In a recent post, the Silicon Graybeard explains what it is like to live in Florida in August. He isn't kidding. On Thursday when I left for work at 0600, the temperature was 78 degrees. At 6 in the morning. But it isn't the temperature that is the biggest problem.
The biggest problem was my windshield fogging over, and not staying clear no matter how high I ran the wipers. The only way to keep the windshield clear is to run the defroster at 80 degrees. What this means is that the dew point is 78 degrees. So now I am sitting in a car that is heated to 80 degrees, with a humidity level of 92 percent.
The dew point temperature is the temperature at which the air can no longer hold all of its water vapor, and some of the water vapor must condense into liquid water. At 100% relative humidity, the dew point temperature and the air temperature are the same, and clouds or fog can begin to form. While relative humidity is a relative measure of how humid it is, the dew point temperature is an absolute measure of how much water vapor is in the air (how humid it is). In very warm, humid conditions, the dew point temperature can reach 75 to 77 degrees F, but rarely exceeds 80 degrees.
Dew point is the best indicator of comfort in a hot climate. Once the dew point of the air exceeds 66 degrees Fahrenheit or so, the air begins to feel hot and uncomfortably stuffy. The reason for this, is that your perspiration can not evaporate to cool you off.
Couple this with heat, and we have what is known as the "heat index," which is a way of saying what the air feels like. In the case of my morning drive, that particular combination of heat and humidity makes for a very uncomfortable drive.
That afternoon, my car said that it was 98 degrees on the way home. That temperature, combined with the dew point of 78 degrees, means that the relative humidity is 53 percent. That makes it feel like it is 116 degrees Fahrenheit (46.4 degC) outside.
THAT is the reason why we need air conditioning in Florida. Now there are people from further north who like to claim that it gets just as hot or hotter up where they are from, but they are not accounting for humidity. On July 6 of this year, New York City recorded a temperature of 91 degreesF, and a dew point of 72 degreesF. That makes the Relative Humidity 54%, and the heat index a relatively comfortable 99 degrees F (37 degC).