Lehman Caves Temp and Humidity before, during, and after Closure

This article was originally published in The Midden – Great Basin National Park: Vol. 21, No. 2, Winter 2021.

By Gretchen Baker, Ecologist

Lehman Caves was closed to the public for 427 days due to COVID, from March 19, 2020 to May 22, 2021. This may have been the longest closure since the cave was opened for tours since 1885. (In 1944, during World War II, there were only 872 visitors for the year, so the cave was likely closed for a few months.)

Figure 1. Electrical relay panel in the Doghouse, Lehman Caves, site for the datalogger recordings in this article.

NPS Photo by Gretchen Baker

Although this closure wasn’t so good from the perspective of sharing one of the Park’s great resources with the public, it did allow for some unique data collection in the cave (Figure 1). We’ve never really had a chance to collect baseline cave climate data, because it has people going in it nearly every day of the year, year after year.

Temperature and humidity dataloggers were installed in several locations in the cave in the summer of 2019 as part of the cave lighting project. They recorded hourly temperature and relative humidity data.

During normal operations (pre-closure), the data showed daily fluctuations (Figure 2). The temperature next to the Doghouse relay station goes from about 52°F to 60°F every day. Humidity drops from near 100% to less than 80% (and sometimes less than 70%) every day. Why? When the electricity to the cave is turned on, it goes to a main transformer at the Giant’s Ear and then goes to four relay panels. A small heater is installed in each relay panel 8 The Midden to help keep the electronics in it dry. So heat is being added to the cave not only due to lightbulbs and human bodies, but also due to these heaters.

Figure 2. Temperature and humidity in Lehman Caves at the Doghouse during “normal” conditions, from September 2019 to December 2019. Note the daily fluctuations in temperature (from about 52 to 60 degrees F) and relative humidity (from near 100% to below 80%) due to the lights on in the

If you take a closer look at Figure 2, you can see that the fluctuations are bigger on the left-side of the graph than the right-side. Can you guess why? The answer lies in the x-axis, the date. On November 1, the number of cave tours goes from 4 to 1, so the lights are left on for less time. This results in smaller temperature and humidity fluctuations.

One last thing to point out on this graph. What happened at the red arrow? The temperature stays high and the humidity low. If you guess that the lights were left on over night, you are correct. It has a big impact on the cave. What about at the purple arrow, where temperature stays low and humidity high? That’s Thanksgiving, one of the three days of the year that the cave is normally closed.

Let’s move now to 2020, when the cave closed. We’re going to specifically look at March through December (Figure 3). First, check out the blue line, the temperature. It stays nice and flat for most of the time period. The relative humidity, the orange line, stays at 100% for a few months. The little blips you see on the right are when the lights were turned on to do cultural resource surveys and quarterly biomonitoring. The stretch from March to October with the lights out provided time for the cave to settle into what we might call its baseline, where it’s not being impacted from tours or infrastructure.

Figure 3. Lehman Caves temperature and humidity at the Doghouse relay panel during cave closure from March to December, 2020. Note how both temperature and relative humidity stay near constant.

Finally, let’s look at what happened in 2021, when the cave reopened to fewer tours (Figure 4). In January and February, the temperature (blue line) is very stable at about 52°F. In March the lights were turned on for a few days for cultural resource work, and the temperature and humidity fluctuate more. Then the lights were off until May, when training occurred. Once regular tours started at the end of May, we saw the same pattern as before, with huge daily fluctuations of both temperature and humidity.

Figure 4. Temperate and humidity at the Doghouse, Lehman Caves from January-July 2021. It is easy to find when regular tours start at the end of May and the pre-closure trend returns

One other thing to note: if you look at the minimum temperature (blue line), once the lights are turned on daily for tours, do you see a trend? There’s a slight increase each day in the minimum (and maximum) temperatures. That means the cave temperature is never really recovering, it just keeps getting warmer over time.

Reducing these temperature and humidity fluxes is one of the goals of the new cave lighting system. Hopefully we can help restore a more natural cave climate by having the lights on less (only when we need them) and by eliminating extra heaters in the cave by using newer technology.