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Education Center |
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Education Centre |
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- General Meteorology
Air Masses
- A for arctic (60o - 90o N)
- P for polar (40o - 60o N or S)
- T for tropical (15o - 35o N or S)
- E for equatorial (15o N - 15o S)
- AA for Antarctic (60o - 90o S)
Air Mass Types
Generalized Map of Global Air Masses
- Continental Arctic (cA)- very cold; very dry
- Continental Antarctic (cAA) - very cold; very dry
- Continental Polar (cP) - cold & dry
- Continental Tropical (cT) - warm & dry
- Maritime Tropical (mT) - warm & moist
- Maritime Equatorial (mE) - very warm; very moist
- Maritime Polar (mP) - cool & moist
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Cold Front
Cold fronts are usually associated with depressions. A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. At a cold front cold air following warm air undercuts the warm air, heaving it upwards with a more violent thrust compared to the steady rise of air at a warm front.
The air associated with a cold front is usually unstable and conducive to cumulonimbus cloud formation. Because the up thrust is delivered along a boundary between the two air masses, the cumulonimbus form a well-defined line in contrast to the well-spaced clouds forming during thermal convection. Usually, rainfall associated with cold fronts is in the form of heavy deluge. More rain may fall in a few minutes as the cold front passes than during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5°C or more within the first hour.
On synoptic (weather) charts a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.
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Fog
Fog is really cloud at ground level that has formed because the air is cold and moist enough.
When air is cooled the amount of water vapour that it can hold decreases. At the dew point temperature, air is saturated. A further fall in temperature will result in condensation of excess water vapour in the form of water droplets. If a sufficiently thick layer of air is moist, condensation can occur throughout giving rise to fog. Visibility is usually reduced to below 1,000 metres.
With no wind at all, fog will form first as shallow streaks near the ground. More usually there is a little prevailing wind serving to spread the fog evenly within one or two hundred metres of the ground. The moister the air, the greater the likelihood of fog forming under clear skies at night when radiation cooling is greatest. As with dew and frost, fog formation is most likely in low-lying grounds and hollows into which colder air sinks, and least likely on hilltops.
Fogs formed as a result of radiation cooling are termed radiation fogs. Advection fogs, in contrast, form when warm humid air from different sources passes over a much colder surface causing condensation. Sea fog in coastal areas is a form of advection fog, formed when warmer sea air comes inland passing over colder land.
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High Pressure
High-pressure area
When air cools, its molecules huddle closer together. The air becomes more dense (higher pressure) and it sinks. This is the nature of a high-pressure area.
The air pushes outward near the surface, seeking surrounding areas of lower air pressure. In the Northern Hemisphere, the Earth's rotation deflects these winds into a clockwise rotation.
In a high-pressure area, weather is generally fair and winds typically light.
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Humidity
The amount of water vapour air can hold before becoming saturated is defined by the absolute humidity, which varies with temperature.
Some water in the form of invisible vapour is intermixed with the air throughout the atmosphere. It is the condensation of this vapour which gives rise to most weather phenomena: clouds, rain, snow, dew, frost, and fog. There is a limit to how much water vapour the air can hold and this limit varies with temperature, When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Warm air can hold more vapour than cold air. In general the air is not saturated, containing only a fraction of the possible water vapour.
The amount of vapour in the air can be measured in a number of ways. The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. This is the absolute humidity of air. Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10°C contains 9.4 g/m3 (grams per cubic metre) of water vapour when saturated. If air at this temperature contains only 4.7 g/m3 of water vapour, then the relative humidity is 50%.
When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Relative humidity is 100%. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point. The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer. A hygrometer has two thermometers, one dry bulb or standard air temperature thermometer, and one wet bulb thermometer. The wet bulb thermometer is an ordinary thermometer which has the bulb covered with a muslin bag, kept moist via an absorbent wick dipped into water. Evaporation of water from the muslin lowers the temperature of the thermometer. The difference between wet and dry bulb temperatures is used to calculate the various measures of humidity.
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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..
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.
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Large Thunderstorms
Large thunderstorms on Earth can even be spotted easily from space.
