Weather, is the state of the atmosphere at a particular time and place. The elements of weather include temperature, humidity, cloudiness, precipitation, wind, and pressure. These elements are organized into various weather systems, such as monsoons, areas of high and low pressure, thunderstorms, and tornadoes. All weather systems have well-defined cycles and structural features and are governed by the laws of heat and motion. These conditions are studied in meteorology, the science of weather and weather forecasting.
Weather differs from climate, which is the weather that a particular region experiences over a long period of time. Climate includes the averages and variations of all weather elements.
Temperature
Humidity
Clouds
Types of Clouds
Precipitation
Wind
Pressure
Scales of Weather
Causes of Weather
Weather Systems
Temperature is a measure of the degree of hotness of the air. Three different scales are used for measuring temperature. Scientists use the Kelvin, or absolute, scale and the Celsius, or centigrade, scale. Most nations use the Celsius scale, although the United States continues to use the Fahrenheit scale.
Temperature on earth averages 15° C (59° F) at sea level but varies according to latitude, elevation, season, and time of day, ranging from a record high of 58° C (140° F) to a record low of -88° C (-130° F). Temperature is generally highest in the Tropics and lowest near the poles. Each day it is usually warmest during mid-afternoon and coldest around dawn. Seasonal variations of temperature are generally more pronounced at higher latitudes. Along the equator, all months are equally warm, but away from the equator, it is generally warmest about a month after the summer solstice (around June 21 in the northern hemisphere and around December 21 in the southern hemisphere) and coldest about a month after the winter solstice (around December 21 in the northern hemisphere and around June 21 in the southern hemisphere). Temperature can change abruptly when fronts (boundaries between two air masses with different temperatures or densities) or thunderstorms pass overhead.
Temperature decreases with increasing elevation at an average rate of about 6.5° C per km (about 19° F per mi). As a result, temperatures in the mountains are generally much lower than at sea level. Temperature continues to decrease throughout the atmosphere's lowest layer, the troposphere, where almost all weather occurs. The troposphere extends to a height of 16 km (10 mi) above sea level over the equator and about 8 km (about 5 mi) above sea level over the poles. Above the troposphere is the stratosphere, where temperature levels off and then begins to increase with height. Almost no weather occurs in the stratosphere.
Humidity is a measure of the amount of water vapor in the air. The air's capacity to hold vapor is limited but increases dramatically as the air warms, roughly doubling for each temperature increase of 10° C (18° F). There are several different measures of humidity. The specific humidity is the fraction of the mass of air that consists of water vapor, usually given as parts per thousand. Even the warmest, most humid air seldom has a specific humidity greater than 20 parts per thousand. The most common measure of humidity is the relative humidity , or the amount of vapor in the air divided by the air's vapor-holding capacity at that temperature. If the amount of water vapor in the air remains the same, the relative humidity decreases as the air is heated and increases as the air is cooled. As a result, relative humidity is usually highest around dawn, when the temperature is lowest, and lowest in mid-afternoon, when the temperature is highest.
Most clouds and almost all precipitation are produced by the cooling of air as it rises. When air temperature is reduced, excess water vapor in the air condenses into liquid droplets or ice crystals to form clouds or fog. A cloud can take any of several different forms—including cumulus, cirrus, and stratus—reflecting the pattern of air motions that formed it. Fluffy cumulus clouds form from rising masses of air, called thermals. A cumulus cloud often has a flat base, corresponding to the level at which the water vapor first condenses. If a cumulus cloud grows large, it transforms into a cumulonimbus cloud or a thunderstorm. Fibrous cirrus clouds consist of trails of falling ice crystals twisted by the winds. Cirrus clouds usually form high in the troposphere, and their crystals almost never reach the ground. Stratus clouds form when an entire layer of air cools or ascends obliquely. A stratus cloud often extends for hundreds of miles.
Fog is a cloud that touches the ground. In dense fogs, the visibility may drop below 50 m (55 yd). Fog occurs most frequently when the earth's surface is much colder than the air directly above it, such as around dawn and over cold ocean currents. Fog is thickened and acidified when the air is filled with sulfur-laden soot particles produced by the burning of coal. Dense acid fogs that killed thousands of people in London up to 1956 led to legislation that prohibited coal burning in cities.
