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Unit 08


Unit Overview

This unit discusses principally the vertical and horizontal variations temperature. The main sections are:

  • What is temperature?
  • The vertical distribution of temperature
  • The horizontal distribution of temperature

Temperature is an index used to measure kinetic energy. Increased molecular motion leads to increased kinetic energy and subsequent increases in temperature. Temperatures in the troposphere typically decrease with an increase in altitude. This rate of temperature decrease with altitude increase is known as a lapse rate. The environmental lapse rate (ELR) is the mean lapse rate of the troposphere, and its value is 0.650° C/100 m. Like most atmospheric variables, it has a central tendency with the possibility of extreme values. Thus, the ELR can be highly variable and extreme positive and negative values can occur.

Lapse rates can also be considered at the theoretical level. These adiabatic lapse rates involve imaginary air parcels that always decrease in temperature with an increase in altitude. Adiabatic refers to a change in temperature without the gain or loss of heat. The two lapse rates are the dry adiabatic lapse rate (DALR) (10 C/100 m) and the saturated adiabatic lapse rate (SALR) (~0.6° C/100 m). An evaluation of the empirical lapse rate (i.e. ELR) and the theoretical lapse rates (i.e. DALR and SALR) enables an assessment of the relative stability of the atmosphere. One can determine whether or not a lifted air parcel (lifting mechanisms are discussed in later units) will continue to rise or will fall once the lifting mechanism is removed.

The three types of stability are stable, unstable, and conditionally unstable. Stable and unstable are sometimes referred to as absolutely stable and absolutely unstable. Temperature also varies across horizontal space. The main controls of these variations are as follows: altitude, latitude, and proximity to large bodies of water, ocean currents, and the degree of cloudiness.

Unit Objectives

  • To discuss the measurement and characteristics of temperature and heat
  • To explain the adiabatic process whereby vertically moving air heats and cools
  • To discuss the global distribution of temperatures and their variation over space and time

Glossary of Key Terms

Adiabatic With air being a poor conductor of heat, a parcel of air at one temperature that is surrounded by air at another temperature will neither gain nor lose heat energy over a short period of time; when such non-transfer of heat occurs, the process is called adiabatic.
Adiabatic lapse rate When a given mass of air is forced to expand, its temperature decreases. If a parcel of air rises to a higher altitude, it expands and cools adiabatically; its lapse rate is therefore referred to as an adiabatic lapse rate.
Advection The horizontal movement of material in the atmosphere.
Annual (temperature) cycle The pattern of temperature change during the course of a year.
Celsius scale Metric temperature scale most commonly used throughout the world (the United States is an exception); the boiling point of water is set a 100°C and its freezing point at 0°C.
Continentality (continental effect) The variation of the continental effect on air temperatures in the interior portions of the world¿s landmasses; the further the distance from the moderating influences of an ocean (known as the maritime effect), the greater the extreme in summer and winter temperatures.
Diurnal (temperature) cycle The pattern of temperature change during the course of a day.
Dry adiabatic lapse rate (DALR) The lapse rate of an air parcel not saturated with water vapour: (1°C/100m, 5.5°P/1000ft).
Dust dome The characteristic shape taken by the large quantities of dust and gaseous pollutants in a city¿s atmosphere.
Environmental lapse rate (ELR) The non-adiabatic lapse rate at any particular time or place; the troposphere¿s (non-adiabatic) normal lapse rate averages .65°C/100m (3.5°P/1000ft).
Fahrenheit scale Temperature scale presently used in the United States; water boils at 212°P (100°C) and freezes at 32°P (0°C).
Isotherms Lines connecting all points experiencing identical temperatures.
Kelvin scale The absolute temperature scale used by scientists, based on the temperature of absolute zero (-273°C/-459.4°P); a Kelvin degree is identical to a Celsius degree, so that water boils at 373°K (100°C) and freezes at 273°K.
Kinetic energy The energy of movement.
Maritime effect The moderating influence of the ocean on air temperature, which produces cooler summers and milder winters relative to inland locations at similar latitudes.
Saturated adiabatic lapse rate (SALR) The lapse rate of an air parcel saturated with water vapour in which condensation is occurring; unlike the dry adiabatic lapse rate (DALR) the value of the SALR is variable, depending on the amount of water condensed and latent heat released.
Stability (of air) A parcel of air whose vertical movement is such that it returns to its original position after receiving some upward force; however, if an air parcel continues moving upward after receiving such a force, it is said to be unstable.
Temperature The index used to measure the kinetic energy possessed by molecules; the more kinetic energy they have, the faster they move. Temperature, therefore, is an abstract term that describes the energy (speed of movement) of molecules.
Temperature gradient The horizontal rate of temperature change over distance.
Temperature inversion Condition in which temperature increases with altitude rather than decreases - a positive lapse rate; it inverts what we, on the surface believe to be the ¿normal¿ behavior of temperature change with increasing height.
Thermometer An instrument for measuring temperature; most commonly, these measurements are made by observing the expansion and contraction of mercury inside a glass tube.

Unit Outline

  • What is temperature?
    • Kinetic energy and its measurement
    • Fahrenheit scale
    • Celsius scale
    • Kelvin scale
  • The vertical distribution of temperature
    • Tropospheric temperature and air stability
      • lapse rate
      • stability vs. instability
    • Adiabatic lapse rates
      • adiabatic processes do not gain heat from or lose heat to surrounding air
      • air rises, expands, and cools as it moves to higher altitudes; it does not gain heat from or lose heat to surrounding air
      • dry adiabatic lapse rate (DALR)
      • environmental lapse rate (ELR)
      • saturated adiabatic lapse rate (SALR)
    • Temperature inversions and air pollution
      • temperature of tropospheric air normally decreases with height, as air rises it expands, and cools
      • temperature inversions show an increase in tropospheric air temperature with height
      • In an inversion, warm air lies on top of cool air and traps dust and pollutants
        • dust domes
        • smog
  • The horizontal distribution of temperature
    • Diurnal cycle
      • balance between radiation and radiant loss
    • Annual cycle
      • similar balance as diurnal cycle between radiation and radiant loss
    • Land/water heating differences
      • land requires less time to heat and cool than water
      • land has greater annual temperature ranges than water
        • maritime effect
        • continental effect
      • air temperature is affected by heat balance and advection, the horizontal movement of air by wind
    • Global temperature variations
      • isotherms, lines connecting all points with the same temperature, used for maps
      • temperature gradient is the horizontal rate of temperature change over distance
      • flow patterns associated with ocean currents and air movement

Review Questions

  1. Compare and contrast the dry adiabatic lapse rate (DALR), saturated adaibatic lapse rate (SALR), and environmental lapse rate (ELR).
  2. Discuss the general trends of the seasonal march of the latitudinal temperature belts.
  3. Define the term temperature and discuss what it actually measures.