It is shaping up to be a pretty active Fourth of July Weekend. A weak offshore low and a strong Four-Corners high will combine with a ton of moisture left over from days of activity to give you better than even chances of getting wet should you spend your day outside Saturday, especially in the mid to late afternoon. Along with the possibility of dangerous lightning, localized flash flooding is also a possibility. High temperatures will drop to the low 90s and stay there well into next week.

Yesterday I mentioned that I often get emails from people convinced that the temperature is much higher than we are reporting from the airport. I very strongly suspect that their thermometers have some siting issues…probably due to sun contamination. I have seen reports that are 10-15 degrees hotter than those at the airport. The airport is a temperature that I trust, as it is set up under some very strict criteria (thermometer in a well ventilated white box, for instance.) The location of your temperature sensor makes a huge difference, and it’s not always just “putting it in the shade.”

Whenever two temperature readings disagree, it is almost always the hotter one that has the issues. And even if you have two weather stations (thermometers), if both sensors are incorrectly placed, you will get abnormally high readings, even if both are working perfectly.

One thing that you need to keep in mind is a thermometer doesn’t measure the temperature of the air…it measures the temperature of the thermometer (stay with me.) The trick is getting the temperature of the thermometer to match the temperature of the air. That can only happen if there is no sunlight contamination of not only the thermometer, but also of any solid object that the thermometer might come into contact with. So if for instance you have mounted your thermometer or sensor under the eaves of your roof, the sun shining on your roof will heat it up, which in turn heats up the thermometer.

The best solution is to hang your sensor in a well ventilated white box well away from a structure 5-6 feet off the ground. Barring that, some of the instrument heat shields available with digital weather stations do a pretty good job.