Catastrophic Weather Perils in the United States

The last 10 years have seen a variety of weather perils cause significant insured losses in the United States. From the wild fires of 2003, hurricanes of 2004 and 2005, to the severe thunderstorm events in 2011, extreme weather has the appearance of being the norm.

The industry has experienced over $200B in combined losses from catastrophic weather events in the US since 2002. While the weather is often seen as a random, chaotic thing, there are relatively predictable patterns (so called “climate states”) in the weather which can be used to inform our expectations of extreme weather events. An oft quoted adage is that “climate is what you expect; weather is what you actually observe.”

A more useful way to think about the relationship between weather and climate is that the climate is the mean state of the atmosphere (either locally or globally) which changes over time, and weather is the variation around that mean. This paper will examine the climate states that drive, to greater or lesser extents, the extreme weather events experienced in the United States, specifically: hurricanes, severe thunderstorm, and wild fire. The aim is not to provide a complete description for extreme weather but a helpful guide to understanding some of the influences on catastrophic weather events.


For hurricanes to form, several atmospheric and oceanic conditions need to be present:

  • Warm Sea Surface Temperatures. Oceanic heat is the energy source for hurricanes. Temperatures in excess of 26.5°C are required for tropical cyclones to form (“cyclogenesis”). This is the primary reason why hurricane formation is generally restricted to the period from June to the end of November – beyond this, the waters of the Atlantic are too cold to support cyclogenesis.
  • Low Wind Shear. Wind shear acts to disrupt the structure of a tropical cyclone.
  • High Atmospheric Moisture. Water vapor in the atmosphere surrounding a tropical cyclone mediates the transfer of energy from the ocean to the tropical cyclone; dry air reduces the ability of the tropical cyclone to draw energy from the ocean. Typically, hurricanes pick up (“entrain”) dry air from over the land as they approach landfall, which is one reason why hurricanes often weaken in the 12-24 hours prior to landfall.
  • Good Outflow. Tropical cyclones draw in warm, wet air at the ocean’s surface, use the energy contained to sustain or intensify the storm, and expel cold, dry air from the top of the storm. If the cold, dry air is not allowed to escape effectively, then the strength of the storm can be significantly reduced.
  • Low Atmospheric Dust. Dust in the atmosphere (in the Atlantic, “dust” usually means sand and soil lifted into the atmosphere from sub-Saharan Africa) acts to reduce the amount of sunlight reaching the ocean and the lower atmosphere by reflecting the sun’s radiation back to space. When there is high dust content, the ocean and the lower atmosphere cool, meaning there is less energy for a tropical cyclone to use.

An analogy can be drawn between a hurricane and an automobile engine. The sea surface temperature represents the fuel going into the engine – the more fuel (heat), the more power the engine produces (stronger storm). The wind shear represents the timing of the engine – if the timing of the engine is off (high wind shear), the engine cannot produce full power (weaker storm). The water vapor represents the oil lubricating the engine – if the engine is not well lubricated (low water vapor content), then the engine is not as efficient as it can be (storm weakens). The outflow represents the engine’s exhaust –an efficient exhaust (good outflow) allows the engine to develop full power (stronger storm). Finally, the dust represents contaminants in the fuel – if there are more contaminants (higher dust), the fuel cannot burn as effectively (weaker storm). We note that of the five necessary conditions for hurricanes to form listed previously, only two of them (sea surface temperature and wind shear) have strong correlations to climate variability. These will be discussed below.