The NWS calculates a “heat index” that takes into account both air temperature and humidity.
You can convert your local air temperature to a heat index value using this NWS chart. Let’s say the air temperature is 90 degrees F and the current humidity is 60% – the head index is 100.
Heat Forecast Tools (weather.gov)
The heat index was first developed in 1979.
The NWS issues a heat alert based on these critieria:
- Excessive Heat Warning—Take Action! An Excessive Heat Warning is issued within 12 hours of the onset of extremely dangerous heat conditions. The general rule of thumb for this Warning is when the maximum heat index temperature is expected to be 105° or higher for at least 2 days and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas not used to extreme heat conditions. If you don’t take precautions immediately when conditions are extreme, you may become seriously ill or even die.
- Excessive Heat Watches—Be Prepared! Heat watches are issued when conditions are favorable for an excessive heat event in the next 24 to 72 hours. A Watch is used when the risk of a heat wave has increased but its occurrence and timing is still uncertain.
- Heat Advisory—Take Action! A Heat Advisory is issued within 12 hours of the onset of extremely dangerous heat conditions. The general rule of thumb for this Advisory is when the maximum heat index temperature is expected to be 100° or higher for at least 2 days, and night time air temperatures will not drop below 75°; however, these criteria vary across the country, especially for areas that are not used to dangerous heat conditions. Take precautions to avoid heat illness. If you don’t take precautions, you may become seriously ill or even die.
- Excessive Heat Outlooks—Be Aware! The outlooks are issued when the potential exists for an excessive heat event in the next 3-7 days. An Outlook provides information to those who need considerable lead-time to prepare for the event.
Heat Watch vs. Warning (weather.gov)
The exact values may vary by forecast office and location – see for example, this NWS page.
A heat alert is, thus, based on a forecast of temperatures in the 2 to 7 day range. However, NOAA may issue “excessive heat” alerts up to 14 days in advance.
The NWS first issued some heat alerts in the 1990s but based on their own web site, these may not have been commonly issued until about 2007, per the NWS history timeline (click on 2006-2009) and thereafter, they expanded the geographic areas where heat alerts were issued. In 2011, the NWS initiated it’s “Weather-Ready Nation” program which greatly expands public communication to alert the public to “severe weather events”.
In July of 2022, NOAA launched it’s heat.gov web site to “reduce the health effects of extreme heat”.
They also began issuing estimates for how many people are affected by their heat messaging: More than 170m Americans under heat alerts as heatwave expands. Estimates for how many people would be effected appear to have originated in the late 2010s, as part of Impact-based Decision Support Services (IDSS), per NOAA.
Thus, if you think you are hearing more “heat alerts” and warnings than you did in the past, this is not your imagination – it is because the NWS did not issue “Excessive heat watches”, “Heat advisories” and “Excessive heat outlooks” in the past – these forecast products are new.
NOAA notes that climate change is just one factor of many that can influence weather events so they may occur more or less often.
The purpose of this post was to answer the question – why did I not remember hearing of heat alerts and warnings in the past? And the answer is because they were not used until the past ten years or so, and their visibility has increased since 2022.
Thus, the incidence of heat alerts is higher than the past primarily because the NWS did not issue them until recent years.
Note – In the 21st century it is tough to practice factfulness, to ask reasonable questions, and to try and understand the actual details. Unfortunately, noting inconsistencies and contradictions is, in the 21st century, forbidden and results in being name called as a “denier”, even if that is not the case.
