What this lesson teaches
I can explain why air temperature changes through the day and year and differs between places.
Syllabus K226, Topic 3.1, Key Question 2. Why does air temperature vary across Earth's surface?
Make a guess
In London, the winter Sun sits low in the sky. Why does that make winter colder?
- The Sun gives off less heat in the winter months.
- The Sun is farther from Earth in the winter months.
- Low-angle sunlight spreads over more ground.
- The Sun shines for longer but more weakly.
Show the answer
Low-angle sunlight spreads over more ground.
The same beam covers a bigger area and passes through more air, so each patch of ground gets less energy.
Low-angle sunlight spreads over more ground, so it heats less; Earth's tilt as it orbits the Sun gives us seasons.
Earth spins on its axis, giving day and night. After sunrise, the Sun warms the ground, and the ground warms the air. At night, the ground loses heat. So it is often coolest around sunrise. The warmest time is usually after noon, because the ground keeps gaining heat for a while.
Earth's axis is tilted by about 23.5 degrees as it orbits the Sun. The half tilted towards the Sun gets higher sunlight and longer days. This is summer. Six months later, it tilts away and has winter. So the two hemispheres have opposite seasons.
Latitude is distance north or south of the equator, in degrees. Far from the equator, sunlight arrives at a low angle. The same beam spreads over more ground and passes through more atmosphere, so each patch gets less energy. Near the equator, the Sun stays high all year.
Other factors also change temperature: height above sea level, cloud, the type of surface, winds and nearby seas. A cloudy afternoon, for example, can be cooler than a sunny morning.
- Rotation
- Earth spinning on its axis gives day and night.
- Tilt of 23.5 degrees
- With Earth's orbit, it causes the seasons.
- Latitude
- Distance north or south of the equator.
Step by step
Sunlight arrives at an angle
A low-angle beam spreads over more ground.
Days change length
The hemisphere tilted towards the Sun has longer days.
Heating changes
Stronger sunlight and longer days make it warmer.
Worked example: June in two hemispheres
In June, the Northern Hemisphere tilts towards the Sun, so it has long days and high sunlight: summer. The Southern Hemisphere has short days and low sunlight: winter. Opposite seasons show that seasons are caused by tilt, not by Earth's distance from the Sun.
Watch out for this
The day must be hottest exactly at noon.
Sunlight is strongest near noon, but the air often keeps warming until mid-afternoon.
Check your understanding
Two equal beams of sunlight hit the ground. One arrives at a lower angle. Why does it heat the ground less?
- The Sun makes less energy at high latitudes.
- The ground must be higher up.
- Its energy is spread over a larger area.
Show the answer
Its energy is spread over a larger area.
Yes. Less energy reaches each patch of ground.