Cooling

As cooling processes are referred to, in which an object or a system of objects, heat is extracted. The items can be solid or semi-solid (eg, loose rock ), but also fluids ( liquids, gases) or a combination thereof.

From cooling is also called the decay violent emotions and mental tensions, see Chill Out.

Theory (thermodynamics )

For solids and liquids, heat transfer (or the extraction of energy ) proceeds according to the temperature gradient. Three processes contribute to varying degrees:

• Heat conduction (primarily for solids and liquids)
• Thermal radiation (in nature, especially sunlight, and nocturnal emission ),
• And (usually weaker) the adsorption or convection.

The goal of these processes is a temperature compensation (see third law of thermodynamics ). It results in spite of sometimes opposite impression always result in an increase of the total entropy - that is, a conversion of various forms of energy into thermal energy.

In a closed system, therefore, a continuous (Ab) is cooling impossible: The thermal energy of an isolated system can only go up to the outside, but do not sink. In the technical field can only cool its interior such as a refrigerator, while the average temperature of the entire space increases.

Other factors in heating or cooling processes are:

• Convection ( in solids zero for gases are usually high in fluids in the central region )
• Coefficient of thermal conductivity and heat capacity
• The adiabatic cooling, for example in the Earth's atmosphere at discounts and thermals
• The mean temperature of the atmosphere ( an average of 5-7 ° C per kilometer ) and the humidity
• Generally, the gas laws, and the Joule- Thomson effect
• The average weather and precipitation
• The albedo ( backscatter coefficient) of soil and plants (also on rainfall dependent)
• Occasionally processes of condensation and the Resublimation.

Cooling processes in nature

In natural systems, the most effective forms of cooling are

• Nocturnal cooling by radiation,
• Advection (contact to cooler objects )
• The cooling by evaporation ( withdrawal of the heat of evaporation )
• And the ventilation (wind, movement).

The speed at which the temperature of the body or object drops depends

• Of the external conditions ( effective temperature difference, humidity, radiation, winds, friction, etc. )
• As well as the thermal properties of the body or fluid ( specific heat, surface finish, porosity, etc.),
• And any possible its isolation (soil, weathering or tree bark, fur, feathers ).

Nocturnal cooling

At night, the air temperature drops to some degree i.a., in extreme climates, even up to 30 ° C. This is caused by the radiation ( infrared or thermal radiation) of the heated soil, which is particularly effective under a clear sky. The daytime heated by solar radiation and the early afternoon thermal equilibrium striving towards the ground cools normally slower than the air from, but more rapidly than water. This leads to thermal inversions or to the development of local wind systems, which are opposite to the Tagwinden approximately (see land-sea wind system near sea coasts ).

The nocturnal cooling can abruptly after or reverses itself even when clouds are gathering in the night sky or the dew point is reached. In the first case reduces the radiation, because the cloud cover acts as an insulator; in the second case the air is supplied to the heat of condensation of the mist.

The difference between day and night temperature is usually detected by a temperature curve, or by specifying maximum and minimum temperature. The maximum temperature occurs in Central Europe to about 14 clock CET ( or 15 clock summer time) to, the minimum temperature usually just before sunrise (see morning dew ).

While the temperature profile near the ground has the mentioned amplitude of a few degrees to ten degrees, the ground temperature is already noticeably balanced by a few centimeters already at a depth. At 1 m depth (see also Frost depth) or in caves, the daily, but also the seasonal temperature change largely disappear, depending on the soil characteristics and geology, the vegetation, the water content and the climate history.

On particularly clear days, the difference between day and night temperature can be almost zero, even be reversed with the passage of a warm front. An example of an unusual weather conditions with exceptionally low nocturnal cooling shows the adjacent figure.

Cooling through technical measures

In art, the proposed cooling is referred to as cooling. You can, for example,

• Used for the production of a state of thermal equilibrium,
• For the removal of friction or heat loss to moving or otherwise heated components
• And in particular carried out to prevent overheating.
• Unwanted cooling may either (if not critical ) to be accepted, as determined by temperature measurement, equalized by thermostat or heating or its effects are modeled by calibration. Conservation purposes in biomaterial.
• During melt spinning, the highest cooling rates achieved ()