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Section 3.3 Seasons and temperature

The difference in the energy budget as a function of latitude shows the importance of

  1. the incidence angle for the Sun, and

  2. the length of a day

The angle is important because higher solar altitudes mean greater intensity; at lower angles the sunlight is spread out over a larger area.

Because of the tilt of the Earth’s axis, the solar angle changes throughout the year and gives rise to seasonal effects.

  • vernal equinox (Mar 21): Sun above equator at noon (\(47^\circ\) altitude at our latitude)

  • summer solstice (Jun 21): Sun above Tropic of Cancer (\(23.5^\circ\) N) at noon (\(70^\circ\) altitude at our latitude)

  • autumnal equinox (Sep 21): Sun above equator at noon (\(47^\circ\) altitude at our latitude)

  • winter solstice (Dec 21): Sun above Tropic of Capricorn (\(23.5^\circ\) S) at noon (\(24^\circ\) altitude at our latitude)

Subsection 3.3.1 Temperature cycles

The temperature high and temperature low are typically recorded for each 24-hour day.

  • From the high and low we can compute the daily mean temperature: the average of maximum temperature and minimum temperature = \(\frac{\textrm{max + min}}{2}\)

  • From the daily mean temperatures we can compute the monthly mean temperature = \(\frac{\text{sum of daily mean temps}}{\#\text{ of days in month}}\)

  • From the monthly mean temperatures we can compute the annual average temperature = \(\frac{\textrm{sum of monthly mean temps}}{12}\)

Subsection 3.3.2 Effects on temperature (both annually and daily):

  • latitude (higher latitude \(\rightarrow\) lower insolation \(\rightarrow\) lower tempeatures)

  • surface type (albedo and thermal properties)

  • elevation and aspect (higher altitude -> lower temperatures; aspect changes insolation)

  • proximity to water (presence of bodies of water moderates temperature variations): higher specific heat, greater absorption of heat via evaporation, and energy distributed over a thicker surface layer than for land.

  • advection (prevailing wind patterns may bring cooler/warmer air)

  • cloud cover (provides cooling in summer and moderates temperatures in the winter)

We can also explore interannual variations in temperature. These allow us to establish climate normals. Climatological temperatures refer to average temperatures over a 30-year period. This is often calculated for 1951-1980 or 1981-2010. The difference between the temperature at a given time for a particular location and the climatological normal is the temperature anomaly.

Subsection 3.3.3 Derived temperature indexes

Derived indexes are used to provide useful information about how the weather feels or to plan and monitor crop growth.

  • wind chill: takes into consideration heat loss from the body due to cold temperature and wind.

  • heat index: takes into consideration how high humidity reduces the rate of evaporation (and therefore cooling) from the body

  • growing degree days (GDD): can be used to track plan maturation based upon daily temperatures; cooler temperatures require longer growth seasons