Optical phenomena, such as rainbows and halos, occur when light shines through cloud particles. Rainbows are seen when sunlight from behind the observer strikes the raindrops falling from cumulonimbus clouds. The raindrops act as tiny prisms, bending and reflecting the different colors of light back to the observer's eye at different angles and creating bands of color. Halos are seen when sunlight or moonlight in front of the observer strikes ice crystals and then passes through high, thin cirrostratus clouds.
Low Level Clouds
Mid Level Couds
High Level Clouds
Cumuliform Clouds
Mesospheric Clouds
Contrails
Severe Weather
Thunderstorms
Fog
A stratified cloud that reaches to ground level. Beautiful surface fog forms by radiative cooling of land during the night; the land cools the boundary layer of air to below condensation, resulting in fog. Other types of fog form by advection of humid air over a cool surface, or by addition of water vapor to the air by precipitation.Fractocumulus
Fractocumulus looks like unorganized and unstructured puffs of cumulus cloud. Dissipating, small convective clouds such as cumulus humilis and medicris often transform into fractocumulus. Fractocumulus can also form near or under thunderstorms and is then termed scud.Nimbostratus
A frontal, low-level stratified cloud producing precipitation of some form or another. Nimbostratus forms when warm humid air is lifted large-scale over a large area, like over a frontal surface. The cloud has enough vertical extend for ice crystals to coalesce into snowflakes. Usually, the snow melts before reaching ground. Nimbostratus can precipitate drizzle to moderate or heavy steady rainfall.Stratocumulus
Stratocumulus is a cloud deck of stratified (flattened) cumulus cloud. Often, there are some gaps between the cumulus cells in the cloud deck, and the cumulus cells are usually easily identified. The typical altitude of a stratocumulus cloud deck is usually between 1 and 2 km. In rare cases, stratocumulus can form sun pillars (if the cloud is frozen) or cloud bows (if the cloud is liquid).Stratus
A low-level, usually thin and featureless layer of cloud, often reminiscent of fog which has evaporated near ground but is still present aloft. Stratus may also form near warm fronts, in valleys during the night, etc. It is largely responsible for "those grey days" without rainfall, but without sunshine either. If there is rain, the cloud is called nimbostratus.
Altocumulus
A detailed, mid-level cumuliform cloud deck, consisting of supercooled and frozen cloud particles. Altocumulus is usually thin (less than 100m thick) and may be oriented along any windshear-direction at that altitude. Altocumulus undulatus is an indication of this.Altostratus
A usually featureless, mid-level stratiform cloud deck, consisting of supercooled and frozen cloud particles. Altostratus will partly or completely block the sun; in this respect it can easily be distinguished from cirrostratus. Altostratus frequently precipitates snow crystals and drizzle, but the precipitation rarily reaches ground. It usually evaporates.Castellanus
Castellanus is a variation of altocumulus clouds, by which the cumuliform cells show some vertical development, or turrets. Castellanus is an indication of instability at middle height (3 - 5 km) and is usually a sign of imminent thunderstorms or an active cold front.Floccus
A subtype of altocumulus cloud, looking like small puffs with no cumuliform structure. Floccus often heralds warm summer thunderstorms or MCSs and may appear together with castellanus.
Cirrocumulus
A high-level cumuliform cloud deck, at an altitude between roughly 6 to 12 km. Cirrocumulus cells show no shadowing, unlike altocumulus, and appear very small. Cirrocumulus can show nice coronas and iridescence, especially the cirriform lacunosus (net-like) structure.Cirrostratus
A high-altitude, thin veil of ice-crystal cloud, not showing much or any structure. Cirrostratus often shows optical phenomena like halos and parhelia (mock suns/moons). The sun is not blocked much by cirrostratus and still appears as a well-defined disk.Cirrus
Delicate fibre-like clouds consisting of ice crystals, at high altitude (typically between 6 to 12 km). Cirrus can take many forms, e.g. spissatus (opaque patch of cirrus, usually remnant of an old cumulonimbus anvil); vertebratus (a herringbone structure) and uncinus (comma-shaped whisps of cloud).
