Dew point, humidity, and what the heat index means here

The heat index is the number most people check before they go outside. In Phoenix it is close to the wrong number, and the reason is not that the equation fails here. It is that the equation is doing what it was designed to do, and what it was designed to do is add humidity to a temperature.

This page works through what the index actually is, what the Phoenix normals do to it, and which number the local forecast office uses instead. Every figure below traces to NOAA’s 1991–2020 hourly normals for Phoenix Sky Harbor, station USW00023183.

What the heat index actually is

The heat index is a humidity correction on top of the air temperature. It is not a measurement of anything: there is no instrument that reads a heat index. It is an equation, fitted to how a body sheds heat in different combinations of temperature and moisture, and then applied to the two numbers a weather station does measure.

The equation the National Weather Service publishes is the Rothfusz multiple-regression, from NWS Technical Attachment SR 90-23, with two adjustments: one that subtracts a little when the air is very dry, and one that adds a little when it is very humid. This site computes its heat index figures with exactly that equation, pairing the normal temperature of an hour with the normal dew point of the same hour.

That pairing is worth flagging, because it is this site’s arithmetic rather than an observation. A normal temperature and a normal dew point do not necessarily occur in the same hour. What the result describes is a typical combination, not a reading anyone took.

The other limit is the equation’s own. The Weather Prediction Center says the regression is not valid for extreme temperature and humidity conditions, and that below a heat index of about 80 °F a simpler formula applies instead. This page’s figures use a straight guard at 80 °F air temperature and return the air temperature below it, which is the same practical effect and not a literal implementation of that rule.

The dew point is the humidity number that moves

Relative humidity tells you how full the air is relative to how full it could be at that temperature, which means it changes when nothing has been added to the air at all. The dew point tells you how much moisture is actually in it.

In Phoenix that distinction is the whole story. A normal August night has a relative humidity of about 44% and a dew point near 60 °F. Nothing was added overnight; the air cooled toward the moisture it already held, and the percentage rose. The dew point is the number that only moves when moisture moves.

What the normals do to the heat index

Now put the two together. The heat index takes the air temperature and corrects it for moisture. In Phoenix, the moisture is low enough that the correction goes the other way: the body can cool itself by sweating, so the same temperature is easier to bear than the number alone suggests.

Read the hottest working hour of the year, 4 p.m. local time:

June is the clearest case. A normal June afternoon is 102.6 °F with a heat index of 96.5 °F, six degrees below the thermometer, because the air is at 10% relative humidity and the correction is subtracting. July and August are the months the monsoon catches up: the moisture nearly closes the gap, and the heat index arrives within a couple of degrees of the air.

That is also why the index is not a disaster-detection number. It rises toward the temperature as the weather gets harder, and it never gets past it here.

The two hours of the year it goes above

Worked hour by hour across all twelve months of the normals, the Phoenix heat index exceeds the air temperature in exactly two hours: 6 a.m. and 7 a.m. in August, by 0.11 °F and 0.12 °F. Two-tenths of a degree, at the hour of the day when the dew point peaks at 60.7 °F and the temperature is still down at 85 °F.

Everywhere else, in every month, at every hour, the index sits at the air temperature or below it. A normal July afternoon at 104.8 °F has a heat index of 102.5 °F. The hottest heat index in the normal year is 102.5 °F, and the hottest air temperature is 104.8 °F.

Two more counts from the same dataset, which are the ones to carry around:

• The heat index reaches 90 °F in four months only: June, July, August and September. July and August have sixteen hours each, June eleven and September eight. The other eight months of the year never reach it once.
• The heat index reaches 100 °F in two months only: July and August, five hours each.

What to use instead

The National Weather Service does not rely on the heat index to trigger its Phoenix heat alerts. It uses HeatRisk, a five-level product that combines temperature with how unusual that temperature is for the date and how long it lasts, and which replaced the old Excessive Heat Warning in March 2025.

That is a better fit for a climate where the humidity correction points the wrong way. A dry 110 °F afternoon in Phoenix can carry a heat index in the low hundreds and still be the kind of day the local office issues an alert for, because the danger is the temperature and the duration, not the moisture.

The practical output for a reader is therefore simple. When you are planning an hour outside in July, the number to read is the air temperature and the number of hours it stays above 100 °F: eight of them in a normal July day. The Heat-Safe Hours Planner works hour by hour from exactly this data, and the Outdoor Event Heat Check answers the narrower question of how often a specific date has actually reached a temperature, from the station record rather than from these averages.

Where this page stops

A heat index at or below the air temperature is not a statement that an afternoon is safe. It means the humidity is not adding to the load, and nothing more. The CDC still describes heat stroke as the state in which the body can no longer control its own temperature, and it still names people without air conditioning as a group at higher risk, a sentence that reads as though it was written about this city.

Two limits on the arithmetic are worth repeating, because both are this site’s rather than NOAA’s. The heat index figures here pair a normal temperature with a normal dew point for the same hour, which is a typical combination and not an observed heat index. And the 80 °F guard is a simplification of the Weather Prediction Center’s own rule, which switches formula on the computed index rather than on the temperature.

If the question is what to do about a person who is unwell in the heat, this page is not the page for it. The Heat-Illness Triage Guide carries the CDC’s symptom lists and first-aid steps, and the comparison article explains why sweating is a poor test for heat stroke. Call 911 for confusion, slurred speech, fainting or a seizure in the heat.