Weather simply refers to the condition of the air on earth at a given place and time - whether it is warm or cold, dry or wet, blowing or calm. The condition of air and how it acts to create weather is influenced primarily by two things - heat (the sun) and water.
Weather forecasts provide critical information about the weather to come. There are many different techniques involved in weather forecasting, from relatively simple observation of the sky to highly complex mathematical models run on computers. Weather prediction can be for the next day, next week, or next few months. The accuracy of weather forecasts however, falls significantly beyond about 10 days. Weather forecasting remains a complex business, because the weather can be so chaotic and unpredictable.
If weather patterns are relatively stable, the persistence method of forecasting provides a relatively useful technique to predict the weather for the next day. If it is hot and sunny on one day, it is likely to be hot and sunny the next. Unfortunately, in many areas of the world the weather is more unpredictable and changeable than that, particularly in the mid-latitudes where depressions influence much of the weather.
With an understanding of how the air moves and how clouds and rain form, some prediction can be made by simply observing the sky overhead, observing wind direction and noting the temperature and humidity of the air. An abundance of cirrus clouds which gradually thicken to lower level clouds on the horizon for example, is a typical indicator of an approaching depression.
Historically, such observations have led to the development of weather folklore, although many have little sound basis in fact. A commonly known, and frequently accurate saying is, 'Red sky at night, shepherd's delight; red sky at morning, shepherds warning.' When the morning Sun illuminates clouds to the west, this may be an indication of an approaching front and depression. In the evening, the setting Sun in the west will illuminate clouds to the east that may have already passed by.
Better predictions of weather require an understanding of the isobaric patterns associated with fronts and depressions, anticyclones and high-pressure ridges. Meteorologists plot isobaric patterns on synoptic charts. Meteorologists used to plot all their synoptic charts by hand. Since the advent of fast powerful computers however, this task has become much easier. Nevertheless, although organisations like the UK Meteorological Office use supercomputers to predict the weather, television weather presenters still use the isobaric patterns of pressure to describe the movement of depressions and other weather phenomena.
Weather on earth starts with the sun. The sun's heat warms our atmosphere and causes two things to happen:
As air rises, its temperature drops. As the moisture in the air cools, it condenses into tiny suspended droplets, forming clouds. The droplets inside the clouds become larger a more moisture is evaporated into the air. Eventually, they are too heavy to remain suspended and fall to earth as precipitation - rain, snow, sleet or hail.
Fronts and Air Masses
The atmosphere consists of air masses, which are different notably in temperature and moisture content. In general the air masses do not mix but are separated by relatively narrow transition zones called fronts. A cold front is part of a system along which cold air is advancing; a warm front is that part of a system along which cold air is retreating. The fronts are frequently characterised by clouds and produce some of the stormiest weather. The type and extent of the clouds depend on the air mass characteristics. Some fronts have few clouds when the air is very dry. Clouds frequently rise to an altitude of 6 km, less frequently to about 10 km, but in the case of very severe storms may extend upwards at least 18 km. Temperatures at these heights fall to -50 to -55°C, even in the summer.
There are few clouds ahead of a cold front to warn of its arrival, although occasionally a layer of altocumulus and/or stratocumulus may spread out in advance. The front itself is usually marked by a line of towering cumulus and showers. In the most severe cases a line or wall of cumulonimbus marks the front and can be accompanied by thunderstorms and hail. The tops of the cumulonimbus often spread out ahead of the front into dense cirrus called an anvil. Temperatures fall and the wind shifts in a clockwise direction while the front passes. The band of cloud and rain is usually quite narrow and passes within a few hours. Afterwards, some non-frontal clouds, such as stratocumulus or cumulus, may develop but just as frequently the skies clear. As the front passes, the wind changes direction. Skies begin to clear, and the temperature usually drops.
