Is there something unique about getting accurate temperature readings in high heat? I notice on the graphic the temperature readings vary 10 or more degrees today across short areas, and that doesn't even reflect the locations near the water that you would expect to be cooler with a bay breeze. On Sunday I was in two different cars that read 100 and 104 in DC - yet the "official" high is much lower. It strikes me that on most days even the car readings are nearly exactly the same as the official reports - so any ideas on why exceptionally high temperatures seem so inconsistent?

And to make the evening commute worse, Major metro delays, as an orange line train is off the tracks between Rossyln and Courthouse! Press Release from WMATA

More news on the derailment: Post story

95.3 in northwest Montgomery.

This didn't even make the news in the Washington area but a CSX train derailed at Point of Rocks on Saturday due to a "sun kink" which causes the track to expand because of excessive heat.

Following up from the live chat . . . it is hot no doubt, but the weather today is simply not following the hysterical morning news reports about a code red day today and air quality unhealthful for even a breath. The air quality today is no where near code red. So what do they base the reports on? Shouldn't there be some more factual reporting done based on the air quality data from the MWCOG or the NPS air quality site. Ceratianly there are risks groups sensitive to this heat and potential for air quality problems and the news perhaps could just leave it at that . . . gawd I'm suprised all the schools didn't close today!

The multi-agency (EPA, NOAA, NASA, etc) Air Quality forecast, using a objectively determined methodology for developing these forecasts, called it a code red day. See AirNow.gov (which the Washington Council of Gov uses)

As a weather weenie, I know I'm supposed to be excited about the prospect of thunderstorms tomorrow. But I'm not. Is this a sign of heat-related stress? Or is it storm fatigue?

wxthr: It might be storm fatigue. It just depends on how many storms you've gotten this year and how badly you were hit by them.

I'm all game for storms tomorrow. I just don't want the weather radio going off every 15 minutes telling me another tornado/squall line is coming.

Whether asks about the accuracy of temperature readings in high heat. Here's a partial answer:

Measuring temperature accurately is hard, because it's hard to build temperature sensors that are accurate across a broad range of temperatures. Good sensors are expensive, cheap sensors aren't particularly accurate. The sensor is generally most accurate in the center of its range -- it gets less accurate when it's really hot or really cold.

The sensor is only part of the problem -- where you mount the sensor is critical. As an example, suppose you have a really accurate thermometor that's carefully placed on a stand in the middle of an asphalt parking lot? During the course of a hot sunny day, the asphalt absorbs heat, until it starts radiating the heat. All of a sudden, your thermometer is being heated from above and below ... and it will continue to be heated by the asphalt long after the sun goes down. What if the parking lot is concrete instead of asphalt? Concrete is more likely to reflect light (and thus heat) so you'll get cooler temperatures in the early evening than with asphalt. What if your thermometer was mounted on the south side of a building with lots of reflective glass windows? Oops -- it gets more reflected heat during the summer (and less in the winter if they designed the building right).

I used to do this sort of thing for a living. Professionals have to take all of these factors into account. They build boxes around the sensors to keep them out of direct light, wind, rain, and snow, and they calibrate the permanent sensors by bring in really expensive, really accurate portable sensors for comparison.

Now let's take your car's thermometer. It's a cheap sensor, not an accurate one (typically +/- 0.5 degrees C). It hasn't been calibrated since it left the factory. It's mounted on a heat source (your car) and when you're using it, it's probably above a lot of asphalt (pavement), and it's never in a consistent location. You're lucky if it gets in the same general neighborhood as the correct temperature on a nice day. In a heat wave or cold snap? Forget about it.

For way too much information about thermometers, see Wikipedia.

Craig, Thanks for the insightful response.