Temperature — Page 72, Lesson 60

I want to walk you through the rest of the factors that affect the diurnal variation of temperature. We've already looked at what happens with cloud cover during the day, so now let's look at the night side of that same coin. Cloud cover by night. At night, the earth's surface radiates heat back out into space — that's terrestrial radiation. When there's cloud cover, those clouds absorb that outgoing terrestrial radiation and then radiate some of it back down to the earth's surface. So instead of all the heat escaping, some of it is returned. The result is that the minimum temperature — what we call T min — is increased. The clouds act like a blanket, keeping the night warmer than it would be under a clear sky. You can see this illustrated in Figure 5.17, Cloud Cover by Night. Now let's talk about the effect of wind, and we'll split this into day and night as well. Effect of wind by day. During the day, the sun heats the ground, and the air in contact with that ground becomes warm. Wind introduces a mechanical mixing process — turbulent mixing. The wind stirs the warm air at the surface together with the cooler air above it. That mixing reduces the maximum temperature, T max, because the warm surface air gets diluted with cooler air from aloft. Additionally, wind reduces the amount of time any given parcel of air stays in contact with the warm ground, so it has less time to heat up. Both effects work to lower T max. Figure 5.18, The Effect of Wind by Day, shows this. Effect of wind by night. At night, the situation is the opposite. Normally, after the sun goes down, the ground cools rapidly and the air just above the surface becomes colder than the air above it. That creates a temperature inversion — a layer where temperature increases with height just above the surface. Now, if wind is present, it turbulently mixes that cold surface air with the warmer air above the inversion. That mixing brings warmer air down to the surface, which increases the minimum temperature, T min. So wind at night actually raises the overnight low. Figure 5.19, The Effect of Wind by Night, illustrates this. So here's the summary: both wind and cloud cover cause T max to be reduced and T min to be increased. When the high is lower and the low is higher, the diurnal variation — the DV, the difference between the daily maximum and minimum temperature — is reduced. Wind and clouds both dampen the daily temperature swing. Now let's move to a completely different surface: the sea. DV over sea. The diurnal variation over the sea is very small — generally less than one degree Celsius. The reason comes down to a property called specific heat. The specific heat of water is defined as unity — that's a value of 1. Other substances have a specific heat that is much less than 1. The temperature rise of a substance is inversely proportional to its specific heat. So if water has a high specific heat, its temperature changes very slowly when heat is added or removed. The sea takes a long time to heat up during the day and a long time to cool down at night. As a result, the diurnal temperature variation over the sea is tiny. Figure 5.20, Diurnal Variation Over the Sea, shows this. Now let's look at the nature of the surface more broadly. Sea. As I just said, the sea heats and cools slowly, giving a very small DV. This small DV over the sea is actually the cause of sea breezes — the temperature difference between land and sea drives that circulation. And because the sea temperature changes so little from day to night, the most common form of fog — radiation fog — never forms over the sea. Radiation fog requires strong nighttime cooling of the ground, and the sea just doesn't cool enough. One more point about the sun's angle: when the angular elevation of the sun is low — that is, when the sun is close to the horizon — much of the solar radiation is reflected back to the atmosphere rather than being absorbed by the surface. So less heating occurs. Land. Different types of land surface heat up at very different rates. Bare rock, sand, dry soil, tarred roads, and concrete runways attain a higher temperature by insolation — that's heating from the sun — than woods, lakes, grasslands, and wet soil do. The temperature difference between the air above a concrete runway and the air above adjacent grass can be as much as four degrees Celsius. Those higher-temperature surfaces provide strong upward currents of air called thermals, or convection currents. These are important for glider pilots and for understanding cumulus cloud formation. Figure 5.21, July Average Temperatures, gives you a visual of how these surface types compare.