The ten most humid of America's major cities have a humidity level that averages over 72.5 percent year round.

New Orleans has the highest relative humidity among big US cities, with an average of nearly 86 percent. The Louisiana city is followed closely by second-ranked Jacksonville, Florida.

These numbers are annual averages based on humidity readings taken every three hours from 1961 to 1990.

The major cities included in the weather rankings here represent the US urban areas with the most people, all those with over one million residents in 2010.

Average relative humidity (%) for the most humid cities City Humidity New Orleans, Louisiana 75.9 Jacksonville, Florida 75.8 Houston, Texas 74.7 Orlando, Florida 74.1 Tampa, Florida 74.1 San Francisco, California 73.7 Seattle, Washington 73.3 Miami, Florida 73.2 Portland, Oregon 73.2 Rochester, New York 72.6

Highest Morning Humidity

Relative humidity fluctuates throughout the day along with the air temperature. At the coolest time of day, humidity usually peaks. This is because cooler air holds less water, and relative humidity measures the percentage of water in the air compared to how much water the air can hold. The time of day when humidity levels are usually highest depends on the city and ranges from 4 am to 7 am local standard time.

The ten large cities that achieve the highest levels daily usually reach 84 to 90 percent humidity. Houston tops the list with an average peak of 89.7 % humidity at 6:00 am.

Highest humidity (%) of the day, on average, for America's most humid cities City Humidity Time Houston, Texas 89.7 6 am Jacksonville, Florida 89.2 4 am Orlando, Florida 89.0 4 am New Orleans, Louisiana 87.6 6 am Tampa, Florida 87.5 7 am Portland, Oregon 84.8 4 am Raleigh, North Carolina 84.6 7 am Nashville, Tennessee 84.1 6 am Birmingham, Alabama 84.1 6 am Miami, Florida 83.9 7 am

Highest Afternoon Humidity

When the temperature rises during the day, the humidity usually drops. For the ten most humid cities, the humidity never gets below 58.5 percent on average, even at its lowest point in the day. The list is topped by Milwaukee, where the humidity often stays at over 62 percent.