Haze and fog both cut visibility and turn the sky gray or milky, but they are very different things. Fog is made of tiny liquid water droplets, and it forms when the air is nearly saturated with moisture. Haze is made of dry particles such as dust, smoke or pollution, and it can form in air that is far from saturated. In the Bay Area, both are common, so mixing them up is easy. The difference matters because marine fog is harmless to breathe, while haze can mean unhealthy air that should change your plans for exercise, driving and time outdoors.
What Is Fog?
Fog is simply a cloud at ground level. It forms when air cools to its dew point, the temperature at which the air can no longer hold all its water vapor as a gas. The extra vapor condenses into tiny droplets that hang in the air. Relative humidity inside fog is at or very near 100 percent. The droplets are too small to fall as rain, but they are big enough to scatter light and block the view.
The Bay Area has two main kinds of fog. Advection fog rolls in from the Pacific in summer and early fall, and radiation fog forms in inland valleys on clear winter nights. Both are made of water droplets, and both feel cool and damp. Both also clear when the air warms or the conditions that formed them change.
What Is Haze?
Haze is a dimming of the air caused by fine particles rather than water droplets. The particles can be dust, smoke, sea salt or pollution. Some form in the air itself, when exhaust and other pollutants react in sunlight to create fine particles. Unlike fog, haze does not need high humidity, and it can appear on dry, sunny days.
The Bay Area's worst haze comes from wildfire smoke. During fire season, smoke from fires in Northern California and the Sierra Nevada often drifts over the region. It can cut visibility to a few miles and push air quality into the unhealthy range at the same time. In November 2018, smoke from the Camp Fire hung over the Bay Area for about two weeks. On September 9, 2020, smoke high overhead turned the sky orange across the region for most of the day.
In winter, temperature inversions trap pollution near the ground during calm, cold spells, and haze can build for days. Much of that winter pollution is smoke from fireplaces and wood stoves, with vehicle exhaust adding to it. That is why the Bay Area Air Quality Management District issues winter Spare the Air alerts that ban wood burning. The haze is most common in inland valleys such as San Jose and the South Bay, where surrounding hills limit air movement on still days.
How to Tell Fog and Haze Apart
A few simple checks will usually tell you which one you are looking at.
Color is the quickest clue, because fog is white or light gray. Haze from smoke or pollution has a brown, yellow or orange tint, especially near a fire. The orange sky of a Bay Area smoke day looks nothing like the clean gray of marine fog.
Temperature and humidity are the next things to check. Fog forms in cool, humid air, usually when the temperature is within a few degrees of the dew point, while haze can hang in hot, dry air. If it is 85°F and the air feels dry but the view is still blurred, you are almost certainly looking at haze.
Your nose can help when smoke is the cause. Wildfire smoke has a distinct woody, acrid smell, while marine fog smells like the ocean. Pollution haze can carry a faint smell of exhaust or wood smoke, and a smell of smoke means you are not looking at clean fog.
Finally, check the Air Quality Index, which the Bay Area Air Quality Management District publishes in real time. If visibility is poor and the AQI is orange, red or purple, the cause is haze and the air is unhealthy. If the AQI is good and visibility is poor, you are almost certainly looking at fog.
Deep Dive: How Does Rising Humidity Turn Haze Into Fog?
Haze and fog are often the same particles at different stages. Many haze particles are salts that soak up water as the air grows more humid. They swell steadily, scattering more light, until the air reaches saturation and some of them grow into true fog droplets.
How Do Salt Particles Swell in Humid Air?
A dry salt crystal stays solid until the humidity crosses a threshold, and then it suddenly dissolves in water it pulls from the air. Chemists call that threshold the deliquescence point. Atmospheric chemistry references put it near 75 percent relative humidity for sea salt and near 80 percent for ammonium sulfate, a common pollution particle.
Once dissolved, the tiny droplet keeps taking up water as humidity rises, because dissolved salt lowers the vapor pressure at its surface. By about 95 percent humidity, such particles can grow to roughly twice their dry diameter. Larger particles scatter much more light, so a hazy sky can turn milky long before any fog forms.
That swelling explains a familiar Bay Area sight. On a humid morning, distant hills look washed out even when the air holds no more pollution than on a clear afternoon. As the air warms and humidity falls, the particles give up their water and the view sharpens again.
When Does a Haze Particle Become a Fog Droplet?
In 1936, the Swedish meteorologist Hilding Köhler described the balance that decides each droplet's fate. Two effects compete. Dissolved salt helps a droplet hold water, while the curved surface of a very small droplet makes water escape more easily.
Below saturation, the two effects settle into a stable size, and the particle stays a swollen haze droplet. Just above saturation, usually by a fraction of a percent, the largest particles pass a tipping point. They then grow freely into fog droplets many times their earlier size, while smaller particles stay behind as haze.
That is why fog and haze often appear together. On a smoky or polluted winter morning in the inland valleys, the air can hold a mix of swollen haze particles and true fog droplets. Visibility can then fall faster than either one alone would cause. The same physics explains why fog in polluted air often looks gray or brownish, not white.
Health and Activity Implications
Marine fog is essentially harmless to breathe, because it is just clean, very humid air, and exercising in it is fine. Its main risks are poor visibility on the road and the cold, damp feel that makes time outdoors uncomfortable. Inland winter fog is slightly different, because it can form under the same inversions that trap pollution. On foggy, still winter days, it is worth checking the AQI too.
Haze is a much more serious matter for your health. Fine particles from smoke and pollution reach deep into the lungs and can cause or worsen breathing and heart problems. Health agencies advise cutting back on hard outdoor exercise when the AQI reaches orange, which means unhealthy for sensitive groups, or higher. A Spare the Air alert is a good signal to check. During the worst smoke events, even healthy adults are told to limit time outside, wear an N95 mask outdoors and keep windows closed.
If you are planning to spend time outdoors and the sky looks gray or milky, take 30 seconds to check both the fog forecast and the AQI. Fog and haze can look alike at a glance, but they call for very different responses.
