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San Francisco summers are getting hotter—and it’s dangerous

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Increased overnight lows might not seem scary, but they represent a health risk for the most vulnerable

Sunlight reflecting on a hazy fog bank over San Francisco’s skyline. Photo by Shutterstock

Mark Twain never said that oft-repeated aphorism about San Francisco summers being a cold winter, but whoever did might have to readjust their standards if the local climate keeps changing.

Climate Central, a self-identified “independent organization of leading scientists and journalists,” published data this week about rising temperatures in major U.S. cities over the past 48 years, with some special attention paid to unexpected variables.

Rather than examine record highs during the days, Climate Central looked at average low temperatures on summer nights.

For example, in 1970 the average low for San Francisco was a little over 53 degrees across summer months. By 2018, that figure was up to a little over 56 degrees.

National Weather Service meteorologist Spencer Tangen tells SFGate a more precise measurement is closer to 2.7 degrees during that period. Both sources cite NOAA data.

Why does this matter?

In climate terms, three degrees is a lot—as Vox explains, when discussing the larger environmental effects of climate change, even fractions of degrees represent huge, often catastrophic shifts in proportion simply because of the volume of space involved.

More importantly, increasing overnight temperatures, while not as obviously worrisome as daytime highs, can be extremely dangerous, because the relief of nighttime temperature drops is the only thing that makes hot spells survivable for many people.

The Weather Channel warned during July’s nationwide heat waves:

Warm nighttime temperatures, especially 80 degrees or warmer, do not allow people to recover from daytime heat. When warm low temperatures are combined with high humidity, conditions can become dangerous, if not deadly, even in the middle of the night.

[...] Over time, heat exhaustion can set in and, if not treated, heat stroke can develop. The elderly, children under four years old, those living in homes without air conditioners and people with chronic disease are at the highest risk for heat stroke, which can damage the brain, heart, kidneys and other muscles.

When temperatures rise, human bodies produce sweat, which provides relief as it evaporates—anyone who has stood in front of a fan on a hot day knows the feeling.

But when humidity rises, sweat evaporates more slowly, and the body’s simple cooling system may become backed up. If this goes on for long enough, illness or even death could result.

The fact that victims may be asleep and not aware of their worsening condition during the night only increases the potential danger.

Of course, San Franciscans are not at risk when nighttime temperatures hit 56 degrees. But the higher the overnight “minimum” gets, the more risky nighttime conditions may become.

For the curious, this weekend’s forecast, according to the National Weather Service, expects overnight lows of between 57 and 60 degrees in San Francisco. Daytime temperatures are expected to max out around 72 degrees.