Zonal Winds

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Figure 12.10 shows the average zonal winds at all levels. The bottom of the diagram indicates surface equatorial easterlies, then a belt of westerlies in each hemisphere, with easterlies again at higher latitudes. There are westerlies of over 30 m/s (i.e. above 108 km/h) in the subtropics at a height of about 12 km, just below the temperature minimum of the tropopause (Figure 1.9). These strong winds result from steep north-south gradients of temperature at that level. Note 12.E explains why there are steep gradients, and Note 12.F accounts for their effect on wind speed.

The tropopause is shown in Figure 12.10 as having discontinuities at midlatitudes. These are

Figure 12.10 The average distribution of zonal-mean winds and atmospheric temperature in January. Shaded areas show east winds. The dark area at the bottom left indicates Antarctica. The most solid bold lines show the tropopause, while the numbered solid lines joining places of equal wind speed (i.e. 'isotachs'') show the speed in metres per second. The dashed lines show the temperature (°C).

Figure 12.10 The average distribution of zonal-mean winds and atmospheric temperature in January. Shaded areas show east winds. The dark area at the bottom left indicates Antarctica. The most solid bold lines show the tropopause, while the numbered solid lines joining places of equal wind speed (i.e. 'isotachs'') show the speed in metres per second. The dashed lines show the temperature (°C).

due to the differences between the temperatures of air masses meeting at the 'polar front', which is discussed below. The warmer surface air of low latitudes raises the tropopause there by promoting convection, and the development of waves in the front involves subsiding air on the poleward side of the front, depressing the tropopause there (Chapter 13).

The asymmetry in Figure 12.10 is partly due to the time of year, i.e. summer in the southern hemisphere, so there is more cooling in the northern hemisphere, increasing the difference from equatorial temperatures. As a result, the high-level westerlies in the winter hemisphere are stronger than in the summer hemisphere (Note 12.F). Over the year as a whole, the westerlies aloft are slightly stronger in the southern hemisphere on account of the low temperatures above Antarctica.

Particularly strong winds occur in winter at heights of 10-25 km near 60°S because of the extremely cold conditions in the polar stratosphere in winter (Figure 1.9). The vortex of these strong winds around the pole is known as the polar night jet. It tends to fend off warmer (ozone-rich) air from lower latitudes, allowing the stratosphere to become cold enough to condense the small amount of moisture present into wispy clouds. These facilitate the destruction of polar ozone by chlorine and bromine compounds arising from human activity (Section 1.4).

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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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