In this part of the country, a heat wave is generally defined as a stretch of three or more days with a high temperature of 90 or above.

When heat becomes a health threat, the National Weather Service sends out alerts called Excessive Heat "Watches" and "Warnings". These are based not only on temperature - but also humidity. Remember, it's the heat index that combines these two.

An excessive heat Watch or Warning is usually issued when the heat index is expected to reach 100oF for at least three hours a day on two straight days. A heat index of 100oF arises, for example, if the temperature's 90oF and the RH is 60%.

These watches and warnings are especially important in highly urbanized areas, where buildings are tightly packed and brick walls and tar roofs tend to trap heat.

The Weather Service's decision to issue an excessive heat watch or warning is also based on factors besides the expected heat and humidity. More on that tomorrow night.

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