Читать книгу Exploring the Solar System - Peter Bond - Страница 75

On balance, regions above latitudes 35°N or S lose more energy than they receive. In contrast, the tropical regions receive more radiant energy than they lose. On a planet with no atmosphere or oceans, this would result in the poles becoming cooler and the tropics becoming hotter. Fortunately, Earth is very efficient at transferring heat towards the poles. One of the main mechanisms for doing this is through large‐scale atmospheric circulation.

Surface temperature differences are largely responsible for Earth's winds. Warm air expands and rises. Colder air is dense and heavy, so it sinks. As a result, air is always in motion. This effect can be clearly seen in coastal areas where sea breezes blow. Air over the warm land rises, to be replaced at ground level by cooler air from offshore. This circulation is often reversed at night, when the land becomes cooler than the sea.

Similar atmospheric motions can also be seen on a global scale, in the form of six large cells of circulating air – three in each hemisphere. Nearest to the equator are the Hadley cells, named after the scientist who first proposed their existence. In temperate latitudes are the Ferrel cells, also named after their discoverer. Finally, there are the polar cells.

Figure 3.9 Hydrogen is continuously escaping into space, creating a cloud of neutral atoms around the Earth that extends beyond the Moon's orbit. This halo of hydrogen is illuminated by reflected solar ultraviolet light, when it is known as the “geocorona.” In this false color ultraviolet image, the geocorona is brighter on the sunlit side (top). The UV camera was operated by astronaut John Young on the Apollo 16 lunar mission.

(NASA‐JSC)

Figure 3.10 Earth's main zonal winds are associated with six large atmospheric circulation systems – three in each hemisphere. In the Hadley cells warm air rises near the equator, moves poleward, and then sinks in the subtropics. The rising section of the Hadley cell causes thick, deep, convective clouds and thunderstorms. Over land, the sinking section is associated with deserts. Over oceans, sinking air results in clear, dry air, and very light winds. Similar circulation systems, called the Ferrel cells and polar cells, occur nearer the poles. Where these cells meet, active uplift of air results in numerous low pressure systems (depressions) and rain belts.

(After Barbara Summey, NASA Goddard VisAnalysis Lab)

Each Hadley cell is driven by hot air rising through convection in equatorial regions, creating low pressure at the surface. This unstable air soars upward until it reaches the top of thetroposphere. The convective air currents result in towering thunderclouds and heavy downpours, particularly in the warmth of the afternoon.

The height of the top of the troposphere varies with latitude – it is lowest over the poles and highest at the equator. It also varies with the seasons – it is lower in winter and higher in summer. It can be as high as 20 km near the equator, and as low as 7 km over the poles in winter.

The air at high level then spreads poleward before sinking at about 30° latitude. This descent warms and dries the air, resulting in cloud‐free skies – perfect conditions for hot deserts, such as the Sahara. The large areas of stable, sinking air have light winds – so they were often dreaded by crews of sailing ships.

At the surface the air then diverges. Some moves back toward the equator, completing the Hadley circulation. Some moves toward higher latitudes until it meets up with colder air spreading from the poles. The boundary between the warm, subtropical air and the cold, polar air is known as the polar front.

The polar front is like a battleground, where a cool air mass is always trying to undercut and displace the warmer, less dense, subtropical air. Air forced upward at the polar front results in the formation of clouds and heavy rain. Low pressure areas known as depressions are driven eastward by the polar jet stream (see Troposphere).⁵

Figure 3.11 A map of tracks followed by tropical storms and cyclones that develop over tropical oceans. Red and orange show the most intense category 5 and 4 storms. As they move westward, driven by upper‐level trade winds, they increase in intensity, then die out when they move over land or cooler water.

(National Hurricane Center / Typhoon Warning Center/ NOAA)