Assignment #2 Energy Transfer in the Atmosphere The Sun’s energy is used for many things. Some of these uses are: Food grown by crops exposed to sunlight, or obtained from animals which eat such crops. Solar cells that power satellites, calculators etc. Solar water heaters on rooftops. Cars use gasoline or other liquid fuels that originate from fossil plants. Electricity may be generated by coal--from fossil plants, too. Windmills are powered by winds, whose motion is caused by solar heat as discussed later. (Ocean wave energy also comes from winds.) Hydro-electric power comes from water descending from mountains: it was lifted into rain clouds by the Sun's heat. In addition, all the water we drink is distilled by sunlight. Were it not for the Sun, all water would be salty. Sunlight dries laundry strung on a line. In some countries, sunlight is used to produce salt from sea-water (in ponds) and to dry tomatoes, figs, fish etc.

How is the temperature of the Sun related to sunlight? The Sun radiates because it is hot. All hot objects radiate, though sometimes the radiation cannot be seen with the human eye. Radiation is the transfer of energy by waves. It is how the Sun’s energy reaches the Earth.

If the Sun constantly heats the Earth, how come the Earth does not heat up? Three reasons are: 1) The Earth also radiates energy back into space 2) Clouds reflect heat before it reaches the ground 3) Air absorbs energy

Of the Sun’s energy that enters the Earth’s atmosphere, about 35% is reflected out. The rest, 65%, is either absorbed by the atmosphere itself, or by the Earth’s surface. This energy is then circulated around the Earth in 3 ways: radiation, conduction, and convection.

Convection Rising air is one way the ground removes heat: it warms up the air near it, which rises. Later, higher up in the atmosphere, the air radiates its heat out to space, cools down and gets denser, then sinks down again, replaced by warm air that is still rising. This is known as the convection of heat.

Convection in the atmosphere stops around kilometers. What is the layer above that called? It is called the stratosphere and is pretty stable and cold. The region below it--where weather is found--is the troposphere, and the boundary between the two is the tropopause. (If you have seen an isolated thunderstorm from a distance, you might have noted that its top is flattened and spread out. It is flattened against the bottom of the stratosphere, which blocks the convection of the storm from rising any further.)

Convection of heat energy also occurs from moving molecules. As you know, part of the energy of sunlight goes into the ground, and some goes into the water. When water evaporates it carries part of the energy of sunlight goes to heating the ground, but another part evaporates water, from the oceans, lakes, rivers and plants. Convection can occur in gases and liquids, but not solids. The particles making up a solid cannot move from place to place.

Conduction Conduction occurs when particles bump into each other. Energy moves through solids by conduction – even though the particles cannot move from place to place, the particles do vibrate back and forth. As temperature increases the movement of the particles increases because they have more energy.

The Water Cycle Water is the driving force that determines our weather. Water absorbs energy from the Sun, and circulates it throughout the Earth. All of the water on Earth is referred to as the Hydrosphere. This includes the water in the oceans, lakes, groundwater, and in the atmosphere.

THE HYDROLOGIC CYCLE as shown below shows the various pathways of (1) water to the oceans (rivers, glaciers, precipitation); (2) water into the atmosphere by evaporation (from falling rain, rivers & lakes, soil, the oceans, transpiration by plants); and (3) onto the landmasses (by rain, snow). Water movement/transport occurs through movement of clouds, by rivers, ocean circulation, groundwater flow, and evaporation.

The bulk of the water is contained in the oceans, which contain about times more water than atmosphere and continents combined, cover approximately 70% of the Earth's surface and are on average 3800 m deep. The remainder of the water is found in ice caps & glaciers (3%), groundwater (1%), and rivers and lakes (0.01%). The latter two reservoirs constitute the terrestrial fresh water supply. Thus, only a very small fraction of the overall water supply is suitable and available for human use. The water transfer between these reservoirs is accomplished by the processes of evaporation, transpiration, precipitation, and flow of water (following gravity).

As we shall see below, cycling of water through the atmosphere is an important factor for energy transfer in the atmosphere. Every year about to cubic kilometers (a cube km in size) of water move across the surface of the continents to the oceans, shaping the surface of the continents. Evaporation by the sun lifts the water into the atmosphere, and gravity forces rain to fall back on the earth as well as causing water to move back to the oceans. The transfer of water vapor from the oceans to the atmosphere goes hand in hand with the transfer of tremendous amounts of energy to the atmosphere and is very important for atmospheric circulation. For this reason atmospheric circulation and winds can be considered part of the hydrologic cycle.

Solar energy is the main force behind this. Because the Earth is a sphere, the Sun’s energy is not uniform across the planet. In equatorial regions, where the sun's rays come in more or less straight on, a maximum amount of heat is received. In polar regions, on the other hand, the sun's rays come in slanted at a shallow angle and considerably less heat is received (see diagram below).

Average Earth surface temperatures. Blue indicates lowest temperatures (polar regions), red indicates highest temperatures (around the equator). The data used for this diagram were collected between January 1985 and December As a consequence, the polar regions stay cooler. The uneven global heat distribution gives rise to convection currents that attempt to equalize the heat distribution. This will be discussed in more detail in the next section.