Heat Index Chart Explained — How to Read the Danger Levels

To read a heat index chart, find the air temperature in one axis and the relative humidity in the other; the cell where they meet is the heat index, the temperature the weather feels like to the body in shade and light wind. The color or label of that cell is its risk category. This article shows how to read the grid, then recomputes it so you can see how steeply it climbs and where the categories begin.

What numbers are in the grid?

The National Weather Service publishes its chart as an image, so the cells could not be copied as text for this article. The grid below was computed instead from the same nine-term regression equation the Heat Index Calculator uses, which the NWS lists as the Rothfusz regression. As a check, the NWS page gives an example of 96°F air temperature at 65% humidity feeling like 121°F, and the formula returns 121.0°F. Whole-degree values from other tables can differ from these by a degree or two, so use any such table as a guide rather than an exact record.

Cells are in °F, rounded to whole degrees. Labels use the cutoffs the calculator applies from the NWS categories: S is below 80 (no category), C is Caution (80 to under 90), EC is Extreme Caution (90 to under 103), D is Danger (103 to under 125) and ED is Extreme Danger (125 or above). The labels were assigned before rounding: the 80°F and 40% cell is 79.9°F unrounded, so it sits just below the Caution cutoff. The NWS lists Danger as 103°F to 124°F and Extreme Danger as 125°F or higher.

How steeply does the chart climb?

• At 90°F air temperature, moving from 40% to 80% humidity adds 22.7°F of felt heat (90.7 to 113.3 before rounding).
• At 60% humidity, rising from 90°F to 95°F adds 13.4°F (99.7 to 113.1).
• At 95°F, going from 50% to 60% humidity adds 7.9°F (105.2 to 113.1).
• The two inputs interact. Because the equation has product terms, humidity matters more the hotter it is: from 40% to 80% it adds 4.3°F at 80°F but 48.9°F at 100°F (a 109.3 to 158.2 climb).

What humidity pushes a given temperature into the next category?

Reading the chart in reverse is often more useful: at this temperature, how humid does it have to be before the category changes? These thresholds were found by scanning humidity in 0.1% steps with the calculator's formula.

An 85°F afternoon stays out of Danger almost whatever the humidity, while at 100°F the Danger category begins at roughly 31% humidity. This is why a hot, apparently dry day can still reach a high category and a mild, steamy one usually does not.

What does the chart leave out?

• Sunshine. The NWS says heat index values were devised for shady, light wind conditions and that full sunshine can increase them by up to 15°F. That is an upper bound, not a fixed amount. At 90°F and 50% humidity the shade value is 94.6°F (Extreme Caution); adding 15°F gives up to 109.6°F, which is in Danger. At 85°F and 60%, 89.3°F becomes up to 104.3°F, also Danger.
• Outside the formula's range. Below about 80°F the NWS says the regression is not appropriate, and gives a separate simpler formula for those conditions. The calculator returns the air temperature itself below 80°F.
• Humidity extremes. The NWS equation page also lists two small adjustments: one for relative humidity below 13% at 80 to 112°F, and one for humidity above 85% at 80 to 87°F. The calculator does not apply them. For example, at 82°F and 95% humidity the regression gives 93.0°F and the adjustment would add 1.0°F; at 100°F and 10% it gives 94.8°F and the adjustment would subtract 0.6°F.
• The person. The chart describes conditions, not people. Age, health conditions, medications, fitness and acclimatization all change how much heat a person can handle, and none of them are in the grid.
• Hard work. For strenuous work or exercise in the sun, other measures such as wet bulb globe temperature are designed for that situation.

Common mistakes when reading it

• Reading the row by temperature alone. At 95°F, humidity separates a 99°F feel from a 134°F one.
• Using whole-degree values near a cutoff as if they were exact. A reading of 102.6 and one of 103.4 round to different sides of a cutoff by less than a degree.
• Using a forecast humidity for the wrong time of day. Relative humidity falls through the afternoon as air warms, so check the hour you will be outside.
• Assuming a lower category means no precautions. Even Caution lists fatigue as possible with prolonged exposure or physical activity.