Q: I note that temperatures reported on The Weather Channel and elsewhere appear to be those recorded in the shade. Why wouldn't it be equally informative to report temperatures as recorded in direct sunlight?
I have a remote thermometer. When the sun hits the thermometer directly, the reading is often above the officially reported highs by ten, or 15 degrees or more. Direct sunlight in mid-afternoon on hot days feels much more oppressive than the reported high temperature.
Is the temperature reading in direct sunlight misleading? If so, why? If not, why isn't it just as informative for me to know how hot I'll be when standing unprotected in mid-afternoon on the Washington Mall or at a baseball field as it would be to know the temperature in the shade at the airport?
A: This question is a good one because it goes to the heart of why we measure the weather, and also how we use the reported measurements.
The photo above is an extreme example of what happens when the sun hits a thermometer directly. It was taken at the National Science Foundation's Summit research camp on Greenland's Ice Sheet, about 10,000 feet above sea level.
The thermometer reads 80 degrees, and I am comfortable without a hot or gloves because no wind was blowing and the sun was warming me as well as the thermometer.
But, even a slight breeze would have chilled me because an official thermometer in the shade at the same time and place would have read in the 20s (above zero Fahrenheit). This is because the air itself was in the 20s.
In this case and the one you describe with your remote thermometer, the reading is the temperature of the thermometer itself, or the metal or maybe a black roof around the thermometer. The reading is not the temperature of the air, however.
Meteorologists need the air's actual temperature at the surface — not the temperature of something warmed by the sun — for many of the calculations they need to do. (Today, it's almost always their computers that do the calculations.)
For example, they need the air's temperature at the surface and upper levels and also the humidity to calculate the air's stability, which is needed to forecast the chance of showers or thunderstorms. (Related information: Temperature profile determines stability)
If warm or cold air is behind a front, a meteorologist needs the air's temperature to calculate how warm, or cold, a place a couple of hundred miles away will be when the front moves past. (Related information: Cold fronts and warm fronts) The air travels behind the front, but the thermometer on the wall in Greenland or at your home stays in the same place, and the sun goes down or behind clouds.
As you point out, the air's temperature isn't the entire story.
In fact, in many cases those involved with different kinds of activities need to know more than the air's temperature.
A good example is noted in a story I did last March on how Auto racers' winning equations include weather. In it, I give examples of how the temperature of the race track itself, the air right above the hot track, and the air 59 inches above the ground can change during the day. This affects the car's handling and the crew has to take these temperatures into account as the adjust the car for top speed.
These temperatures are all likely to be different from an official thermometer at the track, but those who need these different temperatures measure them directly.
Meteorologists have another reason for having standard ways of measure the air's temperature: They want to be sure that a reading of, say, 80 degrees means the same no matter where or when the measurement is taken.
The thermometer in Greenland doesn't meet this requirement because its reading depends on whether the sun is shining on it.
The need for an official thermometer to be in the shade doesn't mean you need to look for a nice shade tree to put your thermometer under. Instead, weather stations, whether they are operated by a weather service or an amateur observer, should include a instrument shelter.
A common kind used by volunteer observers and some weather stations in the USA is called a cotton region shelter.
Such a shelter is a white box — the white reflects away sunlight — with louvers, or slats, in the sides to allow the air to flow freely in and out. The shelter should be between 4 to 6 feet above the ground and at least 2 feet above the deepest snow around it.
The thermometer should not be right on the ground because in cold weather, the temperature there can be much colder than just a couple of feet up, and isn't representative of the larger mass of air right above the ground.
Today, the USA's major weather stations and most airports have automated observing systems. These don't include a cotton region shelter, but they do have shields to keep the sun from shining directly on the thermometers.
Back to the part of your question about comfort: So many things go into determining how comfortable you feel that even a thermometer in the sun isn't likely to be a good guide.
Some of the factors include the humidity, what kind of clothing you are wearing, what you are doing — running a marathon or sitting on the grass at a concert — how fast the wind is blowing and whether it's blowing from a hot or cool place.
The heat index makes it possible to factor in the humidity, but the other variables are so complex that coming up with formulas is difficult.
You can see, even something that seems as simple as temperature can become pretty complicated when you start looking into it in detail.
You can learn more about not only temperature, but also all weather measurements by going to our weather measurements page.
By Jack Williams, USATODAY.com