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The hottest and the coolest days.
July 3, 2008 2:55 PM   Subscribe

What, in theory, are the hottest a coldest days of the year?

I understand that the hottest and coldest times of the year lag the solstices by a few weeks - but I have not been able to find any mention of a more precise date for when the most extreme high and low temperatures should fall. I wondered if it was possible to work this out - either for a whole hemisphere or for a particular place?
posted by rongorongo to Science & Nature (11 answers total)
 
Um... sure. You just collect data over a long period of time. Here is one site that'll give you details for cities.

(You really can't get much broader than that - temperatures vary widely across any large geographic area.)
posted by Tomorrowful at 3:05 PM on July 3, 2008


Hm. Apparently that site has all kinds of records except the one you actually asked for. That'll teach me to try to be fast.
posted by Tomorrowful at 3:06 PM on July 3, 2008


Weather underground has an interface to historical information & averages by airport; that's about the best you're going to be able to do for prediction.
posted by advil at 3:09 PM on July 3, 2008


just to be clear, you want to click on the "seasonal weather averages" link in the middle of the daily view. I didn't link directly to it because you don't seem to be able to switch locations from the graphs.
posted by advil at 3:30 PM on July 3, 2008


Go to weather.com, type in your zip code, click the averages links, then select daily. In southwest Texas, the hottest day of the year in August 4th and the coldest is January 11th. Note, I measure it by the average maximum temperature.
posted by whiskeyspider at 3:42 PM on July 3, 2008


The hottest and coldest days of the year on an average basis are in the southern hemisphere. That's because the Earth is in an elliptical orbit around the sun. We're closest to the sun on January 3 and farthest from the sun on July 4. So that means that our orbit slightly moderates the seasons in the northern hemisphere and exaggerates them in the southern hemisphere.
posted by Class Goat at 4:38 PM on July 3, 2008


The hottest and coldest days of the year on an average basis are in the southern hemisphere. That's because the Earth is in an elliptical orbit around the sun. We're closest to the sun on January 3 and farthest from the sun on July 4. So that means that our orbit slightly moderates the seasons in the northern hemisphere and exaggerates them in the southern hemisphere.

That is true but also a factor is that there is more land area in the northern hemisphere which exaggerates extreme temperatures and more ocean in the southern hemisphere which tends to moderate extreme temperatures. Combine the two effects, the elliptical orbit and the distribution of land masses, and they tend to cancel each other out. This means that we happen to live at a fortuitous time in which the world that has very fortunate moderating tendencies to prevent extreme seasons. 12,000 years ago, due to precession of the earth's axis, the perihelion with respect to seasons would be reversed, which would accentuate extremes in both hemispheres. This is one of many factors causing ice ages.
posted by JackFlash at 8:50 PM on July 3, 2008


Some scientists modeled this here. The data they used show time lags of 1.95, 1.87, 1.02, and 1.24 months for southern mid-latitudes, southern tropics, northern tropics, and northern mid-latitudes, respectively.
posted by lukemeister at 6:09 AM on July 4, 2008


Thanks lukemeister. I guess I was more interested in model that might be applicable for large areas of the globe than I was in daily average temperatures for a particular city (although these are just as good an answer). I have to admit to skimming some of the paper's equations - but here in the UK it looks like January 21st and July 21st would be the days to be interested in.
posted by rongorongo at 8:48 AM on July 4, 2008


rongorongo,

Sure - note that the maximum temperature is flat for +- 15 days around the peak, so all you can really say is that the hottest day will probably be in July or early August at your latitude.

Most places I've lived have been reasonably close to the ocean. Here, in Colorado, I've noticed that the thermal lag is shorter than in other places I've lived. July is hot, but by mid-August it is noticeably cooler, especially in the early morning. In San Francisco the warmest month is September. The paper suggested that naively, one would expect everywhere in the northern hemisphere, because the oceans have so much heat capacity.
posted by lukemeister at 10:16 AM on July 4, 2008


I found this in several places -

Hottest day ever:
UK August 10, 2003
Paris July 28, 1948
Europe August 4, 1881
USA July 10, 1913
World Sep. 13, 1922
posted by lukemeister at 10:35 AM on July 4, 2008


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