Navis.gr - Weather

WEATHER
See also: 'Some Forecasting Rules'   The weather concerns everyone and has some effect on nearly every human activity. It occurs within the atmosphere, the gaseous fluid that completely envelops the Earth. Weather is defined as the momentary, day-to-day state of the atmosphere over any place on the Earth's surface. Climate, on the other hand, refers to weather averaged over a long period. The basic atmospheric conditions that make up the weather include precipitation, humidity, temperature, pressure, cloudiness, and wind.

The air is constantly in movement. There also is a continuous exchange of heat and moisture between the atmosphere and the Earth's land and sea surfaces. These ever-changing conditions can be scientifically analyzed. The science of observing and predicting the weather is known as meteorology.   The Atmosphere and Its General Circulation   Air is compressed by its own weight, so that about half the bulk of the atmosphere is squeezed into the bottom 18,000 feet, or 3 1/2 miles. The bottom layer of the atmosphere, the troposphere, is the site of almost all the world's weather. Above its turbulence and storminess is the calmer stratosphere, which has little moisture and few clouds.

Underlying the great variety of atmospheric motions is a pattern of large-scale air movement over the Earth. The basic cause of these planetary winds, or general circulation of the atmosphere, is the greater heating by the sun of the air over the equator than of the air over the poles. The heated air over the equatorial regions rises and flows poleward in both the Northern and Southern hemispheres. In the polar regions the air cools and sinks and from time to time flows back toward the equator.

The upward movement of air results in a belt of low pressure astride the equator. On either side at about 30 north latitude and 30 south latitude is a belt of high pressure formed as the upper-level flow of air from the equator sinks to the surface. From each of these subtropical high-pressure belts surface winds blow outward, toward both the equator and the poles. They are deflected by the Earth's rotation to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This produces a belt of tropical easterly winds and two belts of mid-latitude westerly winds, one in each hemisphere.

Like the tropical easterlies, or trade winds, the surface winds from the poles are also deflected to the west. Where these polar easterlies meet the westerly winds in each hemisphere at about 60 latitude a belt of low pressure girdles the Earth.

This arrangement of the Earth's wind and pressure belts varies somewhat with the time of the year. They shift northward during the Northern Hemisphere summer and southward during the Southern Hemisphere summer. Both the continuity of the pressure belts and the prevailing directions of the winds are also modified greatly by the differing rates at which the Earth's land and water surfaces exchange heat and moisture with the atmosphere.   Air Masses and Weather Fronts   Air that has acquired a fairly uniform temperature and humidity over a large area of the Earth's surface is called an air mass. Air masses are of four main types arctic (A) or antarctic (AA), polar (P), tropical (T), and equatorial (E). They are of either maritime (m) or continental (c) origin. In general, a maritime air mass is relatively moist and has a moderate temperature. A continental air mass is relatively dry and has either a very hot or very cold temperature.

Every winter, immense, cold continental polar (cP) or continental arctic (cA) air masses accumulate over northern Canada and Siberia. Temperatures may sink as low as -80o F (-62oC). Cold waves occur when a cA air mass sweeps southward in the wake of winter storms. Milder maritime polar (mP) air masses accumulate over the North Pacific and North Atlantic oceans. Maritime tropical (mT) air masses move into the United States from over the Gulf of Mexico, the Caribbean Sea, and the tropical Atlantic Ocean. Maritime tropical air, because of its great moisture-holding capacity, produces heavy rains.

Weather fronts are sharp transition zones between different air masses. A cold front, which is the leading edge of a cold air mass, brings a quick drop in temperature and a rapid rise in pressure. It is often accompanied by thunderstorms in summer and snow flurries in winter. An advancing warm air mass tends to override the rear portion of the cold air mass ahead of it. The trailing edge of a retreating cold air mass along the ground is known as a warm front. Thickening and lowering cloud layers follow, usually with widespread, long-lasting precipitation.

