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9.7 Temperature, Pressure and Humidity

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Terms

Isotherm: An isotherm is a line of equal or constant temperature on a graph, plot, or map; an isopleth of temperature. For example, isotherms are commonly seen on weather maps to show large-scale temperature distributions.

Temperature: A qualitative measure of the amount of heat energy; heat is a form of internal energy which is transferred from one object to another due to a difference in temperature between the objects. Heat is the total energy of motion of all particles (the total kinetic energies of all the particles).

Heat: A quantitative measure of the amount of heat energy; The temperature of a body of matter is a measure of the average kinetic energy of the random motion of its particles. Temperature is the kinetic energy divided by the number of particles. Temperature is that property of a substance which determines whether it is in thermal equilibrium with another object.

Temperature Inversions: When temperature increases with height an inversion is said to exist. Inversions may also be found in the troposphere whenever the lapse rate becomes negative (i.e. temperature increasing with height).

Radiation: Radiation is the transfer of heat by electromagnetic waves. Any matter with a temperature above absolute zero (-273° C) radiates heat energy. This energy travels as electromagnetic waves. Hotter matter has shorter wavelengths. The sun is extremely hot emitting short wave radiation also known as insolation. By comparison, the earth's surface is much cooler, emitting long wave radiation, also known as terrestrial radiation.

Heat transfer by radiation occurs through the vacuum of space, which is why the earth is able to receive the sun’s radiation. Of all the insolation reaching the outer fringes of the atmosphere less than half is finally absorbed by the earth's surface. The rest is lost by reflection from the ground, cloud tops, dust, water vapour, smoke, etc.

Terrestrial radiation has even more difficulty penetrating the atmosphere than insolation. Due to its longer wavelength, water vapour and carbon dioxide in the atmosphere more readily absorb terrestrial radiation. Their molecules then radiate heat back to earth. This is known as the greenhouse effect.

Conduction: Conduction is the transfer of heat through contact. As the surface of the earth is heated by insolation, the air in contact with the surface is heated by conduction. Air is poor at conducting heat so only the air close to the earth's surface is heated. This is why temperature decreases with altitude in the lower atmosphere.

Convection: Convection is vertical movement of the atmosphere that results in mixing the atmospheric properties. When air is heated by contact with the earth's surface it becomes less dense. This warm, less dense air rises and forces the higher cooler denser air above it to move horizontally across termed advection. The advection of high cooler air becomes denser than the surrounding air and starts to sink. Sinking is known as subsidence. Convection, advection and subsidence combine to form a simple circulation pattern.

Isobar: An isobar is a line of equal or constant pressure on a graph, plot, or map; an isopleth of pressure. In meteorology, the pressures shown are reduced to sea level, not the surface pressures at the map locations.

Horizontal Pressure Gradient: Pressure gradient is the change in pressure over a distance. For example, in meteorology the Horizontal Pressure Gradient is the change in atmospheric pressure over a certain distance. The horizontal pressure gradient force is the force of air flow caused by a difference in pressure across an area. The Horizontal Pressure Gradient Force points toward low pressure perpendicular to isobars and geopotential height contours.

Evaporation: Evaporation is the process whereby atoms or molecules in a liquid state gain sufficient energy to enter a gaseous state.

The thermal motion of a molecule must be sufficient to overcome the surface tension of the liquid in order for it to evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore proceeds more quickly at higher temperature and in liquids with lower surface tension.

Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.

If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapour above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapour increases. When the process of escape and return reaches equilibrium, the vapour is said to be "saturated” and no further change in either vapour pressure, density or liquid temperature will occur.

  • Concentration of the substance evaporating in the air. If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
  • Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
  • Temperature of the substance. If the substance is hotter, then evaporation will be faster.
  • Flow rate of air. This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. In addition, molecules in motion have more energy than those at rest, and so the stronger the flow of air, the greater the evaporating power of the air molecules.

Relative Humidity: Relative humidity is the ratio of the current vapour pressure of water in any gas (especially air) to the equilibrium vapour pressure, at which the gas is called saturated at the current temperature, expressed as a percentage. Equivalently, it is the ratio of the current mass of water per volume of gas and the mass per volume of a saturated gas.

Saturated Air: An atmospheric humidity of 100% represents the saturation point, at which the air can hold no more moisture, see also dew point.

Condensation: The transition from the gaseous to the liquid state and the physical process by which water vapour is transformed into dew, fog or cloud droplets.

Dew Point: The dew point of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for the water vapour component to condense into water, called dew. When the dew point temperature falls below freezing it is called the frost point, instead creating frost or hoarfrost by deposition.

The graph shows the maximum percentage of water vapour that can exist in air at sea level across a range of temperatures. Note that with higher temperatures the equilibrium partial pressure of water vapour increases thus more water evaporates. The behaviour of water vapour does not depend on the presence of air. The formation of dew would occur at the dew point even if the only gas present was water vapour.

The most common confusion over the dew point is that it is the temperature at which condensation forms. This is not necessarily true. Actually condensation is always occurring in our air. At the dewpoint temperature however, is when condensation overtakes evaporation, and this is a step in the process of forming dew, clouds, rain, fog, basically water droplets.

The dew point determines relative humidity. When the relative humidity is high, the dew point is closer to the current air temperature. If the relative humidity is 100%, the dew point will be equal to the current temperature. As relative humidity falls, the dew point becomes lower, given the same air temperature.

Freezing: Freezing is a marked temperature point where a liquid solidifies under a set of conditions. By lowering air temperature to a value equal to or less than the freezing point of water (0°C) results in the formation of ice.