Talking about Extreme Heat

The National Weather Service (NWS) uses certain key terms when describing record keeping:

    • Period of record refers to the records kept between 1869 and the present
  • Compared to normal or compared to average refers to data recorded over the 30-year period from 1981 to 2010

What is the Hazard?

During the summer, New York City usually experiences one or more periods of extreme heat. Heat is not only uncomfortable, it can be dangerous – causing dehydration, heat exhaustion, heat stroke, and in severe cases, death. Periods of extreme heat can also lead to pressure on the power grid, causing outages that can create a cascade of negative impacts throughout the city.

Extreme heat events are most likely to occur between June and August, but they sometimes occur as early as May and as late as September. Between 1981 and 2010, temperatures in Central Park reached 90°F or higher for about 18 days per year.

Hazards from extreme heat are made worse when accompanied by high levels of humidity – the amount of moisture in the air. As the temperature climbs, the air is able to hold more moisture. High humidity hinders a person’s body from cooling down naturally, leading people to perceive that the temperature feels hotter. The combination of temperature and humidity – what the temperature “feels like” – is known as the heat index.

Extremely poor air quality is another hazard that often accompanies extreme heat. During New York City’s summer months, stagnant atmospheric conditions trap humid air and pollutants, such as ozone, near the ground.

In New York City, extreme heat events are defined as periods when the heat index is 100°F or higher for one or more days, or when the heat index is 95°F or higher for two or more consecutive days. The risk of people and infrastructure suffering adverse effects from extreme heat increases as the temperature and the heat wave’s duration increases.

Climate change is likely to bring us hotter temperatures and more hot days, leading to far longer and more frequent heat waves.

The National Weather Service (NWS) uses a heat index chart to determine the effects particular combinations of temperature and humidity will have upon the general population.

Source: NWS, 2013

Note that the heat index values used in this chart are calculated in the shade and are not adjusted for different levels of sun exposure. Thus, when people are exposed to full sunshine, they can feel much hotter than the temperature indicated on this chart. During hot weather, the stress a person experiences from the heat depends upon: the temperature, location, and air movement as well as level of physical activity, and exposure to sunlight and radiant heat from nearby surfaces.

The NWS provides New York City with heat-related “products” in the form of Heat Advisories, Excessive Heat Watches, and Excessive Heat Warnings. A Heat Advisory is based on analysis of historical weather and mortality data by the New York City Department of Health and Mental Hygiene (DOHMH).

To help respond to extreme heat events, the NWS worked with DOHMH and New York City Emergency Management (NYCEM) to determine specific criteria for issuing heat products in New York City.

NWS Extreme Heat Products for the New York City Region

Product Criteria
Heat Advisory (NYC) Issued within 24 hours prior to the onset of any of the following conditions:
  • Heat index of 100°F-104°F for any period
  • Heat index of 95°F-99°F or greater for two consecutive days
Excessive Heat Watch Issued 24-48 hours prior to the onset of the following condition:
  • Heat index of at least 105°F for at least two consecutive hours
Excessive Heat Warning Issued within 24 hours of the onset of the following condition:
  • Heat index of at least 105°F for at least two consecutive hours

Source: NWS, 2013

When the NWS issues an extreme “heat product,” it:

  • Includes heat index values and city forecasts.
  • Identifies potential impacts and recommends precautionary and preparedness actions. These actions may include clarifications as to who is at greatest risk, safety rules for reducing risk, and the extent of hazard and heat index values.
  • Assists state and local health officials to prepare civil emergency messages.

Official temperature readings for New York City are taken near Belvedere Castle in Central Park, although temperatures vary throughout the city. The 2015 New York City Panel on Climate Change (NPCC) report estimated that from 1971 to 2000, New York City had an average of 18 days per year with maximum temperatures at or above 90°F and had heat waves lasting an average of four days. This NPCC calculation uses a time period (1971 - 2000) that differs from the time period used by meteorologists (1981 - 2010).

Based on historical NWS data recorded at Central Park, the annual number of days with high temperatures of 90°F and above has been increasing since the late 19th century.

In the future, the number, duration, and intensity of heat waves at or above 90°F are expected to continue to increase as a result of climate change. These trends are discussed further in the Future Environment. In the future, the NPCC projects that New York City will experience between three to four heat waves per year, with an average duration of five days.

The built environment of New York City contributes significantly to the phenomenon known as the "urban heat-island effect." Heat islands develop in urban areas where formerly natural, permeable surfaces are now paved with asphalt or covered by buildings and other structures that absorb and retain heat. This condition slows the cooling process in the city, keeping nighttime air temperatures higher than temperatures in surrounding rural areas.

Waste heat that emanates from city air conditioners, vehicles, and other types of equipment also contribute to the urban heat island effect. The effect is most pronounced during summer nights, when New York City temperatures are, on average, seven degrees Fahrenheit warmer than the surrounding suburban areas. Under certain nighttime conditions, New York City can be as much as ten to twenty degrees warmer than the surrounding region.

The urban heat island effect also causes some neighborhoods to be warmer than others. Not surprisingly, areas with the highest building density (highest concentrations of heat-trapping built structures) and the least vegetation tend to have higher temperatures. The below thermal image of New York City reflects the temperature variation in different neighborhoods. Warmer locations are shown in orange and red, while cooler areas are shown in green and yellow.

New York City Thermal Imagery Taken July 17, 2018

The second map displays the city's vegetative cover. Comparing the two images reveals that New York City hotspots are generally areas that lack any type of vegetation. New Yorkers living in these areas of the city are most exposed to high heat events, particularly among residents who do not have access to air conditioning.

Vegetative Cover of New York City

Source: University of Vermont Spatial Analysis Laboratory

Since the late 19th century, the number of days that New York City has experienced with temperatures of ninety-degrees or higher has increased. The NPCC report notes that climate scientists predict that by the 2050s, New York City could experience as many ninety-degree days annually as Birmingham, Alabama, does today – more than triple the 18 days we experience in an average year.

For more information on historic heat events, use the Hazard History and Consequence Database, an interactive tool developed for this website.