Low-angle sunlight spreads energy over a larger area. Rotation gives us day and night; tilt and revolution give us seasons.
Earth rotates on its axis, producing day and night. After sunrise, sunlight warms the surface, which heats the air above it. At night, the surface keeps losing energy without receiving sunlight. Temperatures are often lowest around sunrise. They usually peak after solar noon because the surface can keep gaining more energy than it loses for a while after noon.
Earth revolves around the Sun with its axis tilted by about 23.5 degrees. The hemisphere tilted towards the Sun receives higher-angle sunlight and longer days: summer. Six months later, it tilts away and receives lower-angle sunlight and shorter days: winter. The two hemispheres therefore have opposite seasons.
Latitude is position north or south of the equator, measured in degrees. At higher latitudes, sunlight usually arrives at a lower angle. The same beam spreads over more ground and passes through more atmosphere. Each unit of surface area receives less energy, helping explain colder conditions. Near the equator, solar angles stay relatively high all year.
Other factors modify these patterns. Altitude, cloud, surface cover, winds and nearby seas also affect temperature. A cloudy afternoon, for example, can be cooler than a sunny morning.
Step by step
Sunlight arrives at an angle
A low-angle beam spreads its energy over more ground.
Day length changes
The hemisphere tilted towards the Sun has longer summer days and higher solar angles.
Heating changes
More concentrated sunlight and longer daylight usually make conditions warmer.
Worked example: June in two hemispheres
In June, the Northern Hemisphere is tilted towards the Sun and has longer days and higher solar angles. The Southern Hemisphere has shorter days and lower solar angles. The opposite seasons show why changing Earth-Sun distance is not the explanation for seasons.
Watch out for this
The day must be hottest exactly at noon.
Solar input is strongest near solar noon under clear skies, but air temperature can peak later because the surface continues gaining net energy.
Check your understanding
Equal beams reach two surfaces. One arrives at a lower angle. Why will it usually heat the surface less strongly?
- Its energy is spread across a larger surface area.
- The Sun stops producing energy at high latitudes.
- The surface is necessarily at a higher altitude.