If you’re startled by a rattlesnake, blame the weather.

An analysis of 5,365 snakebites in California by Stanford University scientists found the number of rattlesnake bites increases after periods of rainy weather — but decreases after a drought.

“It is a very amazing impact that our climate has on snakes and snake-human interactions,” said Dr. Grant Lipman, an emergency medicine physician and long distance runner who conceived of the study after confronting a three-foot-long rattler on a trail above Stanford.

In California, five deaths were reported during the 20 years between 1997 to 2017, according to the study, published in the most recent issue of the journal Clinical Toxicology.

Men suffered more than three-quarters of the reported bites. The average age was 37 years. The vast majority of bites — 80 percent — occurred in yards at home.

Lipman, who routinely treats patients with venomous snakebites and serves as research director for the global ultrarunning competition Racing the Planet, sought to test the conventional wisdom: Snakebites are more common during prolonged dry weather.

“The prevailing understanding is that in a drought, you’ll see more snakes,” said Lipman, “because the hot dry weather forces them out of their regular terrain to forage for food — and they encounter humans.”

But the data surprised them. With computer scientist Caleb Phillips of University of Colorado-Boulder, Lipman collected and examined bite reports — all of them rattlesnakes — from every phone call made to the California Poison Control System from 1997 to 2017. Details included the date and time of the bite; the patient’s age and sex; where the bite occurred on the body; call site; treatment; and medical outcomes. Cases were also grouped by the callers’ ZIP codes to one of California’s 58 counties.

They found that the problem isn’t drought — it’s rain. Snakebite incidence decreased 4 percent following a drought but increased 4 percent following high levels of precipitation.

The incidence of bites peaked following the heavy rain and snowfall years of 2006 and 2011 and fell during two periods of extreme drought: 2002-05 and 2007-10. From 2015 to ’16, the most severe drought on record in California, the number of snakebites reached their nadir during the 20 years of the study, the researchers found.

Biologists say vegetation may be the link. Ample rains cause vegetation to increase, which is likely to give rise to mice and rats. California’s rattlesnake population varies depending on the size of these rodent colonies. During a drought, snakes face higher risks of dying from starvation and dehydration, according to naturalists.

This finding could help guide public health measures, such as determining the best allocation of antivenom supplies, especially if there is an increase in California’s extreme weather caused by climate change, Lipman said.

Rattler season stretches between April and October. The Stanford study found that the majority of bites occurred during the spring or summer — but that could also reflect the increase in outdoor activities during those months.

In April, a 79-year-old man hiking on Mount Tamalpais in the San Francisco Bay Area had to be airlifted to a hospital after picking up a juvenile rattlesnake on the trail and suffering three bites, with at least one on each hand. In May, a 15-year-old girl was bitten by a small rattlesnake near her home in nearby Contra Costa County.

California’s two most recent fatalities occurred in Southern California. William Price was bitten above the right ankle in 2010 while studying steelhead trout in a stream in the San Diego County town of Cuyamaca. Ross Cooke died in 2003 after stepping on a snake he mistook for a log in San Bernardino County’s Lyle Creek.

If you see a snake, Lipman urges staying two “snake lengths” away, and stamping your foot to scare them. Snakes can’t hear, but respond to vibration. If bitten, call 911 immediately, he said.

The greatest concentration of rattlesnakes in the U.S. are in the Southwest, but they are found in many open, rocky areas both in and outside of deserts and forests.