Page 139 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 4 atmosphere and Surface Energy Balances 103
▼Figure 4.10 Energy budget by latitude. Earth’s energy surpluses and deficits produce poleward transport of energy and mass in each hemisphere, through atmospheric circulation and ocean currents. Outside of the tropics, atmospheric winds are the dominant means of energy transport toward each pole.
Daily Radiation Patterns
Figure 4.11 shows the daily pattern of absorbed incoming shortwave energy and resulting air temperature. This graph represents idealized conditions for bare soil on a cloudless day in the middle latitudes. Incoming energy arrives during daylight, beginning at sunrise, peaking at noon, and ending at sunset.
The shape and height of this insolation curve vary with season and latitude. The max- imum heights for such a curve occur at the time of the summer solstice (around June 21 in the Northern Hemisphere and December 21 in the Southern Hemisphere). The air tempera- ture plot also responds to seasons and varia- tions in insolation input. Within a 24-hour day, air temperature generally peaks between 3:00 and 4:00 p.m. and dips to its lowest point right at or slightly after sunrise.
Note that the insolation curve and the air temperature curve on the graph do not align; there is a lag between them. The warmest time of day occurs not at the moment of maximum insolation but at the moment when a maxi- mum of insolation has been absorbed and emitted to the atmosphere from the ground. As long as the incoming energy exceeds the outgoing energy, air temperature continues to increase, not peaking until the incoming energy begins to diminish as the afternoon Sun’s altitude decreases. If you have ever gone camping in the mountains, you no doubt ex- perienced the coldest time of day with a wake- up chill at sunrise.
The annual pattern of insolation and air temperature exhibits a similar lag. For the
Nor th Pole
90°
90°
60° N
Poleward transpor t of energy surplus
–
Equatorial
Polar energy deficit
–+ +
+
+ –+
Polar energy deficit
23.5° Tropic of Cancer
Equator
23.5° Tropic of Capricorn
0°
and tropical 0° energy
surplus
•
At around 36° latitude, a balance exists between energy gains and losses for the Earth–atmosphere system.
90° South
Pole
–
60° S
Poleward transport of energy surplus
90°
The imbalance of energy from the tropical surpluses and the polar deficits drives a vast global circulation pattern. The meridional (north–south) transfer agents are winds, ocean currents, dynamic weather systems, and other related phenomena. Dramatic examples of such energy and mass transfers are tropical cyclones (hurricanes and typhoons) discussed in Chapter 8. After forming in the tropics, these powerful storms mature and migrate to higher latitudes, carrying with them water and energy that redistributes across the globe.
Energy Balance at Earth’s Surface
Solar energy is the principal heat source at Earth’s sur- face; the surface environment is the final stage in the Sun-to-Earth energy system. The radiation patterns at Earth’s surface—inputs of diffuse and direct radiation and outputs of evaporation, convection, and radiated longwave energy—are important in forming the envi- ronments where we live.
Midnight
Sunrise
Noon 3P.M.Sunset
Absorbed insolation Air temperature
Midnight
Coolest time of day
Lag
Surplus
Local noon
Warmest time of day
Air temperature
▲Figure 4.11 Daily radiation and temperature curves. Sample radiation plot for a typical day shows the changes in insolation (solid line) and air temperature (dashed line). Comparing the curves reveals a lag between local noon (the insolation peak for the day) and the warmest time of day.
Absorbed insolation
Radiant energy flow
–Outgoing energy loss +Incoming energy gain
Air temperature