Extreme weather includes unexpected, unusual, severe, or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Extreme events are based on a location's recorded weather history. The main types of extreme weather include heat waves, cold waves, droughts, and heavy precipitation or storm events, such as tropical cyclones. Extreme weather can have various effects, from natural hazards such as floods and landslides to social costs on human health and the economy. Severe weather is a particular type of extreme weather which poses risks to life and property.

Weather patterns in a given region vary with time, and so extreme weather can be attributed, at least in part, to the natural climate variability that exists on Earth. For example, the El Niño-Southern Oscillation (ENSO) or the North Atlantic oscillation (NAO) are climate phenomena that impact weather patterns worldwide. Generally speaking, one event in extreme weather cannot be attributed to any one single cause. However, certain system wide changes to global weather systems can lead to increased frequency or intensity of extreme weather events.

Climate change might make some extreme weather events more frequent and more intense. This applies in particular to heat waves and cold waves. The extreme event attribution sector looks at possible explanations behind extreme events. Climate models indicate that rising temperatures might make extreme weather events worse worldwide.

Extreme weather has serious impacts on human society and on ecosystems. There is loss of human lives, damage to infrastructure and ecosystem destruction. Some human activities can exacerbate the effects, for example poor urban planning, wetland destruction, and building homes along floodplains.

Extreme weather describes unusual weather events that are at the extremes of the historical distribution for a given area. The IPCC Sixth Assessment Report defines an extreme weather event as follows: "An event that is rare at a particular place and time of year. Definitions of 'rare' vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile of a probability density function estimated from observations."

In contrast, the World Meteorological Organization define severe weather as any aspect of the weather that poses risks to life, property or requires the intervention of authorities. Severe weather is thus a particular type of extreme weather.

Definitions of extreme weather vary in different parts of the community, changing the outcomes of research from those fields. Types of extreme weather can include, but are not limited to, heavy precipitation, droughts, heat waves, cold waves, tornadoes, and hurricanes.

Heat waves are periods of abnormally high temperatures and heat index. Definitions of a heatwave vary because of the variation of temperatures in different geographic locations. Excessive heat is often accompanied by high levels of humidity, but can also be catastrophically dry.