2011 Heat Wave Causes

The 2011 heat wave has already broken records in the South, Midwest, and Northeast U.S. and it isn't over yet. Where I live, in a suburb of Dallas, a quick glance at the dying vegetation and the haze above the overheated asphalt is enough to demonstrate the intensity of the heat we are experiencing. As of today, August 7, we have had 37 consecutive days of temperatures above 100 ° (according to the National Weather Service at DFW airport). If temperatures continue to be high, as they are forecast to be during the coming week, we will break the record on Saturday, August 13. What causes these periods of extreme heat?

Every year, summer brings unusual heat to some portion of the U.S. Sometimes it is a relatively small portion, and in other years, such as 2011, large swaths of the country are affected. The primary cause of these extreme heat events is the formation of a strong upper-level ridge of high pressure. High pressure commonly develops over a large area and moves more slowly than low pressure areas. The high pressure ridge can form a blockade, preventing other air masses from moving across an area. This is what has occurred in Summer 2011 '" a huge high pressure area has remained stagnant over the central U.S.

Two blocking patterns that can lead to heat waves are the Omega Block and the Ring of Fire or Cut-off High. In the Omega Block, a strong high forms in the center of the U.S., producing lower pressures on the two coastal regions. The pattern of isobars '" lines connecting areas with the same pressure '" looks like the Greek letter omega ( Ž) . The Ring of Fire or Cut-off High features a large high pressure cell in the southeast U.S. which forms as part of a ridge in the Atlantic Ocean. It is called the Ring of Fire because severe thunderstorms often form along the edges of the circular cell.

This year's exceptional heat wave and, in many areas (especially Texas) extreme drought, may be due to the persistence of a combined Omega Block '" Ring of Fire. It has extended north into the Midwest and Northeast during some parts of the summer, but its greatest effects have been in the Central and Southern states.

You may have heard large cities called "heat islands" during extreme heat events like this. Cities contain less vegetation and more concrete and asphalt, which absorb heat during the day and release it very slowly during the night. Thus, the overnight lows in cities are higher than in suburban and rural areas, and the each night more heat is retained for the next day. Cities do not completely cool off; this is why heat waves are often worse in big cities.

Humidity plays a crucial part in heat waves since a high temperature will feel much hotter when humidity is high than when humidity is low. This effect is called the heat index, which is usually reported along with temperatures on the weather report. It is important to remember that the heat index assumes shade and a light wind. Being in full sunshine can cause heat index values to climb up to 15 º higher!

Sometimes extreme temperatures alone, even without excessive humidity, can be high enough to cause physical injury. This often occurs in the desert Southwest of the U.S.; wind patterns can shift this hot, dry air into parts of Texas as well. Our summer has been hot and dry, due to winds from the South and West.

Everyone should know the signs of heat-related illnesses. These include dehydration, sunburn, heat cramps, heat exhaustion, and heat stroke or sunstroke. Check out the websites below to learn more about these dangerous illnesses, and be prepared! Listen to weather forecasts, especially if you are going to be outside, and make sure you know what to expect. Let's stay cool this summer!