As the “Dew Point” Rises, So Do Our Rain Chances
Water vapor is the power source for precipitation and thunderstorms
Thursday, April 17th, 2014 — I am currently doing my weekly forecast, figuring out the high and low temperatures, wind speeds and rain chances. I noticed that today’s weak “cool” front has brought in a little moisture, but an upper level wave will sweep in a large amount of humid air late tomorrow, Friday into Saturday. I know this by watching the “dew point” temperature on the computer prediction forecast models. The models use all kinds of formulas and algorithms to simulate our weather future. The models have the dew points in the upper 40’s, plenty high enough for area rain and perhaps a few thunderstorms! You hear me and other forecasters talk about the “dew point” all the time, but many people do not quite understand what it represents. In today’s “Weather Talk” I will define the dew point, talk about how the dew point impacts temperatures and how we can forecast upcoming weather using the dew point.
The dew point is our measure of the amount of moisture in the air. The dew point is the temperature at which the air would be saturated with water vapor and the relative humidity would be 100%. If the air temperature is 70º and the dew point is 40º the, the air would have to cool to 40º for it to become saturated. Once the air is saturated it cannot hold any more water vapor so it condenses and forms clouds, fog, rain, snow, frost or dew. Many times when this happens we end up at cloud level or in fog.
Now we know that the dew point indicates the amount of moisture that is in the air, it also determines how quickly the temperature rises and falls. If there is abundant moisture the temperature will rise or fall at a slower rate. A humid night will be warmer while a dry night can see temperatures plummet to a much cooler or colder level. You can really see the difference near a shoreline. The shoreline is naturally humid so it stays warmer overnight than the drier, colder inland. Since the temperature can never be lower than the dew point, the dew point is the ultimate lowest level for where temperatures to drop to on a clear, calm winter night.
In the Borderland, our dew points sometimes can drop to the single digits or even below zeroº! Like it has over the past couple weeks. This is because we are in a dry, desert climate with even drier air to our south and west. When we have a strong southwesterly wind, like we have been experiencing lately the air is quite dry. This happens in our windy season from the end of February until mid May. Weather systems that are carrying humid air with them pull in warmer, dry air from the south west as they track into the Borderland. The moisture is sucked out and all we get from these systems is wind. Once these systems move into the more humid air to our east, they produce showers and thunderstorms.
During the “monsoon season”, (June 15th through September 15th) the dew points rise into the 40’s and 50’s with persistent winds carrying in mainly Gulf of Mexico humid air. When dew points are in the 40º to 50º range there is a better chance of rain showers or thunderstorms. Sometimes we will see dew points push into the 60's, on those occasions we can see severe thunderstorms with hail and possible heavy, flooding rainfall. This is also about the only time of year we can say "It's not the heat, it's the humidity!" Even though we are still nothing close to levels to the east of us! Especially in places like Dallas and Houston where 60º and 70º+ dew points are common place making it very sticky and uncomfortable! Not to mention the southern states and the east coast! For the "muggy" reason alone, I am happy to live in the drier desert climate of the Borderland.
Dew points help us find fronts when there is not a big enough temperature difference or wind change. You can look for a drop in dew point with out even looking at the wind direction, speed or temperature change. Where the dew points drop, that is where you will find the front!
So now the next time you see the current conditions and see the dew point temperature you may be able to determine what type of weather pattern we are in or what weather changes we may expect. Is there enough moisture and are we expected to cool down close to the dew point temperature to produce rare Borderland morning fog? If it is the cold season could we see frost or even some snow flakes? Most importantly here in the desert, if you see the dew point in the 40’s or higher, will we see rain or thunderstorms? Now, there are other factors, most of which I have discussed or will tell you about in future “Weather Talk” articles, that have to come into play along with the dew point before we get rain, snow, fog, frost or dew. At least the watching the “dew point” temperature will put you on the right forecasting track.