Risks of Extreme Summer Heat – Grit

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Consider the dangers of heat waves as the onset of summer brings higher temperatures. As the heat index above 105 creates extreme heat hazards, care should be taken.

As summer sets in across North America, we often have to deal with the extreme weather conditions of heat waves and droughts. Although they are separate events, heat waves tend to occur when conditions are dry, and summer drought is associated with high temperatures.

Heat waves and droughts also have the same meteorological parentage, a ridge of high pressure thousands of feet above the Earth’s surface. A ridge is a huge mound of warm air that effectively blocks storm systems from bringing precipitation. The air sinks, warming and drying as it descends towards the ground, and a few clouds block the hot rays of the sun. For heat waves, the ridge shifts within a few days. For droughts, the pattern is more permanent; often one crest may dissipate to be replaced by another. This pattern can persist for months or even years.

Audio item:

Effects of Extreme Heat Hazards

The National Weather Service (NWS) defines a heat wave as a period of two or more abnormally and uncomfortably hot days.

The heat can be dangerous, even fatal. Mortality rates increase markedly during heat waves. Heat has been the number one cause of weather-related deaths in the United States for the past 30 years, with more than 100 deaths per year attributed to excessively hot weather. Certain groups are at high risk, including the elderly, those with underlying health conditions, and the poor.

grass fire

In 2021, more than 200 people in the country died due to heat waves. We don’t normally associate excessively hot weather with the Pacific Northwest, as ocean breezes generally keep temperatures pleasant. That was not the case in June last year, when an unprecedented heat wave broke many all-time temperature records. Portland, Oregon peaked at 116 degrees Fahrenheit, and Seattle, Washington hit 108 degrees. Across the border in British Columbia, the town of Lytton recorded 121 degrees on June 29, setting a high temperature record for the country of Canada. Hundreds of people have died in British Columbia from excessive heat and more than 100 people have died in Washington and Oregon, many in homes without air conditioners or fans. Washington’s heat wave is considered the deadliest weather disaster in the state’s history. Although the air aloft is dry during a heat wave, an area may experience higher humidity near the surface. Since humidity affects the rate of evaporation of sweat and its cooling effect on the body, meteorologists use the heat index to measure how hot it feels. For example, if the air temperature is 90 degrees and the relative humidity is 60%, the human body will react as if it were at 100 degrees. A heat index of 105 is considered unsafe. The effects on the body of excessive heat can range from painful cramps to life-threatening heatstroke. To protect yourself, wear light-colored clothing made of breathable materials. Avoid strenuous activities during the hottest part of the day, usually the afternoon. If you must be active, take frequent breaks in the shade, which can be 10 to 15 degrees cooler than in the sun. Stay hydrated with plenty of electrolyte-rich fluids. Be especially careful if you are taking medication or if you are elderly or immunocompromised.

Heat waves can cause more severe effects in cities due to a phenomenon called an “urban heat island”, in which building materials absorb the sun’s rays during the day and radiate their heat into the air, especially at night. Nighttime temperatures in a city can be 10 degrees warmer – or more – than in the surrounding suburbs or countryside.

Sweaty farmer standing in front of an olive grove - agriculture

Heat waves can usually be predicted days in advance. In the US, the NWS may issue an Excessive Heat Outlook 3-7 days in advance to give people time to prepare. An Excessive Heat Watch will be issued 1-3 days in advance, and a Heat Advisory is issued 12 hours before hazardous conditions occur. The latter implies a maximum heat index of at least 100 degrees during the day, with nighttime temperatures exceeding 75 degrees for two or more nights. An excessive heat warning is issued when the heat index is expected to reach 105 degrees or more during the day and the night air temperature is expected to exceed 75 degrees for at least two days. (Note that these values ​​vary by region of the country.)

Fortunately, heat waves with excessively high temperatures are usually short-lived, especially in higher latitudes. The upper level ridge tends to break down after a few days.

dry as a bone

A drought is defined by the NWS as “a lack of moisture that results in adverse effects on people, animals, or vegetation over a significant area.” Agricultural drought consists of a significant lack of water for crops or livestock and can develop in a few months. Hydrological drought relates to reductions in the overall water supply and develops over a period of months or even years. Droughts can lead to considerable financial losses, especially for agriculture. According to the National Centers for Environmental Information, 29 separate drought events from 1980 to 2021 caused at least $1 billion in damage each in the United States. The total combined cost to the country was $285.4 billion. The drought that hit the west of the country in 2021 cost an estimated $8.9 billion in damages. Although drought in the United States, Canada and other more industrialized countries was not directly responsible for the deaths, crop failures led to famine in less developed countries around the world.

Climate change drought land and water in the lake

The current drought in the western United States began in 2000 and is now considered a “mega-drought” that will last for decades. Researchers recently examined tree rings and determined that this was the worst drought in the West for at least 1,000 years.

Drought often sets the stage for forest fires, due to dry vegetation. This provides plenty of fuel for wildfires and increases the risk of ignition. (See my article “Wildfire Behavior and Mitigation” for tips on how homeowners can protect their properties from wildfires.)

Drought is seasonal in some places; for example, during summer on the west coast and winter in Florida, especially on the southern peninsula. In these regions, droughts develop when winter rains or summer thunderstorms do not produce normal precipitation, or when the rainy season is delayed or ends prematurely.

drought and bad harvest - parched earth on crop field due to heat

For other locations that generally experience constant rainfall, drought conditions are sporadic and associated with the upper level ridges previously described. Given the size of North America and the magnitude of typical upper-level weather systems, drought is likely to occur somewhere at some point.

Over the years, the Drought Monitor has become the standard for quantifying drought severity. Experts synthesize a variety of numerical inputs and use their own expertise to find a representative value, the results of which are usually displayed as a map. The North American version can be found on Drought Monitor.

Although forecasts of future drought conditions are available, long-term weather conditions are notoriously difficult to predict.

To reduce the impact on your household during a drought, you can purchase high-efficiency washing machines and dishwashers and only use them when fully loaded. Install water-saving showerheads and toilets in the bathroom and take shorter showers. Don’t let the tap water run while brushing your teeth or shaving. Find and fix any leaks. Outside, do not wash your car and limit the watering of plants and your lawn. If dry conditions are common in your area, consider using drought-tolerant native vegetation or plantless landscaping. Be sure to follow the water restrictions issued by your local authorities.


Ed Brotak has been teaching students weather for over 30 years and has led many to pursue careers in meteorology. He lives in Asheville, North Carolina with his wife (also a meteorologist) and two daughters.

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