The sun heats our planet during the daytime. The amount of heat energy that the ground absorbs depends on the albedo of the surface. For example, urban areas tend to have lower albedos and therefore form 'urban heat islands' (see your text for a more in-depth discussion of urban heat islands). The ground absorbs insolation all day, and releases heat to the atmosphere, causing air temperature increases as long as incoming solar energy exceeds outgoing energy. What time of day can you expect the air to be warmest?
You may have noticed that I referred to heat as 'heat energy'. This is because when we discuss 'heat' in Geography, we are not generally talking about the type of heat that you can measure with a thermometer. Heat that can be measured with a thermometer is sensible heat energy. When we are talking about the global energy budget, we tend to focus on heat energy that is absorbed and then stored in the substance as latent heat energy.
Latent heat is a critical part of the global energy budget. Latent heat is released when vapor turns to water, and again when water turns to ice. This creates a 'heating effect'. For example, consider how condensation warms an area, or heat must be released from a freezer to make ice.
Figure 3.4.1 Latent heat "Heating Effect"
Latent heat is absorbed (stored) when ice melts to water, and again, when water evaporates to water vapor. This is a cooling effect as heat is absorbed. Think about how sweat cools us off when it evaporates.
Figure 3.4.2 Latent heat "Cooling Effect"
The amount of latent heat energy required to get to change liquid water to water vapor is called the latent heat of evaporation. This is not necessarily the same as boiling! For example, when you sweat, the sweat does not boil as it evaporates off your skin. Rather, it absorbs latent heat energy from your skin and the sun, and changes phase.
Latent heat is a critical factor in the ocean/continent effect. Different materials heat up at different rates, depending on their heat capacity, or the amount of heat energy that a substance must absorb before it changes temperature. For example, water and concrete have different heat capacities. You may have noticed this if you have every walked barefoot on a pool deck on a sunny day. While the concrete pool deck will burn your feet, the water in the pool is still quite cool, yet both substances have absorbed about the same amount of insolation. This is because concrete has a lower heat capacity (it heats up and cools down faster) than water. Conversely, if you were to visit the same pool at night, the water temperature would likely be about the same, while the concrete pool deck would be much colder. This is because water must loose a lot of energy before it cools down.
Because oceans have a large heat capacity, they are big stores of energy (a concept called the 'Ocean/Continent effect which we will discuss in greater depth next week). Between the tropics where most of the insolation is concentrated, there is an excess of energy. Therefore a lot of latent heat energy stored in evaporation in the tropics. Turn to pages 16-17 in Goodes Atlas and look at the January and July normal temperature maps as well as the normal average temperature range maps. Note that the temperature of the land changes drastically during the year, but the temperature of the ocean remains about the same. We will explore this concept more in Lab 2.
The global energy balance is the balance between flows of energy entering the atmosphere, hydrosphere and lithosphere (also called the geosphere), and leaving it. We have already established that the incoming energy almost exclusively comes from the Sun. The Earth then dissipates this energy in the outer space. What are the mechanisms by which energy enters and leaves the planet?
Intuitively, it is clear that there should be a balance in energy flows in and out of the geosphere. Otherwise, we would have progressive cooling or warming of the planet. Which brings us to the Greenhouse Effect.
K. Allison Lenkeit-Meezan (Foothill College)