Cumulonimbus
A cumulonimbus is a storm cloud with large vertical extend (up to 13 miles maximally, in the tropics). There are different types of thunderstorms: single cells, multicells, supercells and mesoscale convective storms (MCSs). The top of a cumulonimbus usually has the form of an anvil because the tropopause is a strong temperature inversion which the cloud cannot penetrate, so the cloud spreads out laterally. Cumulonimbus is usually precipitating snow, hail, rain or graupel, and may have lightning as well.Cumulus Congestus
A convective cloud that shows large vertical extend, but has not yet reached the precipitation stage. Congestus is Latin for "piled-up". Congestus can show nice crepuscular rays when the sky is a bit hazy and scatters sunlight well.Cumulus Humilis
A very small convective cloud, which forms just after a rising thermal reaches the condensation level. Humilis clouds usually dissipate a few minutes after they form, and not showing much growth, or they grow bigger to cumulus mediocris and congestus.Cumulus Mediocris
A medium-sized convective cloud, showing little vertical growth. Mediocris is the transition cloud between cumulus humilis and congestus. They typically form in spring time, when the sun warms up the land over cold air, making the air unstable.
Noctilucent
Noctilucent clouds are delicate, silvery-blueish clouds far above the troposphere. In summer time, they exist at about 82 km high (50 miles), high enough for the sunlight to illuminate them during the night. They can make a fantastic view. Note the beautiful sea of surface-fog draped like a veil over the countryland.
Contrail Funnels
In uncommon circumstances the ring vortices or dipole vortices, which sometimes occur in contrails, may spontaneously transform into a funnel cloud. This funnel cloud then extends down below the contrail. These funnels typically do not last very long; only a few minutes.Contrail Instability Waves
Shear-flow instability in jet contrails causes them to show undulations which become more extreme (showing larger amplitude) with time. Usually the instability undulations (or waves) last a few minutes after the airplane has created the contrail. After that they break up and become dipole vortices or dissipate.Contrail Shadows
Contrails are clouds with relatively sharp boundaries and hence may cast a sharp shadow on lower-altitude clouds. This shadow is then visible from the underside of the cloud, if the optical thickess of the cloud (the opacity) is not too large. If the lower-altitude cloud is cirrostratus, which is translucent, a three-dimensional shadow will form. This shadow is a plane defined by the sun and the (line-shaped) contrail. As a result, such a shadow is usually only visible if the contrail is in front of the sun for the observer. It is remarkable to see that a contrail shadow usually appears in an odd direction with respect to the sun and realizing that it is being cast on a lower-level cloud: perspective can be really deceptive.Contrail Shearing
Shearing of a contrail occurs when the contrail is deposited in a region of windshear. This windshear can be either directional or velocity-wise. It causes the contrail, which has some extend in height, to deform or twist. This shearing is only visible if the contrail remains long enough in the atmosphere such that the windshear will have visible effect on it (typically longer than 5 to 10 minutes).Contrail
The word contrail is short for (jet) aircraft condensation trail. Contrails can cause a day to be cloudy or overcast that would otherwise have been clear-sky. The presence of contrails is an indication of high moisture in the upper region of the troposphere. Part of a contrail freezes, while the other part descends (like mammatus or virga) and evaporates. The frozen remnant of the contrail may have the appearance of a thin line, or vortex. Usually the contrail will transform into cirrus or cirrocumulus, or dissipate before a transition occurs.Distrail
A distrail, short for dissipation trail, forms when an aircraft flies through a supercooled cloud. The cloud freezes due to the disturbing airflow created by the aircraft, as well as by the addition of plenty of freezing nuclei due to the aircraft's exhaust gases. Altocumulus is usually either frozen or supercooled and can show distrails. They are uncommon, especially the clear distrails. Distrails are usually not very long, since they usually show in altocumulus which is not at a cruising altitude of the aircraft (i.e. the aircraft is either descending or ascending through the cloud). In some cases a distrail may transform into a cloud hole: a large elliptical hole in altocumulus with virga in the middle.Shear (wing) funnel
Certain types of airplanes - the McDonnel Douglas DC-10 and MD-11 are notorious examples - create funnel clouds along their wings when the flaps are deployed, during the landing. The flaps induce a lateral (sideways) component of the airflow under the wing. When this air recombines with the air flowing over the wing backwards, the low pressure and rotational shearing causes a funnel cloud which extends several tens of meters behind the airplane. They can be quite spectacular when seen from the ground near a runway, in humid foggy weather. The funnel clouds cause extreme turbulence.
