absolute zero
boiling point
cryogenics
effects of temperature
freezing point
heat
heat transfer
supercooling
temperature scales

INTRODUCTION

Temperature, property of systems that determines whether they are in thermal equilibrium (see Thermodynamics). The concept of temperature stems from the idea of measuring relative hotness and coldness and from the observation that the addition of heat to a body leads to an increase in temperature as long as no melting or boiling occurs. In the case of two bodies at different temperatures, heat will flow from the hotter to the colder until their temperatures are identical and thermal equilibrium is reached (see Heat Transfer). Thus, the terms temperature and heat, although interrelated, refer to different concepts, temperature being a property of a body and heat being an energy flow to or from a body by virtue of a temperature difference.

Temperature changes have to be measured in terms of other property changes of a substance. Thus, the conventional mercury thermometer measures the expansion of a mercury column in a glass capillary, the change in length of the column being related to the temperature change. If heat is added to an ideal gas contained in a constant-volume vessel, the pressure increases, and the temperature change can be determined from the pressure change by Gay-Lussac's law, provided the temperature is expressed on the absolute scale.

TEMPERATURE AND PRECIPITATION SCALES

Temperature is an important aspect of climate and can be used to grade climatic zones on a scale of five: (1) Tropical, with annual and monthly averages above 20° C (68° F); (2) Subtropical, with 4 to 11 months above 20° C, and the balance between 10° and 20° C (50° to 68° F); (3) Temperate, with 4 to 12 months at 10° to 20° C, and the rest cooler; (4) Cold, with 1 to 4 months at 10° to 20° C, and the rest cooler; and (5) Polar, with 12 months below 10° C.

Within each hemisphere, eight basic climatological zones can also be recognized in terms of precipitation: (1) Equatorial: rain in all seasons; (2) Tropical: summer rain with winters dry; (3) Semi-arid Tropical: slight summer rain; (4) Arid: dry in all seasons; (5) Dry Mediterranean: slight winter rain; (6) Mediterranean: winter rain, summers dry; (7) Temperate: precipitation in all seasons; (8) Polar: precipitation sparse in all seasons.