Heat waves are regarded by the National Weather Service (NWS) as the major cause of weather-related fatalities in the United States in most years (Robinson 2000). While there is no clear-cut numerical definition for what defines a heat wave across the United States (the criteria for defining an official heat wave varies by region), the National Oceanic Atmospheric Administration (NOAA) defines a heat wave as a period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days. The combination of high heat and humidity values (characterized by dewpoint values approaching and exceeding 70F) can be a very dangerous and deadly combination during the late spring through early fall months across the United States. What causes heat waves to occur and what determines how long they last?
Fast facts about Heat Waves
- Heat waves might be considered the most deadly weather phenomena depending on the criteria used to define them.
- High pressure systems that compress air towards the Earth’s surface, and by doing so warm it, are the primary cause of heat waves.
- These weather phenomena typically occur between late spring and early fall, as weather patterns in winter and early spring are more diverse and chaotic.
- The heat wave that occurred in July 1995 was among the most devastating on record.
- It is critical for people (and all animals) to drink water during periods of extended heat.
How do heat waves occur?
From late spring through early fall weather patterns across the United States are much less chaotic and do not change quite as often as they do in the winter and early spring. The precursor to a heat wave is an area of high pressure that develops in the mid-levels of the atmosphere (10,000-20,000+ft) and strengthens. When a region comes under the influence of a high pressure system, air gets pushed towards the surface of the earth (it gets compressed), and it begins to heat up. As these high pressure areas become established, they can strengthen substantially and persist for several days to even weeks. They will also provide a very consistent airmass for areas under their influence. This is what allows for periods of unusually hot weather to occur.
The duration of the heat, characterized by temperatures above the 90F mark and 100F mark across the desert southwest, depends on the strength of the high pressure and how long the high pressure remains in control. To illustrate this, we will examine some of the more recent heat waves of the past 10-years, including one of the deadliest heat waves to occur in July of 1995. We will focus on the 500mb pattern (which is about 18,000ft above the ground on average) and examine where the ridges of high pressure exist as well as their strength.
The following 500mb patterns are for examination:
- Mid-July 1995: This heat wave affected the Ohio Valley region as well as the upper mid-West. Across a five day period over 700 people lost their lives in the state of Illinois due to the combination of high heat and humidity.
- Mid/late July 2006: This heat wave affected a large portion of the continental United States and even into southern Canada. Substantial ridging (high pressure) became established across the majority of the continental United States.
- Late July 2009: In a region that is not prone to high heat, this heat wave affected much of the Pacific Northwest where temperatures soared into the 90’s across numerous locations. This is very uncommon for this region.
- Late June into early July 2013: For a region most prone to high heat across the United States, the desert southwest, experienced several days of extreme heat, even based on their standards. It is not uncommon at all for many locations in this region to flirt with 100 degrees Fahrenheit or even exceed it, however, during this period temperatures would exceed 110F across many locations.
- August 8th-12th, 2006: A brief but intense heat wave impacted a portion of the northeastern United States with temperatures pushing close to 100F in a few spots. At times, high dewpoints made it feel between 105F-110F in some areas.
The common theme that can be established from these graphics is the strong ridging (indicated by the yellow, oranges, and red shadings). The oranges and reds indicate very strong ridging and is a signal that the pattern could also be relatively long lasting. Areas under these higher pressures aloft are dominated by air that sinks and becomes compressed, and therefore warms. When these patterns become sustained, episodes of high heat (temperatures >90F) become more likely and the longevity of the heat increases in likelihood as well.
It is not uncommon, especially across the central and eastern United States to have episodes of high heat (temperatures greater than 90F) occur with dewpoints well into the 60’s or even 70’s, especially during mid-to-late summer. While episodes of high heat can certainly occur with dewpoints much less than 60 or 70 (this is especially true across the western and southwestern United States) the degree of heat is much more dangerous when high dewpoints are introduced.
During mid-to-late summer if an area of high pressure (typically referred to as the Bermuda High) becomes established across the southeastern United States, or just off the southeast United States coast, the wind flow around the high pressure (winds blow clockwise around high pressure centers in the northern hemisphere) allows for high levels of moisture to be transported across the central and eastern United States. This is due to the Gulf of Mexico, where surface water temperatures climb well into the 70’s and even into the 80’s, especially from mid-summer on. While it is very possible to experience high humidity levels in late spring and early summer, it is a more likely occurrence as summer progresses. Therefore, it is more common to have high heat episodes without high levels of humidity earlier in the summer as opposed to later.
Not only does high heat in itself pose a threat to human life but the combination of high heat and high levels of humidity can pose significant threats to human and animal’s lives. It is not uncommon, especially across the central and eastern United States to have high heat occur with dewpoints well into the 60’s and sometimes even 70’s. This occurs when strong southerly winds develop in the lower portion of the atmosphere as they rotate around the high pressure (the wind blows clockwise around high pressure) and if the high pressure is established across the southeastern portion of the United States or just off the coast this pumps in very moist air into the country off the warm waters of the Gulf of Mexico. High dewpoints (leading to high values of relative humidity in the summer) can make it feel much hotter than it really is outside. This is why it is extremely important that people in areas being affected drink water, even if they do not feel thirsty, and pets are not kept outside for extended periods. Young children and the elderly should be checked in on frequently.
Heat Waves in Film and Culture
- A song titled “Heat Wave” was performed by Martha and the Vandellas and released in 1963. The song would reach #1 on the U.S. R&B chart and #4 on the Billboard Top 100.