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Terminologies in Climatology

Updated on February 26, 2016

List of Climatology terminologies covered in this hub.

  • Climatology
  • Urban climatology
  • Urban dust domes
  • Urban thermal plume
  • Microclimate
  • Urban Heat Island Effect
  • Psychrometrics
  • Psychrometric chart
  • Psychrometer
  • Hygrometer
  • Humidity
  • Absolute humidity
  • Relative humidity
  • Specific humidity
  • Humid heat
  • Dry-bulb temperature
  • Wet-bulb temperature
  • Dew point
  • Dew point temperature
  • Specific enthalpy
  • Psychrometric ratio


The meteorological study of climates and their phenomena.

Urban climatology

Urban climatology refers to a specific branch of climatology that is concerned with interactions between urban areas and the atmosphere, the effects they have on one another, and the varying spatial and temporal scales at which these processes (and responses) occur.

Urban dust domes

Urban dust domes are a meteorological phenomenon in which soot, dust, and chemical emissions become trapped in the air above urban spaces. This trapping is a product of local air circulations. Calm surface winds are drawn to urban centers, they then rise above the city and descend slowly on the periphery of the developed core. This cycle is often a cause of smog through photochemical reactions that occur when strong concentrations of the pollutants in this cycle are exposed to solar radiation. These are one result of urban heat islands: pollutants concentrate in a dust dome because convection lifts pollutants into the air, where they remain because of somewhat stable air masses produced by the urban heat island.

The urban heat island which causes a city to heat up, caps the dust and other particulates at a low level in the atmosphere. If there is not a strong enough wind, then this dome that is created remains intact and causes that heated up air within the urban heat island. Though if the wind does blow strong enough, then this dome is blown downwind causing it to move out of the city

Urban thermal plume

An urban thermal plume describes rising air in the lower altitudes of the Earth's atmosphere caused by urban areas being warmer than surrounding areas. Over the past thirty years there has been increasing interest in what have been called urban heat island (UHI), but it is only since 2007 that thought has been given to the rising columns of warm air, or ‘thermal plumes’ that they produce.


A microclimate is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet (for example a garden bed) or as large as many square miles.

Urban Heat Island Effect

An urban heat island (UHI) is a city or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon.

The main cause of the urban heat island effect is from the modification of land surfaces.Waste heat generated by energy usage is a secondary contributor. The less-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.


Psychrometrics or psychrometry or hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchron (ψυχρόν) meaning "cold" and metron (μέτρον) meaning" means of measurement".

Psychrometric chart

A psychrometric chart is a graphical representation of the psychrometric processes of air. Psychrometric processes include physical and thermodynamic properties such as dry bulb temperature, wet bulb temperature, humidity, enthalpy, and air density.

  • The most common chart used by practitioners and students alike is the "ω-t" (omega-t) chart in which the dry bulb temperature (DBT) appears horizontally as the abscissa and the humidity ratios (ω) appear as the ordinates.In order to use a particular chart, for a given air pressure or elevation, at least two of the six independent properties must be known (DBT, WBT, RH, humidity ratio, specific enthalpy, and specific volume).
  • This gives rise to \left(\right) = 15 possible combinations.
  • DBT: This can be determined from the abscissa on the x-axis, the horizontal axis
  • DPT: Follow the horizontal line from the point where the line from the horizontal axis arrives at 100% RH, also known as the saturation curve.
  • WBT: Line inclined to the horizontal and intersects saturation curve at DBT point.
  • RH: Hyperbolic lines drawn asymptotically with respect to the saturation curve which corresponds to 100% RH.
  • Humidity ratio: Marked on the y-axis.Specific enthalpy: lines of equal values, or hash marks for, slope from the upper left to the lower right.
  • Specific volume: Equally spaced parallel family of lines.


A type of hygrometer consisting of two thermometers, one of which has a dry bulb and the other a bulb that is kept moist and ventilated.The difference between the readings of the thermometers gives an indication of atmospheric humidity Also called wet-and-dry-bulb thermometer


an instrument for measuring the humidity of the air or a gas.


Humidity is the amount of water vapor in the air.

There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air at a given temperature expressed in gram per cubic metre.Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature. Specific humidity is a ratio of the water vapor content of the mixture to the total air content on a mass basis.

Absolute humidity

Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. Relative humidity is the ratio of the current absolute humidity to the highest possibleabsolute humidity (which depends on the current air temperature).

Relative humidity

Relative humidity (abbreviated RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature. Relative humidity depends on temperature and the pressure of the system of interest.

Specific Humidity

the ratio of the mass of water vapor in air to the total mass of the mixture of air and water vapor.

Humid heat

(humid heat) the heat capacity (specific heat) of humid air, expressed per unit mass of dry air in the mixture

Dry-bulb temperature

The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. DBT is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature. It indicates the amount of heat in the air and is directly proportional to the mean kinetic energy of the air molecules. Temperature is usually measured in degrees Celsius (°C), Kelvin (K), orFahrenheit (°F).

Wet-bulb temperature

The wet-bulb temperature is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel.A wet-bulb thermometer will indicate a temperature close to the true (thermodynamic) wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only. Wet-bulb temperature is largely determined by both actual air temperature (dry-bulb temperature) and the amount of moisture in the air (humidity). At 100% relative humidity, the wet-bulb temperature equals the dry-bulb temperature.

Dew point

Dew point is a measure of atmospheric moisture. It is the temperature to which air must be cooled to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as dew point temperature, and sometimes written as one word (dewpoint)

Dew point temperature

Dew point temperature (DPT) is the temperature at which a moist air sample at the same pressure would reach water vapor "saturation." At this point further removal of heat would result in water vapor condensing into liquid water fog or, if below freezing point, solid hoarfrost. The dew point temperature is measured easily and provides useful information, but is normally not considered an independent property of the air sample as it duplicates information available via other humidity properties and the saturation curve.

Specific enthalpy

Enthalpy is defined as a thermodynamic potential, designated by the letter "H", that consists of the internal energy of the system (U) plus the product of pressure (p) and volume (V) of the system

Specific enthalpy is a property of the fluid and can be expressed as:

the specific enthalpy of a uniform system is defined as h = H/m where m is the mass of the system. The SI unit for specific enthalpy is joule per kilogram. It can be expressed in other specific quantities by h = u + pv, where u is the specific internal energy, p is the pressure, and v is specific volume, which is equal to 1/ρ, where ρ is the density.

h = u + p v

Psychrometric ratio

The psychrometric ratio is the ratio of the heat transfer coefficient to the product of mass transfer coefficient and humid heat at a wetted surface. It may be evaluated with the following equation

r = \fracwhere:

  • r = Psychrometric ratio, dimensionless
  • h_c = convective heat transfer coefficient, W m-2 K-1
  • k_y = convective mass transfer coefficient, kg m-2 s-1
  • c_s = humid heat, J kg-1 K-1


  • Luke Howard, The climate of London, deduced from Meteorological observations, made at different places in the neighbourhood of the metropolis, 2 vol., London, 1818-20
  • Glossary of Meteorology (2009). "Urban Heat Island". American Meteorological Society. Retrieved 2009-06-19
  • Nall, D. H. (2004-11). Looking across the water: Climate-adaptive buildings in the United States & Europe. In The Construction Specifier, 57, 50 – 56.
  • Hsu Sheng-I (1981). "The urban heat island effect : a case study of metropolitan Phoenix area". Chinese University of Hong Kong
  • Oxford Reference, A Dictionary of Weather, ISBN 978-0-19-954144-7
  • Perry's Handbook of Chemical Engineering, 6th edition; section 12, Psychrometry.

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