What are the U.S. Heat Advisories and What Causes Them? Check Facts Here

As summer progresses across the United States, there are 22 states under heat advisories currently. According to AccuWeather, nearly one-third of the United States' population may experience high temperatures. Extreme heat watches have been issued for Michigan, Ohio, Missouri, West Virginia, Pennsylvania, New Jersey, Illinois, Indiana, and Delaware. These simple warnings are from the National Weather Service (NWS). Heat Advisory carries significant weight, indicating conditions that can pose serious health risks, even fatalities, if precautions are not taken. Understanding the heat advisory meaning goes beyond just knowing it will be hot; it involves recognizing the specific thresholds of temperature and humidity that trigger these alerts and the underlying atmospheric phenomena responsible for them. For public safety, it's crucial to be informed about these weather statements and their implications.   

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What are the U.S. Heat Advisories?

U.S. heat advisories are official notifications issued by local offices of the National Weather Service when a combination of high temperatures and humidity is expected to create conditions dangerous to human health. The primary metric used for these alerts is the Heat Index, which measures what the temperature "feels like" to the human body when relative humidity is factored in with the actual air temperature. 

These advisories serve to warn the public about potential heat-related illnesses like heat exhaustion and heatstroke. They are part of a tiered system of heat alerts, designed to communicate varying levels of risk.

What Do Different U.S. Heat Advisories Signify?

The National Weather Service issues different levels of heat alerts based on the severity and duration of the expected heat. While specific criteria can vary slightly by region due to local climate considerations, the general tiers are:

Heat Advisory

They are issued when the Heat Index is expected to reach levels (e.g., 100-104°F) that could lead to significant health impacts if precautions are not taken. Recently, extreme heat advisories are in effect in Iowa, Arizona, Minnesota, Nebraska, South Dakota, and Wisconsin.

Excessive Heat Watch

Excessive Heat Watch advisories are issued when conditions are favorable for an Excessive Heat Warning in the next 12 to 48 hours. This is a heads-up to prepare. States including Michigan, Ohio, Missouri, West Virginia, Pennsylvania, New Jersey, Illinois, Indiana, and Delaware are on the look out for excessive heat watch. 

Excessive Heat Warning 

The most severe heat alert, issued when extremely dangerous Heat Index values (e.g., 105°F or higher, sometimes for multiple days or with high overnight lows) are expected or occurring. This signifies a high risk of heat-related illness or death.

These alerts are often accompanied by safety recommendations, such as staying hydrated, seeking air conditioning, and avoiding strenuous outdoor activities.

What Meteorological Factors Cause Extreme Heat Advisories?

The primary cause of heat advisory and other extreme heat events in the U.S. is typically a persistent area of high atmospheric pressure. This phenomenon often leads to what is colloquially known as a "heat dome." Here's how it works:

• High-Pressure Systems: A dome of high pressure traps air below it, causing it to sink and compress. As air sinks, it warms up (adiabatic heating) and dries out.

• Clear Skies: High-pressure systems often lead to clear skies, allowing more solar radiation to reach the Earth's surface and further increasing temperatures.

• Stagnant Air: The slow movement of these high-pressure systems means the hot air remains trapped over a region for an extended period, leading to prolonged heat waves.

• Humidity: The presence of high humidity, often from moisture transport from bodies of water like the Gulf of Mexico, prevents sweat from evaporating effectively from the skin, significantly raising the Heat Index and increasing heat stress on the body.

Climate change is also increasingly recognized as a factor intensifying the frequency and severity of heat events.

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Conclusion

In summary, U.S. heat advisories are crucial warnings from the National Weather Service about dangerous heat conditions. These alerts, ranging from advisories to warnings, are triggered by a combination of high temperatures and humidity, quantified by the Heat Index. The underlying cause of heat advisory events is most often a large, stagnant high-pressure system, frequently forming a "heat dome," which traps and heats the air. Recognizing the heat advisory meaning and taking recommended precautions is vital for public health and safety during periods of extreme heat across the U.S. and economy.