The Making of a Heat Wave

In many areas of the country, a heat wave is defined by prolonged periods of excessive heat and humidity. Since warm temperatures feel even warmer when the humidity is high, a heat index has been derived to combine the effects of temperature and humidity. Days in which the heat index reaches values of 100EF or higher are generally considered to put people at a high risk for heat related illnesses. In the Desert Southwest, however, temperatures can exceed 100EF and the effects of humidity do not need to be considered. Additionally, heat waves can not be defined using a specific temperature or heat index threshold. A necessary component of a heat wave is that the temperature exceeds normal or average values for a specific region. A heat wave in the Pacific Northwest could occur at temperatures much less than those of a region in the southern portion of the country.

The atmospheric mechanism responsible for most heat waves are related to regions of "synoptic scale" high pressure. Synoptic scale refers to weather patterns that are typically depicted on weather maps ranging in size from hundreds to thousands of miles and lasting for days or weeks. While cells of high pressure are regularly noted on maps of surface pressure, the pressure patterns at mid and upper levels of the atmosphere are more important during a heat wave.

An example of a high pressure cell, or anticyclone, is depicted in Fig. 1. This pattern illustrates the pressure pattern at a level around 5000 meters (or 15000 feet) above the surface. Within an high pressure cell, the pressure exerted by the atmosphere is greater than that in areas of low pressure. The vertical motion is generally downward, inhibiting the formation of clouds and precipitation. When air is forced to descend, it is compressed which results in warming.

Fig. 1.  Atmospheric pressure pattern associated with anticyclones, or "high" pressure

The horizontal flow around the center of the high is clockwise in the Northern Hemisphere. Because wind speed is proportional to the change in pressure, winds near the center of high pressure cells (the dark red area in Fig. 1) are light. Areas of stronger winds occur away from the center of the high (yellow and orange areas of Fig. 1). This wind pattern also contributes to the dry weather associated with anticyclones since the winds which would normally transport moisture at low and mid levels is diverted around the center of the high. The anticyclones can remain over an area with little movement for days or weeks resulting in dry and hot weather.

Heat waves can be defined by the maximum temperature exceeding a defined value, such as consecutive days equal to or above 100EF in Albuquerque.  While the temperature reached at least 100EF on four consecutive days in July 2003, the record occurred in 1980, when the afternoon temperature in Albuquerque reached at least 100EF on 9 consecutive days (June 22 through June 30).  The average pressure pattern during that 9 day period at the 500 mb level is shown in Fig. 2.  A cell of mid and upper level high pressure extended from Arizona to Louisiana and was centered over the New Mexico/Texas border.

While Fig. 2 shows that high pressure dominated New Mexico, it does not show how far from normal this pattern is.  Fig. 3 illustrates the average pressure pattern (1968-1996) at this same level.  Note that while the average pattern indicates that it is normal to have a general pattern of high pressure over New Mexico, the strength of the high was considerably stronger in 1980.  Another way to examine the unusual strength of this high is to calculate the anomalies, or the difference between the observed and normal fields.  Click here to view an image of the anomalies for this period, which depicts pressure much stronger than normal over New Mexico and Texas.

Similar graphics show the low levels of moisture associated with the strong high pressure cell which occurred in late June of 1980.  Figure are available depicting the relative humidity at 700 mb (1500 m above the surface) during June 22-30, 1980, and how much drier this air mass was than the normal values.

Fig. 2.  Average pressure pattern at 500 mb (a mid atmospheric level) during June 22-30, 1980.

Fig. 3.  Long term average (1968-1996), or normal 500 mb pressure pattern for the nine day period from June 22 through June 30.

Certain precautions should be taken during a heat wave.  It is advised that you should avoid strenuous activity especially during the warmest part of the day.  Stay indoors as much as possible and drink plenty of water regularly and often. Drinks with alcohol or caffeine in them should be avoided.

For more information on the heat index, check out the National Weather Service site: http://www.nws.noaa.gov/om/heat/index.shtml