Some thoughts on extremes

There’s an early summer heat wave in the central US, with temperatures pushing 100F and, due to the humidity, heat stress values into the danger zone across wide swaths of Texas this week …

The old jokes about it being a “dry heat” are true, in that if it’s dry your body’s natural evaporative cooling system (aka sweat) works a lot better than if it is more humid, since the sweat can’t evaporate and carry away heat if the air is too humid to allow it to evaporate. There are several forumlas to calculate the “what it feels like” temperature based on various assumptions. Another factor is what you’re used to; different weather service offices use different criteria for when to issue cautions and warnings. So if you are from, say, Minnesota and visiting Georgia in August (ok, why would you do that??? 😛 ), you should be aware that a combination of temperature and humidity that would generate warnings to stay indoors and prepare for the Apocalypse back home won’t event get a “meh, normal day” down here. The map above uses average national values – generally, if the heat index is over 90, you should probably be careful, drink fluids, etc. and as it approaches and goes over 100 even heat tempered southerners should probably take it easy.

But what about records? Record temperatures are a tricky thing. Generally extreme hot (or cold) values only last a couple days. So random chance says the record on any given day might be a really high value or perhaps an abnormally low value. Here is a chart of the high temperature for each day of the year at Love Field, near Dallas Texas to illustrate the point:

The darker smooth line is the 1991-2010 (30 year) average. The upper dotted line is one standard deviation (sigma, the upper 84th percentile) above normal, while the lower green dotted line is one standard deviation below normal. The upper jagged orangish line is the record high (1950-2022) for that date. The yellowish line that stops about midway is the data for this year through 26 June. Lots of variability! The highs are running at least 1 sigma above normal the last week or so, and yesterday and today spiked up near the record yesterday and today. But as you can see that can be deceptive, since on any given day the record might be really high or really low (relatively speaking) depending on which day the high happened. Another factor is that different stations have different observation periods. So one site might go back to the 1800s while another might only have had a station since 1970. Not having the record setting heatwave of 1936 might bias your records a lot! Airports and stations move, cities grow, and so forth. Sorting through all this to find signals such as climate change, based on record highs is really tough, and subject to argument.

However … record highs aren’t the whole, or even best, story. If you look at stations around the world, night time temperatures are rising much faster than day time temperatures (link to Science Daily story). While there can be some arguments over day time (and lots of arguments over records, for the reasons noted above), there isn’t much debate over the night time warming, which is about twice as much as the day time rate. It’s not as dramatic, but it is a clearer signal our climate is changing.

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