Background Information: Cloud and Rain
Water vapor is an invisible gas that is always present in the troposphere. However, the amount of water vapor which the air can hold depends directly on the air temperature. Warm air can hold much more water vapor than cold air. Consequently, if warm air containing a lot of water vapor rises up into the atmosphere where it is then cooled, it is forced to release all of the water vapor that it cannot hold in the form of gas. This is called condensation.
As the water vapor in the atmosphere changes its state, water droplets, and ice particles are formed. Then they become visible as little droplets of water, which we call clouds. Clouds are continuously in the process of evolving in the atmosphere. Sometimes we see white clouds high in the sky, and sometimes low gray or dark clouds cover large parts of the heavens. Our weather depends greatly on the types of clouds which form in the sky.
If more and more air rises up and is cooled, then more and more drops of water appear in the air, and eventually the drops join together and become too heavy to remain in the atmosphere. They fall down as rain. In fact, cloud droplets are extremely small, about 0.01 millimeter in diameter. This implies that it would be possible to fit approximately 5 cloud droplets on the end of a human hair. A typical raindrop is of the order of 2 mm in diameter. The growth of cloud droplets by condensation is slow, and even under ideal conditions it would take several days for a typical cloud droplet to grow large enough to fall as a rain drop. Yet observations show that clouds can form and begin to produce rain in less than an hour. Thus, there must be some other processes from which cloud droplets can grow large and heavy enough to fall as precipitation. Although the details of how rain is produced are not fully understood, rainfall occurs whenever air is being cooled down by being made to rise up.
Mountain Barrier
Mountains on islands and continents play an important role in the manner in which they affect near-surface winds (movement of air). As surface winds blow across mountains, they are forced upward. Since warm moist air comes in from the sea and rises up a mountain, the air cools, water vapor condenses to form clouds, and droplets coalesce to fall as rain on the windward side. Thus, there is an area of heavy precipitation on the windward side of a mountain.
The winds continue over the mountain, and they descend the other side of the mountain. In so doing, they are warmed as they descend, so that no condensation or rainfall occurs, and the climate is dry because the warmer air has no problem in carrying the residual water vapor. This side of the mountain (the leeward side) is known as a rain shadow.
The pattern can be seen on many large islands of the South Pacific region which lie in the path of the persistent South East Trades.
Temperature of the Surface
When the ground or sea surface is very hot, then the heat is transferred to the overlying air which rises naturally. This is very common in the sunnier Pacific region. There is also a zone of continually rising hot air along the Equator where the South Eeast Trades and North Eeast Trades meet, and this produces dense clouds and heavy rain. This is known as the Inter-Tropical Convergence Zone.
Cold and Warm Air
The forward edge of a moving cold air mass is known as a cold front. Similarly, the forward edge of a warm air mass is called a warm front. Weather forecasters predict local weather changes on the basis of the movement of cold and warm fronts which can now be readily tracked by satellite imaging. From time to time, cold and warm air meet along a front and thunderstorms follow (cold damp air passing over a hot surface also creates thunderstorms without the presence of a warm front). When winds from two different areas meet together, the warmer air is less dense and rises up above the cooler air. This happens around the latitudes 60 degrees north and south of the Equator, where air from the polar regions meets air from the tropics.
Winds and Atmospheric Circulation
Wind can be generally be defined as moving air usually in response to a pressure gradient. Variations in temperature and pressure provide the means to drive the winds of the world. If all points on our planet were at exactly the same temperature and pressure, then we would have no wind. Air always moves from an area of high atmospheric pressure to an area of low atmospheric pressure. The easiest example to understand is that of Sea Breeze and Land Breeze.
Near the coast, an onshore wind frequently sets in during the late morning, rises to a maximum in the early afternoon, and then dies down in the evening. The strength of breeze is greater on warm days, but may be weaker under cloudy conditions. It is called a Sea Breeze.
The basic cause of the air flow lies in the different rate of heating of the land and sea surfaces when exposed to solar radiation. As we know that land and water do not absorb energy in the same way, they heat and cool at different rates. Water takes longer to heat up than the land. Even though the same amount of sunlight hits both the land and water, the land becomes hot much faster than the water. However, the water does not cool as fast as the land areas.
