PLANET EARTH
GROUNDWATER, LAKES, ATMOSPHERE
Groundwater is water under the Earth’s surface. Most groundwater is found in porous rocks, which have tiny holes in them.
If a hole is bored straight down through the rock, groundwater is eventually found at a certain level. This level is called the water table, and it usually rises when rainwater soaks into the ground. A spring is a place where groundwater emerges from a hillside.
EGYPTIAN OASIS
Even in arid (dry) places such as deserts, groundwater sometimes comes to the surface. These lush green areas are called oases. The water at an oasis may have travelled underground from mountains hundreds of kilometres away. Oases are an important source of water, and towns often grow up around them.
LAKES
Lakes form where water fills hollows in the landscape. Some of these hollows are formed by glaciers gouging into the ground, and some are created when river valleys are blocked by dams. Other lakes are formed in volcanic craters, or when land sinks during earth movements.
ATMOSPHERE
The Earth would be as lifeless as the Moon without the atmosphere – a blanket of gases surrounding the planet and extending about 700 km (430 miles) above its surface. This relatively thin layer, held in place by gravity, provides us with oxygen to breathe and separates us from the void of space. It includes the OZONE LAYER, which screens out harmful solar radiation. PRESSURE SYSTEMS in the atmosphere affect the Earth’s weather.
AURORAS
Auroras are shimmering curtains of light seen at night in the polar regions. They are known as the Northern Lights in the Arctic, and as the Southern Lights in the Antarctic. These spectacular displays are caused by charged particles from the Sun striking the upper atmosphere above the poles.
LAYERS OF THE ATMOSPHERE
The troposphere is the lowest layer of the atmosphere and contains 75 per cent of all its gases. Above is the stratosphere, which includes the ozone layer. Higher still is the thin air of the mesosphere, where meteors burn up. The thermosphere contains an electrically charged layer that radio waves bounce off. The exosphere is the top layer, fading off into space.
OZONE LAYER
Ozone is a form of oxygen that gathers in the stratosphere to form a layer. This layer screens out harmful ultraviolet (UV) rays from the Sun, which can cause skin cancer. In the 1980s, scientists discovered that thin areas, or holes, were appearing in the ozone layer over the polar regions each spring. Ozone loss is caused by chemicals called chlorofluorocarbons (CFCs).
PRESSURE SYSTEMS
Pressure systems in the atmosphere are masses of moving air that create winds and the weather. The air is set in motion by changes in temperature and air pressure (the pushing force of air from all directions). Air pressure is greatest at sea level, because there is a larger weight of air pushing down. The higher in the atmosphere you go, the less air and air pressure there is.
MEASURING AIR PRESSURE
Air pressure is measured in units called millibars (mb), using an instrument called an aneroid barometer. Behind the barometer’s dial is a chamber from which some air has been removed. Changes in air pressure cause the air in the chamber to expand or shrink, and this moves the needle around the dial.
HIGH AND LOW PRESSURE
Warm air is lighter than cool air, so it rises above it. Rising warm air creates low-pressure areas called cyclones, or lows. Sinking cool air forms high-pressure anticyclones, or highs. Air moves from higher to lower pressure, bringing different weather systems. Highs usually bring dry, fine weather, and lows bring rain.
Source:
factmonster.com
Other Links:
PLANET EARTH: ICE
PLANET EARTH: COASTS
