Bay Area weather reports sometimes show humidity numbers that sound alarming, such as 90 percent on a summer morning. Yet the air feels nothing like 90 percent humidity in Houston or Miami. The fog is real and the air is damp, but it is not oppressive. During a Diablo wind event, the same reports may show 15 percent humidity, and the air feels parched enough to crack furniture, dry out sinuses and wilt plants. The number that explains both experiences is the dew point, which says more about how the air will feel than relative humidity does.
Relative Humidity vs. Dew Point
Relative humidity compares the moisture in the air with the most the air could hold at its current temperature. A reading of 90 percent means the air holds 90 percent of its maximum. The catch is that the maximum changes a great deal with temperature, because warm air can hold far more moisture than cold air. So 90 percent humidity at 55 degrees holds much less actual water than 90 percent humidity at 85 degrees.
The dew point works differently, as the temperature the air would have to cool to before its moisture starts to condense. At that point relative humidity reaches 100 percent, and fog or dew forms. Because it does not depend on the current air temperature, it is a direct measure of how much moisture the air holds. A dew point of 65°F always means a lot of moisture, and a dew point of 35°F always means dry air. People feel the dew point more than the humidity percentage. Dew points above 65°F feel muggy at any temperature, while dew points below 45°F feel noticeably dry.
Bay Area Dew Points by Season
The Bay Area's marine layer combines high relative humidity with modest dew points. A summer morning at 55 degrees and 90 percent humidity has a dew point of about 52°F. That feels cool and damp, but it is far below the 65°F dew point where air starts to feel muggy. Bay Area fog feels refreshing rather than heavy because the air is nearly saturated with a small amount of moisture at a cool temperature. It is not saturated with a large amount of moisture at a warm temperature, the way Gulf Coast air is.
During Diablo wind events, the picture reverses as hot, dry air sinks from the interior and warms, and relative humidity can fall to 10 or 15 percent. The dew point drops too, to 20 or 25°F and sometimes lower. Air that dry shrinks and cracks wood furniture, dries out contact lenses, brings on nosebleeds and leaves vegetation ready to burn. A dew point of 30°F is far more drying than one of 45°F, and the Bay Area reaches that extreme during fall wind events.
Why Summer Bay Area Doesn't Feel Humid
The Bay Area is surrounded by water and has fog on many summer mornings, yet summer afternoons rarely feel humid. By midday, temperatures reach the 60s or 70s on the coast and the 80s or 90s inland. As the air warms, the same amount of moisture becomes a smaller share of what the air could hold, so relative humidity drops. The dew point stays fairly steady through the day, often around 50 to 55°F near the coast and a bit lower inland. In an inland valley at 90 degrees, a 52°F dew point works out to about 25 percent humidity. That feels dry and comfortable, nothing like a muggy afternoon in the Southeast.
On a classic Bay Area summer day, a cool, foggy morning gives way to a warm, dry afternoon. Even when that afternoon turns hot, it rarely feels sticky. The secret is the low dew point, which stays low because the fog forms over cold water. Ninety degrees with a 75°F dew point in Miami feels suffocating. Ninety degrees with a 52°F dew point in Walnut Creek feels like a perfect summer afternoon.
Deep Dive: How Is Dew Point Calculated, and Why Does It Predict Comfort?
Dew point comes from a single curve, the amount of water vapor air can hold at each temperature. Weather instruments measure temperature and relative humidity, and a short formula based on that curve turns them into a dew point. The same curve explains why dew point, not humidity, tells you how a day will feel.
What Formula Turns Humidity Into Dew Point?
The most vapor air can hold rises steeply with temperature, by roughly 7 percent for every degree Celsius. That relationship comes from a law of physics called the Clausius-Clapeyron equation. Forecasters use a simpler fit to it called the Magnus formula. Oleg Alduchov and Robert Eskridge refined its constants in a 1996 paper in the Journal of Applied Meteorology.
To find the dew point, the formula first works out how much vapor the air actually holds. It then runs the curve backward to find the temperature at which that amount would fill the air completely. By our calculation, a typical foggy-season afternoon in San Francisco, 60°F with 80 percent relative humidity, has a dew point of about 54°F. Cool that air by just 6 degrees, as happens over the cold ocean, and fog forms.
The same math shows why relative humidity can mislead. On a 90°F inland afternoon, air with the same 54°F dew point has a relative humidity of only about 29 percent. The air holds exactly the same water in both places. Only the temperature has changed.
Why Does Dew Point Predict How Sticky a Day Feels?
Your body cools itself by evaporating sweat, and evaporation depends on the difference in vapor between your skin and the air. Skin sits near 95°F when you are warm, and the thin film of air against wet skin is close to saturated. The surrounding air's dew point sets how much vapor it already holds.
By our calculation, air with a dew point of 54°F holds about a quarter of the vapor found next to warm, wet skin. Sweat evaporates easily into it. Air with a dew point of 70°F, common in the Southeast in summer, holds about 45 percent. The drive that pulls sweat off your skin is roughly a quarter weaker, so you stay wetter and feel hotter at the same temperature.
That is why Bay Area summers rarely feel muggy, even on hot inland days. Air reaching the region comes off a cold ocean, which keeps dew points mostly in the 50s. The rare sticky days usually come when tropical moisture drifts in from the south and pushes dew points into the 60s.