A stationary front occurs when the boundary between a cold and a warm air mass does not move appreciably in any direction. Cloudiness and precipitation may then persist for many days on the cold side of the stationary front. An occluded front results when a cold front overtakes a warm front on the ground, lifting the warm air entirely aloft.

Weather fronts are formed as part of eastward-moving low-pressure centers known as wave cyclones or frontal cyclones. Wave cyclones form in the westerly wind belts along the polar fronts that separate polar and tropical air. A wave cyclone develops when a low-pressure area in the upper airflow approaches a stationary front on the ground. This lowers the pressure on the polar front, which then bends to form the typical horizontal wave consisting of a cold front following a warm front. The cold front swings around the equatorial side of the low as it overtakes the slower-moving warm front. As a cold front passes through an area in the Northern Hemisphere the wind generally shifts from the south and southwest to the northwest, in the Southern Hemisphere from the north and northwest to the southwest.

The stormy weather associated with a wave cyclone may affect an area of more than a million square miles. It usually reaches maximum intensity within two days. Storms in North America and Eurasia are usually steered by the upper airflow northeastward, respectively, into the Icelandic or Aleutian lows, semipermanent features of the low-pressure belt in the high latitudes of the Northern Hemisphere. The entire area of circulation is called a cyclone. In the Northern Hemisphere the circulation is counter-clockwise; in the Southern Hemisphere, clockwise.

Wave cyclones usually occur together. As a cyclone matures and moves on, a new one may form along the trailing cold front. When this occurs near an abundant supply of heat and moisture such as along the Atlantic coast of the United States, the secondary cyclone may exceed the primary one in suddenness, wind velocity, and amount of precipitation.

The Pacific Ocean, the Gulf of Mexico, and the Atlantic Ocean are the main sources of moisture for cyclones in the United States. Lows that enter the United States from these bodies of water, or that form over the western interior, may produce strong winds and heavy precipitation. Such storms occurring with a strong winter high may result in a blizzard, with bitterly cold temperatures and driving snow.

The anticyclone is the reverse of a cyclone. It is known as a high (high-pressure center). Highs are usually associated with dry, cool weather. The winds spiral outward around a high in a clockwise direction in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Highs usually originate in high latitudes and take a southeast course in the Northern Hemisphere. Extreme winter cold usually occurs in areas of high pressure, most notably in the semipermanent Siberian High. In North America, highs have carried subfreezing air as far south as the Gulf of Mexico and into Florida.

In summer the slow-moving oceanic anticyclones may influence inland areas in the central and eastern United States, producing cloudless skies, heat waves, and sometimes drought. In autumn, stagnating continental anticyclones may bring spells of summerlike weather (Indian summer). The light winds may lead to an accumulation of pollutants.   Weather Elements   The primary conditions of the atmosphere, or weather elements, are those of wind, pressure, temperature, humidity, clouds, and precipitation.

Wind is the movement of air parallel to the Earth's surface. Winds blow from areas of high pressure toward areas of low pressure, down what is called the pressure gradient. Above 2,000 or 3,000 feet, however, the winds generally blow at right angles to the prevailing pressure gradient of the general atmospheric circulation. Such winds are geostrophic winds. In the Northern Hemisphere lower pressure is to their left and higher pressure is to their right. The opposite is true in the Southern Hemisphere. At 25,000 to 35,000 feet in altitude these westerly winds may reach 200 miles per hour along narrow zones known as jet streams. The direction from which a wind is blowing is very significant in weather prediction. In the Northern Hemisphere, for example, northwest winds usually indicate colder, drier weather; easterly winds, cloudiness and precipitation; and southerly winds, warm, humid weather.

Atmospheric pressure by itself has little significance in weather forecasting. However, changes in pressure are significant if a correction is made for normal diurnal change, or the sharp fall in pressure that usually occurs during the midday hours. Falling pressure generally indicates that a storm is approaching; rising pressure indicates the approach or continuation of fair weather.