Gust Fronts
A cold downdraft of air and precipitation, causing sudden wind changes in direction and velocity, when a storm is about to pass over. Gust fronts can take many shapes and forms, from being much invisible except for a precipitation curtain, to highly turbulent dust clouds near the ground. The gust front is usually very transient in form: it travels fast and changes appearance quickly.Haboob
A haboob is a duststorm or sandstorm as they sometimes occur in the deserts. The outflow of cool air from thunderstorms can create similar duststorms, when the cool downdraft of air reaches ground and has to spread out along the surface. Gustfronts are usually responsible for these sandstorms. More generally, haboobs form late in the evening due to thermal gradients in the lower atmosphere, caused by extreme daytime heating, and last one or more hours.Hail Fallstreak
Near the updraft area of a strong or severe storm, one can usually see hail fallstreaks, as distinct, white streaks. These streaks look different (and have higher brightness) than rain fallstreaks, which tend to be darker. Falling hail need not always be visible: especially if the stones are very large, their number density is low and the hailstreaks are not seen well or at all.Roll Clouds
Storms are sometimes accompanied by arcus, a roll cloud. Cold air rushes out of a downdraft and lifts warmer air in front of the storm complex up to condensation level, making both a fascinating and menacing sight while it is approaching at high velocity.Scud Cloud
Fractocumulus-like cloud rags, sometimes appearing in the precipitation region of a storm. They are caused by condensation of water vapor in air below the cloud base, due to the high air moisture in the precipitation region.Shelf Cloud
A shelf cloud is similar to lenticularis cloud, but on a much larger scale. A shelf cloud sometimes forms ahead of a severe, long-lasting thunderstorm such as a mesoscale convective system (MCS), multicell or supercell. Cold downdraft air surges out along the surface and lifts the (usually conditionally stable) warm air ahead of the storm up to condensation level. A smooth shelf-like cloud forms, skirting the thunderstorm. The shelf cloud is usually accompanied by a gustfront and sometimes also a roll cloud.Storm Skyscapes
This shows some unusual, spectacular or beautiful skies connected to stormy weather. The clouds before or after the passage of a storm can exhibit quite strange forms and shapes, especially when they are seen around sunset.Tail Cloud
A tail cloud is sometimes seen attached to the wall cloud of a severe thunderstorm. The tail cloud originates from the area of the downdraft, which has a high moisture content due to the precipitation.Tornado
A tornado is a funnel cloud, or vortex, of spinning air, water vapor and sometimes dust and debris. The word tornado is usually reserved for the funnel cloud spawned by supercell storms; other funnel clouds are called spouts. Tornadoes can form out of supercell storms if the updraft (the mesocyclone) rotates and creates enough air vorticity with respect to the flanking downdraft area. This vorticity, if the airflow pattern in the storm is balanced well, can be enhanced and concentrated, forming a low-pressure vortex at the perimeter of the mesocyclone and ultimately the tornado. The tornado becomes visible below cloud base due to condensation of the lower pressure air.Updraft Cumulus
At well-organized storms, the updraft is usually visible at the flank of the storm, where the flanking line connects to the storm tower. If the storm is severe, one can usually visually see the updraft cumulus grow towards the anvil of the storm explosively. Updraft towers are sometimes accompanied by clear hail fallstreaks. Generally, the stronger the updraft, the larger the hail which will fall out of it. Next to the updraft is usually a downdraft (rainfall).Updraft Turrets
In some cases of explosive updraft velocities, the cumuliform cloud exhibits so-called turrets: grooves in between cumulus. The cloud then grows so quickly that the cumuliform cloud has no time to expand in all directions at once, and the turrets are formed. Turrets are most frequently seen high up in the updraft towers in severe thunderstorms, which are in their maturing phase.Wall Cloud
Severe thunderstorms have a well-defined structure of updraft and downdraft (with precipitation), situated next to eachother. Cool humid air from the downdraft may flow to the updraft, moistening the air and lowering the cloud base under the updraft. This lowering, the wall cloud, may be enhanced by large-scale rotation of the updraft itself. Well-defined wall clouds usually have the form of a straight wall, more commonly wall clouds look like triangle-shaped sockets of cloud under the thunderstorm base.Whale's Mouth
The whale's mouth-effect is the weird-looking sky sometimes appearing when the first gust front of a storm is passing over. The cold downdraft of air of the storm rushes outward and forward along the surface, and lifts the warmer air in the direct vicinity of the storm over its condensation level. One gets to see the back side of this mini cold front, and the inside of the gust front cloud. It looks somewhat like mammatus and other forms of turbulent cloud masses, sometimes showing very neat ordering.
