question: Clouds are made up of water that has evaporated, in Summer when it is hotter and more water is evaporating there is often far less clouds than in Winter when it is cold and there are lots of clouds. What causes this?
comment: Truly evaporated water is invisible, it’s basically air. Think about the totally clear area right at the top of a boiling kettle. Clouds are tiny droplets if water, white and visible, like steam. The summer heat evaporates lots of water, but it only forms clouds in certain areas where it’s gets pushed up to colder parts of the sky. That’s part of why some places have summer thunderstorms, with hail sometimes.
passages: Section::::Polar stratospheric.
  
  Polar stratospheric clouds (PSC's) form in the lowest part of the stratosphere during the winter, at the altitude and during the season that produces the coldest temperatures and therefore the best chances of triggering condensation caused by adiabatic cooling. Moisture is scarce in the stratosphere, so nacreous and non-nacreous cloud at this altitude range is restricted to polar regions in the winter where the air is coldest.
   Some clouds can form at very high to extreme levels above the troposphere, mostly above the polar regions of Earth. Polar stratospheric clouds clouds are seen but rarely in winter at altitudes of 18 to 30 kilometers, while in summer, noctilucent clouds occasionally form at high latitudes at an altitude range of 76 to 85 kilometers. These polar clouds show some of the same forms as seen lower in the troposphere.
  
  Homospheric types determined by cross-classification of forms and levels.
   Section::::Description.:Precipitation.
  
  Most often, stratocumulus produce no precipitation, and when they do, it is generally only light rain or snow. However, these clouds are often seen at either the front or tail end of worse weather, so they may indicate storms to come, in the form of thunderheads or gusty winds. They are also often seen underneath the cirrostratus and altostratus sheets that often precede a warm front, as these higher clouds decrease the sun's heat and therefore convection, causing any cumulus clouds to spread out into stratocumulus clouds.
  
  Section::::Description.:Comparison altocumulus.
   Based on data taken from the United States using these methods, cirrus cloud cover was found to vary diurnally and seasonally. The researchers found that in the summer, at noon, the cover is the lowest, with an average of 23% of the United States' land area covered by cirrus. Around midnight, the cloud cover increases to around 28%. In winter, the cirrus cloud cover did not vary appreciably from day to night. These percentages include clear days and nights, as well as days and nights with other cloud types, as lack of cirrus cloud cover. When these clouds are present, the typical coverage ranges from 30% to 50%. Based on satellite data, cirrus covers an average of 20% to 25% of the Earth's surface. In the tropical regions, this cloud covers around 70% of the region's surface area.
   Noctilucent clouds can form only under very restricted conditions during local summer; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. They are a relatively recent classification. The occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent.
  
  Section::::Formation.
   Sometimes the inversion layer is at a high enough altitude that cumulus clouds can condense but can only spread out under the inversion layer. This decreases the amount of sunlight reaching the ground and prevents new thermals from forming. As the clouds disperse, sunny weather replaces cloudiness in a cycle that can occur more than once a day.
   After the passage of the cold front, the sky usually clears as high pressure builds in behind the system, although significant amounts of cumulus or stratocumulus, often in the form of long bands called "cloud streets" may persist if the air mass behind the front remains humid. Small and unchanging amounts of cumulus or cirrus clouds in an otherwise clear sky are usually indications of continuing fair weather as long as the barometric pressure remains comparatively high.
  
  Section::::Precipitation.
   Ultraviolet radiation from the Sun breaks water molecules apart, reducing the amount of water available to form noctilucent clouds. The radiation is known to vary cyclically with the solar cycle and satellites have been tracking the decrease in brightness of the clouds with the increase of ultraviolet radiation for the last two solar cycles. It has been found that changes in the clouds follow changes in the intensity of ultraviolet rays by about a year, but the reason for this long lag is not yet known.
   Cloud formation and climate change
  
  Nephology (; from the Greek word "nephos" for 'cloud') is the study of clouds and cloud formation. British meteorologist Luke Howard was a major researcher within this field, establishing a cloud classification system.
  
