere at one place and time: today's temperature, rain, wind and cloud. Climate is the long-term pattern of weather at a place, including its averages and its usual extremes. Environment and Climate Change Canada calculates climate normals, which are 30-year averages; the current set covers 1991–2020. The climate figures in this lesson are approximate values rounded from those normals.
LOWERN: the six climate factors
| Factor | How it works | Canadian example |
|---|---|---|
| L: Latitude | Near the equator, sunlight strikes at a high angle and concentrates its energy; toward the poles it arrives at a low angle and spreads out, and winter days are short | Iqaluit (about 64° N) averages about −26 °C in January and about 8 °C in July; Toronto (about 44° N) about −5 °C and 22 °C |
| O: Ocean currents | Warm currents warm the air above them; cold currents cool it | The warm North Pacific Current keeps the BC coast mild; the cold Labrador Current chills Newfoundland and Labrador |
| W: Wind and air masses | Prevailing westerly winds carry the qualities of the air mass they come from (cold or warm, dry or moist) | Westerlies bring mild, wet Pacific air to Vancouver but dry continental air to St. John's |
| E: Elevation | Air cools by roughly 6.5 °C for every 1000 m you climb | Banff, high in the Rockies, is cooler in summer than Calgary on the plains below |
| R: Relief | Air forced up a mountain cools, its moisture condenses and it rains on the windward side; the air sinks, warms and dries on the leeward side, the rain shadow | Vancouver gets about 1160 mm of precipitation a year; Kamloops, behind the Coast Mountains, under 300 mm |
| N: Nearness to water | Water heats and cools slowly, so large lakes and oceans keep nearby land milder in winter and cooler in summer | Lake effect snow and milder winters along the Great Lakes shores; small temperature ranges on the coasts |
Continental versus maritime climates
- Far from the ocean; land heats and cools quickly
- January about −16 °C, July about 20 °C
- Large annual temperature range: about 36 °C
- Low precipitation, about 520 mm a year
- Wettest in summer, from thunderstorms
- Beside the ocean, with westerly winds blowing in off the Pacific
- January about 4 °C, July about 18 °C
- Small annual temperature range: about 14 °C
- High precipitation, about 1160 mm a year
- Wettest in late fall and winter, driest in summer
Air masses and the jet stream
An air mass is a huge body of air that takes on the temperature and moisture of the region where it formed. Canada's weather is a contest among a few of them: very cold, dry continental Arctic air from the frozen north, mild, moist maritime Pacific air from the west, and hot, humid maritime tropical air from the Gulf of Mexico that brings sticky summer days to southern Ontario. The jet stream is a fast river of air about 10 km up that flows from west to east along the boundary between cold polar air and warmer air. When it dips far south, Arctic air floods in and we get a cold snap; when it bulges north, heat builds. A slow, wavy jet stream can lock a weather pattern in place for days, as in the heat dome over British Columbia in June 2021.
Reading a climate graph
- Read the axes first: temperature (°C) on one side, precipitation (mm) on the other. The line is temperature; the bars are precipitation.
- Annual temperature range = warmest month − coldest month. If the coldest month is below zero, subtracting a negative means adding.
- Mean annual temperature = the sum of the 12 monthly temperatures ÷ 12.
- Total annual precipitation = the sum of the 12 bars.
- Look at the shapes: a range of about 25 °C or more suggests a continental climate; a small range and a winter rain peak suggest a maritime one. Months averaging below 0 °C mean snow and frozen lakes.
- Finally, explain the pattern with LOWERN: latitude, water, winds and relief usually tell the story.
| City | January (°C) | July (°C) | Range (°C) | Precipitation (mm/year) |
|---|---|---|---|---|
| Vancouver, BC | 4 | 18 | 14 | 1160 |
| Kamloops, BC | −3 | 22 | 25 | 270 |
| Winnipeg, MB | −16 | 20 | 36 | 520 |
| Toronto, ON | −5 | 22 | 27 | 810 |
| St. John's, NL | −4 | 16 | 21 | 1540 |
| Iqaluit, NU | −26 | 8 | 35 | 360 |