A mist droplet is one of the tiny drops of water that hang in the air as mist. Most are between about 4 and 20 micrometers across, and a typical droplet is about 10 micrometers, a hundredth of a millimeter. A micrometer is a thousandth of a millimeter.

Size

Mist and fog are made of the same thing: water droplets so small that they stay suspended in the air instead of falling. The World Meteorological Organization (WMO) tells them apart by how far an observer can see. In fog, visibility at the ground is less than 1 kilometer (0.6 miles), and in mist it is 1 kilometer or more. In practice, mist is light fog.

Scientists measure the droplets with instruments that count them and sort them by size. In 1992, in a study for NASA and the U.S. Federal Aviation Administration, an aircraft flew down through fogs at five places in the United States. Near the ground, the most common droplets were 4–8 micrometers across, and higher up, 12–20 micrometers.

A 2022 study of 42 fogs near Paris found droplets most often around 11 micrometers across. Some fogs also had a second group of larger droplets, around 22 micrometers.

Size comparisons

A typical mist droplet, about 10 micrometers across, is a little wider than a red blood cell, which is about 7.5 micrometers across.
A thou, one-thousandth of an inch, is exactly 25.4 micrometers, the width of about two and a half typical droplets.
About 10 typical droplets side by side would span the width of a human hair, about 0.1 millimeters.
About 100 typical droplets side by side would span 1 millimeter, the smallest mark on a metric ruler.
A drizzle drop can be up to about 0.5 millimeters across. A drop that size is 50 times as wide as a typical mist droplet and holds about 125,000 times as much water.

Mist, fog and drizzle

According to the WMO, mist usually forms a thin, grayish veil, while fog forms a whitish veil over the landscape. When light shines through fog, single droplets can often be seen with the naked eye, drifting in swirls.

Drizzle is different, because it falls. It is precipitation made of very fine drops, close together, falling from a low layer of cloud. Its drops are usually smaller than 0.5 millimeters across. The drops seem almost to float, so even slight movements of the air show up in them.

How mist forms

Mist forms when moist air cools until it is saturated, holding as much water vapor as it can. Radiation fog forms when the ground loses heat, usually at night, and chills the layer of air closest to it. Advection fog forms when warm, moist air moves over a cooler surface, such as a cold sea.

Each droplet starts on a tiny particle floating in the air, called an aerosol particle, as water vapor condenses onto it. In one fog near Paris, the most common aerosol particles were about 0.4 micrometers across, and the most common droplets about 11 micrometers. Droplets can also grow by bumping into each other and merging.

Sources

Fog compared with Mist. In International Cloud Atlas. World Meteorological Organization, 2017.
Fog. In International Cloud Atlas. World Meteorological Organization, 2017.
Drizzle. In International Cloud Atlas. World Meteorological Organization, 2017.
Radiation fog. In International Cloud Atlas. World Meteorological Organization, 2017.
Advection fog. In International Cloud Atlas. World Meteorological Organization, 2017.
Drop Size Distributions and Related Properties of Fog for Five Locations Measured From Aircraft. Zak, NASA Contractor Report 4585, 1994.
Experimental study on the evolution of droplet size distribution during the fog life cycle. Mazoyer et al., Atmospheric Chemistry and Physics, 2022.
18.3 Erythrocytes. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.
NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. National Institute of Standards and Technology.
Just How Small Is "Nano"?. National Nanotechnology Coordination Office.

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