Dangerous Summer Heat Waves
When Temperatures Soar, Heat Index Values Define Excessive Heat
As summer begins next month in the Northern Hemisphere, let's explore heat waves and the heat index. The United States' National Weather Service (NWS) urges people to pay attention to heat index values and heat warnings. Excessive heat and solar exposure can be deadly.
What is a Heatwave?
A heatwave is defined as a prolonged period or two days of excessive heat and humidity, according to the National Weather Service. Heat waves are a result of stationary high pressure areas hovering over a region. The definition of a heat wave varies by location and climate, as temperatures over 100 F in Arizona are different than in New York. Humidity can cause a large difference in how heat feels.
The Heat Index
The National Weather Service defines a heat wave by the official Heat Index value. The NWS developed the heat index in order to represent how temperature readings actually feel. The heat index represents the interaction of both heat and humidity. The heat index is also known as the apparent temperature. The National Weather Service defines a heat wave as two days with a heat index value from 105-110 F.
Weather forecasts issue alerts for heat waves based on monitored air and surface temperatures. Meteorologists look at a temperature profile by examining the temperatures at different heights in the atmosphere. Surface temperatures are recorded in the shade, in order to provide more uniform readings. This results in locations in direct sun having higher heat and sun exposure.
Temperature Inversion
These temperature measurements tell meteorologists if temperature inversion will happen. Temperature inversion occurs when the air overhead is warmer than the surface. This prevents the warming air from escaping into higher altitudes, acting like a lid. Temperature inversion prevents cooling during the night and also traps pollution. As temperature inversion occurs, pollution cannot rise and escape into the upper atmosphere.
Hot Times in the City
Heat in cities can be higher than in rural areas. This phenomenon is known as the heat island effect. The surfaces in cities, like pavement, sideways, and roofs absorb heat, holding it in. This causes temperatures in cities to be several degrees hotter than outlying areas. Increased levels of pollution due to inversion can also add to poor air quality during heat waves. Stagnant air can cause additional health risks. Power failures are common, as electrical power plants experience increased stress from air conditioning usage.
Deadly Heat
The National Weather Service issues alerts and special bulletins providing heat index values for weather forecasts in most areas. These warnings can help keep people safe in heat waves. The NWS provides special attention to heat and heat waves as heat is the number one weather killer. According to the NWS, during a "40-year period from 1936 through 1975, nearly 20,000 people were killed in the United States by the effects of heat and solar radiation." Heat not only affects human lives but also kills animals. Excessive heat causes crop damage, increased fire danger, drought and other heat related illnesses.
