More than a few people have a tough time understanding the difference between the meaning of relative humidity (RH) and dewpoint (DP). So, Rebecca and I thought we would write a short discourse on the difference between humidity and dew point:
Dew point is also called the saturation temperature. It tends to be a better indicator of moisture in the atmosphere. Changes in dew point temperature reflect changes in the moisture content of the air.
The higher (lower) the DP the more (less) moisture in the air. DP's in the 60s or more indicate very moist air. DPs in the 50s or less indicate dry air. Since dew point is the saturation temperature of the air, when the DP temperature is reached, the relative humidity is 100%.
Here is a general summertime guideline for dew points...
Below 50: very dry
60-64: moderately humid
65-69: very humid
80: forget it and stay in bed.
Humidity is relative to changes in the air temperature. As it warms the humidity goes down, as it cools the humidity goes up. Here's another way to explain it: if the RH is less than 40%, it will feel dry outside. On the other hand, when the RH is greater than 80% it feels moist outside. The question is how moist, this is where dewpoint comes in. The DP will determine if it is uncomfortably moist or just regularly moist. Generally, as long as the temperature is comfortable the RH feels comfortable, as long as RH is Between 40 and 80%.
Here's an example: Let's say the air temp and dew point are 60 degrees then humidity is 100%. Now let's say the air temp goes to 80 degrees and the dew point stays at 60 then the humidity is 50%
This is why dew point is a better gauge of how comfortable or uncomfortable a day in summer is.
The heat index (HI) combines air temperature and dew point yielding an apparent or "feels-like" temperature. The HI is to summer what the windchill factor is to winter. The body cools down when it perspires and this "sweat" evaporates. When the HI is high it means there is a lot of moisture in the air. The more moisture in the air then evaporation is slower or decreased, thus when you perspire, if the HI is very high it becomes more difficult for your body's "sweat" to evaporate and therefore it becomes more difficult for your body temperature to stay cool. If our internal body temperature starts to rise during hot and humid weather this can lead to an assortment of heat related illnesses. Here is an overview of heat related illnesses: Types of Heat related Illnesses
An Excessive Heat Was has been issued by NWS for the lower elevations of Eastern NY State and Western New England for Thursday afternoon through Friday night. The Heat Index may rise to between 100 and 110 degrees tomorrow afternoon and again on Friday afternoon!
So remember to drink plenty of water...also remember the hot weather safety tips mentioned here Severe Weather Safety