What this lesson teaches
I can explain why air temperature changes through the day and year and differs between places.
Syllabus K329, Topic 3.1, Key Question 2. Why does air temperature vary across Earth's surface?
Make a guess
On a sunny afternoon, which spot is usually cooler: a shaded, well-watered park or a nearby car park?
- The car park, because cars block the Sun.
- Both are the same, because they are close together.
- The park, because shade and plants cool the air.
- The car park, because concrete reflects most sunlight.
Show the answer
The park, because shade and plants cool the air.
Shade cuts heating, and plants use energy to turn water into vapour. Dark paving absorbs sunlight and stores heat.
Higher ground is usually cooler, plants and shade cool the air, and the sea keeps coastal temperatures steady.
Temperature usually falls with height in the troposphere, the lowest layer of the atmosphere. Higher up, there is less air above, so air pressure is lower. Rising air expands in this lower pressure and cools. The lower air is also heated mainly by the ground, so air far above the ground is cooler. The lapse rate describes how temperature changes with height. It is only an average: sometimes, in a temperature inversion, air higher up is warmer.
Albedo is the share of sunlight a surface reflects. Snow has a high albedo and reflects much of the sunlight. Darker surfaces usually absorb more energy and heat up. Keep surface and air temperatures apart: a hot road is much hotter than the air above it.
Plants cool their surroundings with shade and evapotranspiration, which uses energy to turn water into vapour. Wet soil also cools more than dry, bare soil, because more water evaporates. Buildings and roads store heat and release it later, at night. With fewer plants and heat from human activity, city air is often warmer. The effect depends on moisture, materials and the time of day.
The sea reduces temperature extremes nearby. Water can store more heat than land and mixes heat down through its depth, so it warms and cools slowly. This maritime effect gives coasts cooler summers and milder winters. Places far inland often have a bigger yearly range: the continental effect. To see the sea's effect fairly, compare places at similar latitudes and heights.
- Height
- Air usually cools as you go higher.
- Albedo
- How much sunlight a surface reflects.
- Maritime effect
- The sea keeps coastal temperatures steadier.
Step by step
Height
Rising air expands and cools.
Surface
Shade and plants cool; dark surfaces heat up.
Sea
Water heats and cools slowly, so coasts change less.
Worked example: A coastal comparison
Suppose two places at the same latitude and height have yearly temperature ranges of 12 and 26 degrees C. If the first is on the coast and the second far inland, the sea explains the smaller range. These are teaching numbers, not real stations.
Watch out for this
A mountain is cold because it is far from the equator.
That confuses height with latitude. A mountain can be near the equator and still be cold because it is high.
Check your understanding
Why might a shady, well-watered park be cooler than nearby paving?
- Green leaves reflect most of the sunlight that reaches them.
- The park is at a lower latitude, so it gets less direct sun.
- Shade cuts heating, and plants use energy to release water.
Show the answer
Shade cuts heating, and plants use energy to release water.
Yes. Both effects cool the air.