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How Do We Measure Air Temperature?

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In talking about this oppressive heat wave, I have mentioned on several occasions that the temperature is measured in the shade and if you are out working in the direct sunlight it will seem like it is another 10-15 degrees or so hotter. There is nothing new about this, but I have received several correspondences lately asking why the temperature is measured in the shade. So, thought a brief explanation might be appropriate.

The reason why air temperature is measured in the shade has to do with the nature of the sun's radiation and the atmosphere's response to the radiation. The air is largely transparent to the sun's radiation which has a very short wavelength, so technically the sun does not heat the air. The sun heats the ground and whatever other objects it strikes and those objects re-radiate back toward space, but at a much longer wavelength. It is this radiation that the atmosphere absorbs which is why the hottest part of the day is not when the sun is directly overhead but late in the afternoon after the ground and other stuff has had all day to absorb the sun's heat and re-radiate it back into the air. So, if a thermometer is placed in direct sunlight it will not measure the air temperature but will measure hot much heat the thermometer itself has absorbed from the sun. In other words, it will measure how hot the thermometer is and not how hot the air is.

As far as the forecast goes, it is still going to be hot, but there is reason for cautious optimism. For several days now, Alan and I have been mentioning a weak disturbance aloft that will be moving around the upper level ridge. That will have the effect of lowering temperatures somewhat aloft, will bring a little more moisture our way resulting in more clouds, and provide a slight chance of showers or storms. This combination of factors will also knock a few degrees off the daily maximum temperatures.

Even though we started this morning with a record warm morning low, temperatures aloft have already dropped off somewhat and that should keep us below record levels this afternoon. Tue and Wed will see the primary effects of the system aloft and there is even the possibility we will stay below triple digits on Wednesday. More importantly, there will also be at least a slight chance of showers and storms. Although the areal coverage is not expected to be all that great, any rain is welcome. Also, those storms that do occur will have the potential for brief, but localized heavy rainfall and damaging downburst winds.

After that, it appears we will remain in triple digit territory through the coming weekend with only isolated late afternoon rain chances. There are some indications that the ridging aloft will drift further west early next week. If so, that will allow for a NW wind flow aloft which is typically a wetter and somewhat cooler pattern for us. Let us hope and pray that will indeed be the case.

In the meantime, stay tuned, stay cool, and check back for updates.

Dick Faurot

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