Chapter 5 - Global Temperature Patterns

Basic Temperature Concepts

Temperature is a measure of the average kinetic energy of the molecules/atoms in a substance (gas, liquid, or solid).  When a material absorbs energy, its molecules/atoms move or vibrate more energetically.  Its temperature rises.

Thermometers are thin glass tubes that contain a liquid, normally mercury.  As the thermometer warms, the mercury atoms move more energetically; they spread out.  The mercury expands, rising up the tube.

Daniel Fahrenheit (1686-1736) produced the first useful temperature scale for use in thermometers.  He chose the lowest temperature that he was able to produce in the lab as 0 °F and the (approximate) human body temperature as 100 °F.  On the Fahrenheit scale the freezing point of water is 32 °F and the boiling point is 212 °F.

Anders Celsius (1701-1744) created a temperature scale based on the freezing point 0 °C and boiling point 100 °C of water.  The Celsius scale is also called centigrade.

Lord Kelvin (1824-1907) introduced a modified version of the Celsius scale by using absolute zero (no kinetic energy) as the starting point, but maintaining the 100 degree span between the freezing point (273 K) and boiling point (373 K) of water.

Temperature Controls

Latitude

Temperature  At low latitudes more solar radiation falls on each square meter of the Earth's surface than at high latitudes because the sun is more directly overhead at low latitudes and at a lower angle in the sky at higher latitudes.  The result is that low latitudes generally have warmer annual average temperatures than high latitudes.

Seasonality  At low latitudes (in thr tropics) the sun is never very far from being directly overhead and the daylength is always close to 12 hours so there is little temperature variation during the course of a year.  At high latitudes the noontime altitude of the sun changes from being low in the sky in the winter (below the horizon inside the Arctic and Antarctic circles) to moderately high in the sky during the summer and the length of day changes drastically.  So there is a very large swing between winter and summer temperatures.

Altitude  Recall that temperatures decrease on average by 6.4 °C per 1000 m (3.5 °F per 1000 ft) above the Earth's surface in the troposphere. Part of the reason is that much of the energy absorbed from the sun is at the surface and that energy most strongly warms the air near the surface.  Another reason is that the density (or pressure) of the air decreases with increasing altitude so that half of the mass of the atmosphere lies below about 18,000 ft (high mountains.  Thin mountain air cannot hold as much sensible heat as dense sea level air  because there aren't as many molecules per volume of air to hold the heat.  The thinner air does less to block filter incoming radiation so insolation is more intense.  Surfaces heat more quickly.  But when the sun goes down surfaces cool quickly since the air can't hold much heat.

Cloud Cover  Clouds reflect sunlight and reduce insolation at the Earth's surface, though some light does diffuse through clouds to reach the surface.  Clouds also absorb infrared radiation coming from the surface thereby reducing the loss of heat and decreasing temperature drop during the nighttime hours.  Regions with frequently cloudy weather generally have a more moderate range of temperatures, both daily and seasonal.

Land-Water Differences
      Land heats and cools rapidly, daily and seasonally - continental climates have extreme temperature ranges
      Water heats and cools more slowly, daily and seasonally- marine climates have moderate temperature ranges

Evaporation  Much more evaporation occurs over ocean surfaces than over land surfaces so over the oceans more of the surplus radiation (shorwave and longwave) is expended in latent heat of evaporation (LE) and less in sensible heat (H).  Over land, in general, more of the surplus radiation is used in producing sensible heat (temperature change).

Transparency  Water is transparent and sunlight filters down to depths of 100 meters or more.  So the energy absorbed by the water is distributed throughout the upper 100 m, not just at the very surface.  This distributed energy allows the temperature to rise and fall gradually (because large volumes of water are heated in the day and all summer and cooling at night and all winter).  The soil and rock of solid ground is essentially opaque.  Solar energy is absorbed and held directly at the ground surface.  It is only slowly transmitted underground by conduction.  Consequently, the ground surface heats rapidly in the daytime and the upper few feet of the ground heat much more rapidly in summer than the ocean does.  The land also cools much quicker at night and in the fall-winter.

Specific Heat  Water can hold more heat than soil and rock.  It also takes more heat to change the temperature of the ocean compared to the land.  So ocean temperature rises more slowly in the spring and summer and cools more slowly in the fall and winter and the day-night and summer-winter temperature range is much less for the oceans and the air over the oceans compared to the land and the air over the land.

Movement Water can mix vertially and flow laterally (ocean currents) so heat can be distributed to a larger volume of water than just where the heating takes place. This is another reasong that water temperature doesn't change as quickly or as much as land temperature.

Earth's Temperature Patterns

Looking at a temperature map of the Earth will show that temperatures are warm in the tropics and generally get cooler towards the poles.  The lines on a temperature map are called isotherms, lines of equal temperature.  The lines generally run east-west.  The warmer temperature isotherms are closer to the equator and the cooler temperature isotherms are closer to the poles.  This is a zonal pattern, a pattern that depends on latitude.

The position of the isotherms (and the temperature bands) changes with the seasons.  During the northern hemisphere summer, the thermal equator, the line the connects the warmest temperatures on the globe, lies north of the geographic equator.  The thermal equator shifts to south of the geographic equator for the southern hemisphere summer.

Comparing the pattern of summer temperatures with winter temperatures, another map can be produced for the global annual temperature ranges.  As one might predict from a knowledge of the Earth's temperature controls (see above), the smallest annual temperature ranges are found in the equatorial and marine realms (especially in the tropical oceans).  The greatest temperature ranges occur in large continental areas and at high latitudes (especially high latitude continental areas).

Sensation of Temperature

Wind Chill  When it is cold our skin feels cold because heat is going out of our body.  It feels colder to us when it is cold and the wind is blowing because the moving air is able to carry more heat away from our body.  The wind chill factor is a determination of how much heat is lost from exposed skin (so how cold it will feel) depending on the actual temperature plus the wind.  For example, if it is 20 °F out and the wind is blowing at 20 miles per hour, it feels like (the heat lost from exposed skin is equivalent to) 4 °F above zero.  On the other hand, wearing a wind-proof outer layer over a warm inner layer of clothing largely nullifies the wind chill effect.

Heat Index  Everyone knows that hot and humid feels hotter than hot and dry.  The reason is that our bodies cool, in part, through evaporation of perpsiration.  Heat energy is taken from our skin to evaporate perspiration (energy goes into latent heat of evaporation).  When the air is humid, since there is already a lot of water vapor in the air there is less room for additional water molecules to evaporate from our skin.  Evaporation is much slower.  So our skin remains sweaty and the heat stays with our body.  For example, a temperature of 85°F at 90% relative humidity feels like 100 °F (at 25% relative humidity).