Humidity Variation during Heat Waves in WA State

By Nick Bond

Summer is approaching, and while there will surely be some spells of hot weather, we cannot say much about their severity. Our most extreme heat waves tend to occur in July and August, but they certainly can occur earlier in summer and occasionally well into September. People that are outside and active for extended periods such as agricultural and construction workers are particularly exposed to this weather hazard; the heat stresses they endure are related to not just temperature but also the humidity. The purpose of this piece to show how much the latter can vary over short distances.

A picture is worth a thousand words and towards that end shown here are a pair of maps with measurements collected during the extreme heat wave that occurred in eastern WA near the end of June 2015. Maps were made with a rather slick app maintained by NOAA’s National Operational Hydrologic Sensing Center.

Focusing on the Yakima Valley and Columbia Basin, the station observations of air temperature and dewpoint at 2300 UTC (4 pm local time) on 28 June 2015 are shown in Figures 1a and 1b, respectively. This was the hottest day of the event, with the 113°F at Walla Walla and Chief Joseph Dam setting the warmest June temperature on record for anywhere in the state. While there were spatial variations in air temperature during this blistering-hot afternoon, they were dwarfed by those in dewpoint, which ranged from the mid-40s to 70°F at this time of day. Similar results featuring larger variations in dewpoint than in air temperature were found for the afternoon of 10 August 2018 during one of the hottest days last summer. The large differences in humidity presumably relate to local surface conditions and especially whether a source of moisture is directly upwind. The region includes a patchwork of dryland and irrigated fields, and evapotranspiration from the latter can be a significant source of atmospheric humidity. Conceivably the spatial patterns in summertime humidity are more-or-less systematic, but we have not carried out the kind of analysis required to determine the extent to which that is the case. It would seem worthwhile to do so because the heat stress experienced by humans (and livestock) depends to a great deal on these kinds of variations in humidity.

The wet-bulb globe temperature (WBGT) is a widely-used measure of the threat of heat stress for individuals exposed to direct sunlight. The WBGT is a function of the air temperature, humidity, wind speed and solar radiation and can be estimated using a widget provided by the National Weather Service (NWS) forecast office in Tulsa, OK. Using values representative of the late June 2015 heat wave, and with all other parameters held constant, the WBGT was 82°F where the dewpoint was 45°F and 89°F where the dewpoint was 65°F. The higher value of 89°F is much more hazardous; guidelines indicate that humans working or exercising in this kind of environment become heat stressed in just 20 minutes. Long-term trends show that summer time values of absolute humidity as well as temperature are creeping up in WA state, and so unfortunately, this type of regional weather hazard is liable to continue to increase in severity. The National Weather Service debuted a new HeatRisk forecast product last summer to supplement their heat advisories, watches, and warnings, which was featured in our June 2018 newsletter. With the warm season upon us, we thought it could not hurt to provide a reminder of the existence of this resource.