Dewpoint Climatology

Dewpoint Climatology For The Wichita, KS Forecast Area

Mary-Beth Schreck and Kenneth R. Cook

National Weather Service, Wichita KS

1.0 Introduction

The science of the planetary boundary layer (PBL) is one of the less understood subject areas of meteorology. Within this region of the atmosphere is something that we forecast daily but have little skill with - dewpoint temperature. Dewpoint temperatures are forecast by the National Weather Service (NWS) hourly out to 7 days. They often become very important as they directly affect fog and fire weather forecasting. To improve forecasts, it is imperative to develop an understanding of dewpoint temperature climatology and how synoptic and local effects play a role in dewpoint temperature behavior. This document will assess the climatological averages and trends of observed dewpoint temperatures at Russell, Salina, Wichita, Medicine Lodge, and Chanute Kansas.

2.0 Methodology

Data were gathered using a 30 year archive of hourly observations from the National Climatological Data Center (NCDC). This dated from 01 June 1976 to 01 June 2006. The data were stratified by hour over the period of record for each of the 5 sites investigated. The average hourly dewpoint temperature was then calculated for each month. A group average was calculated by averaging the data from each of the five sites. A "perturbation" or anomaly (Td') was then created by subtracting the site dewpoint temperature (Td) from the group average (Td avg.). Equation 1 illustrates this:

Eq. 1: Td' = Td avg - Td

This perturbation, or Td', was used to indicate where the dewpoint temperature varied from the group average, depicting local or site specific microclimatic differences more clearly.

Finally, the data were examined to extract cases of wet and dry years. Once isolated, these cases were examined against the average dewpoint temperature. Contrasts were made between the wet and dry cases as well. The results were then graphed with significant differences noted in the behavior of the hourly dewpoint temperature during these extremes.

3.0 Results and Discussion

See this link for a webpage with all monthly graphs for each site.

Examination of the data shows that when synoptic influences, such as fronts, are present these will dominate the behavior of the dewpoint temperature regardless of the time of year. This has a higher frequency of occurrence and is more pronounced in the winter months given the known increase in the synoptic scale influence on the weather.

During the summer months, the local effects tended to show more of an influence. This is likely due to the exchange of soil moisture into the atmosphere via evapotranspiration. Contributions to evapotranspiration, such as variations in rainfall, subsequent soil moisture, and level of vegetative greenness have a direct impact on its amount, and therefore affect, on the overall daily dewpoint temperature fluctuations.  This manifested itself in this study when the subtle effects of sunrise, afternoon mixing, sunset, and overnight condensation were frequently noted.

There is a larger variation in the dewpoint temperature during a 24 hour period in the winter, generally on the order of 4 degrees Fahrenheit. This is due to there being more deposition than condensation during the winter, a result of dormant plants and lack of crops. Deposition is a drying process (Glossary of Meteorology, 2000) unlike condensation (which occurs in summer), which adds more latent heat to the atmosphere. Thus it could be argued that the deposition thereby causes the dewpoint temperature to drop a greater amount overnight during the winter months.  In contrast, condensation during the summer months tends to keep the dewpoint temperature variation smaller.  In this study, it averaged between 1 and 2 degrees Fahrenheit during the summer.

In the spring, there was a transition from 1 peak to 2, and in the fall a transition from 2 peaks to 1 was observed. This is likely related to the amount of evapotranspiration at different times of the year. In the area studied, an afternoon drop off in dewpoint temperature is observed by July, likely due to the wheat harvest (McPherson et al., 2005).

In addition to the evapotranspiration previously discussed, the type of crop also plays a role, perhaps a significant one, in the behavior of hourly dewpoint temperatures observed during the growing season of these crops. To examine this further, a comparison was drawn between a site located in the Iowa corn belt ( Waterloo, IA) and a site in the Kansas wheat belt ( Salina, KS). Significant differences in hourly dewpoint temperature behavior were found.

The reason the type of crop has such a bearing on the behavior of the hourly dewpoint temperature is directly related to the plant's stomata. Stomata are any of the minute openings in the epidermis of a plant organ (as a leaf) through which gaseous interchange takes place (Mirriam-Webster, 2003). The stomata on a corn plant close during the overnight hours (COMAP 2006). This precludes any transpiration from occurring from these crops, thereby facilitating a several degree fall in the dewpoint temperature by morning.

By contrast, the stomata on a wheat plant do not close, and therefore continue to transpire moisture into the atmosphere overnight (COMAP 2006). This, in turn, allows the dewpoint temperature to fall only a few degrees overnight. To further illustrate this point, examining the data during the month of June in Salina, Kansas suggests much more stability in the hourly dewpoint temperature over the diurnal cycle than any other month. This is likely due to peak transpiration rates from the winter wheat crop, as it is in its most mature phase at this time.

Some additional trends that were also extracted from the data include the observation that after an afternoon drop off, the dewpoint temperature begins to rise approximately 1 to 2 hours before sunset. This occurs in all months except for December and January and is likely the result of a decrease in mixing, thereby allowing moisture to be “trapped” below a forming inversion.

Additionally, during the warm season, the lowest dewpoint temperature occurs in the evening due to the longer day and longer period of mixing. This is in contrast to the cool season, where the lowest dewpoint temperature tends to occur in the early morning due to the longer night.

While the PBL is not well understood, it is important that we continue research to become better a versed in this region of the atmosphere. Forecasting dewpoint temperatures often becomes very important as they directly affect fog and fire weather forecasting. By gaining an understanding of the land use and typical boundary layer behavior for a given location, one can determine a typical dewpoint curve for that location throughout the year.

4.0 References

COMAP: Boundary Layer Symposium. 2006.

Cox, Robert: Discussion of Boundary Layer Meteorology, Personal communication.

“Deposition.” American Meteorological Society Glossary of Meteorology. Second Ed. 2000.

McPherson, R.A., D.J. Stensrud, 2005: Influences of a Winter Wheat Belt on the Evolution of the Boundary Layer. Mon. Wea. Rev., 133, 2178-2199.

“Stomata.” Def. 2. Merriam-Webster’s Collegiate Dictionary. Eleventh Edition. 2003.