Temperature changes also are significant in weather forecasting. In the Northern Hemisphere rising temperatures are associated with southerly winds, falling temperatures with northerly winds. The opposite is true in the Southern Hemisphere. Under cloudless skies temperatures fall sharply at night. Nighttime temperatures may be 10oF lower in rural areas than in urban areas because of concentrations of man-made sources of heat in the cities.

Water vapor in the air is what clouds, fog, rain, and snow are made from. The amount of moisture in the air is known as humidity. Warm air can hold more moisture than cold air can. The maximum amount of moisture that air of a specific temperature can hold is known as its saturation value. Relative humidity is the proportion of water vapor actually in the air at a given temperature as compared with the maximum amount that the air could hold at that temperature. It may vary from almost none over deserts to as much as 100 percent in thick fog or rain.

Clouds often signal an imminent weather change. Rising cloud levels indicate clearing weather, while thickening and lowering clouds signify precipitation. Clouds form when water vapor is cooled below its dew point and condenses into tiny but visible droplets or ice crystals. The cloud base indicates the level at which rising air reaches its dew point. The main cloud types are the high, wispy cirrus, the layered stratus, and the massive, billowy cumulus. The terms alto, meaning "high," and nimbus, meaning "rain," further describe clouds.

Fog is a cloud whose base is on the ground. Like clouds, it forms when moist air cools below its dew point. Dew is formed when moist air is in contact with a surface such as grass that has been cooled below the air's dew point by nighttime radiation. When the temperature is below freezing, frost forms instead of dew.   Precipitation and Types of Storms   When warm, moist air cools to its dew point, condensation occurs if there are dust particles or salt crystals to serve as nuclei of condensation. When moist air is lifted by the collision of warm and cold air masses or by movement up a mountain slope, cooling and condensation result in extensive precipitation. In thunderstorms, updrafts hurl raindrops up and up again until they are heavy enough to fall. A hailstone grows like a raindrop, as it is exposed alternately to temperatures below and above freezing before it falls to the ground. Sleet is frozen rain.

If air is lifted above the freezing level aloft, the moisture may condense as ice crystals. When ice crystals invade a supercooled cloud, the water vapor condenses on them, forming snow crystals. As a snow crystal floats into lower, warmer air it joins with other snow crystals and becomes a snowflake. Snow flurries are caused by sudden cooling as a cold front moves in. Snowstorms occur when a flow of polar air lifts a warm air mass and when the mean temperature of the air through which the snow falls is below freezing. Big storms occur when the two air masses are blocked by a dawdling high.

When hot, moist air is carried above the freezing level by the strong updraft in a cumulonimbus cloud, thunder and lightning occur along with strong gusts of wind, heavy rain, and sometimes hail. This is a thunderstorm. The West Indies hurricane and the Pacific typhoon, powerful storms with torrential rains and winds of 75 miles per hour or more, originate over tropical seas in late summer and early fall when surface temperatures are highest and tropical air reaches farthest from the equator. Airplanes penetrate hurricanes to gauge their intensity and to plot their courses.

The tornado has a narrow, funnel-shaped trunk that reaches down from a dark thundercloud and whirls at speeds up to 300 miles per hour. A tornado moves to the northeast in the Northern Hemisphere, to the southeast in the Southern Hemisphere. Tornadoes appear most frequently in spring and early summer when, in the United States, for example, cold, dry air flows over the Rocky Mountains and overrides the warm, moist air flowing from the Gulf of Mexico. Turbulence is caused by the sinking cold air and rising warm air.   Weather Instruments   Weather conditions are measured by standard instruments. Surface wind speeds are usually measured by an anemometer. An anemometer consists of three or four wind-driven cups mounted on a vertical axis whose rate of rotation varies with wind speed. Wind direction is indicated by a vane, a pointer that swings with the wind and is mounted on a vertical axis attached to a compass rose. For weather measurements, wind direction always refers to the direction from which the wind is blowing.

Atmospheric pressure is measured by an aneroid barometer, a flexible metal vacuum box that expands or contracts with changes in pressure. Atmospheric pressure is also measured by a mercury barometer, a glass tube in which the height of a column of mercury varies with pressure changes. Continuous pressure is recorded by a barograph, a barometer that moves a pen against a rotating drum.

