A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of the Earth or
another planetary body. A cloud is also a visible mass attracted by gravity (clouds can also occur as masses of
material in interstellar space, where they are called interstellar clouds and nebulae.) The branch of meteorology in
which clouds are studied is nephology or cloud physics.
On Earth the condensing substance is typically water vapor, which forms small droplets of ice crystals, typically
0.01 mm in diameter. When surrounded by billions of other droplets or crystals they become visible as clouds.
Dense deep clouds exhibit a high reflectance (70% to 95%) throughout the visible range of wavelengths: they thus
appear white, at least from the top. Cloud droplets tend to scatter light efficiently, so that the intensity of the solar
radiation decreases with depth into the gases, hence the gray or even sometimes dark appearance of the clouds at
their base. Thin clouds may appear to have acquired the colour of their environment or background, and clouds
illuminated by non-white light, such as during sunrise or sunset, may be coloured accordingly. In the near-infrared
range, clouds would appear darker because the water that constitutes the cloud droplets strongly absorbs solar
radiation at those wavelengths.
Fog is a cloud bank that is in contact with the ground. Fog is usually the only clouds that touch the ground and it only
differs slightly from other clouds in that it touches the surface of the Earth. The same cloud that is not fog on lower
ground may be fog where it contacts higher ground such as hilltops or mountain ridges. Fog is distinct from mist
only in its density. Fog is defined as a cloud which reduces visibility to less than 1 km, whereas mist is that which
reduces visibility to more than 2 km.
The foggiest place in the world is the Grand Banks off the island of Newfoundland. Fog is frequent there as the
Grand Banks is the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream
from the south. The foggiest land areas in the world are Point Reyes, California, and Argentia, Newfoundland and
Labrador, both with over 200 foggy days a year. Even in generally warmer southern Europe, thick fog and localized
fog is often found in lowlands and valleys, like the lower part of Po Valley and the Tiber Valley, especially in late
autumn and winter.
A vapor (American spelling) or vapour (see spelling differences) is a substance in the gas phase at a temperature
lower than its critical temperature. This means that the vapor can be condensed to a liquid or to a solid by
increasing its pressure, without reducing the temperature.
For example, water has a critical temperature of 374°C (or 647 K) which is the highest temperature at which liquid
water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water is known as water vapor and
will condense to liquid if its partial pressure is increased sufficiently.
A vapor may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas
pressure will equal the equilibrium vapor pressure of the liquid (or solid).
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