WRAL WeatherCenter Blog

What Will the Dew Point Do?

Posted July 20, 2009

Dew Point forecast for 2 pm, July 20th 2009, from the Rapid Update Cycle model available on the NCAR web site.

The title sounds a bit like a line from Dr Seuss, but a question we get from time to time when humidity is especially high or pleasantly low (as we enjoyed through the weekend just past) is "How can I find out what the dew point will be in the next few days?"

As we've discussed before, during warm or hot weather, dew point, which changes directly in step with the amount of water vapor in the air, can give a nice, simple sense of whether we will perceive humidity levels as being comfortably low (dew points in the 40s and 50s), moderate (low to mid 60s), a bit humid (upper 60s to around 70), or sticky and steamy (low 70s and up). Making these determinations based on relative humidity values is a good bit more difficult, since that quantity depends not just on the amount of water vapor in the air, but also on the temperature.

For those of you who might like to see how dew point will be changing during the day, or over the next few days, there are some resources online that provide computer model projections of dew point. I attached an image with a dew point map of the U.S. that is available from the National Center for Atmospheric Research web site. Here, you can see dew point contours projected into the future at 1-3 hour intervals for the next 12 hours or so from the Rapid Update Cycle (RUC) model, or switch to the Eta or GFS model to see longer range projections at less frequent time intervals of 6-12 hours. Mapping in this way can provide some nice insight about where humid or dry air is located and how it is moving and may change with the passage of  fronts and other weather systems.

Another way to see how dew point may be changing is more specific to a particular location. It uses a text table called "Model Output Statistics" (MOS) that interpolates model results to a location of interest, usually at airports around the country. For example, there are MOS tables for the Raleigh-Durham airport (RDU), the Fayetteville airport (FAY), the Wilmington airport (ILM) and many others in our state. These tables (called MOS Bulletins) include many other pieces of information, like temperature, wind direction and speed, precipitation probability and so on, and include a line with dew point for every three hours for the first two days of the forecast and every six hours farther along. I've included a link where you can choose any station or multiple stations from any state in the U.S., and will then see the MOS output from the GFS (Global Forecast System) and NAM (North American Mesoscale) models.  Once you've selected the stations you want, click "Submit Query" at the bottom of the page.

The bulletins are fairly self-descriptive (DPT is the dew point line), noting that the times are given in Universal Time (subtract 4 hours for Eastern Daylight and 5 hours for Eastern Standard times). In case you need help, I've also included links to descriptions of the tables that give an example and explain each element of the message.

After a pleasantly dry weekend, those maps and tables are showing an increase in humidity for this week, with dew points largely back in the mid 60s to around 70 for our area...

 

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