This is the happiest day of your life. Today is your wedding day. You have planned an elaborate outdoor wedding, and have spared no expense to have and wear perfect and memorable clothes. No rain is forecast for your day, so you put aside your fear that the weather will ruin your mid-afternoon wedding.
But, something is wrong. You cannot see it, but you feel clammy and uncomfortable. So does everyone in your wedding party. The humidity is high. The clammy feeling is upon your skin, under your wedding clothes and on your exposed skin too. The moisture of your sweat will not evaporate. This is uncomfortable, and everyone has trouble getting their mind off of the muggy feeling. What is happening?
One feels hot when the temperature is 90 degrees, but not necessarily uncomfortable if the humidity measure is at 20%. Air is a gas that expands as it heats up. If the water vapor in the air does not expand, you will not feel the heat so much. Think of the rising temperature in a desert, which is often referred to as a dry heat.
However, if the atmosphere is influenced by a massive inflow of moisture, the humidity level will rise. A temperature of 90 degrees that has a 90% humidity measure, is nearly saturated with water. You feel hot and sticky. You may see thunderstorms rapidly forming as weather patterns change to an up flow of moisture laden air from the ground and a down flow of falling cooler air from the high levels of the clouds.
Humidity is the measure of the amount of water vapor in the atmosphere, a measure of parts water per parts air. Sometimes you see it as dew on the ground or as fog in the air. The term dew point is the measure of the temperature in which dew forms. You may see a haze, and hear people say that the air is thick. Many times, you will not see humidity, but you will feel it, and especially so if the air is stagnant (Air that is still. There is no breeze). The preferred weather instrument for measuring humidity is the psychrometer.
A psychrometer consists of two thermometers. One is a standard thermometer that measures air temperature. The second one, which is affixed next to and parallel with the first one, is a wet thermometer that features a small wet cloth over the mercury-filled bulb. As the weather dries the moisture over this cloth, the temperature measure in the wet thermometer will fall (it is being cooled by the evaporation of the water in the cloth). The measure of humidity is determined by comparing the temperatures of the two thermometers on a humidity chart (a graphical representation of learned measures that has been developed over the centuries).
Get on with your wedding, but be prepared to take shelter, The feeling on your skin is a warning of a pending change in the weather. Perhaps you have been blessed with air that you can wear.