The Relationship Between Air Temperature & Humidity
By Joan Whetzel
Temperature and humidity become important when considering grain drying, or air circulation in livestock housing and human homes. For livestock, controlling temperatures and dampness helps manage odors and toxic gases. For grain drying, heat is added to attract moisture which can then be extracted when the heat is pumped out. Keeping humans comfortable, means adding moisture in dry environments or removing it in humid environments, and controlling temperatures with air conditioning or heating.
What Are Relative Humidity, Dew Point and Dew Point Temperature?
Relative Humidity measures the amount of moisture that's actually in the air, then compares it to the amount of moisture it would take to completely saturate the air. Since completely saturated air contains 100% humidity, when the relative humidity holds less moisture, it is shown as a smaller percentage of moisture, like 50 percent. When the air is saturated, the moisture condenses forming dew. Dew point temperature is the temperature that the air would have to be in order for dew to form. The closer the air temperature is to the dew point temperature, the more likely it is that dew will form. So dew formation is dependent on the amount of moisture in the air and the air temperature.
Temperature, Relative Humidity, and Perception of Temperature
Elevated Relative Humidity (RH) levels generate the required conditions for the production of mold and mildew, dust mites, bacteria and fungi. These conditions increase the risks of illness in humans and animals.
Increased RH levels in conjunction with the radiant heat created by the summer sun makes the air temperature see much hotter. The heat index chars illustrate how the RH changes our perception of the actual air temperature. RH affects our perception of hot temperatures in much the same way as the wind chill factor affects our perception of cold temperatures in the winter. Both illustrate the "feels like" temperature after the RH or the wind effects have been factored in.
Calculating RH and Dew Point.
Psychometric charts use a set of curving lines to show the temperature calculations. Dew point measurements are obtained by measuring a wet bulb temperature (thermometer with a wet cloth over the bulb, with air blown over it) and dry bulb temperature (regular thermometer reading). The wet bulb, which is usually lower, is subtracted out from the dry bulb temperature. The median temperature between the two thermometer readings is the dew point temperature. Example: the wet bulb temp is 87 degrees and the dry bulb temperature is 95 degrees, so 95 - 87 = 8 degrees. The median temperature of 91 degrees is the dew point. This is also the temperature at which the RH would be 100 percent.
Temperature, RH, and Comfort Levels
Everyone of us has a personal preference for the range of temperature and humidity levels that feel most comfortable to us. When the temperature and RH combination exceed comfort levels leads to complaints of feeling too hot. (Well, you can't please all of the people all of the time.) Indoor temperature and humidity levels can be controlled to the point of keeping the majority of people comfortable. In humid climates, AC units that remove the excess moisture are used, whereas in dry environments, a water cooling system is used. Those who are still a little warm can wear fewer layers and lighter clothing, while those who feel a little too chilly can add an extra layer, like a sweater.
How hot or cold the air feels, depends on the air's moisture content. Whilewe can't control the outdoor temperature and RH, we can at least manage the indoor temp and humidity levels enough to keep the majority of people comfortable at one time. Controlling indoor temp and humidity levels will also lower the rates of illness due to mold, mildew and other moisture related issues.
Check Out These Sites:
· Psychometry Conversion Table, from the National Weather Service
· NOAAs National Weather Service Heat Index Chart