1. WeatherSpark.com
  2. United States
  3. Wyoming
  4. Thermopolis
  5. May

Average Weather in May in Thermopolis Wyoming, United States

Daily high temperatures increase by 10°F, from 64°F to 74°F, rarely falling below 49°F or exceeding 85°F.

Daily low temperatures increase by 9°F, from 40°F to 49°F, rarely falling below 31°F or exceeding 56°F.

For reference, on July 27, the hottest day of the year, temperatures in Thermopolis typically range from 60°F to 89°F, while on December 31, the coldest day of the year, they range from 13°F to 32°F.

Average High and Low Temperature in May

The daily average high (red line) and low (blue line) temperature, with 25th to 75th and 10th to 90th percentile bands. The thin dotted lines are the corresponding average perceived temperatures.

The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on May. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.

Average Hourly Temperature in May

Average Hourly Temperature in May in Thermopolis18152229112233445566778899101011111212131314141515161617171818191920202121222223232424252526262727282829293030313112 AM4 AM8 AM12 PM4 PM8 PM12 AMAprJunNowNowvery coldcoldcoolcomfortablewarmvery cold
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
The average hourly temperature, color coded into bands. The shaded overlays indicate night and civil twilight.

Ashnak, Armenia (6,388 miles away) is the far-away foreign place with temperatures most similar to Thermopolis (view comparison).


The month of May in Thermopolis experiences gradually decreasing cloud cover, with the percentage of time that the sky is overcast or mostly cloudy decreasing from 50% to 43%.

The clearest day of the month is May 31, with clear, mostly clear, or partly cloudy conditions 57% of the time.

For reference, on March 20, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 54%, while on July 27, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 77%.

Cloud Cover Categories in May

Cloud Cover Categories in May in Thermopolis1815222911223344556677889910101111121213131414151516161717181819192020212122222323242425252626272728282929303031310%100%10%90%20%80%30%70%40%60%50%50%60%40%70%30%80%20%90%10%100%0%AprJunMay 150%May 150%May 3157%May 3157%May 1151%May 1151%May 2153%May 2153%NowNowclearmostly clearpartly cloudymostly cloudyovercast
0% clear 20% mostly clear 40% partly cloudy 60% mostly cloudy 80% overcast 100%
The percentage of time spent in each cloud cover band, categorized by the percentage of the sky covered by clouds.


A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Thermopolis, the chance of a wet day over the course of May is gradually increasing, starting the month at 19% and ending it at 21%.

For reference, the year's highest daily chance of a wet day is 22% on May 23, and its lowest chance is 3% on December 20.

Over the course of May in Thermopolis, the chance of a day with only rain increases from 17% to 21%, the chance of a day with mixed snow and rain remains an essentially constant 1% throughout, and the chance of a day with only snow remains an essentially constant 0% throughout.

Probability of Precipitation in May

The percentage of days in which various types of precipitation are observed, excluding trace quantities: rain alone, snow alone, and mixed (both rain and snow fell in the same day).


To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.

The average sliding 31-day rainfall during May in Thermopolis is essentially constant, remaining about 1.3 inches throughout, and rarely exceeding 3.2 inches or falling below 0.3 inches.

The highest average 31-day accumulation is 1.5 inches on May 22.

Average Monthly Rainfall in May

The average rainfall (solid line) accumulated over the course of a sliding 31-day period centered on the day in question, with 25th to 75th and 10th to 90th percentile bands. The thin dotted line is the corresponding average liquid-equivalent snowfall.


Over the course of May in Thermopolis, the length of the day is rapidly increasing. From the start to the end of the month, the length of the day increases by 1 hour, 2 minutes, implying an average daily increase of 2 minutes, 4 seconds, and weekly increase of 14 minutes, 29 seconds.

The shortest day of the month is May 1, with 14 hours, 10 minutes of daylight and the longest day is May 31, with 15 hours, 12 minutes of daylight.

Hours of Daylight and Twilight in May

Hours of Daylight and Twilight in May in Thermopolis1815222911223344556677889910101111121213131414151516161717181819192020212122222323242425252626272728282929303031310 hr24 hr4 hr20 hr8 hr16 hr12 hr12 hr16 hr8 hr20 hr4 hr24 hr0 hrAprJunnightnightdaydayMay 114 hr, 10 minMay 114 hr, 10 minMay 3115 hr, 12 minMay 3115 hr, 12 minNowNow
The number of hours during which the Sun is visible (black line). From bottom (most yellow) to top (most gray), the color bands indicate: full daylight, twilight (civil, nautical, and astronomical), and full night.

The latest sunrise of the month in Thermopolis is 6:05 AM on May 1 and the earliest sunrise is 31 minutes earlier at 5:34 AM on May 31.

The earliest sunset is 8:15 PM on May 1 and the latest sunset is 31 minutes later at 8:46 PM on May 31.

Daylight saving time is observed in Thermopolis during 2019, but it neither starts nor ends during May, so the entire month is in daylight saving time.

For reference, on June 21, the longest day of the year, the Sun rises at 5:31 AM and sets 15 hours, 27 minutes later, at 8:57 PM, while on December 22, the shortest day of the year, it rises at 7:43 AM and sets 8 hours, 56 minutes later, at 4:39 PM.

