![]() The difference between the dry bulb (no cloth covering) and wet bulb thermometer readings is a measure of the relative humidity of the atmosphere on that day. Inside Madonna's Legendary Performance at the First VMAs It eventually reaches some new, lower equilibrium temperature. There’s also a little rope attached to the instrument that allows you to “sling” it around in the air, speeding up the evaporation of the wet bulb thermometer, which cools off in response to the heat lost due to evaporation. One had a cloth wick covering on the bulb, which you saturate with water. It consisted of two thermometers attached side by side. Among the more interesting instruments was the sling psychrometer. One of my favorite labs was the weather lab, where we would take the students up on the roof of the Kline Geology Laboratory and introduce them to a standard meteorological station, containing various types of weather instruments inside a little white shed elevated a few feet off the ground. When I was a graduate student at Yale, I was a teaching assistant for the very popular undergraduate course “Oceans and Atmospheres” taught by my former Ph.D. In fact, the best measure of susceptibility to heat stress combines temperature and humidity into a single variable. The old cliché is that “it’s not the heat, it’s the humidity,” but as anyone who has been to Las Vegas in August will tell you, that’s not true. Over a large part of the planet, it would have been both very hot and very humid. Warmer air holds more moisture, so when conditions are favorable for rainfall-which they would have been over much of the planet-you get even more of it. Many other regions-particularly in the tropics and subpolar latitudes-likely saw increased precipitation. Climate model simulations of the PETM, using elevated CO2 levels consistent with the paleo data, suggest that western North America was likely one of the exceptions to the rule, one of the handful of continental mid-latitude regions that saw drying, primarily during summer, due to high surface pressure and the poleward migration of the jet stream. Fossil pollen evidence suggests that tropical forests flourished and spread at this time. Was the drying part of a widespread trend? Probably not. There’s evidence that some tropical ocean regions became so hot that they were abandoned by many organisms. Hippos, alligators, and palm trees graced Ellesmere Island, off the northwestern coast of Greenland, suggesting lush, balmy conditions near the North Pole. Mangroves and rainforests reached Arctic latitudes. What was the PETM actually like? Of course, we don’t have photos or documentary evidence, but a rather striking picture emerges from the fossil record of plants and animals. Michael Mann looks to this episode in the distant past to show us what our future may bring. In an excerpt from his illuminating new book “Our Fragile Moment: Lessons from Earth’s Past to Combat Climate Crisis“, Dr. The average temperature of the planet reached an infernally hot 90F-and large parts of the planet would have been unlivably hot for us, had we existed back then. ![]() If we’re looking for a paleo-analog for the deadly heat we might face if we continue to warm our planet through fossil fuel burning and carbon pollution, the so-called “PETM” (Paleocene-Eocene Thermal Maximum) of 56 million years ago might do. After having witnessed the warmest summer on record, many are understandably wondering just how hot it could get? Perhaps there are some clues in Earth’s past.
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