Seasonal and Daily Temperatures

Temperature variations - diurnal and seasonal

Seasonal Variations

Caused by length of daylight and tilt of the earth.

Is the earth closer to the sun in January or July?

Tilt of earth is 23.5 degrees.

Summer and Winter Solstice, Autumnal and Vernal Equinox.: June 22 Dec 22 Sept 23 March 20

Over a year and averaged over the globe, the planet's energy gain (Solar energy) is equal its energy losses (terrestrial radiation). This is true for the globe as a whole, but not for individual latitude bands. Tropics gain more energy then they lose, while polar regions loss more than they gain.

The tilt of the earth also influences the length of day.

Notice also that when the solar beam is inclined, it as more atmosphere to pass through, more atmosphere - more absorption and scattering.

Over Wisconsin we get our greatest solar input near June 21 - yet maximum temperatures aren't until late July or early August. Why? Energy imbalances. Solar energy gains is important for global distributions, also local scale:

South versus north side of mountains - trees, ski slopes and grapes

Large windows face south - overhangs

Diurnal Variations

Around noon is when the earth's surface receives the most intense solar rays - not when maximum temperature is - solar input still overcomes energy loss - so surface heats up. How warm it gets depends on the surface - soil, moisture content and vegetation cover. Dry sand poor heat conductor - heat doesn't go deep, low specific heat so it heats up fast. Water good conductor plus mixing also energy to be mixed over a deeper layer also it has a high specific heat.

The effects of winds on vertical profile of temperature - mixing.

Night - radiational cooling, ground losses more energy then it gains, ground cools quicker than air - nocturnal or radiation inversion. -- windless night, long night, clear sky and dry snow, ground condition.

Cold air pools in low lying areas.

Crop protection - cover; heaters; wind machines; flood the field.

Temperature Range - difference between the daily maximum and minimum temperatures. Largest diurnal range occurs in deserts, humid regions - small diurnal temperature range, cities near large bodies of water.

Mean daily temperature - average over 24 hour period

normal - 30 year mean

Annual temperature range in temperature, land versus water mean annual temperature

Isotherm - line of constant temperature.

Controls of Temperature

  1. Latitude
  2. Land and Water
  3. Ocean Currents
  4. Elevation

Air temperature and human comfort - we loss heat through radiation emission at terrestrial wavelengths, also gain from absorption and gain/loss from conduction. Insulating air is blown away by the wind - how cold this makes us feel is expressed as the wind chill factor.