• Temperature defn:  a measure of the average speed of the atoms and molecules that comprise air.  Higher temps correspond to faster average speeds.

• air temperature is measured on a regular basis from thermometers mounted in an "instrument shelter" (Fig. 14.8)
• these shelters, keep the thermometer from direct sunlight, and allow for a free flow of air
• max and min temperature thermometers are usually mounted in the shelter as well

• Daily Mean Temp = (max + min) / 2

• Daily Temp Range = (max - min)

• Monthly Mean Temp = sum of the daily means/ # days in month

• Annual Mean = average of 12 montly means

• Annual Temp Range = diff. between highest and lowest montly mean temp

B. Daily Radiation Curves and Temperature

• the daily variation in air temperature is controlled (primarily) by insolation

• when insolation exceeds outgoing longwave, the air temperature rises; when outgoing longwave exceeds insolation, the temperature falls

• as insolation input decreases toward sunset, the energy lost (longwave) exceeds the energy input (insolation) - temps begin to drop

• (the warmest time of the day occurs not at the moment of maximum insolation (solar noon), but at the moment when a maximum of insolation is absorbed)

• the warmest time of the day thus occurs 3-4 hours after solar noon - lag between peak insolation & maximum temperature

C. Temperature Controls

Defn: any factor that causes air temperature to vary from place to place and from time to time

(1) Latitude:

• insolation receipt is the most important influence on temperature variations

• recall that direct insolation is more concentrated (provides more heating) than oblique insolation; and the equatorial areas are the only latitudes to receive direct insolation;

• thus, there is a decrease in insolation/ heating/temperature from the equatorial regions northward and southward toward the poles

• from the equator poleward: continually warm; seasonally variable; always cold

(2) Altitude:

      Recall: temperature decreases w/ht. in the troposphere. Why?

• the distance from the heat source (the ground) increases;

• and the air becomes less dense (thinner) w/ht., & thus it's ability to absorb and radiate energy lessens w/ht.

• average temps are lower

• nightime temps (minimums) are lower

• diurnal temperature ranges are greater

• UV component is greater - sunburn hazard

cloud type, height, and thickness determine how insolation is reflected

• low thick clouds have albedo of ~ 90%

• high, thin (ice crystal) clouds have albedo of ~ 50%

• clouds are moderating influences on temp - they produce lower daily maximum temps & higher nightime minimums

• clouds result in slightly lower Earth-atmos. system temperatures

4) Land-Water Heating Differences

• more moderate (less extreme) temperature patterns are associated with water bodies vs. land masses

• marine (maritime) - locations that are dominated by the moderating effects of the ocean - they exhibit a smaller range of diurnal and annual temperatures than continental locations

• continental (continentality) - stations that lack the temperature-moderating effects of large water bodies - they exhibit a greater range of diurnal and annual temperatures than maritime locations

                                                                                                                                                    
Mean Monthly Temps for Vancouver, British Columbia & Winnipeg, Manitoba                        Same for Eureka, CA and NYC
 

What causes these differences?

Land-Water Temperature Controls:

• Evaporation:
• over water surfaces more of the net radiation (NET R) is expended for evaportion - thus, less energy available for sensible heating
• Transmissibility:
• in water, light penetrates to a greater depth, and thus the heat energy is spread over a larger area
• over land, the energy is concentrated and contained in the top most layer, & thus higher temps are achieved there

• Specific Heat:
• water requires more heat energy to raise its temperatures than does land
• mixing (movement):
• the movement of water (currents and movement due to density differences) causes the available heat to be spread over a larger area; heat energy is redistributed

5) The Effects of Currents & Sea Surface Temps (SST's)

• influence the temperatures of adjacent land masses (e.g., Gulf Stream & Japan Current produce milder temps for Eastern U.S. & Japan)

"Isotherms" - connect locations having the same temperature - show temp patterns

                
          Isothermal Map

Highlights:

January:  isotherms trend zonal (E-W); S. Hemisphere has the hottest temps; coldest temps are over the landmasses in N. Hemisphere ; coldest temps are in NE Siberia - because it's clear, dry, & calm; ocean temps are more moderate

                            
World Mean Sea-Level Temperatures in January
 

July: : winter now in S. Hemisphere, but since there is more ocean area in S. Hemisphere it has milder winters than N. Hemisphere winters; warmest temps found in N. Hemisphere deserts - clear skies, dry air & land, & strong insolation.

                            
World Mean Sea-Level Temperatures in July
 

Annual Range of Tempertures at a Geographic Location : (recall that a combination of daylength and angle of insolation determine heating);

• an equatorial station - which experiences little variation in daylength and sun angle throughout the year- has a small annual temp range
• a mid-latitude station - which experiences a greater variation in daylength and sun angle throughout the year- has a larger annual temp range
• thus: the annual temperature range increases with an increase in latitude
• since a continental location vs. a marine location experieinces hotter summers and colder winters, the annual temperature range increases with an increase in continentality as well
• e.g., Yakutsk, Siberia = 62.2 C (112 F) range

E. The Human Body & Temperature
 

• Wind Chill - an indication of the enhanced rate at which body heat is lost to the air. As wind speed increases, heat loss from the skin also increases.

• Heat Index (HI) - an indication of how the air "feels" to an average person. It takes the humidity of the air into consideration. The relative humidity (RH) of the air affects the rate at which perspiration can be removed from the skin.

F. Anthropogenic Effects on Global Temperature

• Significant climate change has occured in the past & will most will likely occur in the future - from ice ages to warmer periods - humans have little influence over these natural cycles

• However, society may now be significantly influencing global temperatures, by:

• adding greenhouse gases (CO2, methane, CFC's, & water vapor) to the atmosphere & thus enhancing the greenhouse effect
• it appears as though average global temperatures are rising & human activities appear to be affecting short-term temperature trends