Storm Station 247

The Field Book/Vol. VI, The Events/FB-EVT-130

Heat wave and heat dome

A heat wave is a spell of days hotter than the place is used to, often humid, often with warm nights. Its usual engine is a heat dome: a strong ridge of high pressure aloft that parks over a region, sinks the air beneath it, clears the sky and keeps the storms away.

Plate FB-EVT-130Heat, cold, air, dust and fireRevision 1, 2026-09-25Status draftAlso called heat wave, heat dome, heat advisory, extreme heat warning, extreme heat watch, heat index, ridge

Section through a heat dome, west to east: the ridge high aloft; air sinking beneath it and warming as it is squeezed; clear sky and strong sun heating the ground; a capping inversion at the top of the hot layer that holds storms down; the hot layer near the ground, hottest over the city; and a ring of storms around the edge of the dome, where the air can rise.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-EVT-130-ASECTION THROUGH A HEAT DOME, WEST TO EAST; NOT TO SCALEFIG. A UNDER A HEAT DOMETHE RIDGE ALOFTSINKING AIR WARMS 5.4 °F EACH 1,000 FT (9.8 °C A KM)THE CAPTHE CITY STAYS HOTTEST AT NIGHTCLEAR SKY, STRONG SUNSTORMS AT THE RIMSTORMS AT THE RIMKEYThe hot layerThe cap, an inversionSinking air123456TITLEHeat wave and heat dome, under a heat domeVOL. VI THE EVENTS · HEAT, COLD, AIR, DUST AND FIRETYPESECTIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 1 of 3DATE2026-09-25IDFB-EVT-130-ADRAWN AS linework on paperSOURCES AMS, NWS
Fig. A Under a heat dome. Section through a heat dome, west to east; not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The ridge aloft
  2. Sinking air, warmed as it is squeezed
  3. Clear sky and strong sun
  4. The cap that holds storms down
  5. The hot layer at the ground
  6. Storms around the rim

What it is

A heat wave is a run of days hotter than the place is used to. There is no one number for it: 95 °F (35 °C) is a heat wave in Seattle and an ordinary July week in Phoenix. What makes it dangerous is how long it lasts, how humid it is, and whether the nights cool off. The Weather Service counts heat as the deadliest weather in the United States, and many of its victims die indoors, in homes that never cool down.

Most long heat waves have the same engine: a heat dome, a strong ridge of high pressure high in the atmosphere that settles over a region and stays. Fig. A draws one in section and Fig. B from above.

What it is not

It is not one hot afternoon. A heat wave is days of heat, and the warm nights between them are part of what makes it dangerous.

It is not a drought, though the two feed each other. A heat dome keeps the rain away, and dry ground heats faster than wet ground, because none of the sun's energy goes into evaporating water.

It is not measured by the thermometer alone. Humid air stops sweat evaporating, and the body cannot cool. The heat index below measures how hot it feels; the next plate, Extreme heat, wet bulb and WBGT FB-EVT-131, measures how much heat a body can shed at all.

Lookalikes

A hot day
One hot afternoon is weather; a heat wave is days of it, and the nights matter as much as the days.
Drought
A long lack of rain; a heat dome can cause one and dry soil can make the heat worse, but they are different events.
Extreme heat, wet bulb
How much heat a body can shed, measured by the wet bulb temperature and the wet bulb globe temperature: the next plate's subject.

The machine

The ridge

High above the ground, around 18,000 to 20,000 ft (5.5 to 6 km), the atmosphere has waves: troughs where the air is cool and lifting, ridges where it is warm and sinking. The jet stream runs along the boundary between them. In summer a ridge can grow strong and stall, the jet stream bent far to the north around it. Under the ridge the weather stops changing. That stalled ridge is the heat dome.

Warming by sinking

Air under the ridge sinks slowly, day after day. Sinking air is squeezed by the higher pressure below, and squeezed air warms: 5.4 °F for every 1,000 ft it sinks (9.8 °C a kilometre), the second equation below. Air that sinks 3,000 ft (914 m) arrives 16 °F (9 °C) warmer, before the sun has done anything. Sinking air is also drying air, so clouds cannot form, and the sky stays clear.

The sun, the ground and the cap

With no cloud, the long summer days put the sun's full energy onto the ground, and the ground heats the air above it. The sinking air aloft is warm too, which puts a lid on the hot layer: a capping inversion, where the temperature stops falling with height. Air heated at the ground rises only as far as the cap and no farther, so the thunderstorms that would cool a summer afternoon never form, and the heat builds day after day.

Cities make it worse. Pavement and brick soak up the day's heat and give it back all night, so a city can stay 10 °F (6 °C) or more warmer than the country around it after dark, and the people in it get no relief.

