Category Science

          Rain can form in two ways. In tropical areas, where temperatures are warm, any water droplets in the clouds join together to form raindrops that are heavy enough to fall from the clouds. Elsewhere, rain starts life as snow in the freezing temperatures of the high clouds. As the snow falls nearer the ground, it will turn to rain if the temperature is above freezing.

          While it may be tempting to say that rain comes from clouds, you can also say that rain is clouds, giving up on their dreams of being water vapor and falling back down to Earth, where they start their journey through the precipitation cycle again. If you want a better understanding of why rain comes down from clouds, start with that precipitation cycle, the mechanism through which water moves from the Earth to the atmosphere and back again.

          The amount of water available on Earth never changes. But its state (liquid or gas/vapor) does, and that’s all thanks to thermal energy from the sun. As liquid water is heated by the sun, it receives enough energy to break its molecules apart and transform into water vapor.

          The warmer the air, the more water vapor it can hold. That warm, moisture-saturated air rises, along with the water vapor it contains, and as it rises it cools. Once the air has cooled past the “dew point,” it condenses around “condensation nuclei,” which are usually teeny-tiny particles of dust, smoke or even salt that are suspended in the air. (If you’ve ever looked through a shaft of sunlight and seen dust particles dancing in the air, that’s a great visual.)

          The tiny water droplets that initially form are what you see as clouds – and if you pay close attention to clouds in the sky, you’ll see that they’re constantly shrinking and growing in response to the warring forces of evaporation and condensation.

          Water vapor that has condensed into tiny droplets and formed clouds is well on its way to becoming rain – but it’s not there yet. For now, the water droplets are so tiny that the air currents keep them aloft, just as swirling particles of dust can stay in the air. But as those droplets continue to rise, buoyed by rising bodies of warm air, they have two routes for making it back to Earth.

          The first is when water droplets collide and coalesce with other droplets, eventually becoming heavier than the uplift of the air around them, at which point they fall down through the cloud. Or, through something called the Bergeron-Findeisen-Wegener process, the ice process of precipitation or simply the Bergeron process, the droplets rise high enough to freeze into ice crystals, attracting more water vapor to themselves and growing quickly until they’re heavy enough to fall as snow or melt and fall as rain.

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          Aircraft flying at high altitudes will leave a white trail behind them when the air is very cold. This is caused by the exhaust gases expelled by the aircraft’s engines. The gases contain a large amount of water vapour, which condenses and freezes in the cold, high-altitude air, leaving behind cloud-like trails called contrails.

          The condensation trails left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and “see your breath”. The figure below represents how saturation vapor pressure varies as a function of temperature. The blue line is the saturation vapor pressure for ice as a function of temperature (in degrees Kelvin). Air parcels in the region labeled saturated will form a cloud. Imagine two parcels of air, A and B as located on the diagram. Both parcels are unsaturated. If B represents the engine exhaust, then as it mixes with the environment (parcel A) its temperature and corresponding vapor pressure will follow the dotted line. Where this dotted line intersects the blue line is where the parcel becomes saturated.

          If you are attentive to contrail formation and duration, you will notice that they can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long a contrail remains intact, depends on the humidity structure and winds of the upper troposphere. If the atmosphere is near saturation, the contrail may exist for some time. On the other hand, if the atmosphere is dry then as the contrail mixes with the environment it dissipates. Contrails are a concern in climate studies as increased jet aircraft traffic may result in an increase in cloud cover. It has been estimated that in certain heavy air-traffic corridors, cloud cover has increased by as much as 20%. An increase in cloud amount changes the region’s radiation balance. For example, solar energy reaching the surface may be reduced, resulting in surface cooling. They also reduce the terrestrial energy losses of the planet, resulting in a warming. Jet exhaust also plays a role in modifying the chemistry of the upper troposphere and lower stratosphere. NASA and the DOE are sponsoring a research program to study the impact contrails have on atmospheric chemistry, weather and climate. In this series of satellite images we will investigate the duration of contrails. The satellite images are from the GOES-8 visible channel. Each image is separated in time by approximately 15 minutes. The GOES-8 image has a spatial resolution of approximately 1 km. The satellite image is a view of upper mid-west including southern Wisconsin and northern Illinois. Madison is located on the image. Contrails were observed from the ground during this period. At this wavelength, the GOES-8 imager is measuring the amount of radiant energy reflected by the surface and the clouds. The whiter a given portion of the image, the larger the amount of reflected visible light. White portions of the image represent thick clouds and dark regions are water or heavily vegetated regions. Contrails show up on the image as white streaks, similar to how they appear from a surface view.

