Most of the world's weather systems are powered by areas of high and low air pressure, and how they interact. Heat waves begin with a mass of high atmospheric pressure. The air from the upper atmosphere is pulled toward the ground, creating this system of high pressure. This compresses the air near the surface, which also increases its temperature. Because there is no upward flow of air, clouds do not form, which limits rain and shade. Areas of high pressure also tend to be less windy, which keeps the air mass stagnant. This is why heat waves can last for long periods of time.
Heat waves are generally characterized by periods of unseasonable warmth. This is defined as being at least 10 degrees Fahrenheit higher than the average temperature of a region during the same time of year. Although this type of anomaly is common and can also happen during the cooler months, it is generally considered a heat wave during the warm months when the air becomes uncomfortably warm. For example, a period of 50-degree weather in February is generally not considered a heat wave.
As much as the heat is a factor in a heat wave, so is the humidity. Humidity measures how much moisture is in the air. Warm air holds more moisture than cold air, and at higher temperatures, humidity can be extremely uncomfortable. The reason for this is that the human body cools itself by the evaporation of sweat. When the humidity is too high, sweat does not effectively evaporate, and people can easily overheat. The effect of humidity is measured by the heat index, which describes what the apparent temperature is in high humidity. For example, at 65 percent humidity, a 96-degree day can feel like it's 121 degrees outside.
In addition to unseasonably warm temperatures and high humidity, a heat wave is characterized by its duration. One aberrant warm day does not make a heat wave. However, when that heat stays around for two or more days, it is considered a heat wave. The longer a mass of high atmospheric pressure sits on an area, the warmer it can get. In this respect, heat waves that last many days or even several weeks can be particularly dangerous.
The main health effects of a heat wave result from the body not being able to effectively cool itself. Without proper cooling, a person can suffer from heat exhaustion and eventually heat stroke. Without enough fluid intake, a person can suffer dehydration. This often occurs when the body sweats out more water than it takes in. Drinking diuretic drinks such as alcohol and caffeinated beverages can exacerbate this. The elderly, particularly those with respiratory and cardiac problems, can be especially vulnerable during a heat wave.