Fog is made of tiny water droplets suspended near the ground, while haze is made of dry particles like dust, smoke, or pollution hanging in the air. The simplest way to tell them apart: fog forms when humidity is 95% or higher, and haze occurs when humidity is below that threshold. They look different, form through different processes, and have very different implications for your health.
What Each One Is Made Of
Fog is essentially a cloud sitting at ground level. It consists of microscopic water droplets (or occasionally ice crystals in very cold conditions) suspended in the air. Because these droplets scatter light in all directions, fog appears white or gray and can reduce visibility to near zero in dense patches.
Haze is a different phenomenon entirely. Rather than water droplets, haze consists of tiny solid or liquid aerosol particles: dust, smoke, pollutants, or chemical residues. These particles scatter light in a way that gives haze its characteristic brownish, yellowish, or bluish tint, depending on the type and concentration of particles involved. NASA researchers have noted that haze can also form as a secondary effect of fog. When fog droplets evaporate, they leave behind residue from pollutants like sulfur oxides and nitrogen oxides that were absorbed into the water. Those leftover particles then linger in the air as haze.
How They Form
Fog forms when air near the ground cools to its dew point, the temperature at which water vapor condenses into droplets. This happens most commonly on clear nights when the ground radiates heat away quickly, cooling the air just above it. It also forms when warm, moist air moves over a cold surface (like a cold ocean current) or when moisture evaporates from a warm body of water into cooler air above it. The key ingredient is always moisture: relative humidity needs to reach at least 95% for fog to develop.
Haze builds up through a completely different mechanism. It’s driven by the accumulation of particles rather than the condensation of water. Natural sources include wildfire smoke, volcanic emissions, and wind-blown dust. Human sources include vehicle exhaust, industrial emissions, and agricultural burning. These particles accumulate when weather conditions trap them near the surface, particularly during temperature inversions, where a layer of warm air sits on top of cooler air and acts like a lid. Weak winds and high atmospheric pressure make it worse by preventing the particles from dispersing horizontally or vertically.
How to Tell Them Apart by Looking
Fog is thick and opaque. When you’re inside a fog bank, everything around you looks washed out in a uniform white or light gray. Objects disappear gradually as distance increases. Fog tends to form in patches and can be localized, hugging valleys, coastlines, or low-lying areas.
Haze is thinner and more translucent. You can usually still see objects through it, but they appear washed out or discolored. Distant mountains or city skylines take on a milky, brownish, or bluish cast. Unlike fog, which often burns off within hours as the sun warms the ground, haze can persist for days or even weeks when weather patterns keep trapping particles near the surface.
Humidity is the most reliable indicator if you’re unsure. Fog feels damp on your skin and can leave moisture on surfaces. Haze feels dry. If you’re looking at a weather report, anything listed with relative humidity above 95% and visibility below 1,000 meters (about 3,300 feet) is fog. Between 95% humidity and visibility above 1,000 meters, it’s classified as mist. Below 95% humidity, it’s haze.
Visibility and Aviation Standards
Both fog and haze reduce visibility, but fog is far more disruptive. In aviation weather reports, which follow international standards set by the ICAO, fog is reported when visibility drops below five-eighths of a mile (roughly 1 kilometer). Haze gets its own separate reporting code because it poses a different kind of visibility challenge: less severe but often more widespread and persistent.
For pilots, fog can create “instrument flight rules” conditions where flying by visual reference becomes impossible. Haze more commonly creates “marginal visual flight rules” conditions, where visibility is reduced but not eliminated. For drivers, the distinction matters too. Fog can cut visibility to a few car lengths with almost no warning, especially in patches. Haze degrades visibility gradually and predictably, making it easier to adjust.
Health Effects of Haze vs. Fog
Pure fog, made only of water droplets, poses no direct health risk beyond the accidents it can cause from poor visibility. However, fog in polluted areas can actually concentrate harmful chemicals. As fog droplets form, they absorb pollutant gases from the surrounding air. These gases undergo chemical reactions inside the droplets, producing sulfates, nitrates, and other harmful compounds at a faster rate than they would in open air. When that fog lifts, it can leave behind a residual haze that’s more toxic than the original pollution.
Haze, particularly in urban and industrial areas, is a direct health concern. The particles that create haze often include PM2.5, particles smaller than 2.5 micrometers in diameter that are small enough to pass deep into your lungs and even enter your bloodstream. Prolonged exposure to haze-level pollution is linked to respiratory illness, cardiovascular problems, and worsened asthma symptoms.
Air quality indexes provide a practical guide for haze conditions. When the AQI reads 0 to 50, air quality is excellent and outdoor activity is safe for everyone. At 51 to 100, most people are fine, though those sensitive to pollution may notice symptoms. Between 101 and 150, sensitive groups should reduce time outdoors. At 151 to 200, everyone should start limiting outdoor exertion. Above 200, outdoor activity becomes inadvisable for most people, and above 300, staying indoors is recommended for everyone.
When Fog and Haze Overlap
In practice, fog and haze aren’t always neatly separated. In heavily polluted regions, the two frequently combine. Pollutant particles in the air serve as condensation nuclei, tiny seeds that water vapor clings to when forming droplets. This means polluted air can actually produce fog more easily than clean air, and the resulting fog is dirtier. Researchers studying winter episodes in industrial cities have documented cycles where haze transitions into fog as humidity rises overnight, then fog evaporates back into haze during the day, each cycle concentrating pollutants further.
This fog-haze cycle is most common in late fall and winter, when temperature inversions are strongest and sunlight is too weak to break them up quickly. If you live in an area where morning fog lifts to reveal a persistent brownish sky, you’re likely seeing this process in action. The fog itself isn’t the main concern in these situations. It’s the particulate load it leaves behind.