Geography · Ch 14 — Solar Radiation, Heat Balance and Temperature
Temperature
Temperature
What temperature really is
Heat and temperature are often used interchangeably, but they are not the same thing. Heat is the energy created by the molecular movement of the particles that make up a substance. Temperature, on the other hand, is simply the measurement, in degrees, of how hot or cold a thing or a place is. When insolation interacts with the atmosphere and the earth's surface, it produces heat, and that heat is what we measure as temperature.
Factors controlling temperature distribution
The temperature of the air at any place is not random. It is controlled by three main factors:
- Latitude of the place
- Altitude of the place
- Distance from the sea (maritime influence)
Of these, latitude has the most visible and consistent effect, and this shows up clearly on maps that use isotherms.
Isotherms and what they reveal
An isotherm is a line joining places that have equal temperature. Maps of surface air temperature for January and July are drawn using these lines, and they tell a clear story about how temperature is distributed across the globe.
In general, the effect of latitude is well pronounced: isotherms run roughly parallel to the lines of latitude. But this pattern is not perfect. The deviation from the general trend is much more pronounced in January than in July, especially in the northern hemisphere.
Why the northern hemisphere behaves differently
The northern hemisphere has a much larger land surface area than the southern hemisphere. Because of this, the effects of landmasses and ocean currents are strongly visible there.
In January, isotherms bend northward over the oceans and southward over the continents. The North Atlantic Ocean is the classic example. Warm ocean currents — the Gulf Stream and the North Atlantic Drift — make the northern Atlantic warmer than it would otherwise be, so the isotherms bend towards the north over this ocean. Over land, however, temperature drops sharply, and the isotherms bend towards the south across Europe. This bending is especially pronounced over the Siberian plain.
A striking illustration of this: along the 60° E longitude, the mean January temperature is minus 20° C at both 80° N and 50° N latitudes. That means two places separated by 30 degrees of latitude have the same January temperature, purely because of the contrast between the warm ocean influence and the frozen continental interior.
January temperature patterns across the globe
The book gives a clear picture of mean monthly temperatures for January:
| Region | Mean January temperature |
|---|---|
| Equatorial oceans | over 27° C |
| Tropics | over 24° C |
| Middle latitudes | 2° C to 0° C |
| Eurasian continental interior | –18° C to –48° C |
The southern hemisphere: a smoother pattern
The effect of the ocean is far more pronounced in the southern hemisphere, where land is scarce. Here the isotherms run more or less parallel to the latitudes, and temperature variation is gradual rather than sharp. The isotherms of 20° C, 10° C and 0° C run parallel to 35° S, 45° S and 60° S latitudes respectively.
July: a different picture
In July, the isotherms generally run parallel to the latitude. The equatorial oceans record temperatures warmer than 27° C. Over land, the subtropical continental region of Asia, along the 30° N latitude, records more than 30° C. Along the 40° N latitude runs the isotherm of 10° C, and along the 40° S latitude the temperature is also 10° C. …