  While this branch of meteorology still exists today, the term nephology, or nephologist is rarely used. The term came into use at the end of the nineteenth century, and fell out of common use by the middle of the twentieth.
   Distribution in the mesosphere is similar to the stratosphere except at much higher altitudes. Because of the need for maximum cooling of the water vapor to produce noctilucent clouds, their distribution tends to be restricted to polar regions of Earth. A major seasonal difference is that convective lift from below the mesosphere pushes very scarce water vapor to higher colder altitudes required for cloud formation during the respective summer seasons in the northern and southern hemispheres. Sightings are rare more than 45 degrees south of the north pole or north of the south pole.
  
  Section::::Extraterrestrial.
   Polar mesospheric clouds form at an extreme-level altitude range of about . They are given the Latin name noctilucent because of their illumination well after sunset and before sunrise. They typically have a bluish or silvery white coloration that can resemble brightly illuminated cirrus. Noctilucent clouds may occasionally take on more of a red or orange hue. They are not common or widespread enough to have a significant effect on climate. However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.
   Clouds surrounding mountains display a seasonal variability. Clouds at Olympus Mons and Ascreaus Mons appear in northern hemisphere spring and summer, reaching a total maximum area of approximately 900,000 km and 1,000,000 km respectively in late spring. Clouds around Alba Patera and Pavonis Mons show an additional, smaller peak in late summer. Very few clouds were observed in winter. Predictions from the Mars General Circulation Model are consistent with these observations.
  
  Section::::Polar caps.
   The general seasonal characteristics of polar mesospheric clouds are well established from the five years of continuous SME data. Over that period, data for four cloud ‘seasons’ in the north, and five ‘seasons’ in the south were recorded. In both hemispheres, the season begins about one month before summer solstice and ends about two months afterwards. Since there are no biases due to such factors as changing number of hours of visibility, weather conditions, etc. this is a ‘true’ behaviour. It is believed to be a result of the fact that summertime mesopause region becomes coldest during this period causing water-ice to form, in contrast to most other regions of the atmosphere which are warmest in summer. Temperatures at latitudes equatorward of the boundary of detection never get low enough for water-ice to form.
   There are five main ways water vapor can be added to the air. Increased vapor content can result from wind convergence over water or moist ground into areas of upward motion. Precipitation or virga falling from above also enhances moisture content. Daytime heating causes water to evaporate from the surface of oceans, water bodies or wet land. Transpiration from plants is another typical source of water vapor. Lastly, cool or dry air moving over warmer water will become more humid. As with daytime heating, the addition of moisture to the air increases its heat content and instability and helps set into motion those processes that lead to the formation of cloud or fog.
   Divergence is the opposite of convergence. In the Earth's troposphere, it involves the horizontal outflow of air from the upper part of a rising column of air, or from the lower part of a subsiding column often associated with an area or ridge of high pressure. Cloudiness tends to be least prevalent near the poles and in the subtropics close to the 30th parallels, north and south. The latter are sometimes referred to as the horse latitudes. The presence of a large-scale high-pressure subtropical ridge on each side of the equator reduces cloudiness at these low latitudes. Similar patterns also occur at higher latitudes in both hemispheres.
   The noctilucent clouds AIM is to study, also known as Polar mesospheric clouds, occur in the Earth's atmosphere at high altitudes of roughly above the surface, far higher than other clouds. The AIM mission will help determine what factors — temperature, water vapor, and dust particles — lead to the formation of these clouds. The clouds seem to be a relatively recent phenomenon: they were first seen in 1885, and lately seem to be occurring more frequently. The clouds always occur during the summer season near the poles and the Northern season always starts around the same time. Scientists have found that the start of the Southern season can vary up to a month however.
   Snow clouds usually occur in the context of larger weather systems, the most important of which is the low pressure area, which typically incorporate warm and cold fronts as part of their circulation. Two additional and locally productive sources of snow are lake-effect (also sea-effect) storms and elevation effects, especially in mountains.
  