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          Meteorologists measures cloud cover in oktas — the number of oktas indicates how much of the sky is covered with cloud. On a scale of 0 to 8, 0 oktas means that there are no clouds; 8 oktas means the sky is completely covered.

          In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). In addition, in the SYNOP code there is an extra cloud cover indicator ‘9’ indicating that the sky is totally obscured, usually due to dense fog or heavy snow.

          When used in weather charts, okta measurements are shown by means of graphic symbols (rather than numerals) contained within weather circles, to which are attached further symbols indicating other measured data such as wind speed and wind direction.

          Although relatively straightforward to measure (visually, for instance, by using a mirror), oktas only estimate cloud cover in terms of the area of the sky covered by clouds. They do not account for cloud type or thickness, and this limits their use for estimating cloud albedo or surface solar radiation receipt.

          Cloud oktas can also be measured using satellite imagery from geostationary satellites equipped with high-resolution image sensors such as Himawari-8. Similar to traditional approaches, satellite images do not account for cloud composition.

          Oktas are often referenced in aviation weather forecasts and low level forecasts: SKC = Sky clear (0 oktas); FEW = Few (1 to 2 oktas); SCT = Scattered (3 to 4 oktas); BKN = Broken (5 to 7 oktas); OVC = Overcast (8 oktas); NSC = nil significant cloud; CAVOK = ceiling and visibility okay.

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          The air contains millions of microscopic dust particles, which absorb water from rivers, lakes and seas. This happens when the water is heated. The heat turns the water into an invisible gas called vapour — a process called evaporation. When the warm, moist air cools down, it condenses (turns back into a liquid) on the surface of the dust particles. When the tiny droplets of water group together, a cloud forms. Clouds can be formed in several different ways, such as by warm air rising up through thermals, or when warm air is forced over hills and mountains. They can also be formed when two air masses meet and the cold air pushes under the warm air, forcing it up.

          Clouds are created when water vapor, an invisible gas, turns into liquid water droplets. These water droplets form on tiny particles, like dust, that are floating in the air.

          You hang up a wet towel and, when you come back, it’s dry. You set out a bowl of water for your dog and when you look again, the water level in the bowl has dropped even though Woofy has been nowhere near the bowl.

          Where did the missing water go? It evaporated. That means some of the liquid water in the towel or bowl changed into an invisible gas called water vapor and drifted away into the atmosphere. (Notice that “evaporated” contains the word “vapor.”) The same thing is constantly happening with oceans, lakes, rivers, swamps, swimming pools – and everywhere water is in contact with air.

          Liquid water changes into a gas when water molecules get extra energy from a heat source such as the Sun or from other water molecules running into them. These energetic molecules then escape from the liquid water in the form of gas. In the process of changing from liquid to gas, the molecules absorb heat, which they carry with them into the atmosphere. That cools the water they leave behind.

          The air can only hold a certain amount of water vapor, depending on the temperature and weight of the air – or atmospheric pressure – in a given area. The higher the temperature or atmospheric pressure, the more water vapor the air can hold. When a certain volume of air is holding all the water vapor it can hold, it is said to be “saturated.”

          What happens if a saturated volume of air cools or the atmospheric pressure drops? The air is no longer able to hold all that water vapor. The excess amount changes from a gas into a liquid or solid (ice). The process of water changing from a gas to a liquid is called “condensation,” and when gas changes directly into a solid, it is called “deposition.” These two processes are how clouds form.

          Condensation happens with the help of tiny particles floating around in the air, such as dust, salt crystals from sea spray, bacteria or even ash from volcanoes. Those particles provide surfaces on which water vapor can change into liquid droplets or ice crystals.

