Environmental conditions at different locations on Earth can vary quite significantly - from freezing cold and snowing at the poles to blisteringly hot and dry in the deserts. Between these two extremes there are both daily and seasonal variations. Winds blow from different directions, bright sunshine erupts from behind clouds and rain cascades from the sky. All these factors create conditions to which different plants and animals have adapted over time. Humans, on the other hand, tend to live pretty much everywhere and often need artificial means to handle the extremes of their environment.
Select Map to Display: :: Ground Cover :: Vegetation Density :: Population Density :: Surface Reflectivity
This topic deals mainly with the Earth's climate, as opposed to the weather. The difference between the two is essentially time. The weather for a specific location varies day-to-day, sometimes hour-by-hour. The climate of the location is a statistical encapsulation of all these weather patterns into a description of its annual and seasonal cycles. Thus the average behaviour of a climate tends to be given as values for each calendar month. However, you still need hourly weather data from which to extract the monthly data, so you will find that there is always some overlap between the two in discussions on this site.
Understanding the way the modern climate system works is a real challenge as anthropogenic activity on Earth is introducing significant changes in atmospheric composition and surface properties (such as increases in CO2, aerosol concentrations and deforestation).
No Animation :: Surface Temperature :: Monthly Rainfall :: Cloud Cover
At a fundamental level, atmospheric motion is driven by the uneven distribution of incident solar radiation on the Earth's surface. Areas around the equator receive a greater amount of incoming solar radiation during the day than they lose as long-wave infrared radiation at night. Whilst this is balanced partially by increased moisture evaporation, the result is an overall net heat gain around the tropics. At high latitudes the incident radiation is much less, resulting in a overall net heat loss. Both the atmosphere and the oceans respond to this imbalance by transporting heat away from the equator towards the poles.
Global Air Movement
Air at the warmer equator expands and rises, taking with it the moisture vapour it has collected and forming the large vertical cumulus clouds so typical in the tropical sky. This carries large amounts of latent heat energy high into the upper atmosphere where the pressure differences split the air and draw it pole-wards.
As this warm moisture-laden air moves away from the equator, the Earth rotates underneath it which deflects its direction, a process known as the Coriolis Effect. The further towards the poles the air gets, the greater the Coriolis forces. This begins to deflect the air in the direction of the Earth's rotation, resulting in an easterly flow of air - creating stronger and stronger westerly winds. This deflection reaches its maximum at mid-latitudes, by which time the air has cooled and become more dense, creating large downward eddies where the pole-ward flow breaks down. As the air is sucked towards the surface, it continues its westward motion and forms the Trade Winds. This downwards flow creates the first of a series of six giant rotating tori that circulate the Earth running parallel to the equator, three to the north and three to the south. This is commonly known as the tri-cellular model of global airflow.
In the tri-cellular model, after crossing the warm oceans in the trade winds and becoming warm and moist, air arrives at the Equator and is heated, causing it to rise. This forms very high cumulonimbus clouds, afternoon thunderstorms and low-pressure systems. Because most of the air is rising, the equator is an area with very gentle winds called the doldrums.
As the air rises it cools and moves away from the Equator. Further cooling occurs and increasing density and diversion by the Coriolis force cause the air to slow down and subside, bringing an area of high pressure. The latitude at which this occurs is about 30°. This area has clear skies and stable weather conditions. When the air reaches the ground the Hadley cell is completed and some of the air returns back to the Equator as the North-east and South-east trade winds and some continues towards the poles.
The air which continues towards the poles forms the bottom of the Ferrel cell. This picks up moisture as it crosses the seas and meets cold polar air at a latitude of about 60°, this is known as the polar front. This air is then forced upwards, causing an area of low pressure and bringing unstable conditions which produce cyclonic rainfall. The rising air at this stage goes one of two ways. It either travels back towards the equator along the top of the Ferrel cell, or travels up towards the poles where, having cooled down, it descends forming an area of high pressure. The air then returns to the polar front as cold Easterlies.
In addition, the surface of Earth is far from homogenous. Land masses interrupt large ocean basins and interpose high mountain ranges. The surface properties of the land mass also varies with vegetation, desert sands and snow-capped peaks, all affecting heat absorption and creating thermal currents in one direction or another. As an example, the differential absorption of solar radiation between the oceans and the land on a daily basis gives rise to local sea breezes. On a seasonal basis, this same effect gives rise to longer-lasting local pressure difference because air over the land is generally warmer than air over the sea in summer, but colder in winter.
All of this leads to quite different regional wind and climate patterns than would be predicted by the the smooth flow of heat away from the tropics towards the poles. Add to these the El Niño and La Niña effects, as well as man's own climate changing activities, and you can understand why weather forecasting is not a job for the faint-hearted or, apparently, even a Cray super-computer
El Niño refers to the periodic warming of water in the eastern Pacific Ocean near the equator. El Niño events occur every 3 to 7 years and often begin to develop around Christmas, hence the name which means The Little Boy or Christ Child in Spanish. During El Niño, normal west-blowing trade winds relax in the central and western Pacific. As these winds slacken, the surface water off the coast of Central and South America warms up. El Niño also affects global weather patterns.
It's energy, in the form of increased thunderstorm activity, increases the strength and speed of upper-atmosphere winds moving West to East. These winds tend to sink over western Africa, where they inhibit rainfall. These dry conditions, in turn, tend to produce fewer hurricanes. During El Niño, more water evaporates from the tropical Pacific and the large amounts of water vapour release heat when the vapour condenses into clouds and rain. Thus, El Niño events tend to transfer heat from ocean to atmosphere.
La Niña on the other hand is caused when wind blows westward across the water of the eastern Pacific Ocean near the equator. Cool water is pulled up from below and eventually causes changes in the atmosphere. La Niña is marked by cool equatorial waters in the western Pacific Ocean. It is referred to as El Niño's cooler sister (La Niña means The Little Girl in Spanish). During La Niña, the trade winds blow westward across the Pacific (from the Americas toward Indonesia), piling up warm surface water; the sea surface can be more than a foot higher at Indonesia than at Ecuador. The sea surface temperature in the western Pacific can be 8 degrees higher, while an up-welling of cold water from below causes cool water temperatures off South America. La Niña tends to encourage the formation of hurricanes in the Atlantic. It can also mean heavy precipitation in the Pacific Northwest and upper Midwest of the United States.
- Guide to the Science of the Atmosphere - USA Today
- A New Model for Climatic Change
- Climate Effects on Human Health
- Atmosphere & Climate Links
- Astronomy 161 - The Solar System