Dewpoint

[Guest guest]

Hey guys,

I found out that dewpoint is a better indicator of when you can put

the windows open (if your allergies permit), since it is more

indicative of humidity than relative humidity. I thought to look it

up when local weather person said we'd be comfortable for a few days

since the dewpoint is going to be around the upper 40s-low 50s, so

looked it up. As temperature goes up and down, dewpoint stays the

same. Dewpoints below 55 about are considered very dry for summer.

I'm a newby on subject so some information may be wrong. I don't

know about others, but letting fresh air into house really helps me

if it isn't humid. In general I read it's better to go by dewpoint

temperature than relative humidity since as temperatures go up and

down during day, relative humidity will continually be changing.

However if dewpoint is low, can count on more consistent dry feeling

even while temperatures go up and down. I wonder if there is any

place in country where dewpoints tend to be low. I imagine not

since they probably come and go with wind currents, but not sure.

<http://en.wikipedia.org/wiki/Dew_point>

<http://weathersavvy.com/Q-dew_point1.html>

Here's one excerpt from accuweather:

Dew point is simply the saturation point of the air, or the

temperature to which the air has to be cooled in order to get

condensation. It is really a true measure of how much water vapor is

in the air...the higher the dew point, the more water there is in

the air, and the lower the dew point, the drier the air mass is. In

the summer, dew point is an excellent tool to use in determining how

comfortable, or uncomfortable, it feels outside. Here is a general

summertime guideline for dew points...

Below 50: very dry

50-59: comfortable

60-64: moderately humid

65-69: humid

70-79: oppressive

80 and above: steam bath!

Now you may be wondering...how does dew point relate to relative

humidity? Well, the relative humidity is dependent on the

temperature and the dew point. The closer together the dew point and

temperature are, the higher the relative humidity is. The farther

apart they are, the lower the relative humidity is. Here's an

example of what I mean, and also an illustration why the term

relative humidity does tell you anything about how comfortable or

uncomfortable it is outside.

Let's say we've got a very humid air mass in place, and the dew

point is 70. It's daybreak, and it's very murky outside...hazy, and

perhaps even foggy. The grass, car tops, etc. are all covered with

heavy dew. Let's say the temperature at that time is 70 degrees.

With a temperature of 70, and a dew point of 70, the air is

saturated, and the RELATIVE HUMIDITY at that time is 100%. The sun

comes up, burns off the fog and evaporates the dew. There is no

front coming through that day...no air mass change whatsoever. So,

the air mass will have basically the same amount of water vapor in

it all day long, i.e., the dew point will stay close to 70. Now with

the sun out, the air heats up, and the temperature rises. Let's say

the high temperature that day, around 4pm, is 90 degrees. At that

time, the hottest part of the day, the RELATIVE HUMIDITY is only

50%!!! That doesn't tell the average person anything about how it

feels outside!!!

The dew point is a good tool to use all year long. In the winter

time, the lower the dew point, the drier the air mass is, and the

more likely precipitation will fall as snow, even if the temperature

initially is above freezing. That's because the air temperature will

cool as the precipitation initially evaporates.

The heat index is simply a combination of temperature and dew point,

and how it feels. When the dew point is relatively low...say in the

low 60s or lower, then the heat index is basically the temperature.

However, as it gets more humid (higher dew point), then it feels

hotter than the actual temperature. There really is something to the

expression... " it's not the heat but the humidity " . When the

temperature is quite hot, and the dew points are very high, the heat

index is extremely high. Sometimes the heat index can go into the

dangerous category of 110 degrees or more.

The Real Feel Temperature is something we developed in-house here at

AccuWeather. It takes into consideration other factors besides just

temperature and dew point. It factors in the time of day, the sun

angle, and amount of cloud cover, the winds or lack thereof, etc.

For example, a temperature of 90 and a dew point of 70 will feel

worse at noon or 1 pm with the sun high in the sky than it will at 5

pm or 9 am. Also, a temperature of 90 and a dew point of 70 will

feel worse when it is dead calm than if there is a 15 mph wind

blowing. The same thing goes for 90/70 with bright sunshine vs. 50%

cloud cover. For all of these examples, the heat index will be

identical...no difference

whatsoever, since it is based solely on temperature and dew point.

The Real Feel Temperature, however, will be different for each

case.

I hope I answered your question. If so, please close out the

question and give me a rating. Thanks!

Sincerely,

Dave Dombek

Expert Senior Meteorologist

AccuWeather.com