The information here reveals how often heat combines with humidity at America's largest cities to create uncomfortably muggy weather.

When humidity climbs above 40 percent, the damp air makes any temperature over 80 degrees Fahrenheit (26.7 degrees Celsius) feel hotter than what the thermometer actually reads.

The apparent temperature, also known as the Heat Index, measures how hot the weather really feels, considering both temperature and humidity. For instance, a temperature of 90 °F (32.2 °C) along with 60 % humidity pushes the apparent temperature to a sweltering 100 °F (37.8 °C).

Days of High Apparent Temperature

The table here lists the number of days when the apparent temperature or Heat Index equalled or rose above 95, 105 or 115 °F (35, 40.6 or 46.1 °C) for at least an hour.

The totals cover the 30 years from 1978 to 2007 for the largest cities in the United States. Major cities missing from the list due to lack of data are Austin TX, Boston MA, Detroit MI, Las Vegas NV, Milwaukee WI, Portland OR, Riverside CA and San Jose CA.

Number of days with apparent temperature, 1978 to 2007 City 95+ °F 105+ °F 115+ °F Atlanta, Georgia 804 55 5 Baltimore, Maryland 603 75 2 Birmingham, Alabama 1367 106 2 Buffalo, New York 39 3 0 Charlotte, North Carolina 758 24 0 Chicago, Illinois 375 60 3 Cincinnati, Ohio 407 27 0 Cleveland, Ohio 182 14 1 Columbus, Ohio 285 15 2 Dallas, Texas 2467 352 4 Denver, Colorado 7 0 0 Hartford, Connecticut 251 20 2 Houston, Texas 2917 460 11 Indianapolis, Indiana 462 53 2 Jacksonville, Florida 2484 300 11 Kansas City, Missouri 1030 211 7 Los Angeles, California 12 0 0 Louisville, Kentucky 831 108 4 Memphis, Tennessee 1832 363 11 Miami, Florida 3098 72 0 Minneapolis, Minnesota 262 35 0 Nashville, Tennessee 991 74 1 New Orleans, Louisiana 2560 463 18 New York, New York 332 31 1 Oklahoma City, Oklahoma 1606 185 1 Orlando, Florida – 120 2 Philadelphia, Pennsylvania 550 72 4 Phoenix, Arizona 3370 1060 12 Pittsburgh, Pennsylvania 134 10 0 Providence, Rhode Island 194 21 0 Raleigh, North Carolina 1002 88 0 Richmond, Virginia 1005 146 3 Rochester, New York 150 11 0 Sacramento, California 683 68 2 Salt Lake City, Utah 7 0 0 San Antonio, Texas 2659 89 1 San Diego, California 13 2 1 San Francisco, California 5 0 0 Seattle, Washington 6 0 0 St. Louis, Missouri 1068 212 7 Tampa, Florida 3129 204 3 Virginia Beach, Virginia 927 107 1 Washington, DC 609 56 3

Hot Days Due to Humidity

Humidity takes hot weather to the extremes more often in some cities than others.

The table below gives the difference in number of days between apparent temperature and actual temperature. This shows how often high humidity pushed the Heat Index above a certain level from 1978 to 2007.

For example, on 54 days in Atlanta, although the thermometer hadn't reached 105 °F, the humidity was so high that the temperature felt at least like 105 for an hour or more.

Negative numbers mean that more commonly dry air brought the apparent temperature below the actual thermometer reading. In San Diego, for instance, although the thermometer climbed to 95 °F on 16 days, the humidity was low enough on three of those days to make the heat feel cooler than 95.

When compared with the table above, you'll see that all the days when the apparent temperature reached 115 °F or more are due to humidity at every city except Phoenix. That means the thermometer never actually reached 115 at those cities, but the muggy air sure felt at least that hot.

Similarly, humidity has the biggest influence on making the weather feel 95 °F or hotter in Miami and Tampa, Florida. The sultry air there takes the Heat Index to at least 95 degrees on an average 100 days a year. In comparison, most of the days above 95 in San Antonio get there from temperature alone.