Vertical and horizontal heat transfer in the atmosphere

Overview

Vertical and horizontal heat transfer in the atmosphere and how convection transfers heat upwards.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Distinguish between vertical and horizontal heat transfer in the atmosphere.

   Model answer

   Vertical transfer moves energy upwards from the Earth's surface through the depth of the troposphere, by radiation, conduction, convection and latent heat. Horizontal transfer, called advection, moves energy sideways across the globe from the low-latitude surplus zone to the high-latitude deficit zone, by winds and ocean currents. Vertical transfer distributes heat through the atmosphere; horizontal transfer distributes it around the planet.

2. 2

   How does radiation transfer heat vertically in the atmosphere?

   Model answer

   The warmed ground and sea emit long-wave terrestrial radiation upwards. Greenhouse gases and cloud droplets absorb this energy and re-emit it in all directions, so it is passed upwards step by step through the troposphere before eventually escaping to space. Radiation needs no medium and works both by day and by night, and it is the reason clear nights cool quickly while cloudy ones do not.

3. 3

   Why is conduction a very minor way of moving heat upwards through the atmosphere?

   Model answer

   Conduction passes heat by direct contact between molecules, and air is a poor conductor because its molecules are far apart. It therefore only warms an extremely thin layer of air, a few millimetres deep, in immediate contact with the ground. Its importance is that it starts the process off: that thin warmed layer then becomes buoyant and is carried upwards by convection.

4. 4

   Explain how convection transfers heat upwards.

   Model answer

   A parcel of air in contact with a warm surface is heated, expands and becomes less dense than the air around it, so it becomes buoyant and rises as a thermal. As it rises it cools, and cooler denser air sinks to replace it at the surface, setting up a circulating cell. Convection is the most important mechanism for moving heat through the depth of the troposphere and is strongest over hot land surfaces in the afternoon and over warm tropical oceans.

5. 5

   How does turbulence contribute to vertical heat transfer?

   Model answer

   Wind blowing over a rough surface of hills, trees and buildings is broken into irregular eddies that mix air mechanically between levels. Warm air from near the ground is carried upwards and cooler air is dragged downwards. This mixing works even when the surface is not hot enough to drive strong thermals, and it is why windy conditions keep the lowest air well mixed and prevent sharp temperature differences near the ground.

6. 6

   Why is latent heat transfer so effective at moving energy upwards over the oceans?

   Model answer

   Evaporation at a warm ocean surface takes up a very large quantity of energy per kilogram of water without raising the temperature of the air. The vapour is then lifted by convection to several kilometres, where it condenses into cloud and releases that stored energy as sensible heat. In this way huge amounts of energy are moved from the sea surface into the middle and upper troposphere. It is the mechanism that powers thunderstorms and tropical cyclones.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.