The first sign of an approaching warm front is usually a layer of cirrus that begins to cover the western or south-western sky. This layer thickens to a cirrostratus and perhaps, also altocumulus begin to form. Light rain may begin at this point becoming more intense in the dark grey nimbostratus with fog often developing. Stratus cloud and some thick stratocumulus may also form. Sudden heavy downpours may occur, along with thunder or lightning and sometimes hail, indicating cumulonimbus are embedded in the solid layer of nimbostratus. The thunderstorms may last for hours. Temperatures rise slowly with the advance of the front, but more rapidly as the front passes and wind shifts in the clockwise direction. Then the cloud cover begins to break up but the fog often lingers. When the front passes, the sky clears and the air pressure rises. Temperatures also rise as warm air replaces cold air.
Occlusion (or occluded front)
Occlusions form when the cold front of a depression catches up with the warm front, lifting the warm air between the fronts into a narrow wedge above the surface. On a synoptic chart an occluded front appears as a purple line with a combination of triangles and semi-circles. The direction in which the symbols point is the direction in which the front is moving.
Even though air masses often move, sometimes they don't, resulting in a stationary front. Precipitation and weak winds usually occur in areas with stationary fronts.
Unstable - summer
Unstable - winter
Stable type clouds
High and Low Air Pressure
Certain weather conditions are associated with high and low pressure systems.
High Pressure Areas
High pressure areas, or highs, are shown by "H" symbols. In a high pressure system, air pressure is greater than the surrounding areas. This difference in air pressure results in wind, or moving air. In a high pressure area, air is more dense than in areas of lower pressure. The result is that air will move from the high pressure area to an area of less density, or lower pressure. Winds blow away from high pressure areas toward areas having lower air pressure.
Low Pressure Areas
Low pressure areas, or lows, are shown by "L" symbols. Winds tend to blow into low pressure areas because air, like other gases, moves from areas of higher pressure into areas of lower pressure. As winds blow into a low, the air moves up. This upward flow of air can cause clouds and precipitation to form.
Air pressure is measured by barometers. In general, weather will improve when pressure increases and worsen when pressure decreases.
Precipitation: Water in the Air
Water is one of the few substances that can be found in all three states--solid, liquid and gas--at normal temperatures and pressures. You might not be able to see all the forms of water, but they're there.
All the forms of water that fall from the air to the earth's surface are called precipitation. Whether the precipitation is snow, rain, sleet or hail depends on the temperature of the air that the water falls through. If the air is above freezing, the precipitation will most likely be rain. If the air is below freezing, the precipitation will most likely be snow.
But sometimes, different temperature layers exist within the air through which the water is falling. For example, if snow falling from a high cloud passes through a warmer layer, then it could melt into rain. But if the rain passes through yet another layer that's colder near the ground, then it becomes sleet. Hail is also the result of water passing through different temperature layers.
Water Vapour and Humidity
When people say "It's not the heat--it's the humidity!" they're really referring to the relative humidity, or the amount of water in the air at a certain temperature compared to how much water the air can hold at that temperature.
Air, like all gases, expands at higher temperatures. So air can hold more water vapour at higher temperatures than it can at lower temperatures. The amount of humidity in the air affects how quickly water can evaporate from surfaces such as roads, lakes, even your skin! For example, at an air temperature of 80 degrees, 60 percent humidity might be very comfortable. But at 90 percent humidity, sweat evaporates from your skin very slowly, and you feel hot and sticky.
Wind is caused when air moves from an area of high pressure to one of low pressure. The greater the difference between the areas, the stronger the wind. Since the equator is constantly hot and the poles are cold, there is a general pattern to air circulation on Earth. In many areas the wind usually blows from the same direction. Wind speed is measured with an anemometer; wind direction is indicated with a wind vane.
The heat of the atmosphere is largely influenced by the sun and the degree to which it warms the air, land and oceans. The land and water retain heat and continue to heat the atmosphere after the sun disappears. Air temperature is measured with thermometers.