1. Looking Out the Window: Is the White Morning Scene Fog or Mist?
When you open the curtains in the morning, the town may look white and blurry, and distant buildings are only faintly visible. Some people call this scene fog, while others call it mist. In fact, both are caused mainly by tiny water particles floating in the air, and their formation is very similar [1]. So, what is the thing that separates them? Is it how white they look? Or is it how thick they are?
2. Visibility: Can You See 1 km Away?
The answer is "how far away you can see horizontally." This visible distance is called "visibility." The Japan Meteorological Agency decides that if visibility is less than 1 km, it is fog. If visibility is 1 km or more but less than 10 km, it is mist [1]. A children’s explanation from the same agency also uses 1 km as the boundary [2]. If you can see farther than 10 km, it is not called mist. One kilometer is a distance an adult can walk slowly in about 15 minutes. If you cannot see a building that far away, it is fog. This rule is not just for Japan. The World Meteorological Organization’s International Cloud Atlas also divides "fog" and "mist" at 1 km. If horizontal visibility at the surface is less than 1 km, it is fog. If it is not that low, it is mist [3].
3. Is Fog a Cloud That Has Come Down to the Ground?
Fog is explained as "a cloud touching the ground" [4]. Both clouds and fog are made of tiny water droplets (clouds may also include ice particles). They are called fog if they touch the ground, and clouds if they do not. This is a practical distinction. When you climb a mountain and enter a cloud, everything becomes white and visibility becomes poor. From below, it looks like a cloud. But to someone standing inside it, it feels just like fog. This helps you imagine it. Note that saying "fog equals cloud" means both are made of water droplets. The reason they have different names is where they are in the sky. This raises another question: why do clouds form high in the sky, but fog forms near the ground?
4. How Fog Forms: Clear, Calm Nights and Other Types
Fog often forms on clear, calm nights. If there are no clouds, the heat from the ground that warmed up during the day escapes into the air during the night. The ground and the air just above it cool down. When air cools, it cannot hold water vapor anymore, so it appears as tiny water particles. This type of fog is called "radiation fog." It tends to form from midnight to early morning and is common in basins and valleys [6]. Other types include "advection fog," which forms when warm air flows over cold ground or sea, and "steam fog," which forms when cold air touches a warm water surface, such as rivers or lakes in winter [6]. So, when fog gets thick, how far can you see?
5. Dense Fog: How Far Can You See in Very Thick Fog?
Fog that is especially thick is called "dense fog." It is defined as fog with visibility of about 100 meters (about 328 feet) or less on land, and 500 meters (about 1,640 feet) or less at sea [5]. 100 meters is one-tenth of 1 km. It is a state where you cannot see beyond twice the length of a 50-meter running track. This value is also used as a guideline for dense fog advisories. For example, in Hyogo Prefecture, advisories are issued when visibility is 100 meters or less on land, and 500 meters or less at sea [5]. Advisory standards differ by region, so this is just one example. The reason the standard is longer at sea is not written in the materials. You might imagine it is related to the speed of ships and the distance needed to stop, but we will leave that as speculation here.
6. Haze: Why It Is Not a Weather Term
When distant mountains look blurry in spring, people say "haze is hanging." However, haze is not a meteorological term and is not used in weather forecasts. It is a general term for when the air looks whitish because many tiny dust or smoke particles are floating in it [2]. Some private explanations say haze is when distant things look blurry due to fine particles like dust, pollen, or yellow sand [4]. In other words, while fog and mist are made of water particles, haze is mainly made of particles other than water. The Japan Meteorological Agency also has a term called "suspended dust" (enmū). This is a state where visibility is less than 10 km due to dry fine particles. The difference from mist is whether the particles are wet (water) or dry [1].
7. Investigate It Yourself: Measuring How Far You Can See
If you want to investigate further, you can do this today: Decide on the farthest building or mountain you can see from your window. Use a map to find out how many kilometers away it is. From the next day, keep a note of the days you can see that landmark and the days you cannot, and compare them. If you go out, try observing how far you can see on days when fog or mist appears. Prepare a white piece of paper and a gray piece of paper. Compare which color the scenery is closer to. Avoid roads and waterside areas; observe from a safe place. To read official information, look at the Japan Meteorological Agency’s forecast terms (terms related to visibility) and their Q&A for children. They explain the differences between fog, mist, suspended dust, and haze in short sentences.