Temperature is measured by a thermometer, a glass tube in which the height of a column of mercury or alcohol varies with changes in temperature. Continuous temperatures are recorded by a thermograph, a special thermometer consisting of a heat-sensitive metal coil that, like a barograph, moves a pen against a rotating drum.

Humidity data, including relative humidity, vapor pressure, and dew point, are secured with the use of a hygrometer. The hygrometer used most often, a psychrometer, consists of a wet-bulb and a dry-bulb thermometer. The differences between the temperatures recorded by the two thermometers are related to the amount of moisture in the air.

The ceiling, or base height of cloud layers, is measured by an automatic ceilometer, which uses a beam of pulsed light and a photoelectric telescope. The ceilometer can measure in the daytime or at night. Other methods sometimes used are the ceiling light and the ceiling balloon; both measure the base height by triangulation.

Precipitation is usually measured by a rain gauge, an open-mouthed container that catches the rain. One inch of rain generally means one inch of water over one acre of surface. Rain gauges are the primary instruments for measuring the quantity of precipitation. Radar is used to measure the intensity of rainfall or snowfall.

Soundings of upper-level pressure, temperature, humidity, and winds are made by radiosondes carried aloft to 100,000 feet or more by balloons. The radiosonde transmits data to ground recorders. The speed and direction of upper winds are obtained by tracking a radiosonde with a radio direction finder, or radiotheodolite. Such an observation is known as a rawinsonde. Upper-wind information is also obtained by tracking an ascending balloon visually with a surveying instrument. Rockets are used to obtain data up to altitudes of about 200,000 feet, heights that cannot be reached by balloons.

Doppler radar continuously measures the wind, moisture, and temperature in the upper atmosphere. Doppler profiles record the apparent shift in frequency with respect to the observation point of waves emitted by a moving source, a phenomenon known as the Doppler effect.   Methods of Weather Forecasting   One of the most common methods of weather forecasting today is synoptic forecasting. It is based on a summary, or synopsis, of the total weather picture at a given time. The development and movement of weather systems is shown on a sequence of synoptic charts, or weather maps. These weather systems are then projected into the future. The weather observations used for synoptic charts are made at thousands of weather stations around the world four times a day at midnight, 6 A.M., noon, and 6 P.M., Greenwich mean time (GMT). The most common synoptic chart is the surface weather map. Various upper levels of the atmosphere also are charted.

Another method, statistical forecasting, employs mathematical equations based on examination of the past behavior of the atmosphere. Still another, numerical forecasting, uses mathematical models based on the physical laws that describe atmospheric behavior. For forecasts of up to about five days, numerical methods are most often used; for somewhat longer periods, statistical methods are more accurate. Beyond about 90 days, weather events can be predicted just as well through climatological forecasting, or by using the averages of past weather records.

Computer-drawn maps now predict wind, temperature, and humidity patterns for many atmospheric levels from the ground up to about 50,000 feet. Statistical methods are then used to map probable maximum and minimum temperatures and precipitation.

In weather analyses, isobars are drawn on a map. These are lines connecting points of equal atmospheric pressure. Charts showing the height of constant-pressure (isobaric) surfaces and other sets of isolines aloft also are drawn. This was a major time consuming task when it was done manually. Analysis is now largely done automatically on computers as part of numerical prediction. The computer-drawn maps show all isolines and centers of maximum and minimum value. The maps are issued as paper copy for further study and manual modification, as microfilm for archiving and retrieval, or as signals that go out over the facsimile networks to all receiving stations for continuous mapping.
See a typical Weather Facsimile Chart [93 KB]
See a typical Weather Facsimile Machine [66 KB]

Numerical weather prediction is essentially a problem in fluid dynamics. Complete and precise data on the initial state of the Earth's atmosphere, water bodies, and land surfaces, plus a complete understanding of the physical laws describing the transfer of heat and moisture, theoretically could yield near-perfect numerical weather forecasts. Such information, however, is not fully available.