Sunrise & Sunset with Twilight in May

Sunrise & Sunset with Twilight in May in Thermopolis1815222911223344556677889910101111121213131414151516161717181819192020212122222323242425252626272728282929303031312 AM4 AM6 AM8 AM10 AM12 PM2 PM4 PM6 PM8 PM10 PM12 AMAprJun5:34 AM5:34 AMMay 318:46 PMMay 318:46 PM6:05 AM6:05 AMMay 18:15 PMMay 18:15 PM5:46 AM5:46 AMMay 168:32 PMMay 168:32 PMSolarMidnightSolarMidnightSolarNoonSunriseSunsetNowNow
The solar day over the course of May. From bottom to top, the black lines are the previous solar midnight, sunrise, solar noon, sunset, and the next solar midnight. The day, twilights (civil, nautical, and astronomical), and night are indicated by the color bands from yellow to gray.


We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.

The chance that a given day will be muggy in Thermopolis is essentially constant during May, remaining around 0% throughout.

Humidity Comfort Levels in May

dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
The percentage of time spent at various humidity comfort levels, categorized by dew point.


This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.

The average hourly wind speed in Thermopolis is gradually decreasing during May, decreasing from 8.2 miles per hour to 7.5 miles per hour over the course of the month.

For reference, on March 26, the windiest day of the year, the daily average wind speed is 8.9 miles per hour, while on August 6, the calmest day of the year, the daily average wind speed is 6.3 miles per hour.

Average Wind Speed in May

The average of mean hourly wind speeds (dark gray line), with 25th to 75th and 10th to 90th percentile bands.

The hourly average wind direction in Thermopolis throughout May is predominantly from the north, with a peak proportion of 35% on May 1.

Wind Direction in May

Wind Direction in May in ThermopolisWNW1815222911223344556677889910101111121213131414151516161717181819192020212122222323242425252626272728282929303031310%100%20%80%40%60%60%40%80%20%100%0%AprJunNowNowwestsouthnortheast
The percentage of hours in which the mean wind direction is from each of the four cardinal wind directions, excluding hours in which the mean wind speed is less than 1.0 mph. The lightly tinted areas at the boundaries are the percentage of hours spent in the implied intermediate directions (northeast, southeast, southwest, and northwest).

Growing Season

Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).

The growing season in Thermopolis typically lasts for 5.1 months (156 days), from around May 4 to around October 7, rarely starting before April 15 or after May 21, and rarely ending before September 17 or after October 26.

During May in Thermopolis, the chance that a given day is within the growing season is very rapidly increasing rising from 42% to 99% over the course of the month.

Time Spent in Various Temperature Bands and the Growing Season in May

Time Spent in Various Temperature Bands and the Growing Season in May in Thermopolisgrowing season1815222911223344556677889910101111121213131414151516161717181819192020212122222323242425252626272728282929303031310%100%10%90%20%80%30%70%40%60%50%50%60%40%70%30%80%20%90%10%100%0%AprJunMay 142%May 142%99%May 3199%May 3168%May 1168%May 1190%May 2190%May 21Apr 1510%Apr 1510%NowNowfreezingvery coldcoldcoolcomfortablewarmhot
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
The percentage of time spent in various temperature bands. The black line is the percentage chance that a given day is within the growing season.

Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.

The average accumulated growing degree days in Thermopolis are increasing during May, increasing by 234°F, from 137°F to 370°F, over the course of the month.

Growing Degree Days in May

The average growing degree days accumulated over the course of May, with 25th to 75th and 10th to 90th percentile bands.

Solar Energy

This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.

The average daily incident shortwave solar energy in Thermopolis is increasing during May, rising by 1.0 kWh, from 6.4 kWh to 7.4 kWh, over the course of the month.

Average Daily Incident Shortwave Solar Energy in May

The average daily shortwave solar energy reaching the ground per square meter (orange line), with 25th to 75th and 10th to 90th percentile bands.


For the purposes of this report, the geographical coordinates of Thermopolis are 43.646 deg latitude, -108.212 deg longitude, and 4,587 ft elevation.

The topography within 2 miles of Thermopolis contains significant variations in elevation, with a maximum elevation change of 669 feet and an average elevation above sea level of 4,503 feet. Within 10 miles contains significant variations in elevation (2,605 feet). Within 50 miles contains large variations in elevation (8,038 feet).

The area within 2 miles of Thermopolis is covered by shrubs (71%) and grassland (19%), within 10 miles by shrubs (58%) and grassland (34%), and within 50 miles by shrubs (72%) and grassland (20%).

Data Sources

This report illustrates the typical weather in Thermopolis year round, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.

Temperature and Dew Point

There are 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Thermopolis.

For each station, the records are corrected for the elevation difference between that station and Thermopolis according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.

The estimated value at Thermopolis is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Thermopolis and a given station.

The stations contributing to this reconstruction are: Boysen Thermopol (24%, 25 kilometers, southwest); Worland Municipal Airport (67%, 41 kilometers, northeast); and Casper–Natrona County International Airport (9%, 164 kilometers, southeast).

Other Data

All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Tables of the Sun, Moon and Planets , by Jean Meeus.

All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.

Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.

Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.

Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .

Time zones for aiports and weather stations are provided by AskGeo.com .

Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.


The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.

We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.

We further caution that our travel scores are only as good as the data that underpin them, that weather conditions at any given location and time are unpredictable and variable, and that the definition of the scores reflects a particular set of preferences that may not agree with those of any particular reader.