Heat Information Page

Overview

Heat can be a killer. The combination of hot weather and high humidity can quickly cause heat-related illness to set in. The National Weather Service issues advisories for high heat to warn you of the dangers. These alerts are issued when an excessive heat event is occurring, is imminent, or has a very high probability of occurrence.

NWS Heat Products
Heat Index
Wet Bulb Globe Temperature
Heat Injuries
Heat Safety
Heat Index Forecast
Wet Bulb Globe Temperature Forecast
Heat Risk

NWS Baltimore/Washington Heat Products:

National Weather Service criteria for issuing heat-related products varies from region to region, is based on climatology of peak heat experienced, and adjusted in some areas due to public and core partner needs. Listed below are the heat-related products and their issuance criteria for the NWS Baltimore/Washington Weather Forecast Office:

An Excessive Heat Watch is issued when conditions are favorable for an excessive heat event in the next 12 to 48 hours. This is defined as a heat index of 110 or greater EAST of the Blue Ridge, and 105 or greater WEST of the Blue Ridge. A watch is used when the risk of excessive heat wave has increased, but its occurrence and timing are still uncertain. It is intended to provide enough lead time so those who need to set their plans in motion can do so, such as established individual city excessive heat event mitigation plans.

An Excessive Heat Warning is issued when an excessive heat event is expected in the next 24 to 36 hours. This is defined as a heat index of 110 or greater EAST of the Blue Ridge, and 105 or greater WEST of the Blue Ridge. The warning is used for high heat conditions that pose a significant threat to life.

A Heat Advisory is issued when heat indices reach 105 or higher EAST of the Blue Ridge, and 100F or higher WEST of the Blue Ridge. Advisories are issued when excessive heat is expected in the next 24 to 36 hours. An advisory is for less serious conditions that cause significant discomfort or inconvenience and, if caution is not taken, could lead to a threat to life.

Heat Island Effect: Large urban cities, such as Washington D.C. and Baltimore, are equally at risk for a large number of heat deaths. People with few resources living in a highly urbanized area with the potential for long periods of high heat and humidity can be a deadly mix. The thresholds above could be lowered for prolonged heat waves, or the first heat event of the year as many people are not yet accustomed to the summer heat.

Heat Index

The NWS heat alerts are based mainly on the Heat Index. The heat index takes into account heat and humidity.

How to read the chart: find the temperature on the left hand side, then move to the right until you find the column for the approximate relative humidity. That number will be the temperature that it will "feel" like. Example: A temperature of 95 and relative humidity of 50% will "feel" like 107 degrees.

	0%	5%	10%	15%	20%	25%	30%	35%	40%	45%	50%	55%	60%	65%	70%	75%	80%
115	103	107	111	115	120	127	135	143	151
110	99	102	105	108	112	117	123	130	137	143	151
105	95	97	100	102	105	109	113	118	123	129	135	142	149
100	91	93	95	97	99	101	104	107	110	115	120	126	132	136	144
95	87	88	90	91	93	94	96	98	101	104	107	110	114	119	124	130	136
90	83	84	85	86	87	88	90	91	93	95	96	98	100	102	106	109	113
85	78	79	80	81	82	83	84	85	86	87	88	89	90	91	93	95	97
80	73	74	75	76	77	77	78	79	79	80	81	81	82	83	85	86	86
75	69	69	70	71	72	72	73	73	74	74	75	75	76	76	77	77	78
70	64	64	65	65	66	66	67	67	68	68	69	69	70	70	70	70	71

IMPORTANT: Heat index values were devised for shady, light wind conditions. Exposure to full sunshine can increase values by up to 15 degrees! Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.

If you know the temperature and the dewpoint, then you can figure out the heat index and relative humidity (RH) using this calculator.

Wet Bulb Globe Temperature

Extreme heat is a major weather-related hazard. Even a short period of exposure can be so taxing on the body that it results in a heat-related illness. Although outdoor physical activity may be unavoidable, heat-related illnesses and fatalities are preventable by using NWS forecast tools like Heat Index (which tells you how temperature feels to the human body in a shady area) or alternatively, Wet Bulb Globe Temperature (WBGT), before heading outside.

What is WBGT?

WBGT is an experimental forecast tool indicating expected heat stress on the human body when in direct sunlight. It estimates the effect of temperature, relative humidity, wind speed, and solar radiation on humans using a combination of temperatures from three thermometers:

A Wet bulb measures the temperature read by a thermometer covered in a wet cloth. As water evaporates from the cloth, evaporation cools the thermometer. This mirrors how the human body cools itself with sweat.
A black globe is used to measure solar radiation. Solar radiation heats the globe and wind blowing across it cools the globe.
A Dry bulb calculates the air temperature measured in the shade. It is the temperature you would see on your thermometer outside.