Numerical weather prediction was not practical at all before high-speed computers were developed in the late 1940s. Six basic equations expressing the three dimensions of motion and the conservation of heat, moisture, and mass are used in numerical mathematical models. These equations are solved by the computers to obtain instantaneous changes at thousands of regularly spaced grid points. The changes are repeatedly computed for successive short time intervals of 20 minutes each for the desired time range of the forecast. This marching forward in time is the essence of numerical prediction. Calculations are made for several levels in the atmosphere. The entire numerical forecast procedure out to 48 hours is repeated every 12 hours. Once a day a forecast out to five days ahead is calculated as a basis for preparing daily extended outlooks.   Collection and Distribution of Weather Data   Weather stations transmit coded weather data every hour for aviation use, every six hours for general forecasting, and daily for climatological records. Surface weather data are included on precipitation, temperature, atmospheric pressure, change in pressure, wind direction and speed, humidity, dew point, cloud type, sky cover, visibility, ceiling, and current weather. In addition, daily measurements of temperature extremes and precipitation are made by volunteer observers at thousands of substations. Other weather networks are operated for warning of floods, fire weather, fruit frost, and tornadoes.

Professional weathermen communicate directly with the public by telephone, radio and television broadcasts, and teletypewriter. In some cities there are continuous VHF-FM radio weather broadcasts.
An international system of teletypewriter and facsimile networks distributes weather information. Numerically coded data from around the globe is relayed by collection stations to central processing offices.

Weather surveillance satellites, beginning with the Television and Infra Red Observation Satellite (Tiros) in 1960, have made it possible to detect weather systems from the time they begin. No longer is a destructive storm larger than a tornado likely to strike without warning. Polar-orbiting pairs of satellites, first launched in 1966, were the first operational satellite system of the United States.

The Applications Technology Satellites (ATS) of the National Aeronautics and Space Administration (NASA) were launched in 1966. These so-called synchronous satellites, 22,300 miles up, seemed to hang motionless in the sky because their orbits coincided with the rotation of the Earth below. They provided pictures of almost half the Earth every 26 minutes during daylight. Features as small as 2 miles across could be identified. Hurricanes and even individual clouds within them showed up clearly.

NASA's Nimbus III satellite, launched in 1969, was equipped with a remote sensor called the satellite infrared spectrometer (SIRS). From the SIRS measurement of infrared radiation, vertical temperature soundings could be calculated over large areas. A high-resolution infrared radiometer (HRIR) on Nimbus III was used to distinguish between warm and cold ocean currents, clouds and snow, and ice and water. In cloud-free areas the location and extent of ice fields and snow cover could be determined. This was useful in predicting floods and also for calculating thermal radiation in numerical weather forecasting.

In the 1970s a polar-orbiting satellite system, called the Improved Tiros Operational Satellites (ITOS), and the Geostationary Operational Environmental Satellites (GOES) were launched by the United States. The ITOS carried a vertical temperature profile radiometer and the GOES provided wind data from cloud movements. In the late 1970s and early 1980s these systems were replaced by the National Oceanic and Atmospheric Administration (NOAA) series (6-10) of polar orbiters and the GOES series (2-6) of geostationaries. Numbers for the NOAA series began at 6 as a continuation of the ITOS series.

The polar orbiters continue to provide data for numerical weather models. They carry advanced instruments for temperature sounding and microwave channels to facilitate retrievals in cloudy areas. Newer satellites measure both the Earth's absorption and reflection of solar radiation and the amount of ozone in the atmosphere.

GOES-5 monitored the eastern Pacific and western United States while GOES-6 monitored the western Atlantic and eastern United States. When GOES-5 failed in July 1984, GOES-6 alternately covered the eastern region during the hurricane season and the western region during the winter storm season. Another geostationary satellite was launched to replace the disabled GOES-5 in February 1987. The new satellite, called GOES-East, reported weather over the Atlantic and Gulf of Mexico. GOES-6, renamed GOES-West, monitored the eastern Pacific. GOES-West failed early in 1989, and GOES-East was moved to a more westerly position to compensate. GOES-7 had been launched by the end of 1991 and was the only operational GOES at that time. The entire GOES system provides near-continuous cloud pictures and full-Earth disc pictures through the day and night.