A thunderstorm is rain or hail accompanied by thunder and lightning and gusty winds. Thunderstorms usually develop when there is sufficient heating of air near the Earth's surface which rises in a very unstable atmosphere. Thunderstorms are a violent example of atmospheric convection, with uplift and cooling of air, and subsequent cloud formation. As the cloud forms, water vapour changes to liquid and/or frozen cloud particles. This results in a large release of heat that takes over as the principal source of energy for the developing cloud. Once the cloud starts to form by other forces, this release of heat helps keep it growing. The cloud particles grow by colliding and combining with each other, forming rain, snow, and hail. When the droplets become heavy enough to fall against the updraft, precipitation begins, which may be short-lived but very heavy.
Having reached its final stage of growth, the towering cumulonimbus cloud may be several miles wide and often 10,000 metres or more in height. High level winds shear the cloud top into the familiar anvil shape. When the Sun illuminates these cloud towers, they appear as huge white mountains. When moving several abreast they may form a squall line.
Once precipitation begins the updraft which initiated the cloud's growth weakens and is joined by a downdraft generated by the precipitation. This updraft-downdraft couplet constitutes a single storm "cell". On the ground the updrafts and downdrafts of air are felt as rapid gusts of wind. Most storms are composed of several cells that form, survive for about half an hour, and then die. New cells may replace old ones, and it is possible for some storms to continue for several hours.
lightning always accompanies the thunderstorm. lightning arises from a discharge of electrical energy which has built up within the cumulonimbus cloud as a result of repeated separation and splitting of water and ice particles in the turbulent conditions that prevail. Although air is a fairly good insulator, eventually the separation of electric charge becomes so great that the insulation breaks down and a lightning strike results. lightning discharge may occur entirely within the cumulonimbus cloud or between the cloud and the ground. The lightning strike causes a rapid heating of the surrounding air, resulting in a sudden expansion and contraction of air that is heard as thunder. Close to the lightning strike the thunder may be heard as a short loud crack. Further away, the thunder rumbles or echoes, because sound from different parts of the lightning strike are not all heard at the same time. One can work out how far away the lightning strike was by counting the time taken for the thunder to arrive. A 5 seconds difference is roughly equal to a distance of 1 mile.
In warmer regions of the world thunderstorms can be particularly violent because they contain so much energy, made available from the strong surface heating by the Sun. Such thunderstorms may also be accompanied by tornadoes, rapidly spinning columns or spouts of air. Tornadoes are particularly common in the central United States.
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lightning
During any given minute, there are more than a thousand thunderstorms around the Earth causing some 6,000 flashes of lightning. Every minute!
Thunderstorms are caused by rapidly rising and falling currents of air. The friction from this moving air creates electrical charges within a cloud. Water droplets and ice pellets fall, carrying charged electrons to the lower portion of the cloud, where a negative charge builds. A positive charge builds up near the top of a cloud.
Most of the electrical energy in a thunderstorm is dissipated within the clouds, as lightning hops between the positively and negatively charged areas. lightning becomes dangerous, though, when it reaches for the Earth.
How lightning strikes
When the negative charge in the cloud becomes great enough, it seeks an easy path to the positively charged ground below. The current looks for a good conductor of electricity, or a tall structure anchored to the ground (such as a tree or a tall building). The negative charge sends out a feeler, called a stepped leader, which is a series of invisible steps of negative charges.
As the stepped leader nears the ground, a positive streamer reaches up for it. Only then, once this channel is made, does the visible lightning happen. A return stroke runs from the ground to the clouds in a spectacular flash.
Though the bolt appears continuous, it is actually a series of short bursts. Most lightning strikes occur in less than a half second and the bolt is usually less than 2 inches in diameter.
Thunder
The air around a lightning bolt is superheated to about 54,000 degrees Fahrenheit (five times hotter than the sun!). This sudden heating causes the air to expand faster than the speed of sound, which compresses the air and forms a shock wave; we hear it as thunder. Since the bolt is actually several short bursts strung together, multiple shock waves are created at different altitudes; this is why thunder seems to rumble -- each shock wave takes a different amount of time to reach your ear.
