Thursday, August 19, 2010
The Big Question
I have repeatedly gotten asked one question--including tonight--so let me answer it for all.
Why is the warmest period of the year in early August, when the sun is strongest on June 21st? How could that be?
Or a similar question: Why are the highest daily temperatures in summer around 5 PM when the sun is strongest at 1 PM (PDT)?
The temperatures should be highest when the sun is the strongest, right?Nope...that isn't correct.
The temperature of the earth depends primarily on two things. The amount of radiation coming IN and the amount of radiation going OUT. Everyone is familiar with what is coming in--radiation from the sun. Nighttime, no solar warming. During the day, the sun's radiation peaks at solar noon (12 PM PST, 1 PM PDT).But there is ALSO radiation going out. That is INFRARED radiation. The earth (and even you) emit infrared radiation. The warmer you are, the more infrared radiation you emit. (no jokes about "hot dates" please). This radiation is going on 24-h a day.
Whether the earth warms up or cools down depends on the sum of what is coming in and what is going out. More coming in than going out...the earth warms. More going out than coming in..the earth cools.
Consider the figure below for the daily situation (standard time). The curve with the yellow fill shows the solar radiation--which of course goes from zero at sunrise and sunset to a maximum at noon. The blue shows the infrared outgoing radiation, which varies as temperature changes. The red line is the temperature. You will notice for a period after noon, the incoming is still greater than the outgoing and thus the air temperature warms. Temperature rises until the outgoing equals the incoming...that is the time of maximum temperature--sometime in the afternoon. (Remember that the amount of infrared radiation depends on temperature, so it is greatest when temperature is highest).
Interestingly the minimum temperature each day often occurs AFTER the sun rises, because it takes a while for the incoming solar warming to exceed the outgoing infrared cooling (see figure).
The same idea works with the annual temperature variation. Yes, the sun is strongest on June 21st, but for a period of time the solar radiation is greater than the outgoing radiation. The crossover is in early August.
And there is something else that influences the time lag between maximum heating and surface temperature..the thermal inertia or heat capacity of the planet: the surface...and particularly water...takes a while to heat up during the summer. Over land, the thermal inertia is relatively small, while for water the thermal inertia is very large. Shallow bodies of water heat up more rapidly than the deep oceans.
Another way to think of all this is to consider you bank account. As long as more money is being deposited than being taken out, your bank balance will increase...even if your deposits are falling off!
Or your bathtub...as long as more water is coming in than draining out, the water level rises.
Makes sense?...let me know if I have confused things even more!
Posted by Cliff Mass at 9:26 PM