The ITOS and GOES systems were the United States contributions to the World Weather Watch and the Global Atmospheric Research Program (GARP). These programs improve the collection and processing of global weather data by linking international meteorological centers via computer-to-computer high-speed channels.

One of the best devices for continuous detection and tracking of hurricanes, thunderstorms, tornadoes, and other severe storms at distances up to 250 miles is radar. At Kansas City, Mo., radar summaries of the weather over the country are prepared and distributed by facsimile networks. The new radar capability of the United States Weather Service is the Weather Surveillance Radar-1988 Doppler (WSR-88D, or NEXRAD) which, when completed, will employ 137 radar stations to identify low-level wind shears associated with tornadoes.   Long-Range Weather Forecasting   Numerical weather prediction, such as atmospheric modeling on computers, is one of the most accurate methods of weather forecasting. But no matter what method is used, day-to-day forecasting decreases in reliability as the time range increases.

The increase in forecast errors over time is due to the unreliability of measurements of initial atmospheric conditions over many areas, the wide spacing of data points, and an insufficient understanding of why the atmosphere acts as it does. Such errors can cause errors in computer-calculated forecasts. They grow larger as the computations move forward in time until the numerical forecasts become useless. Persistent or systematic errors are reduced by manual corrections. A typical error of atmospheric models is that the weather systems usually move faster than predicted.

In providing public forecasts, the increasing uncertainty with time is taken into account. The range of predicted temperatures, for example, is increased as the time range increases, and precipitation is usually forecast as a probability percentage.

Continuous weather elements such as temperature can be forecast with greater accuracy than discontinuous ones such as precipitation. Forecasts for the higher levels of the atmosphere, with their smoother patterns, are more accurate than for the surface zones. Beyond about five days, daily weather cannot be accurately predicted, though average weather departures from normal can be predicted to some extent. Long-range forecasts deal with the total effects of weather systems not yet born, unlike forecasts for up to about five days. But useful inferences can still be made about the future evolution of atmospheric circulations.

The averaging of successive daily flow patterns in the atmosphere smoothes and filters out temporary disturbances, revealing broad westerly wind currents that meander between high and low latitudes. At any one time these currents form three to five large waves around each hemisphere. They move slowly and sometimes remain stationary for long periods, steering lows and highs along preferred tracks. The locations and sizes of these large waves determine the longer-period average weather anomalies such as cold spells, warm spells, and droughts.

In monthly forecasts the future locations of large-scale circulation meanders are estimated by considering how they evolved and by evaluating the effects of normal seasonal changes and of abnormal oceanic heating or cooling. Numerical predictions for the first few days of the month are also used. The resulting circulation pattern is interpreted in terms of expected temperature and precipitation anomalies by using statistical methods and climate records.

For periods beyond 30 days, forecasting methods are largely historical or statistical. In using a historical weather analogue an attempt is made to find the past period in which the weather pattern most closely corresponds to present conditions. The resulting long-range forecast is a duplicate of the weather that actually occurred in that period. Successful attempts have also been made to find some statistical correlation between a given weather condition and one or more other phenomena, even though no physical relationship is known. The Indian monsoon rains, for example, have been found to be statistically correlated with atmospheric pressure over South America.   Weather Modification   Smaller efforts at controlling or modifying the weather, such as the use of smudge pots to prevent orchard frosts, utilize relatively manageable engineering. A major breakthrough in weather modification occurred in 1946, when it was discovered that seeding supercooled clouds with dry ice pellets could produce precipitation. The same effect is produced by seeding with silver iodide smoke. Seeding is based on the artificial provision of nuclei for the condensation or freezing of water vapor in the air. Most seeding is done from aircraft. Other means include airborne ramjets, rocket and artillery shells, and ground-based generators. Cloud seeding is mainly used to increase precipitation in order to fight drought. Some seeding inhibits cloud formation and thus diminishes precipitation. This may be valuable in flood prevention.

