Sun, Seasons and Temperature
You will understand why our summers are warm and our winters cold, why the hottest part of the day comes well after noon, and why the river valley can be the coldest spot in town on a still night.
In this lesson
- Why the Earth’s tilt, not its distance from the sun, gives us seasons
- How high the sun climbs over Lethbridge in June compared with December, and why that matters so much
- Why the warmest time of day is mid-afternoon and the warmest time of year is after the longest day
- How clear, calm nights cool the ground, and how cold air pools in the Oldman River valley and coulees
It’s the tilt, not the distance
A lot of people assume summer happens because the Earth is closer to the sun. It is a reasonable guess, but it is wrong. The Earth is actually closest to the sun in early January, right in the middle of our winter.
The real reason is tilt. The Earth spins on an axis that leans about 23.4 degrees. As we travel around the sun over the year, that lean stays pointed the same way in space. For half the year the Northern Hemisphere leans toward the sun, and for the other half it leans away.
When we lean toward the sun, two things happen at once: the sun climbs higher in the sky, and it stays up longer. Both mean more energy reaching the ground. When we lean away, the sun stays low and the days are short. That is the whole story of the seasons, and it is why Australians have summer at Christmas.
How high the sun gets over Lethbridge
Lethbridge sits at about 49.7 degrees north. At noon on the longest day in June, the sun climbs to roughly 64 degrees above the horizon. At noon on the shortest day in December, it only reaches about 17 degrees, barely above the rooftops. At the spring and fall equinoxes it sits in between, at about 40 degrees.
Why does the angle matter so much? Think of a flashlight. Shine it straight down at the floor and you get a small, bright circle. Tilt it and the same light spreads into a long, dim oval. The total light is the same, but each spot gets less.
The sun works exactly the same way. A square metre of ground at noon in June gets roughly three times the sunshine that same square metre gets at noon in December. Add the fact that the June day lasts about 16 hours here, while the December day lasts just over 8, and the difference in daily energy is enormous.
When the sun is low, its light also has to travel through much more of the atmosphere to reach you. More of it is scattered and absorbed along the way. That is part of why winter sunshine feels so weak, and why sunsets look orange.
Why the heat lags behind the sun
The daily cycle
The sun is strongest around solar noon, when it is highest in the sky. Yet the warmest part of a typical summer day comes mid-afternoon, often around 3 to 5 pm. Why the delay?
Think of filling a bathtub. As long as more water flows in than drains out, the level keeps rising, even after you start turning the tap down. The ground and air are the same. After noon the sunshine starts to weaken, but for several more hours the ground is still gaining more heat than it loses. The temperature only peaks when incoming and outgoing heat finally balance.
The coldest time is usually right around sunrise, after a whole night of losing heat with nothing coming in. On a typical day the temperature keeps sliding until the sun has been up long enough to start winning again.
Plan outdoor summer work for the morning. By mid-afternoon the day has had hours to “fill the bathtub”, and that is when the heat peaks. On winter mornings, expect the coldest reading close to sunrise.
The yearly cycle
The same lag happens over the year. The longest day is around June 21, but the warmest stretch of summer typically comes in July, after the ground, lakes and oceans have soaked up weeks of strong sunshine. Likewise, the shortest day is around December 21, but the deep cold usually comes after that, once the land has had time to give its stored heat away.
A typical day, of course, is not every day. On the prairies, a passing cold front, a chinook or a thick deck of cloud can flip the daily pattern on its head, with temperatures falling all afternoon or climbing in the middle of the night.
Clear nights, cold ground
Everything warm gives off invisible heat, called infrared radiation. All night long the ground is radiating heat up toward the sky. What happens to that heat depends on what is above you.
- Cloudy night: clouds absorb much of that heat and send part of it back down, like a blanket. Temperatures drop slowly.
- Clear, calm, dry night: the heat escapes almost straight to space. The ground cools quickly, and it chills the air touching it. This is radiational cooling.
Wind matters too. A breeze keeps stirring warmer air from above down to the ground. When the wind dies, the coldest air settles right at the surface. That is why the biggest temperature drops, and the first frosts, come on clear, still nights.
When cold air sits at the ground with warmer air above it, we have a temperature inversion, “inverted” because normally it gets colder as you go up. Inversions are common on clear nights and can persist for days in winter under a strong high.
Cold air is heavier than warm air, so on calm, clear nights it slides downhill like slow, invisible water and pools in low spots. In Lethbridge that means the Oldman River valley and the coulees. On those nights the valley bottom can be several degrees colder than the tablelands of West Lethbridge or the north and south sides, and frost or fog can form down there first. Our dry prairie air makes radiational cooling especially strong, because there is little water vapour to hold the heat in. A chinook or a good west wind usually scours the cold pool out.
On cold, clear mornings, roads and paths through the river valley and coulees can be icier or foggier than up top. Frost-tender plants in low corners of a yard are also the first to suffer in spring and fall.
Try it yourself
Note the time of today’s highest temperature on the live station. Is it closer to 1 pm or 4 pm? Then, on the next clear, calm evening, watch how fast the temperature falls after sunset compared with a cloudy evening. If you drive into the river valley early on a still morning, glance at your car’s thermometer on the way down.
West Lethbridge now (3:14 am): 0°C, wind S 2 km/h, humidity 95%. Live station ›
Key terms
- Axial tilt
- The roughly 23.4 degree lean of the Earth’s spin axis. It causes the seasons.
- Sun angle
- How high the sun is above the horizon. The higher it is, the more concentrated its energy on the ground.
- Solar noon
- The moment the sun is highest in the sky. In Lethbridge it falls well after 12 o’clock on the clock.
- Solstice
- The longest day (around June 21) or the shortest day (around December 21) of the year.
- Seasonal lag
- The delay between the strongest sunshine and the warmest temperatures, caused by land and water storing heat.
- Radiational cooling
- The ground and air cooling off as heat radiates away to the sky, strongest on clear, calm, dry nights.
- Temperature inversion
- A layer where air gets warmer with height instead of colder, often with cold air trapped near the ground.
Check your understanding
1. What mainly causes our seasons?
The tilt changes how high the sun climbs and how long it stays up. The Earth is actually closest to the sun in early January.
2. About how high does the sun get at noon in Lethbridge on the shortest day in December?
At about 49.7°N the December noon sun reaches only about 17 degrees. About 64 degrees is the June figure, and about 40 is the equinox.
3. Why is the warmest part of a summer day usually mid-afternoon rather than noon?
Like a bathtub that keeps filling while the tap is still running, the temperature rises until incoming and outgoing heat balance.
4. Which night is most likely to bring the biggest temperature drop?
With no cloud blanket, no wind to mix the air and little water vapour, heat escapes fastest to space.
5. On a still, clear night, where in Lethbridge is it likely to be coldest?
Cold air is dense and drains downhill, pooling in low spots like the river valley and coulees.
Weather School is for learning and enjoyment. It is not official guidance and may contain errors. For watches and warnings, always use Environment Canada (WeatherCAN app or weather.gc.ca).