Multicell Thunderstorm
Multicell thunderstorms are storms that have some organized structure, making their lifetime longer. Multicell storms consist of several cells of storms, which occur one after another: each cell has a lifetime of 30 to 40 minutes, typically. In severe multicell storms, there are usually several cells active at any one time. Lightning is a good indication of an active thunderstorm cell.Single-Cell Thunderstorm
A single-cell thunderstorm forms when there is not much windshear (neither in direction nor strength) with height. The storm will be poorly organized, and the updraft is followed by a downdraft of cool air and precipitation, cutting off the updraft and terminating the storm growth cycle. A single cell storm will typically last less than one hour. If there is windshear with altitude, the storm usually becomes multicellular, having several growth cycles and lasting longer.Supercell Thunderstorm
The supercell is the most severe of all thunderstorms. It can form out of a generic multicell storm, if there is enough directional windshear in the atmosphere to slant the updraft and separate the cool downdraft from the updraft. The main updraft can strengthen and start rotating due to the air vorticity and mesoscale inflow/outflow pattern; rotation in a storm is an indicator for supercellular behaviour. Supercells create their own small low-pressure systems with fronts (usually in the form of a flanking line and gustfront). They can last several hours and spawn tornadoes, wall clouds and drop very large hail (in rare cases up to 10 cm or 4").
Precipitation is produced when the droplets and crystals in clouds grow large enough to fall to the ground. Clouds do not usually produce precipitation until they are more than 1 km (0.6 mi) thick. Precipitation takes a variety of forms, including rain, drizzle, freezing rain, snow, hail, and ice pellets, or sleet. Raindrops have diameters larger than 0.5 mm (0.02 in), whereas drizzle drops are smaller. Few raindrops are larger than about 6 mm (about 0.2 in), because such large drops are unstable and break up easily. Ice pellets are raindrops that have frozen in midair. Freezing rain is rain that freezes on contact with any surface. It often produces a layer of ice that can be very slippery.
Snowflakes are either single ice crystals or clusters of ice crystals. Large snowflakes generally form when the temperature is near 0° C (32° F), because at this temperature the flakes are partly melted and stick together when they collide. Hailstones are balls of ice about 6 to 150 mm (about 0.2 to 6 in) in diameter. They consist of clusters of raindrops that have collided and frozen together. Large hailstones only occur in violent thunderstorms, in which strong updrafts keep the hailstones suspended in the atmosphere long enough to grow large.
Precipitation amounts are usually given in terms of depth. A well-developed winter storm can produce 10 to 30 mm (0.4 to 1.2 in) of rain over a large area in 12 to 24 hours. An intense thunderstorm may produce more than 20 mm (0.8 in) of rain in 10 minutes and cause flash floods (floods in which the water rises suddenly). Hurricanes sometimes produce over 250 mm (10 in) of rain and lead to extensive flooding.
Snow depths are usually much greater than rain depths because of snow's low density. During intense winter storms, more than 250 mm (10 in) of snow may fall in 24 hours, and the snow can be much deeper in places where the wind piles it up in drifts. Extraordinarily deep snows sometimes accumulate on the upwind side of mountain slopes during severe winter storms or on the downwind shores of large lakes during outbreaks of polar air.
Wind is the horizontal movement of air. It is named for the direction from which it comes—for example, a north wind comes from the north. In most places near the ground, the wind speed averages from 8 to 24 km/h (from 5 to 15 mph), but it can be much higher during intense storms. Wind speeds in hurricanes and typhoons exceed 120 km/h (75 mph) near the storm's center and may approach 320 km/h (200 mph). The highest wind speeds at the surface of the earth—as high as 480 km/h (300 mph)—occur in tornadoes. Except for these storms, wind speed usually increases with height to the top of the troposphere.
