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What is precipitation? • Precipitation is any form of moisture which falls to the earth. This includes rain, snow, hail and sleet. Precipitation occurs when water vapour cools. When the air reaches saturation point (also known as condensation point and dew point) the water vapour condenses and forms tiny droplets of water. These tiny droplets of water from clouds. • All rain is the same. It happens as the result of warm, moist air being cooled, leading to condensation and in turn rain.
Relief Rainfall • Stage 1.Warm wet air is forced to rise over high land. • Stage 2.As the air rises it cools and condenses. Clouds form and precipitation occurs. • Stage 3.The drier air descends and warms. • Stage 4.Any moisture in the air (e.g. cloud) evaporates.
Convectional Rainfall • Stage 1.The sun heats the ground and warm air rises. • Stage 2As the air rises it cools and water vapour condenses to form clouds. • Stage 3.When the condensation point is reached large cumulonimbus clouds are formed. • Stage 4.Heavy rain storms occur. These usually include thunder and lightening due to the electrical charge created by unstable conditions.
Frontal Rainfall • Stage 1.An area of warm air meets and area of cold air. • Stage 2.The warm air is forced over the cold air • Stage 3.Where the air meets the warm air is cooled and water vapour condenses. • Stage 4.Clouds form and precipitation occurs
Climatic Zones • The world has several climatic zones. These are summarised on the map below. • The classification is based on maximum and minimum temperatures and the temperature range as well as the total and seasonal distribution of precipitation.
Summary of climatic zones: • Polar - very cold and dry all year • Temperate - cold winters and mild summers • Arid - dry, hot all year • Tropical - hot and wet all year • Mediterranean - mild winters, dry hot summers • Mountains (tundra) very cold all year
What is pressure? • Pressure is the weight of air pressing down on the earth's surface. Pressure varies from place to place and and results in pressure systems.
What is low pressure? • At times of low pressure the air is usually rising. As the air rises, it cools, condenses and forms clouds. Areas of low pressure are known as depressions. • Depressions, or 'lows', bring rain, strong winds and changeable conditions. Changeable weather is a feature of British weather, depressions are responsible for much of this. • Strong winds blow in an anticlockwise direction.
Front • Depressions occur when warm air meets cold air. The point where warm air meets cold air is called a front. Along the front there is usually cloud and rainfall. This occurs because the warm air cools and condenses when it meets the cold air
What is high pressure? • Areas with above average pressure are called anticyclones. Anticyclones occur when air is sinking. As a result there are usually few clouds in the air. In the UK anticyclones bring cold clear days in winter and hot and sunny days in summer. • Light winds blow in a clock wise direction.
Weather experienced during a winter anticyclone • In winter the skies are cloudless so heat is allowed to escape. Therefore temperatures are usually very cold. The ground cools rapidly at night so frost often forms. Fog can also form as the cold air makes water vapour condense into tiny droplets. Fog can last long into the day as there is insufficient heat from the sun to evaporate the water droplets away.
Weather experienced during a summer anticyclone • Summer anticyclones bring very different weather. As the air descends it is heated causing water in the air to evaporate. Therefore there are few clouds in the air. The skies are clear allowing the suns rays to reach the surface of the earth. This causes temperatures to rise. Heat waves can occur if anticyclones remain over Britain for a number of weeks.
Air mass: • A large body of air that has similar temperature, pressure and moisture properties.
Anticyclone: • High pressure system in which air descends to give calm conditions and clear skies. Associated with summer heatwaves and winter frosts and fogs.
Atmosphere: • The air surrounding the Earth and bound to it by gravity.
Atmospheric pressure: • Pressure produced by the atmosphere on any surface by its weight.
Clear sky: • Sky with a total cloud cover of less than one okta.
Climate: • long-term (30 year) weather averages.
Cold front: • The "leading edge" of a relatively cold air mass.
Continental climate: • A climate with a high temperature range away from the influence of the sea. Winters will be colder and summers warmer compared to a coastal location for the same latitude.
Depression (cyclone, low-pressure): • Area in the atmosphere in which the pressures are lower than those of the surrounding region at the same level. In its development a depression usually has the following phases. A wave (young) depression forms and moves along a front. Mature depressions have well-developed warm sectors and both cold and warm fronts. An occluded depression is that within which there has developed an occluded front.
Dew point (dew-point temperature): • The temperature to which certain air must be cooled in order for saturation to occur. When this temperature is below 0 °C, frosts form.
Evaporation: • The physical process by which a liquid or solid substance is transformed to a gas; the opposite of condensation.
Fog: • Saturated air with visibility below one kilometre. Fog differs from cloud only in that the base of fog is at the Earth's surface while clouds are above the surface.
Front: • The meeting point between two air masses of different density. Since the temperature is the most important regulator of the atmosphere density, a front almost invariably separates air masses of different temperature. When warmer air replaces the colder, it is a warm front, and a front is a cold one when the opposite occurs.
Humidity: • Water vapor content of the air.
Isobar: • A line of equal or constant pressure. Measured in millibars (mb).
Maritime climate: • A climate with a low temperature range influenced by proximity to the sea. Winters will be warmer and summers cooler compared to a continental location for the same latitude.
Occluded front: • Afront that is formed as a cold front overtakes a warm front and lifts the warm air completely off the ground.
Okta: • A measure of cloud cover (in fractions of eight) on a synoptic chart.
Precipitation: • Any of all of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The forms of precipitation are: rain, drizzle, snow, hail, and ice pellets.
Prevailing wind • Is the most common wind direction for a particular location.
Relief rainfall: • Formed when air is forced to rise over relief features such as hills or mountains. Cooling and condensation occurs as the air rises.