Seeding experiments are aimed at dispersing fog at airports, reducing the formation of crop-damaging hailstones, preventing forest fires by suppressing lightning, and reducing the fury of hurricanes. Seeding may also be used to redistribute precipitation and to diminish heavy snowfalls. Because there are large, unexpected natural variations in the weather, it is almost impossible to measure the effectiveness of human intervention. In attempts to increase rainfall, for example, it is difficult to determine how much rain would have fallen without seeding.

By the late 1980s, success in controlling the weather was almost entirely limited to increasing precipitation and dispersing fog. Intentional modification of the weather remains speculative. Research in the 1980s emphasized the numerical modeling of small-scale weather systems and cloud physics. By modeling these systems with sufficient accuracy, researchers were able to determine the kind of action necessary to bring about the desired change in weather.

Inadvertent weather modification is another area that has stirred a great deal of interest. This involves the study of changes in weather and climate that are brought about by changes in land use. The building or expansion of cities and the conversion of farm land to industrial use can cause changes in weather. Whether such changes in land use cause significant changes in climate remains unknown. (For further study of weather and meteorology).   Weather Terms   Air Mass: A large body of air with the uniform temperature and humidity of its source region.
Anticyclone: A large area of high atmospheric pressure, characterized by outward-spiraling winds. A high.
Atmospheric Pressure: The force exerted by the atmosphere's weight on a surface of unit area.
Climate: The long-term average weather of a region.
Cyclone: A large area of low atmospheric pressure, characterized by inward-spiraling winds. A low.
Front: The surface between two different air masses.
High: The center of an area with high atmospheric pressure.
Humidity: The amount of water vapor in the air.
Isobar: A line through points of equal atmospheric pressure.
Low: The center of an area with low atmospheric pressure.
Polar Front: The surface between polar and tropical air masses, along which cyclonic disturbances form.
Synoptic Chart: A map depicting the weather in an area at a given moment.
Wave Cyclone: A storm or low-pressure center that moves along a front.   World Weather Extremes   Highest temperature. 136 F (57.8 C) (El Azizia, Libya, Sept. 13, 1922).
Highest average annual temperature. 94 F (34.4 C) (Dallol, Ethiopia).
Lowest temperature. -128.6 F (-89.2 C) (Vostok, Antarctica, July 21, 1983).
Lowest average annual temperature. -70 F (-56.7 C) (Plateau Station, Antarctica).
Greatest 12-month rainfall. 1,042 in (2,646.7 cm) (Cherrapunji, India, August 1860-July 1861).
Lowest average annual precipitation. 0.03 in (0.08 cm) (Arica, Chile).
Highest sea-level atmospheric pressure. 32.01 in (81.31 cm) (Agata, U.S.S.R., Dec. 31, 1968).
Lowest sea-level atmospheric pressure. 25.90 in (65.8 cm) (Typhoon Ida, Philippine Sea, Sept. 24, 1958).   Weather Sayings   Mackerel sky and mares' tails make lofty ships carry low sails. Mackerel sky (cirrocumulus clouds) and mares' tails (cirrus clouds) indicate that windy, stormy weather is coming.

One would rather see a wolf in February than a peasant in his shirtsleeves. Crops made to sprout prematurely by early warm weather (when a person would dress lightly) may be killed by a temporary return of cold weather.

Rainbow in the morning, travelers take warning; rainbow at night, traveler's delight. As storms usually come from the west in the mid-latitudes, a morning rainbow, seen in the west, indicates humid air and the coming of stormy weather. An evening rainbow, seen in the east, indicates the passing of stormy weather.

The higher the clouds the better the weather. High clouds indicate the dry air and high pressure of fair weather.

When ropes twist, forget your haying. Twisting rope fibers indicate increasing humidity and the coming of rain.