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Old 3rd Jan 2004, 01:23   #1 (permalink)
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Red face Why is January the coldest month?

We all know that the time when the earth' nothern hemisphere is farthest from the sun is the winter solstice, around the 22nd of December, and is closest to the sun, six months later in June.
Then why is it that the coldest time in the Northern hemisphere is not December, but later in January-February, and the hottest time is not June, but July-August?
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Old 3rd Jan 2004, 01:30   #2 (permalink)

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The temperature is not only related to "how close we are to the sun". (It's actually more to do with angles than distances, but that's a whole other subject.) It's also affected by the current ambient temperature.

In December, all other things being equal, the earth will receive the minimum amount of heat from the sun, and it will also have been cooling steadilly for the last 3 months or so. By January, the earth will be receiving very slightly more heat from the sun, but will have had 4 months or so of overall cooling.

The same effect can be seen on a daily basis - it's exactly the same as the reason why the hottest part of the day is mid-afternoon, and not noon.

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Old 3rd Jan 2004, 02:38   #3 (permalink)
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Don't forget also that the surrounding water/landmasses greatly affect our temps and weather. Currents differ, and cold dry air (in a widley generalised term) is a frequent occurance from the N/NE, where lies a huge cold icy block of land. Low in moisture and temp, we receive the air mass as it moves south/west. The dominant air flow in deep winter is polar continental, and summer changes to tropical maritime - prevailing SW.

Obviously these are prevailing, not permanent, hence crappy mild wet days dominated by dry cold crisp days. Well, that's the pre-global warming trend.

Regards and Happy new year
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Old 3rd Jan 2004, 03:35   #4 (permalink)
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Er, is it me, or am I reading something based on a totally wrong premise. Check the time of year when the Earth is at the greatest distance, and, how little difference the elliptic makes.
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Old 3rd Jan 2004, 04:58   #5 (permalink)

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The ambient temperature depends on where the airmass has been before as much as anything.

In UK we are kept warmer than our latitude deserves, by the fact that the GulfStream warms our seas. If the air comes from the same direction, i.e. from the SW, we generally have warm, moist conditions. Conversely, if the wind brings air from the north, it becomes colder.

If we get cold air from the east or north east, watch out at this time of year, that's when we tend to get the really cold conditions (heat lost due to long track over very cold land) plus heavier snowfall at lower elevations. Conversely, in the summer, hotter easterly winds (lots of solar heating over a long land track) will result in "Scorchio" conditions in UK.
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Old 3rd Jan 2004, 05:35   #6 (permalink)
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Propellor, FFF has it right.

I'm a bit rusty but....
Although the day is now lengthening and the sun is getting 'higher' there is still not enough sunlight hitting our latitudes to achieve thermal 'equilibrium'. Over an average 24hrs more heat is being radiated into space than is received from the sun - hence its still getting colder. Equilibrium is attained about a month after the solstice, both winter and summer, and corresponds with the coldest and hottest days respectively. Of course we're talking strict averages here, but the coldest day is Jan 21st and the hottest July 21st, [ish].
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Old 4th Jan 2004, 01:54   #7 (permalink)
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Propellor, your original assertion that the earth is furthest from the sun in December is not true. It is CLOSEST tomorrow, Jan 4th. Astonomers call that perihelion. Difference between max and min is, I believe (but don't quote me), about 5 million km.

Even if that assertion were true, it would not explain the southern hemisphere summer which is now.

Seasons are caused by the inclination of the earth's axis, and the change in heating effect of the sun's radiation by being higher or lower in the sky. The word for this, which you should look up on this medium, is insolation.
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Old 4th Jan 2004, 02:48   #8 (permalink)
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The rate of heat input reaches its lowest at the solstice. However for the next month or so that rate does not rise sharply. Through January, although rising, it is insufficient to stem net cooling. It is only later that the amount of insolation reaches the level required to stop net heat loss. In the late summer the same principle applies - although insolation is falling it still exceeds heat loss so the temperature continues to rise -although it slows - and then goes into reverse.

There are other damping affects (eg sink effect of the sea) but the main factor is that the land temperature does not depend on the rate of change of insolation but rather on whether insolation is greater or less than radiation.
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Old 4th Jan 2004, 11:44   #9 (permalink)
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Not the coolest month here of course - 46C on the METAR on 2.1.4!
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Old 5th Jan 2004, 04:37   #10 (permalink)
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41C in Darfield, in NZ's South Island a couple of days ago...
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Old 5th Jan 2004, 05:05   #11 (permalink)
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Anyone know the ratio of heat generated by Gravity i.e. internal pressure, v solar radiation?
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Old 6th Jan 2004, 06:03   #12 (permalink)
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No idea what the ratio is, but a study of the following data would suggest it is very low:

Mercury (close to Sun), gravity 0.38, mean surface temp 350C(day)

Earth, gravity 1, mean surface temp 20C

Jupiter (far from Sun), gravity 2.53, mean surface temp -110C
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Old 7th Jan 2004, 19:29   #13 (permalink)
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Only a tiny amount of atmospheric heat comes from inside the earth. The overwhelming majority comes from radiation from the sun.

In general terms there is a lag in surface temperatures behind the sun's movement, so min and max temps are usually after the times for min and max incoming solar radiation.

Coldest days are after the winter solistice, and hottest after midsummer. Hurricanes, for example, are heat driven, and the hurricane season is about July through October. not May through August.

Bristol hands refer to your Met notes.

Dick W
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