Estimating distance
Because light travels faster than sound (186,291 miles per second vs. 1,088 feet per second) you see lightning before you hear the thunder. When you see it, count the seconds before the thunder arrives. Divide this number by 5, and you'll know approximately how many miles away the lightning was (5 seconds = 1 mile).
lightning facts
- A lightning charge contains 30 million volts at 100,000 amperes.
- The total energy in a large thunderstorm is more than that in an atomic bomb.
- About a hundred U.S. residents are killed by lightning every year.
- Benjamin Franklin's famous kite experiment, in 1752, showed that lightning was electricity.
- The Empire State Building in New York City is struck by lightning about 25 times every year.
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Low-Pressure
Low-pressure area
When air warms, its molecules scatter. The air becomes lighter and rises.
The formation of low-pressure systems is more complicated, however, and involves a wavelike action that occurs between two areas of high pressure. The wave becomes stronger until it breaks and a low-pressure system is born, developing a rotation that is counter clockwise in the Northern Hemisphere.
In a low-pressure area, weather is generally cloudy and winds typically strong.
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Meteorology
The science of the study of weather is called meteorology. Meteorology is the study of the changes in temperature, air pressure, moisture, and wind direction in the lowest part of the atmosphere in which most of the observed weather phenomena occur. Meteorologists investigate these day-by-day variations in the weather.
Weather phenomena are governed by a set of physical and chemical processes which are determined by simple mathematical relationships. The way these processes interact however, creates a much more complex system, which is why the weather can be so unpredictable and hard to forecast. Meteorology combines the disciplines of mathematics, physics, chemistry and geography to try to simplify the understanding of an inherently complex atmospheric system.
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Occlusion Fronts
Mid-latitude depressions are usually associated with warm and cold fronts separating warm and cold sectors of air. The lighter warm air rises above the heavier cold air, more gently at a warm front but more vigorously at the cold front following behind. Cold fronts usually travel faster than warm fronts, and therefore at some stage of depression development, the cold front catches up with the warm front. In cross section, the warm air is lifted right off the ground, so that the observer on the surface misses out the warm sector stage. This is known as an occlusion or occluded front.
On synoptic (weather) charts an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple
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Rain
How Rain Is Formed
The oceans are the main source of rain, but lakes and rivers also contribute to it. The sun's heat evaporates the water. It remains in the air as an invisible vapour until it condenses, first into clouds and then into raindrops. Condensation happens when the air is cooled.
Air cools either through expansion or by coming into contact with a cool object such as a cold landmass or an ice-covered area. When air passes over a cold object, it loses heat and its moisture condenses as fog, dew, or frost. Air also cools as it rises and expands. The water vapour in the cooling air condenses to form clouds and, sometimes, rain.
Air rises for several reasons:-
- In orographic lift, the air is forced upward as it encounters a cooler, denser body of air or when it meets raised landforms such as mountains.
- In convective lift, air coming into contact with a warm surface, such as a desert, is heated and becomes more buoyant than the surrounding air.
- Convergent lift occurs in storms such as tornadoes. Air whirling toward the centre of a cyclone collides with itself and is forced upward.
For raindrops to form there must be particles in the air, such as dust or salt, at temperatures above freezing. When the particles are cooled to temperatures below freezing point, water condenses around them in layers. The particles become so heavy they fall through the clouds. In a thunderstorm, the rain particles may become very large and fall from the cloud as hail. When the air temperature is at or below freezing all the way to the ground, the particles will fall as snow.
The formation of rain clouds may be very local. During a hot summer day, air rising over a moist region may cause cumulus, or woolpack, clouds to form in the cooler air above the surface. These clouds darken to rain clouds as more moisture condenses. Frequently, the rain cloud is the only cloud in the area, the rest of the sky remaining sunny. Such rainstorms occur almost constantly in the doldrums--the hot, calm areas near the equator. Cumulus clouds can sometimes be forced to release rain by "seeding" them with particles of dry ice or silver iodide. Commercial rainmakers have claimed success using these methods. (See also Clouds)
Rainbows
When light from a distant source, such as the sun, strikes a collection of water drops--such as rain, spray, or fog--a rainbow may appear. It appears as a multicoloured arc whose "ends" seem to touch the Earth. Rainbows are seen only when the observer is between the sun and the water drops, so rainbows appear in the part of the sky opposite the sun. Rainbows are frequently seen in the early morning or late afternoon, when the sun is low in the sky.