The rim

Around the edge of the dome the cap is weaker and the air can rise. Storms form there and are carried round the rim by the wind aloft, and they can be severe. Where the dome ends, so does the heat.

How hot it feels

The body cools by sweating, and sweat cools only as it evaporates. In humid air it evaporates slowly, and the body holds its heat. The heat index, the first equation below, is the Weather Service's measure of how hot the air feels in shade: at 96 °F (35.6 °C) and 55% humidity it feels like 112 °F (44.3 °C). Fig. C works the equation over a range of temperature and humidity and grades each result on the Weather Service's four levels. In full sun it can feel up to 15 °F hotter still.

Schematic plan of a heat dome from above: closed height contours of the ridge centred over the middle of the country, the jet stream bent north around it, the heat under its centre, and a ring of storms carried round its edge by the wind.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-EVT-130-BPLAN OF A RIDGE OVER THE MIDDLE OF THE COUNTRY AT ABOUT 19,000 FT (5.8 KM), SCHEMATICFIG. B THE DOME FROM ABOVE19,290 FT (5,880 M)19,390 FT (5,910 M)19,490 FT (5,940 M)19,590 FT (5,970 M)HTHE JET STREAMSTORMS CARRIED ROUND THE RIMNKEYThe heat under the centre12345TITLEHeat wave and heat dome, the dome from aboveVOL. VI THE EVENTS · HEAT, COLD, AIR, DUST AND FIRETYPEPLANSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 3DATE2026-09-25IDFB-EVT-130-BDRAWN AS linework on paperSOURCES AMS, NWS
Fig. B The dome from above. Plan of a ridge over the middle of the country at about 19,000 ft (5.8 km), schematic NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The centre of the ridge
  2. Its height contours
  3. The jet stream, pushed north
  4. The heat under the centre
  5. Storms riding round the rim
Table of the heat index for air temperatures from 80 to 110 °F and humidities from 40 to 100 per cent, each cell worked by the Weather Service's equation and graded caution, extreme caution, danger or extreme danger, with 96 °F at 55 per cent marked at 112 °F.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-EVT-130-CTHE HEAT INDEX FOR EACH TEMPERATURE AND HUMIDITY, BY THE WEATHER SERVICE'S EQUATION, AND ITS FOUR CATEGORIESFIG. C THE HEAT INDEXAIR TEMPERATURE, °F80°82°84°86°88°90°92°94°96°98°100°102°104°106°108°110°HUMIDITY40%808183858891949710110510911411912413013645%8082848789929610010410911411912413013750%8183858891959910310811311812413155%81848689939710110611211712413060%828488919510010511011612312913765%828589939810310811412112813670%8386909510010611211912613475%8488929710310911612413280%84899410010611312112985%85909610211011712613590%86929810511312213195%8694101109117127137100%8796104112121132CAUTION, 80 TO 90 °FEXTREME CAUTION, 90 TO 103 °FDANGER, 103 TO 124 °FEXTREME DANGER, 125 AND ABOVE °FBLANK: PAST 137 °F, OFF THE WEATHER SERVICE'S CHART. IN SHADE; IN FULL SUN UP TO 15 °F HOTTER.1234567TITLEHeat wave and heat dome, the heat indexVOL. VI THE EVENTS · HEAT, COLD, AIR, DUST AND FIRETYPECLASSIFICATIONSCALETO SCALEREVREV 1 DRAFT SHEET 3 of 3DATE2026-09-25IDFB-EVT-130-CDRAWN AS linework on paperSOURCES AMS, NWS
Fig. C The heat index. The heat index for each temperature and humidity, by the Weather Service's equation, and its four categories TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The air temperature
  2. The humidity
  3. Caution
  4. Extreme caution
  5. Danger
  6. Extreme danger
  7. 96 °F at 55%, a heat index of 112 °F

Ingredients

  • A strong ridge aloft that stays in place for days, the jet stream pushed north around it
  • Air sinking under the ridge, warmed as it is squeezed, drying and clearing the sky
  • Long summer days of strong sun on ground that holds the heat
  • Often humid air near the ground, which keeps the nights warm and makes the heat harder on the body

Scales

time
several days to weeks
horizontal
a region hundreds to a thousand miles (hundreds to 1,600 km) across, under one ridge
vertical
the whole depth of the lower atmosphere, the ridge strongest near 18,000 to 20,000 ft (5.5 to 6 km)
orlanski
synoptic

Equations

The heat index

HI=−42.379+2.04901523 T+10.14333127 R−0.22475541 TR−0.00683783 T2−0.05481717 R2+0.00122874 T2R+0.00085282 TR2−0.00000199 T2R2HI = -42.379 + 2.04901523\,T + 10.14333127\,R - 0.22475541\,TR - 0.00683783\,T^2 - 0.05481717\,R^2 + 0.00122874\,T^2R + 0.00085282\,TR^2 - 0.00000199\,T^2R^2
HIHI
the heat index, how hot it feels in shade to a person, °F
TT
the air temperature, °F
RR
the relative humidity, %

Assumes The Weather Service's fit to Steadman's model of the body, in shade and a light wind; used where the index is 80 °F or more, with small corrections at very low and very high humidity. In full sun it can feel up to 15 °F hotter.

