Extreme weather includes weather phenomena that are at the extremes of the historical distribution, especially severe or unseasonal weather. The most commonly used definition of extreme weather is based on an event's climatological distribution. Extreme weather occurs only 5% or less of the time. According to climate scientists and meteorological researchers, extreme weather events are rare. 
Heat waves are long periods of abnormally high temperatures. There is generally no universal definition of a heatwave because of the variation within temperatures are different in geographic locations. Along with the excessive heat, they are often accompanied by high levels of humidity. These two characteristics increase the relative temperature or heat index to dangerous levels.
Because heatwaves are not visible as other forms of severe weather are, like hurricanes, tornadoes, and thunderstorms, they are one of the less known forms of extreme weather. This severe weather phenomena can damage populations and crops due potential dehydration or hyperthermia. Heat cramps, heat expansion, heat stroke, and dehydration can result in human populations. The dried soil are more susceptible to erosion, decreasing lands available for agriculture. Outbreaks of wildfires can increase in frequency as dry vegetation has increased likeliness of igniting. The evaporation of bodies of water can be devastating to marine populations, decreasing the size of the habitats available as well as amount of nutrition presented within the waters. Livestock and other animal populations may decline as well. Power outages can also occur within areas experiencing heatwaves due to the increased demand for electricity. The urban heat island effect can increase temperatures even more, particularly overnight.
A cold wave is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in temperature within a 24 hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. The precise criterion for a cold wave is determined by the rate at which the temperature falls, and the minimum to which it falls. This minimum temperature is dependent on the geographical region and time of year. Cold waves generally are capable of occurring any geological location and are formed by large cool air masses that accumulate over certain regions, caused by movements of air streams.
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