The colours that make up the rainbow are red, orange, yellow, green, blue, indigo and violet.
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Sea Breaze & Land Breaze
A sea breeze develops when the land become much warmer than the sea on a sunny day.
A land breeze develops when the land cools down much more than the sea during the nighttimes.
Sea Breeze
Temperature differences at the Earth’s surface occur wherever there are differences in surface substances. Water for example, has a much greater heat capacity than soil and rock. When the Sun heats it, it takes much longer for its temperature to rise. On a warm summer day along the coast, this differential heating of land and sea leads to the development of local winds called sea breezes.
As air above the land surface is heated by radiation from the Sun, it expands and begins to rise or convict, being lighter than the surrounding air. To replace the rising air, cooler air is drawn in from above the surface of the sea. This is the sea breeze, and can offer a pleasant cooling influence on hot summer afternoons when further inland the heat may become oppressive. A very hot summer Sun may cause a sea breeze of up to 15 mph along the coast, felt in decreasing strength 20 to 25 miles inland.
Since the sea breeze owes its existence to the enhanced heating of the land under the Sun, it follows that at night, when the land cools faster than the sea, a land breeze may develop. In this case, it is air above the warmer surface water that is heated and rises, pulling in air from the cooler land surface.
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Temperature
The hotness or coldness of a substance is called its temperature and is measured with a thermometer. The ordinary thermometer consists of a hollow glass bulb attached to a narrow stem with a thread-like bore. The bulb is filled with liquid, usually mercury, but also alcohol when very low temperatures need to be measured, which expands when the temperature rises and contracts when the temperature falls. The amount of expansion and contraction is measured by a calibrated scale.
Whilst thermometers are really measuring their own temperature, they are usually needed to measure the temperature of the surrounding air. To ensure that the temperature of the surrounding air is the same as the thermometer, it must be shaded from sunlight and be exposed to adequate ventilation. These conditions are provided by enclosing the thermometer within a white wooden box with louvered sides, called a Stevenson screen
Most temperature scales today are expressed in degrees Celsius (°C), although one will sometime see Fahrenheit (°F) in use, particularly in the United States. The Celsius scale is fixed by two points, the freezing and boiling point of water, which at normal atmospheric pressure are 0°C and 100°C respectively. The scale is then divided into 100 units. 0°C is equivalent to 32°F and 100°C to 212°F. The Kelvin temperature scale is the absolute temperature scale. Absolute zero, the coldest temperature possible in the universe is 0K or -273°C. Because one Kelvin is equivalent to one degree Celsius, 0°C is the same as 273K. 15°C is the same as 288K.
Special thermometers are used to indicate the maximum and minimum temperatures reached over a period, usually one day. For the amateur, a popular combined maximum and minimum thermometer is the U-shaped thermometer. Thermometers are also used to measure the temperature of the ground at night, which may fall several degrees below that of the air above, and to calculate the humidity of air.
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The Water Cycle
The water cycle, of evaporation, condensation and precipitation, drives much of the world's weather.
Water covers 70% of the Earth's surface. Almost all of this is stored in the oceans (97.5%) and in freshwater lakes, rivers and streams on land (2%). The atmosphere holds less than .001% in the form of water vapour. If all this water vapour was precipitated completely and evenly over the whole Earth, it would yield only about 25mm or 1 inch of rainfall. Nevertheless, water vapour in the atmosphere plays a very important role in the weather.
There is always water vapour present in the atmosphere. When the air becomes saturated, excess water vapour is released as condensation. This condensation is the source of all clouds and rain. Water vapour enters the atmosphere by evaporation from surface bodies of water. These include puddles, ponds, streams, rivers, lakes and oceans. Water also enters the atmosphere by evapotranspiration from plants and trees. The water vapour is returned to the Earth's surface as precipitation (rain, hail, sleet or snow), and is received by soil, vegetation, surface streams, rivers and lakes and ultimately the sea. This cycle of evaporation, condensation and precipitation is called the water cycle of the Earth and atmosphere. Annual precipitation for the Earth is more than 30 times the atmosphere's total capacity to hold water. This indicates the rapid recycling of water that occurs between the Earth's surface and the atmosphere.