With WBGT forecasts available by region up to 7 days in advance, it is a useful tool for planning ahead.

Who should use WBGT?

This experimental tool is most useful for active, acclimatized people such as outdoor workers, athletes, and anyone else performing strenuous outdoor activities — and has been used for decades by military agencies, OSHA, and marathon organizers.

What should you do if WBGT deems outdoor activity dangerous?

Always follow the advice of your local officials before engaging in outdoor activities, as WBGT guidelines vary geographically since temperatures fluctuate by region and so does the body’s response to exercising or working in heat. OSHA recommends the following protective measures for outdoor workers:

Acclimatize workers starting the first day working in the heat and after any extended absences
Provide shade for outdoor work sites
Schedule work earlier or later in the day
Use work/rest schedules
Limit strenuous work (e.g., carrying heavy loads)

WBGT Guidelines for the General Population:

Heat Injuries

Heat Index	Possible heat disorders for people in higher risk groups
130 or higher	Heatstroke/sunstroke highly likely with continued exposure.
105-130	Sunstroke, heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
90-105	Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or physical activity.
80-90	Fatigue possible with prolonged exposure and/or physical activity.

Heat Disorder	Symptoms	First Aid
Sunburn	Redness and pain. In severe cases swelling of skin, blisters, fever, and headaches.	Ointments for mild cases if blisters appear and do not break. If breaking occurs, apply dry sterile dressings. Serious, extensive cases should be seen by a physician.
Heat Cramps	Painful spasms usually in muscles of the legs and abdomen possible. Heavy sweating.	Firm presure on the cramping muscles, or gentle massaging to relieve the spasm. Give sips of water. If nausea occurs, discontinue use.
Heat Exhaustion	Heavy sweating, weakness, skin cold, pale and clammy. Pulse thready. Normal temperature possible. Fainting and vomiting.	Get victim out of sun. Lay down and loosen clothing. Apply cool, wet cloths. Fan or move victim to air conditioned room. Sips of water. If nausea occurs, discontinue use. If vomiting continues, seek immediate medical attention.
Heat Stroke (sunstroke)	High body temperature (106 F or higher). Hot dry skin. Rapid and strong pulse. Possible unconsciousness.	Heat stroke is a severe medical emergency. Summon emergency medical assistance or get the victim to a hosiptal immediately. Delay can be fatal. Move the victim to a cooler environment. Reduce body temperature with cold bath or sponging. Use extreme caution. Remove clothing, use fans and air conditioners. If temperature rise again, repeat process. Do not give fluids.

Types of Heat Injuries

Heat cramps are painful, often severe, involuntary spasms of the large muscle groups used in strenuous exercise. Heat cramps tend to occur after intense exertion. They usually develop in people performing heavy exercise and replenishing fluid loss with non-electrolyte containing water. Sport drinks are a good source of electrolytes.
Heat rash, also known as prickly heat, is a rash accompanied by acute inflammation and blocked sweat ducts. The sweat ducts may become dilated and may eventually rupture. Heat rash affects areas of the body covered by tight clothing. Prevention is the best therapy. It is also advised to wear loose-fitting clothing in the heat.
Heat exhaustion is considered to be the forerunner of heat stroke. Heat exhaustion is marked by excessive dehydration and electrolyte depletion. Symptoms may include headache, nausea, and dizziness. Definitive therapy includes removing patients from the heat and replenishing fluids. If this advice is not followed it may lead to heat stroke.
Heat stroke occurs during periods of sustained high temperature and humidity. Sweating is absent from those affected, which will send the body’s core temperature soaring, causing brain injury and eventually death.

Heat Safety

Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time of the day. Individuals at risk should stay in the coolest available place, not necessarily indoors.
Dress for summer. Lightweight, light-colored clothing reflects heat and sunlight, and helps your body maintain normal temperatures.
Foods (like proteins) that increase metabolic heat production also increase water loss.
Drink plenty of water or non-alcoholic fluids. Your body needs water to keep cool. Drink plenty of fluids even if you don't feel thirsty.
Do not drink alcoholic beverages.
Spend more time in air-conditioned places. Air conditioning in homes and other buildings markedly reduces danger from the heat. If you cannot afford an air conditioner, spending some time each day (during hot weather) in an air conditioned environment affords some protection.
Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.