Heat wave. It's a term that immediately evokes southern climates: the aridity of Phoenix or Palm Springs, the subtropical humidity of Houston or Miami. Yet, extreme heat is a weather emergency that can affect all of us.

According to Stu Ostro, a senior meteorologist at The Weather Channel, extreme heat occurs when the temperature reaches extremely high levels, or when the combination of heat and humidity causes the air to become oppressive and stifling.
Extreme heat can manifest in several ways:

A spell of sweltering humidity, which reaches levels commonly associated with moist tropical regions. Stress on the body can be exacerbated when atmospheric conditions cause pollutants to be trapped near the ground.

An excessively dry condition, in which strong winds and blowing dust can worsen the situation.

A rise in the heat index, the body's perception of the "apparent" temperature based on both the air's real temperature and the amount of moisture present in the air. Humidity and mugginess makes the temperature seem higher than it is. In high humidity, an 85 degree day may be perceived as 95 degrees.