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What is a heat advisory? Excessive heat warning? Heat wave? Why they feel even hotter in NYC.

Officials warn New Yorkers about expected heat wave
Officials warn New Yorkers about expected heat wave 04:54

NEW YORK -- The Tri-State Area is baking under high heat so far season. . 

Summers can sizzle around the New York City area, where the hottest normal high is 85 degrees in July. The hottest temperature ever recorded in Central Park measured 106 degrees on July 9, 1936. 

Because heat can be deadly -- in fact, it's the number one weather-related killer in the U.S., taking more than 1,000 lives every year -- understanding heat related advisories, watches and warnings can save lives.

Heat advisories & watches

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Heat advisory: The National Weather Service issues a heat advisory when the heat index is forecast to rise to between 95 and 99 degrees for at least two days in a row. A heat advisory is also issued if the heat index is forecast to rise between 100 and 104 degrees. 

Excessive heat watch: Excessive heat watches are issued a day or two days before the heat index is forecast to hit 105 degrees or higher. 

Heat index: The heat index represents the "real feel" taking both temperature and humidity into consideration. The higher the humidity, the hotter it will feel outside.

Heat dome: A heat dome happens when hot air is trapped by the atmosphere. One is expected to set up over the eastern half of the country this week, making for multiple days of soaring temperatures. 

What is a heat wave?

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When high temperatures reach or exceed 90 degrees for three consecutive days or more, that is defined as a heat wave.

While New York City is no stranger to hot days, it's when those hot days become incessant and successive that problems start to arise.

Heat waves in NYC

CBS New York

On average, the city sees two heat waves per summer, lasting around four days each. The longest heat wave ever recorded in the city lasted 12 days from Aug. 24 to Sept. 4 in 1953. 

In a typical summer, a high temperature of 90 degrees or above occurs 15 times. Climate projections indicate that this number may rise to 69 by the 2050s, with the duration increasing to six days.

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Exacerbating the effects of heat waves is the city's infrastructure. The concrete jungle, filled with towering buildings and endless paved surfaces, allows for more efficient heating during the daytime due to a phenomenon known as the '"Urban Heat Island Effect."

However, the effects of this phenomenon are most pronounced at night. This is because the infrastructure of the city traps in all of the heat that built up during the day, thus preventing a normal nocturnal cooldown seen in non-urban settings. Some nights there can be as much as a 20-degree difference in temperature between the city and surrounding suburbs. High heat during the day is stressful enough on the human body, but when it can't cool down even at night, that becomes a major health concern. 

What is an excessive heat warning?

CBS New York

Although not always the case, excessive heat warnings usually coincide with heat waves. The National Weather Service issues excessive heat warnings 24 hours in advance of a forecasted heat index of at least 105 degrees for at least two consecutive hours.

Excessive heat warnings are the highest form of heat alerts that are issued. They are not issued as regularly as heat advisories, being reserved for only the most sweltering days. 

Excessive heat warnings in NYC 

High heat indices are usually associated with higher levels of humidity, and New York City certainly sees a lot of that during the summer, especially during the month of August. The high levels of humidity work in conjunction with the Urban Heat Island Effect to produce higher heat indices. An average summer typically yields between one to three of these such warnings. Some summers have none, such as in 2022, while others have much more. 

Nelson Vaz shared the chart he produced using the Iowa Environmental Mesonet (IEM), which shows distinct excessive heat warnings issued for NWS Upton's area of responsibility since 2006. Please note this data is not "official," according to Vaz. Nelson Vaz, with the National Weather Service at Upton

Nelson Vaz, with the National Weather Service at Upton, N.Y., says excessive heat warnings have been issued on average less than once per year in New York City over the last 18 years.

How to stay safe in high heat

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It is during prolonged periods of heat that the most heat related illnesses and deaths occur. Three hundred and seventy deaths are attributed to heat each year in the city, with most of them happening during heat waves. Heat is also the number one weather related killer across the country. 

CBS New York

Neighborhoods in the South Bronx, Upper Manhattan and Central Brooklyn are the most susceptible due to a variety of factors, including a lack of air conditioning and trees and higher rates of pollution. Elderly people are most at risk, regardless of any of these factors.

CBS New York

When an excessive heat warning is issued, extreme caution is needed. This includes limiting time outdoors, drinking plenty of water (even if you're not thirsty), not drinking too much alcohol or coffee, exercising during cooler portions of day, eating light meals and staying in the shade or air-conditioned places.

The U.S. Centers for Disease Control and Prevention has more tips for staying safe in high heat.  

Stick with CBS New York's First Alert Weather team for the latest forecast and weather alerts.

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