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Tornadoes
Tornadoes don't pack near the energy of a hurricane. It's the way the energy is concentrated that makes twisters so deadly.
How tornadoes form
Scientists don't completely understand how tornadoes work. Some tornadoes occur inside tropical cyclones and hurricanes. But most form where cold, dry air meets warm, most air. (The southern and central U.S. are typical places where this occurs.) The moist air is lifted rapidly, creating tremendous instability and large thunderheads.
Violent updrafts and strong winds sometimes combine to form a whirling vortex of air. For the vortex to form, the air must be blowing in the same direction (usually counter clockwise when viewed from above) at all levels of the atmosphere. If the vortex reaches the ground, it is called a tornado. It turns brown or gray because of the dirt and other material it sucks up. A funnel cloud is a vortex that has not touched down. A waterspout is a tornado over water.
Fujita scale
The Fujita wind damage scale provides a measure of a tornado's potential destruction:
| Scale |
Wind (mph) |
Damage |
| F-0 |
72 or less |
Light |
| F-1 |
73-112 |
Moderate; roofing peeled |
| F-2 |
113-157 |
Considerable; roofs torn off |
| F-3 |
158-206 |
Severe; roofs and walls destroyed |
| F-4 |
207-260 |
Devastating; strong houses destroyed |
| F-5 |
261-318 |
Incredible; houses moved |
| F-6 |
319-380 |
Unlikely to occur |
Tornado facts
Some 800 tornadoes strike the United States every year, most often in the lower Mississippi Valley.
Tornadoes
- create Earth's fastest winds, believed to exceed 300 mph.
- usually travel to the northeast.
- occur most frequently in the U.S. between 4 and 6 p.m.
- average 5 to 10 minutes on the ground.
- can stand still or move forward at 70 mph.
- can be up to a mile wide at ground level.
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Warm Front
A warm front exists when warm air is rising over cold air. In vertical cross-section, the boundary takes the form of a gradual slope (roughly 1:100) and lifting is slow but persistent. As the air lifts into regions of lower pressure, it expands, cools and condenses water vapour as flat sheet cloud (altostratus), from which rain can start to fall once cloud has thickened to about 2,500 metres from the ground. Cloud continues to lower towards the boundary at ground level, known as the surface front. This lower level cloud is called stratus or nimbostratus, from which appreciable amounts of rain may fall. Sometimes, nimbostratus cloud may be only a few hundred feet above the ground, and can completely cover hilltops and mountains.
Because frontal systems have a velocity of there own, an observer on the ground will witness a succession of cloud types with cloud gradually thickening before rain arrives. These telltale signs can be used by the observer to predict the onset of bad weather within a few hours. When the surface warm front arrives, there may be a burst of rather heavier rain, and this offers a hopeful sign that a drier interlude is on the way. Clouds will break, rain cease, and there may be a noticeable rise in temperature as the warm air engulfs the observer.
On synoptic (weather) charts a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.
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Wind
The air is nearly always in motion, and this is felt as wind. Two factors are necessary to specify wind, its speed and direction. The direction of wind is expressed as the point of the compass from where the wind is blowing. Air moving from the northeast to the southwest is called a northeast wind. It may also be expressed in degrees from true north. A northeast wind would be 45°. A southwest wind would be 235°. The wind speed can be expressed in miles or kilometres per hour, meters per second, knots or as a force on the Beaufort scale..
Wind develops as a result of pressure or temperature differences between two locations on the Earth’s surface. Sea breezes for example, develop due to the differential heating of land and sea at the coast during warm sunny days. Winds also blow out from high-pressure regions or anticyclones and into low-pressure regions, for example depressions. The wind however, does not blow in a straight line, but follows a spiralling path because large movements of air are deflected by the Carioles force as a result of the Earth’s rotation. At a global scale, the temperature and pressure differences across the latitudes of the Earth generate the global wind belts.
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