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Equator

Equator

Northern winter

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Equator

Figure 1.10. How the monsoon rains move north then south of the equator during the year, following the zone where the sun is directly overhead.

of solar heating is north or south of equator, at different points during the year (Figure 1.10).

The band of rising air near ihe equator (the ITCZ) follows this zone of greatest heating. In the northern summer, it is slightly north of the equator—although its precise position depends on the layout ofland and sea surfaces that can help to drag it either slightly farther south or farther north. In the southern summer, the ITCZ moves to the south. During spring or autumn, it moves between these two extremes, usually crossing the equator itself at these times of the year. Each time the I TCZ passes over the equator, there an increase in rainfall there so equatorial rainforest climates have two peaks in rainfall each year. However, because they at least get the edge of the ITCZ throughout the year these equatorial locations tend to be quite rainy all the time; the seasonal peak just makes them extra-rainy! Farther away north or south from the equator towards the edges of the tropics, the summer "monsoon" is caused by the arrival of the ITCZ as the sun's summer heating pulls it north (into the northern hemisphere) and then south (into the southern hemisphere). In these places the dry descending air is replaced for a few months by the equatorial climate. In satellite images one can see a "green wave" traveling up through northern Africa in early summer, as the vegetation starts to grow again with the rains.

Desert climate

Cooled air moving inland Air warms, humidity falls

Warm land surface

Cooled air moving inland Air warms, humidity falls

Warm land surface

Figure 1.11. Where cold seawater wells up off the coast, air cools and then is warmed as it passes over land. This prevents rainfall, bringing about a coastal desert. In addition to this, the cold sea surface prevents upwards movement in the atmosphere, likewise supressing rain formation.

Figure 1.11. Where cold seawater wells up off the coast, air cools and then is warmed as it passes over land. This prevents rainfall, bringing about a coastal desert. In addition to this, the cold sea surface prevents upwards movement in the atmosphere, likewise supressing rain formation.

In India the summer monsoon is especially strong because the mountains of the Himalayan Plateau heat up and feed rising air straight into a belt of upper-level winds known as the jet stream. This pulls up more air to replace itself from lower altitudes, dragging the ITCZ especially far north in this region during the summer, way up into northern India. The pulling effect of the Himalayas on the ITCZ also means that it gives rain to other mid-latitude areas such as Japan and Korea, that would otherwise be much too far north to see an effect from the monsoon.

In winter, when the ITCZ has gone south, there is a "winter monsoon" wind traveling from the north in Asia. In most areas this is dry and cold, but it can carry rain-bearing winds from temperate latitudes if it sucks in some air that has traveled over a moist sea surface.

Winds off the oceans transport water vapor, so areas that get ocean winds tend to be wet. But if the ocean is cold, colder than the air. there may be an arid belt along the coast (Figure 1.11). For example, such desert belts occur close to ocean upwelling areas off Peru (Figure 1.12*) and Namibia, where winds pulling the surface water away draw up cold deep water to the surface. How does this cause aridity? Because to get rain, there needs to be a cooling effect on already-moist air causing water droplets to condense out to cloud and then raindrops. If the air actually warms as it moves over land, the water vapor is held more tightly in the warmer air and cannot condense out. As an additional influence, over the cold sea surface where water up-wells, the cooling of air above tends to cause sinking within the atmosphere. This too makes rain unlikely, because strong upwards convection is necessary for producing rain.

The basic climate system explained in this chapter is the blank canvas on which

* See also color section.

V

Figure 1.12. A view off the coast of Peru. Cool seawater welling up nutrients from the deep supports a very active marine ecosystem, which feeds the abundant sea birds. Desert cliffs on the coast are also influenced by the the cool water suppressing the formation of rain clouds. Source: Axel K lei don.

Figure 1.12. A view off the coast of Peru. Cool seawater welling up nutrients from the deep supports a very active marine ecosystem, which feeds the abundant sea birds. Desert cliffs on the coast are also influenced by the the cool water suppressing the formation of rain clouds. Source: Axel K lei don.

we will now paint a complex picture of the ways in which plants both respond to and actually modify their environment. In Chapter 2 the broad patterns in vegetation produced by this background of climate will be described, and in Chapter 3 the ways in which vegetation can move in response to changes in this background. Chapter 4 will deal with the ways in which plants both respond to and produce their own local climatc. the microclimatc. Chapters 5 and 6 cross over to how vegetation itself can help to make climatc on the broader scalc. over hundreds and thousands of kilometers. Chapters 7 and 8 will deal with some other important ways in which plants both modify and respond to their natural environment, in terms of the carbon-containing gases in the atmosphere.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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