          A large accumulation of such droplets or ice crystals is a cloud. We usually think of clouds as being up in the sky, but when conditions are right, a cloud can form at ground level, too. Then it’s called “fog.” If you’ve ever walked through fog, you’ve walked through a cloud.

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          Cumulus Clouds form at different heights, although they are most often seen in the middle of the cloud layer. Fluffy in appearance, cumulus clouds are often grey on the bottom and a very bright white at the top. Sometimes known as cauliflower clouds, they are usually seen on dry, sunny days.

          Cumulus clouds are clouds which have flat bases and are often described as “puffy”, “cotton-like” or “fluffy” in appearance. Their name derives from the Latin cumulo-, meaning heap or pile.[ Cumulus clouds are low-level clouds, generally less than 2,000 m (6,600 ft) in altitude unless they are the more vertical cumulus congestus form. Cumulus clouds may appear by themselves, in lines, or in clusters.

          Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Normally, cumulus clouds produce little or no precipitation, but they can grow into the precipitation-bearing congestus or cumulonimbus clouds. Cumulus clouds can be formed from water vapour, supercooled water droplets, or ice crystals, depending upon the ambient temperature. They come in many distinct subforms, and generally cool the earth by reflecting the incoming solar radiation. Cumulus clouds are part of the larger category of free-convective cumuliform clouds, which include cumulonimbus clouds. The latter genus-type is sometimes categorized separately as cumulonimbiform due to its more complex structure that often includes a cirriform or anvil top. There are also cumuliform clouds of limited convection that comprise stratocumulus (low-etage), altocumulus (middle-etage) and cirrocumulus (high-etage). These last three genus-types are sometimes classified separately as stratocumuliform.

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          Cirrus clouds form at heights above 600m ( 20,000ft ). At this altitude, it is so cold that water inside the clouds is frozen into crystals of ice. They have a feathery, wispy appearance and are sometime called “mares’ tails”. A large number of cirrus clouds will occasionally form a complete layer of white cloud.

          Cirrus is a genus of atmospheric cloud generally characterized by thin, wispy strands, giving the type its name from the Latin word cirrus, meaning a ringlet or curling lock of hair. This cloud can form at any altitude between 16,500 ft (5.0 km; 3.13 mi) and 45,000 ft (14 km; 8.5 mi) above sea level. The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of “mares’ tails”.

          From the surface of Earth, cirrus typically appears white, or a light grey in color. It forms when water vapor undergoes deposition at altitudes above 5,500 m (18,000 ft) in temperate regions and above 6,400 m (21,000 ft) in tropical regions. It also forms from the outflow of tropical cyclones or the anvils of cumulonimbus clouds. Since cirrus clouds arrive in advance of the frontal system or tropical cyclone, it indicates that weather conditions may soon deteriorate. While it indicates the arrival of precipitation (rain), cirrus clouds only produce fall streaks (falling ice crystals that evaporate before landing on the ground).

          Jet stream-powered cirrus can grow long enough to stretch across continents while remaining only a few kilometers deep. When visible light interacts with the ice crystals in cirrus cloud, it produces optical phenomena such as sun dogs and halos. Cirrus is known to raise the temperature of the air beneath the main cloud layer by an average of 10 °C (18 °F). When the individual filaments become so extensive that they are virtually indistinguishable from one another, they form a sheet of high cloud called cirrostratus. Convection at high altitudes can produce another high-based genus called cirrocumulus, a pattern of small cloud tufts that contain droplets of supercoiled water. Some polar stratospheric clouds can resemble cirrus, while noctilucent clouds are typically structured in a way that is similar to cirrus.

          Cirrus clouds form on other planets, including Mars, Jupiter, Saturn, Uranus, and possibly Neptune. They have even been seen on Titan, one of Saturn’s moons. Some of these extraterrestrial cirrus clouds are composed of ammonia or methane ice rather than water ice. The term cirrus is also used for certain interstellar clouds composed of sub-micrometer-sized dust grains.

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