The Weather Guys

The heat index indicates how hot it feels. It is expressed as a function of air temperature and the relative humidity.

The heat index temperature is for standing in the shade and light winds; when exposed to direct sunlight, the heat index value can be increased by up to 15 degrees F.

When our bodies get hot we cool down by sweating. The sweating does not directly cool our bodies; it is the evaporation of the sweat that cools us down.

If the air has a high humidity, then the rate of evaporation is reduced. That hampers the body’s ability to maintain a nearly constant internal body temperature. That is why we are uncomfortable on hot, muggy days.

When the temperature is high but the relative humidity is low, the heat index can be less than the actual temperature. That’s because cooling by evaporation of sweat is very efficient in these situations.

However, high relative humidities prevent evaporation and make it seem hotter than it really is because our bodies cannot cool down. In these cases, the heat index is greater than the actual temperature.

The National Weather Service (NWS) will initiate alert procedures when the heat index is expected to exceed 105° to 110°F (depending on local climate) for at least two consecutive days.

The NWS will issue a heat advisory when the heat index is predicted to be 100°F. You are then advised to limit vigorous outdoor activity and drink plenty of fluids. Never leave children or pets in a locked car.

A heat wave is a period of abnormally and uncomfortably hot and usually humid weather.

The World Meteorological Organization defines a heat wave as when the daily maximum temperature for more than five consecutive days exceeds the average maximum temperature by 9°F.

On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees about 1 mile above the ground as a measure of the extremity of winter.

In the middle of January, about 68 million square kilometers of the Northern Hemisphere are covered by air that cold at that level. Between about July 5 and July 20 that area shrinks to zero, and the complete absence of such air lasts only a very few days.

We are just past the point in this summer where that area is beginning to increase again. Thus, despite the fact that we have been through a couple of really hot (and humid) weeks this July, the return of winter has already begun in terms of this measure. This is largely because the number of daylight hours has already begun to decrease — noticeably here in Madison.

At every latitude north of 66.5N, however, the sun was out for 24 hours long on the day of the Summer Solstice (June 21). Ever since, the specter of nighttime has been creeping poleward with the daylight decreasing ever so slightly just above the Arctic Circle. The associated nighttime cooling is greater during a longer night, resulting in the gradual increase in the amount of cold air covering the Northern Hemisphere.

So, if these past couple of weeks have been a bit too much for you, perhaps some solace can be found in the fact that the tide is already, however imperceptibly, beginning to turn.