  Section::::Precipitation.:Cloud formation.:Low pressure areas.
   Observed from the ground, this phenomenon is known as noctilucent clouds. From satellites, PMCs are most frequently observed above 70°–75° in latitude and have a season of 60 to 80 days duration centered about a peak which occurs about 20 days after the summer solstice. This holds true for both hemispheres. Great variability in scattering is observed from day-to-day and year-to- year, but averaging over large time and space scales reveals a basic underlying symmetry and pattern. The long- term behaviour of polar mesospheric cloud frequency has been found to vary inversely with solar activity.
   Since the saturation vapor pressure is proportional to temperature, cold air has a lower saturation point than warm air. The difference between these values is the basis for the formation of clouds. When saturated air cools, it can no longer contain the same amount of water vapor. If the conditions are right, the excess water will condense out of the air until the lower saturation point is reached. Another possibility is that the water stays in vapor form, even though it is beyond the saturation point, resulting in supersaturation.
   Section::::Formation.
  
  The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their physical state and chemical composition.
  
  Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk.
   In meteorology, its role is crucial in the formation of rain. As droplets are carried by the updrafts and downdrafts in a cloud, they collide and coalesce to form larger droplets. When the droplets become too large to be sustained on the air currents, they begin to fall as rain. Adding to this process, the cloud may be seeded with ice from higher altitudes, either via the cloud tops reaching , or via the cloud being seeded by ice from cirrus clouds.
   Stratocumulus Opacus is a dark layer of clouds covering entire sky without any break. However, the cloud sheet is not completely uniform, so that separate cloud bases still can be seen. This is the main precipitating type, however any rain is usually light. If the cloud layer becomes grayer to the point when individual clouds cannot be distinguished, stratocumulus turn into stratus clouds.
  
  Stratocumulus Perlucidus is a layer of stratocumulus clouds with small spaces, appearing in irregular pattern, through which clear sky or higher clouds can be seen.
   Air masses can be modified in a variety of ways. Surface flux from underlying vegetation, such as forest, acts to moisten the overlying air mass. Heat from underlying warmer waters can significantly modify an air mass over distances as short as to . For example, southwest of extratropical cyclones, curved cyclonic flow bringing cold air across the relatively warm water bodies can lead to narrow lake-effect snow bands. Those bands bring strong localized precipitation since large water bodies such as lakes efficiently store heat that results in significant temperature differences (larger than 13 °C or 23 °F) between the water surface and the air above. Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes.
   Section::::Other effects of cloud feedback.
  
  In addition to how clouds themselves will respond to increased temperatures, other feedbacks affect clouds properties and formation. The amount and vertical distribution of water vapor is closely linked to the formation of clouds. Ice crystals have been shown to largely influence the amount of water vapor. Water vapor in the subtropical upper troposphere has been linked to the convection of water vapor and ice. Changes in subtropical humidity could provide a negative feedback that decreases the amount of water vapor which in turn would act to mediate global climate transitions.
   Section::::Forecast.
  
  Nimbostratus is generally a sign of an approaching warm or occluded front producing steady moderate precipitation, as opposed to the shorter period of typically heavier precipitation released by a cold-frontal cumulonimbus cloud. Precipitation may last for several days, depending on the speed of the frontal system. A nimbostratus virga cloud is the same as normal nimbostratus, but the rain or snow falls as virga which doesn't reach the ground. Stratus or stratocumulus usually replace the nimbostratus after the passage of the warm or occluded front.
  
  Section::::Origin of name.
  
presuppositions: Clouds are made up of evaporated water.
corrections: Clouds are tiny droplets of water because evaporated water is invisible.
raw_presuppositions: Evaporation forms clouds. The only visible forms of evaporation are clouds. Clouds are made up of evaporated water.
raw_corrections: Evaporation only forms clouds in certain areas where the sky is colder.  More water is evaporated in the summer but there are less clouds than in the winter because truly evaporated water turns into air and is invisible. Clouds are tiny droplets of water because evaporated water is invisible.