What are temperature extremes?
Although they happen more slowly and are more difficult to see than a tornado or an earthquake, "heat waves" and "cold snaps" are deadly natural hazards. Extreme heat and cold occur somewhere in the world every year and can afflict nearly every location on Earth. Heat waves are periods of unusually high temperatures, usually lasting three days to three weeks. Typically, heat waves are characterized by temperatures of 35°C (95°F) or higher, although lower temperatures accompanied by high humidity levels can also be considered a heat wave. Cold snaps are commonly three days to three weeks in duration, with temperatures usually falling below -15°C (5°F).

Prolonged exposure to heat can cause our body's temperature to increase by a few degrees, resulting in heat exhaustion. If our body's temperature gets too high, our brain can no longer operate our cooling mechanisms, which can be a life-threatening condition known as heat stroke. Besides this immediate hazard, long heat waves can also cause drought. This is because most heat waves happen during periods of little or no precipitation, and high temperatures allow for more evaporation of water from soil. Prolonged exposure to extreme cold temperatures can lead to frostbite, which can cause permanent damage to our skin. Such exposure may also cause hypothermia, a condition that occurs when the body loses more heat than it can produce. Although usually treatable, hypothermia kills thousands of people every year.

When combined with high humidity, heat becomes a much greater hazard for humans. High humidity reduces our body's ability to cool itself. For this reason, the Heat Index was developed by the National Weather Service to show how humidity makes the air feel even warmer on a hot day. Similarly, cold poses a much greater danger to humans when combined with wind. Exposed skin cools much more rapidly when the wind is blowing because the flowing air pulls heat away. The compliment to the Heat Index, the Wind Chill indicates how much colder the air feels to us when the wind is blowing.

Where do temperature extremes occur?
Temperature extremes are most common in the mid-latitude regions, especially near the interior of large continents, such as North America. Here, without the moderating effects of the oceans, winter minimum temperatures can drop below -20°C (-4°F) and above 40°C (104°F) for several weekss. In 2003, an estimated 35,000 people died in Europe when high temperatures exceeded 35°C (95°F) for several weeks. Such heat waves have doubled in their duration over the last century in this region, most likely due to global warming.

When do temperature extremes occur?
In the mid-latitude regions, temperature extremes are most common June through August, and December through February. In polar regions and the higher mid-latitudes, extreme low temperatures can occur anytime between late fall and early spring. In the lower mid-latitudes, extreme high temperatures are common from late spring through early fall. Much like high latitudes, high altitudes are frequently subject to extreme low temperatures. In alpine areas, which are typically above 3500 m (11,500 ft) depending on latitude, extreme low temperatures can occur for nine months or more during a year.

How do we cope with temperature extremes?
Forecasting can help prepare communities for extreme temperature events. In the United States, the National Weather Service issues heat advisories, wind chill advisories, and winter weather advisories when extreme temperatures are likely. When such advisories are issued, coping with extreme temperatures can involve simple adjustments to daily activities. On hot days, limiting sun exposure during the warmest afternoon hours and drinking a lot of water to stay hydrated are important steps. Loose-fitting and light-colored clothing can help reduce body temperatures during periods of excessive heat. On extremely cold days, wearing extra warm clothing and staying indoors as much as possible can prevent frostbite and hypothermia. Reducing wind exposure also works to keep our body temperatures higher for longer periods of time. As with most natural hazards, preparation is the most effective form of coping with these extremes.