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Is the temperature (even a bit) warmer in a forest?

For example, there no frost on the ground in a forest when there is some in a field just outside the forest. Is it because the temperature is a bit warmer? Or simply because there is no dew on the ground in a forest?

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    $\begingroup$ it's because the effective surface is the canopy top... below the canopy provides some insulation and so the forest floor has less exposure to frost. $\endgroup$
    – f.thorpe ♦
    Jan 1, 2015 at 2:34
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    $\begingroup$ Here's a schematic of what housing insulation does to the daily temperature cycle - ignoring evapotranspiration, the insulating properties of a forest would have a similar, but probably less pronounced effect - less extreme minimum and maximum temperatures. $\endgroup$
    – naught101
    Jan 2, 2015 at 6:07
  • $\begingroup$ @naught101 - That's exactly what I notice when I walk my dogs through a nearby woods. Cooler in summer, warmer in winter. The extremes aren't quite as extreme in the woods. $\endgroup$
    – David Hammen
    Jan 3, 2015 at 10:36
  • $\begingroup$ have you considered the albedo effect? $\endgroup$
    – user4955
    Dec 8, 2015 at 16:07

3 Answers 3

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It will depend on the time of year and the time of day. As you noted, you often see more frost in an open field than in a forest. A comment indicated the forest insulated the surface but I believe it will also reduce radiant heat loss at night, compared to the open field. During the day, the ground surface is more shaded and the trees reduce the advective movement of warmer air into the forest floor so the forest will often be cooler than the open field.

Another factor is evapotranspiration. Forests will tend to be cooler in drier summer conditions because, in general, you get more evapotranspiration from trees than from shallow-rooted vegetation and annual grasses die off in summer. The evapotranspiration cools the area.

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  • $\begingroup$ I didn't know about evapotranspiration and I didn't think about radiant heat loss too. Thanks for your insights! $\endgroup$
    – MagTun
    Jan 3, 2015 at 10:23
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Don't forget the protection from wind in colder seasons as well. Basic survival skill is to always make shelters under cover (as in a forest) rather than in an open meadow. On a cold day it may not feel much warmer in still air but getting out of the wind does make a significant difference.

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  • $\begingroup$ You're right, for the wind, but does it also count for the soil. For example at the same (below zero) temperature, does a windy field will freeze faster than a non-windy one? $\endgroup$
    – MagTun
    Jan 3, 2015 at 10:26
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    $\begingroup$ You don't have to get very deep for it not to make too much difference. But at the surface, yes - you can definitely have frost in fields with very little or none in adjacent forest. Another example of this is black ice on roads where they are exposed to wind. $\endgroup$
    – geogaffer
    Jan 3, 2015 at 20:05
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    $\begingroup$ This is a good point. Rate of cooling isn't the same as temperature but it is important in frost development and frost damage. But wind turbulence can also prevent frost development by preventing the formation of a stagnant cold air layer. $\endgroup$
    – haresfur
    Jan 3, 2015 at 22:42
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In addition to what's mentioned in the other answers, there's also the fact that the biological activity in forest soils is typically greater than elsewhere, especially further away from the equator where nutrient cycling is not as rapid; this is essentially a practically permanent state of composting, and produces a significant amount of heat, even during the colder parts of the year.

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  • $\begingroup$ Do you have a reliable source regarding this "significant amount of heat"? Plus even if it somehow were a significant amount of heat, that biological energy has to come from somewhere $\endgroup$
    – JeopardyTempest
    Apr 19 at 0:54
  • $\begingroup$ @JeopardyTempest: No single source stating it explicitly, but it's based on established facts; it's well known how compost produces a lot of heat, often enough for piles to keep themselves warm even when exposed to external sub-zero temperatures, and it's also well known how forest soils without very rapid nutrient cycling are full of humus, which is essentially the end-product of many years of continuous natural composting. Put these two together and you see how forest microbial activity is producing a lot of heat. The energy comes from the organic matter, ultimately derived from sunlight. $\endgroup$
    – Outis Nemo
    Apr 19 at 8:49
  • $\begingroup$ I'm not certain it's wrong. But keeping compost itself warm (especially as it insulates itself) is a whole lot different than keeping the air warm. A warm human (or a quite a few) doesn't rapidly heat a huge area, and humans put off much more heat I'd think. $\endgroup$
    – JeopardyTempest
    Apr 20 at 4:40
  • $\begingroup$ @JeopardyTempest: Quite a few humans actually very noticeably heat areas; even in a large room, such as an auditorium or concert hall, if there are a lot of people there you're quickly going to need to find a way to cool it down. Of course it's not the same outside, but if you were to line the forest floor with humans about half a meter deep I suspect you'd find the heat produced to be quite significant. $\endgroup$
    – Outis Nemo
    Apr 20 at 8:12

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