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Sunday, August 16, 2015

tamino: July 2015 is the hottest month of any month in the record, not just July

by tamino, Open Mind blog, August 14, 2015

Now that NASA has released their data updated through July, we know that in that data set, this July was the hottest July on record with a temperature anomaly of 0.75 C, i.e., it was 0.75 C above “climatology” (which is what’s usual for the given month). It’s not the hottest temperature anomaly in the data set, however; that record still belongs to January 2007, at 0.96 C above climatology.
Yet it does seem that this July, while not the hottest temperature anomaly on record, is the hottest month on record.
Every year, the global average temperature goes through an annual cycle — not just the temperature at a given location. In the Northern Hemisphere, we tend to be hottest in July and coldest in January but, in the Southern Hemisphere, the seasons are reversed, hottest in January and coldest in July. The seasons are definitely hemisphere dependent.
But what about the global average? My first instinct, many years ago, was that the Earth would, overall, be hottest in January simply because we’re closer to the Sun (at the perihelion of our orbit). But it turns out (as was quickly pointed out by a blog commenter) the Earth is actually hottest in July. That’s because when the Southern Hemisphere is tilted toward the Sun in January, all that solar heat mainly strikes the oceans, which dominate the Southern Hemisphere rather than land. The thermal inertia of the oceans is much greater than that of the land masses, so it heats up more slowly, and just doesn’t get that hot even at the peak of summer.
But in July, it’s the Northern Hemisphere that’s tilted toward the Sun. The lower thermal inertia of land (mostly in the Northern Hemisphere rather than the Southern) means it can heat up quickly, so the Northern Hemisphere reaches higher temperatures at its summer peak than the Southern Hemisphere does at its summer peak.
Why not translate those temperature anomalies into actual temperature estimates?
The catch is that from thermometer records it’s hard to determine climatology— what the actual absolute temperature is, globally averaged. That’s why the major temperature data providers track the Earth’s temperature with anomalies; anomalies can be determined more accurately, and they give us just as good a picture of how temperature has changed (which is, after all, what we’re mainly interested in).
But there’s one source which seems to give pretty good estimates of absolute temperature — reanalysis data. I don’t think anyone would trust them to be precise enough to rank individual months, but they just might be our best source of data to estimate climatology.
There are two reanalysis products that are well-known and easy to get: an American version from NCAR/NCEP and a European version called “era-interim.” Let’s use both to define climatology, then add the anomaly values from NASA to estimate actual absolute temperature.

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