During the day, the land becomes warmer than the sea. The warm air rises and air pressure becomes lower over the land than over the sea. Air then blows from sea to land as a Sea Breeze. Meanwhile warm air tends to descend on cooling over the sea.
The reverse system operates at night. In coastal regions at night, Land Breezes may develop. In general land breezes are not as strong as Sea Breezes. They are developed more markedly in tropical regions, where they may sometimes force moist unstable air to rise over the sea, leading to thunderstorms off the coast towards dawn.
During the night, the land cools more quickly than the sea. Higher pressure, therefore, develops over the land. Air blows from land to sea as a Land Breeze, and rises over the sea on warming.
On a global scale, there is a circulation of the air in the lower troposphere. Let us first look at an idealized Earth that isn't spinning or tilted over on its axis, and try to understand what form the general circulation of the atmosphere would take. We have already noted that the equatorial regions of the world receive more solar energy than the polar regions. The warm air from the equatorial region will rise and spread out towards higher latitudes. As the air rises and spreads, it cools. When the air reaches the polar regions, it has cooled enough so that it sinks. The region where air is rising tends to result in a low pressure area, and the region where air is sinking is generally a relatively higher pressure. At the surface, air will move from an area of higher pressure to an area of lower pressure. That is, at the surface air will move from the poles towards the equator, but in the upper part of the atmosphere the movement is from equator towards the poles. This type of circulation has the name Hadley Cell circulation.
However, when we look at the winds of the world, we also have to remember that the Earth is a spinning planet. This spin results in an apparent deflection of the winds. This is known as the Coriolis Effect. Because of the Coriolis Effect, winds in the Southern hemisphere curve to the left and winds in the Northern hemisphere curve to the right. Although the wind system is very complex, one can briefly say that the spin also breaks the wind system down into a series of bands, so that instead of a large Hadley Cell in each hemisphere, there are now several cells in each hemisphere.
In short, air rises at the Equator because it is hot and less dense. This air moves outwards and sinks down at approximately 30° north and south of the Equator. Some of this air moves back to the surface as the Trade Winds, while the rest moves towards the poles as the Westerlies. Meanwhile, the cold air at the poles sinks downwards and moves outwards as the Polar winds, which meet the Westerlies along the Polar Front (approximately 60 degrees north and south of the Equator).
Although we know that the winds of the world do follow this type of general pattern, this is still an idealized situation. The actual winds in any given place depend on many factors. Large land masses, in particular, affect local wind pattern. These patterns also shift somewhat in latitude as the earth orbits the sun. However, over large expanses of ocean, these patterns are fairly consistent.
Climate Variation with Time
Climate has not displayed present patterns throughout earth's history, and it is likely to continue to change. Climate can be thought of as a machine that is driven by solar energy, but it is also affected by many other factors such as:
- Changes in the amount of solar radiation
- Changes occurring in the oceans, such as the salinity and density of water in the oceans, their shape, etc.
- Changes in land features, such as the amount of land ice in glaciers, the type of vegetation present, etc.
- Changes in the atmospheric composition, such as the amount of carbon dioxide which is present or the amount of sulfur dioxide and dust which is injected into the atmosphere by volcanoes.
Some examples of changes in climate brought about by the above factors can be summarized as follows:
- Changes in the amount of energy emitted by the Sun, for example, during the early 1980s, the solar output decreased by about 0.1 percent over an 18-month period.
- Violent volcanic eruptions, for example, the eruption of Mt. Pinatubo in the Philippines in 1991 sent so much sulfur into the stratosphere that the earth's global temperature decreased by about 0.5 degrees Celsius in 1992.
Human activities, for example, the increased burning of coal and oil since the Industrial Revolution started in the 1800s has caused an increase in atmospheric carbon dioxide from about 280 ppm (parts per million)] in 1750 to 350 ppm today. The carbon dioxide absorbs more of the outgoing long-wave radiation from the earth and re-radiates it back into the troposphere, causing an increase in temperature (global warming).