Working form At 96 °F (35.6 °C) and 55% humidity, the heat index is 112 °F (44.3 °C): danger, on the Weather Service's chart.

Warming by sinking

ΔT=Γd Δz,Γd=5.4 ∘F per 1,000 ft (9.8 ∘C km−1)\Delta T = \Gamma_d\,\Delta z, \qquad \Gamma_d = 5.4\ ^\circ\mathrm{F}\ \text{per}\ 1{,}000\ \mathrm{ft}\ (9.8\ ^\circ\mathrm{C}\ \mathrm{km}^{-1})
ΔT\Delta T
the warming of air as it sinks, °F
Γd\Gamma_d
the dry adiabatic lapse rate
Δz\Delta z
how far the air sinks, ft

Assumes Unsaturated air sinking without gaining or losing heat to its surroundings; air under a real ridge also loses a little heat by radiation.

Working form Air that sinks 3,000 ft (914 m) under the ridge warms by 16 °F (9 °C) on the way down, without any sun at all.

Signatures

sounding
warm; dry; sinking air aloft; a capping inversion at the top of the hot layer; which holds storms down
surface
highs well above normal for days on end; nights that stay warm; above 75 °F (24 °C) in humid places; light wind
satellite
clear skies under the ridge; a ring of storms around its edge
radar
little or nothing under the dome; storms along its rim; where they move round it with the wind

The numbers

QuantityValue, and the kind of number it is
Heat waveA period of abnormally and uncomfortably hot, and usually humid, weather, lasting at least a day and conventionally several daysStandard, Glossary of Meteorology
Heat AdvisoryA heat index high enough to be dangerous, set by each office for its climate; commonly around 105 °F (41 °C) in much of the countryTypical, Directive 10-515
Extreme Heat WarningA heat index higher and longer lasting, set by each office; commonly around 110 °F (43 °C) in much of the countryTypical, Directive 10-515
Heat index categoriesCaution 80 to 90 °F (27 to 32 °C); extreme caution 90 to 103 °F (32 to 39 °C); danger 103 to 124 °F (39 to 51 °C); extreme danger 125 °F (52 °C) and aboveStandard, The Heat Index Equation (the Rothfusz regression
Dry adiabatic lapse rate5.4 °F per 1,000 ft (9.8 °C per km)Standard, Glossary of Meteorology

How the station sees it

Airport stations report the temperature and the dew point every minute, and the heat index is worked from them. Cooperative observers keep the long daily record of highs and lows that a heat wave is judged against, and the lowest temperature of each night is watched as closely as the highest of each day. Road weather stations add the air temperature along the highways between the airports, where the heat can differ by several degrees.

A standard station measures the air in shade, in a shield at a fixed height over grass, Stevenson screen and siting FB-INS-009. The heat on a sunny parking lot, or in an upstairs room, is higher than any station reports.

How it is warned

Each Weather Service office sets its own heat criteria for its climate. A Heat Advisory is issued when the heat index is expected to be dangerous, commonly around 105 °F (41 °C) in much of the country. An Extreme Heat Watch is issued a day or more ahead of worse heat, and an Extreme Heat Warning when it is expected, commonly around a heat index of 110 °F (43 °C).

In a heat wave, drink water before you are thirsty, stay in air conditioning in the hottest hours, and check on anyone old, sick or alone. Never leave a child or a pet in a parked car, even for minutes.

VTECPhenomenonThe alerts that carry it
HTheatHeat Advisory
XHextreme heatExtreme Heat Warning; Extreme Heat Watch

Every alert in force now

See also

Sources

  1. American Meteorological Society. Glossary of Meteorology.
  2. National Weather Service. Directive 10-515, WFO Non-Precipitation Weather Products Specification.
  3. National Weather Service. The Heat Index Equation (the Rothfusz regression, 1990) and the heat index chart.
  4. National Weather Service. Watch, Warning, Advisory definitions.

Definition after the Glossary of Meteorology. Plate FB-EVT-130, revision 1, 2026-09-25. The number is permanent; cite it.