Seasonality: • Periodic fluctuations in the climate related to seasons of the year e.g. wet winters, drier summers.
Smog: • A word currently used as a synonym for general air pollution. It was originally created by combining the words "smoke" and "fog."
Synoptic chart: • A weather chart reflecting the state of the atmosphere over a large area at a given moment.
Temperature: • A physical quantity characterizing the mean random motion of molecules in a physical body. In other words, it is a measure of the degree of hotness or coldness of a substance.
Temperature range: • Maximum minus the minimum temperature for a particular location.
Warm front: • The forward edge of an advancing warm air mass that is rising over cooler air in its path.
Warm sector: • The zone of warm air within a depression.
Water vapour: • Water substance in vapour (gaseous) form; one of the most important of all constituents of the atmosphere.
Weather: • The state of the atmosphere, mainly with respect to its effects upon life and human activities. As distinguished from climate, weather consists of the short-term (minutes to about 15 days) variations of the atmosphere state.
Wind: • Movement of air caused by changes in temperature and air pressure. Winds are always identified by the compass direction from which they blow.
Clouds • A cloud is a visible aggregate of tiny water droplets and/or ice crystals in the atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied by precipitation; rain, snow, hail or sleet.
Introduction to Weather • Meteorology is the study of weather. Weather is caused by the movement or transfer of energy. Energy is transferred wherever there is a temperature difference between two objects. Many weather phenomena result from a transfer of energy that occurs via the movement of air in the atmosphere. This is known as convection. • Air contains water vapour from the evaporation of liquid water sources on the Earth's surface, including oceans, lakes and rivers, and from evapotranspiration by plants. When air is moved about the Earth, either vertically when uplifted or horizontally as part of air masses, it may cool and release water vapour as condensation in the form of clouds and eventually rain and other forms of precipitation, which is returned to Earth. This cycle of evaporation, condensation and precipitation between the Earth and the atmosphere is known as the water cycle.
Introduction to Weather • The physical transfer of heat and moisture by convective processes is the basis for the formation of many meteorological patterns and features, including anticyclones, depressions, fronts, monsoons, thunderstorms, hurricanes and tornadoes. Heat however, may also radiate directly from a hot object to a colder one, without involving the movement of air. Many small-scale weather phenomena are the result of this form of heat transfer, including dew, frost and fog. • Weather can be simply measured by observing and recording temperature, rainfall, pressure, humidity, sunshine, wind and cloudiness. It is also possible to identify and name different types of clouds, which are associated with different patterns of weather. Commonly observed cloud types include cirrus, cumulus, cumulonimbus and stratus. To make predictions and forecasts about what the weather will do in the future however, it helps to draw synoptic charts, composed of special weather symbols and isobars that reveal patterns of weather. The use of sophisticated technology such as weather radar and satellite imagery also assist with weather forecasting.
Anticyclones • The cloudy rainy weather of low-pressure depressions is due to rising air, which is most pronounced near frontal regions. The anticyclone on the other hand is produced by a large mass of descending air. This takes place throughout a depth of the atmosphere up to 12km. This means that the air is very stable and atmospheric pressure is high. In addition, winds associated with an anticyclone are usually very light if present at all, especially close to the centre of the high-pressure system. • Subsidence warms the air by compression. Any clouds present quickly evaporate as the temperature of the air rises above its dew point. For this reason, anticyclones usually bring fine, dry and settled weather, particularly in the summer. • Sometimes, subsidence and compression of the air can produce a temperature inversion at one or two thousand metres above the ground. Such phenomena act as caps to rising air heated by the ground under the influence of the Sun, preventing extensive air cooling and cloud formation. Unfortunately, if the air is moist below the temperature inversion, a dreary formless layer of cloud can form which becomes difficult to disperse owing to the light winds.
Anticyclones • Winter anticyclones, if clear of cloud, bring with them further problems. A short cloudless day is the forerunner of a long night with more radiation cooling than a low-angle Sun can counteract the next day. The second night of cooling therefore starts with a lower air temperature than the first. Such conditions, if persistent, can lead to successive nights of frost, which become progressively harder. When the air is particularly moist, cooling at night soon results in fog. Britain in particular can experience episodes of anticyclonic fog from late September through to May. • Anticyclones move, but not quite in the same purposeful way as travelling depressions. They nudge their way into position and can be incredibly stubborn about leaving, perhaps persisting for weeks, diverting depressions to different routes. Such persistent anticyclones are known as "blocking highs". In winter they can lead to long spells of very cold weather, especially if their airflow comes from Russia and Siberia. In summer they can lead to long hot spells and sometimes drought. • A ridge of high pressure is a wedge-shaped extension of an anticyclone or belt of high pressure. The weather associated with ridges is similar to that in an anticyclone. In temperate latitudes as in the British Isles, ridges of high pressure often occur between two depressions and move with them. They give rise to intervals of fair weather between the cloud and rain of the low-pressure systems.
Clouds • A cloud is a visible aggregate of tiny water droplets and/or ice crystals suspended in the atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied by precipitation; rain, snow, hail or sleet. • All clouds form as a consequence of rising air. Sometimes air is forced to rise over mountains. More usually, warm air, being less dense, will rise above cold air. At fronts for example, warm air masses rise over cold air masses when they converge. At much smaller scales, columns of rising warm air may be generated by daytime heat from the Sun. • When air rises, it expands, causing cooling and a drop in temperature. As the temperature falls, the humidity (or water vapour content) of air increases towards 100%. Finally, after sufficient cooling, the air becomes saturated, and water vapour begins to condense out as tiny water droplets, forming cloud.
