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 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!


  1. You could tie this into climatology as well: the 6 month lag between ENSO cycles and the global air temperature response, the lagging respnse due to volcanic forcing, etc.

  2. Cliff, you didn't explain why radiation going out lags behind the radiation going in. I think it's because of earth heating inertia (not sure if it's the right term). If radiation going in warmed up earth instanteneously, the peaks would match and max temp would be at noon (1pm pdt). In reality, it takes time to warm up earth, and it takes time for earth to cool down. Hence the difference in time when the in and out peaks happen.

  3. This does clarify why the heat of the day happens so late in the day. Can you explain why the heat of the day comes at a different time in the Pacific Northwest than elsewhere in the country? I have lived in Indiana, Ohio, Tennessee, and New York, and in all of those places, the heat of the day happens around 2:00 in the afternoon - by 5:00 things have cooled down noticeably. So why is it later here?


  4. Interesting graph, but it appears to suggest the warmest time is 1500 (that would be 3 pm in wintertime, and 4 pm in summertime). I've been following the weather for 25 years and I've observed the day's high to occur somewhere between 2:30 and 4:00 pm, pretty consistently. I've only seen it happen around 5 pm a handful of times each year. There also factors like afternoon breezes that can cause a drop in temperature during the afternoon.

  5. Here is what I heard: over population is causing global warming.

  6. Response to MoJo:
    I think the reason the highs happen later in the PNW than in the Midwest has to do what what Cliff said about thermal inertia's being higher for water than for land.

    As for the difference between the PNW and the East Coast: The dominant factor could be where the local air mass comes from. Normally, PNW air mass comes from over the ocean, whereas East Coast air mass often comes from over land.

  7. Response to Fluffyblue:

    I think the graph is a simplified illustration of what Cliff is saying about temperature lag as a function of in and out radiation. It is not intended to be precise. (You can observe that by comparing the plots of the blue and red lines at 00:00 time versus 24:00. "Logically", they should be the same!)

    The graph also doesn't take into account weather changes--e.g., movement of cooler or warmer air systems into the area--or other factors. It's just an idealized pattern.

  8. I'm with Alex... you "beg the question" of why infrared out lags solar in. I like Alex's explanation derived from yours, but is it correct?

  9. As a non-geologist, non-climatologist, I am also wondering what causes the August slower leak of heat -- like A. Mineev said, it's difficult to explain in exact terms.

    Thanks for your blog, love the informed and interesting weather!


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