Many visitors to the Bay Area make the same mistake in July. The forecast says 60°F and sunny, so they dress for a mild summer day. An hour later, standing at Ocean Beach or halfway across the Golden Gate Bridge, they are shivering. The temperature has barely changed, but the wind has picked up. Wind steals heat from the body quickly, and along the coast it blows hardest on summer afternoons. That is why a breezy afternoon in San Francisco can feel far colder than the number on the thermometer. It also explains why the same temperature feels so different in Berkeley and on the Marin Headlands.
How Wind Chill Works
Your body is always giving off heat, and on a calm day a thin layer of warm air forms right next to your skin. That layer works like a light blanket until wind blows it away and replaces it with cooler air. Your body then has to keep producing heat to stay warm. The stronger the wind, the faster that warm layer disappears and the colder you feel.
Wind chill is a way to put a number on that effect. The National Weather Service calculates it from the air temperature and the wind speed. The result is the temperature that would cause the same heat loss from bare skin on a calm day. The formula is meant for temperatures of 50°F or lower with winds of at least 3 mph. Above 50°F, the wind still makes you feel cooler, but the effect is smaller and the official formula does not apply.
In most of the country, wind chill is a winter concern. In the Bay Area, temperatures near 50°F are common on summer mornings and evenings along the coast. Here the wind matters in July in a way it does not in Sacramento or Los Angeles.
The table below shows wind chill values from the Weather Service formula at a few temperatures and wind speeds that are common along the Bay Area coast.
| Air temperature | 10 mph wind | 20 mph wind | 30 mph wind |
|---|---|---|---|
| 50°F | 46°F | 44°F | 42°F |
| 45°F | 40°F | 37°F | 35°F |
| 40°F | 34°F | 30°F | 28°F |
Bay Area Wind Chill by Location
Some of the Bay Area's windiest places are also its most visited. The Golden Gate Bridge, the Marin Headlands and the shoreline at Crissy Field often see afternoon winds of 15 to 25 mph in summer, with stronger gusts. Ocean Beach catches the full ocean breeze, and its sand can sting your legs on the windiest days. Point Reyes, especially near the lighthouse, is one of the windiest spots on the Pacific coast.
Those places feel coldest in the evening, when the sun goes down and the air drops toward 50°F while the wind is still blowing. At that point, the table above starts to apply. A 50°F evening with a 20 mph breeze on the bridge has a wind chill near 44°F, which feels much more like winter than summer.
Good timing helps you avoid the worst of the chill. Along the coast, the summer wind is usually lightest in the early morning and strongest from mid-afternoon into the evening. If you want to walk the bridge or the beach in comfort, go before noon, when the air may be just as cool but much calmer. Late afternoon is the classic time for visitors to get caught out, because the wind is at its peak and the fog is often rolling back in.
The contrast with the inland East Bay is striking. On the same afternoon that San Francisco sits in the upper 50s with a brisk sea breeze, Walnut Creek may be near 90°F with only a light wind. The marine air keeps the coast cool and windy, while the inland valleys are hot and only a little breezy by late afternoon. The thermometer may show a 30-degree difference, and the wind on the coast makes the gap feel even wider.
Inland towns get wind chill too, mostly in winter. On a 42°F winter afternoon in Livermore with a 20 mph wind, the formula gives a wind chill in the low 30s.
Dressing for Bay Area Wind Chill
The classic Bay Area advice to always carry a jacket is really advice about wind. A thin layer that blocks the wind often does more good than a thick sweater that lets air through. The windproof layer keeps the warm air next to your body in place. A windbreaker over a T-shirt can feel warmer at Ocean Beach than a heavy knit sweater on its own.
Fog adds to the chill in a different way. The official formula does not include humidity, but fog drip slowly dampens your hair, skin and clothes. Wet fabric loses much of its insulation, and the moisture on your skin cools you as the wind evaporates it. Fog also blocks the sun, which would otherwise warm you. A foggy 58°F afternoon with a strong breeze can therefore feel much colder than a sunny 58°F afternoon with the same wind.
Choose fabrics with fog in mind, since cotton soaks up moisture and stays cold, while wool and synthetic layers keep more of their warmth when damp. A hat and a light pair of gloves can make an evening walk across the bridge far more pleasant, even in August.
Wind Chill and Bay Area Microclimates
Wind is one reason Bay Area microclimates feel even more different than temperature data suggest. Two neighborhoods at the same temperature can feel nothing alike if one is sheltered from the wind and the other is not.
The Mission District in San Francisco shows this well. It sits in the shelter of Twin Peaks, which blocks much of the fog and wind. On a summer afternoon it often runs 10 to 12 degrees warmer than the Sunset District, and with less wind, the difference you feel is even larger.
Weather apps show air temperature and, sometimes, a "feels like" number, but your comfort depends on exactly where you stand. If you find a spot out of the wind, such as behind a building, a hill or a row of trees, the same afternoon can feel noticeably warmer.
Deep Dive: How Was the Wind Chill Formula Built?
The wind chill numbers on today's forecasts come from a model of heat loss from the human face. That model was tested on volunteers in a cold wind tunnel and adopted by the United States and Canada in 2001. It replaced an older index from the 1940s that made windy days sound much colder than they felt.
Where Did the Original Wind Chill Index Come From?
The first wind chill index came from Paul Siple and Charles Passel, who worked in Antarctica in the early 1940s. They measured how fast a plastic cylinder of water froze in different combinations of wind and cold, and published their results in 1945. Weather services used versions of their index for decades.
Researchers later concluded that the old index exaggerated the effect of wind. A container of water is not much like a person, and the index produced wind chill values that were too cold, especially at high wind speeds.
How Was the Modern Formula Tested?
In 2000, a joint U.S. and Canadian group of meteorologists and scientists set out to replace the old index. They chose an approach by Randall Osczevski of Defence Research and Development Canada and Maurice Bluestein of Indiana University-Purdue University Indianapolis. It modeled heat flowing out through the skin of an adult face, the part of the body most often left bare in the cold.
According to the National Weather Service, the team tested the model in early summer 2001 at the Defence and Civil Institute of Environmental Medicine in Toronto. Twelve volunteers, six men and six women, walked at 3 mph on a treadmill inside a chilled wind tunnel. Sensors on their cheeks, forehead, nose and chin measured heat flow. Each person completed four 90-minute sessions at different winds and temperatures.
The final formula assumes a person walking into the wind, with that walking speed added to the measured wind. It also adjusts the wind from the standard measuring height of 33 feet down to about 5 feet, the height of a typical face. In the formula, wind enters only through a small power of its speed, so each extra mile per hour adds less chill than the last. By our calculation, going from 10 to 20 mph at 50°F lowers the wind chill by about 2 degrees, matching the table on this page.
The formula also has limits worth knowing. It assumes no sunshine, so a bright, windy afternoon on the coast usually feels warmer than its wind chill suggests. It also leaves out humidity and wet clothing, which can make a foggy, windy evening feel colder than the number shows.