Pressure plays a vital role in all weather systems. Pressure is the force of the air on a given surface divided by the area of that surface. In most weather systems the air pressure is equal to the weight of the air column divided by the area of the column. Pressure decreases rapidly with height, halving about every 5.5 km (3.4 mi).
Sea-level pressure varies by only a few percent. Large regions in the atmosphere that have higher pressure than the surroundings are called high-pressure areas. Regions with lower pressure than the surroundings are called low-pressure areas. Most storms occur in low-pressure areas. Rapidly falling pressure usually means a storm is approaching, whereas rapidly rising pressure usually indicates that skies will clear.
Weather systems occur on a wide range of scales. Monsoons occur on a global scale and are among the largest weather systems, extending for thousands of miles. Thunderstorms are much smaller, typically 10 to 20 km (6 to 12 mi) across. Tornadoes, which extend from the bases of thunderstorms, range from less than 50 m (55 yd) across to as much as 2 km (1.2 mi) across.
The vertical scale of weather systems is much more limited. Because pressure decreases so rapidly with height and because temperature stops decreasing in the stratosphere, weather systems are confined to the troposphere. Only the tallest thunderstorms reach the stratosphere, which is otherwise almost always clear.
All weather is due to heating from the sun. The sun emits energy at an almost constant rate, but a region receives more heat when the sun is higher in the sky and when there are more hours of sunlight in a day. The high sun of the Tropics makes this area much warmer than the poles, and in summer the high sun and long days make the region much warmer than in winter. In the northern hemisphere, the sun climbs high in the sky and the days are long in summer, around July, when the northern end of the earth's axis is tilted toward the sun. At the same time, it is winter in the southern hemisphere. The southern end of the earth's axis is tilted away from the sun, so the sun is low in the sky and the days are short.
The temperature differences produced by inequalities in heating cause differences in air density and pressure that propel the winds. Vertical air motions are propelled by buoyancy: A region of air that is warmer and less dense than the surroundings is buoyant and rises. Air is also forced from regions of higher pressure to regions of lower pressure. Once the air begins moving, it is deflected by the Coriolis force, which results from the earth's rotation. The Coriolis force deflects the wind and all moving objects toward their right in the northern hemisphere and toward their left in the southern hemisphere. It is so gentle that it has little effect on small-scale winds that last less than a few hours, but it has a profound effect on winds that blow for many hours and move over large distances.
In both hemispheres, the speed of the west wind increases with height up to the top of the troposphere. The core of most rapid winds at the top of the troposphere forms a wavy river of air called the jet stream. Near the ground, where the winds are slowed by friction, the air blows at an acute angle toward areas of low pressure, forming great gyres called cyclones and anticyclones. In the northern hemisphere, the Coriolis force causes air in low-pressure areas to spiral counterclockwise and inward, forming a cyclone, whereas air in high-pressure areas spirals clockwise and outward, forming an anticyclone. In the southern hemisphere, cyclones turn clockwise and anticyclones, counterclockwise.
The air spreading from anticyclones is replaced by sinking air from above. As a result, skies in anticyclones are often fair, and large regions of air called air masses form; these have reasonably uniform temperature and humidity. In cyclones, on the other hand, as air converges to the center, it rises to form extensive clouds and precipitation.
During summer and fall, tropical cyclones, called hurricanes or typhoons, form over warm waters of the oceans in bands parallel to the equator, between about latitude 5° and latitude 30° north and south. Wind speed in hurricanes increases as the air spirals inward. The air either rises in a series of rain bands before reaching the center or proceeds inward and then turns sharply upward in a doughnut-shaped region called the eye wall, where the most intense winds and rain occur. The eye wall surrounds the core, or eye, of the hurricane, which is marked by partly clear skies and gentle winds.
In the middle and high latitudes, polar and tropical air masses are brought together in low-pressure areas called extratropical cyclones, forming narrow zones of sharply changing temperature called fronts. Intense extratropical cyclones can produce blizzard conditions in their northern reaches while at the same time producing warm weather with possible severe thunderstorms and tornadoes in their southern reaches.
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