In modern meteorology, visibility is the measure of a distance at which light or an object can be clearly seen.
The term “visibility” was first used by John Dalton in his 1801 paper on the composition and properties of air.
He defined it as: “the length of time that a body may be seen to move through the atmosphere”.
The definition has been modified over time to include other factors such as cloud cover, atmospheric conditions, etc.
Visibility measurements are typically performed with instruments called “visibility meters”.
These devices use various methods to determine how far objects (such as aircraft) can be seen from a given position.
Some of these methods involve measuring the intensity of reflected sunlight (or moonlight), while others simply record the number of times that an observer sees something moving past him/her.
In this guide, we’ll be taking a look at how visibility is measured. So, keep on reading to find out more!
How Do We Measure Visibility?
Visibility is measured in two ways:
1) Directly from the observer’s point of view (e.g., using a telescope). This is called direct visibility.
2) Indirectly from the observer’s perspective via reflections off clouds, water vapor, ice crystals, etc. This is called reflected visibility.
In both cases, the measurement is made relative to the observer’s position. For example, if you are standing next to a lake, then your visibility will be less than if you were standing on top of a mountain.
In direct visibility, the visibility is determined by measuring how long it takes for a light source to travel from one location to another.
If the light source is a plane flying directly overhead, this method is known as direct line-of-sight visibility.
It is also possible to measure the angle between the sun and the horizon, and calculate the corresponding visibility.
In reflected visibility, the visibility is calculated based on the amount of sunlight reflected back into the sky.
The amount of sunlight reflected depends on the size and shape of the particles suspended in the atmosphere.
Smaller particles scatter more light, so they reduce the amount of visible light reaching the ground. As the particles get larger, they reflect more light away from the viewer.
The result is that smaller particles tend to increase visibility, whereas large particles decrease visibility.
What Factors Affect Visibility?
Clouds affect visibility in several ways.
First, they block out part of the sky, reducing the amount of sunlight available to illuminate the surface below.
Second, they create their own shadows, which reduces the amount of sunlight falling on any particular area of the earth’s surface.
Third, clouds often produce rain or snow (see also “Can Planes Fly In Snow?“), which increases the amount of moisture in the air.
All three of these effects work together to reduce visibility.
The atmospheric conditions at the time of observation play a major role in determining visibility. In general, when there is little wind, visibility tends to be better.
When there are strong winds, visibility tends to be worse.
Wind speed and direction also have a big impact on visibility. Winds blowing toward the observer tend to reduce visibility, since they blow the small airborne particles closer to the ground.
Conversely, winds blowing away from the observer tend to improve visibility, since they push the tiny particles higher into the atmosphere where they become less likely to cause problems.
The type of weather also affects visibility. Fog and heavy precipitation can make things very difficult.
Snowfall and sleet can also reduce visibility because they form layers of ice on the ground.
Finally, high humidity can make visibility worse because it causes condensation on objects such as windows and windshield (see also “How To Clean Plexiglass Windows“)
Sunrise/sunset times also influence visibility.
Sunrise and sunset times vary throughout the year, but generally occur around 6:00 AM and 4:00 PM respectively.
When sunrise occurs before 7:00 AM, visibility is usually good. However, if sunrise occurs after 8:00 AM, visibility will typically be poor.
Similarly, sunset occurring before 5:00 PM will normally lead to good visibility. Sunset occurring later than 6:00 PM will most often result in reduced visibility.
What Unit Is Used To Measure Visibility?
MOR (or meteorological visibility) is expressed in either meters or kilometers. It can also be expressed in Statute Miles, but this depends on the country concerned.
Measurement range can also vary depending on the application of the measurement.
What Is Considered “Good” Visibility?
When reporting on visibility in an aeronautical context, in a take-off and landing report for example, this is typically done in meters up to and inclusive of 5000 m, and in kilometers above that value.
In many countries, the minimum requirement for pilots flying by instrument procedures is 500 m MOR.
This means that aircraft must be able to see at least 500 m ahead of them during approach and departure.
However, the actual visibility required by the pilot may differ greatly depending on the situation.
For instance, if the pilot is approaching a runway with low cloud cover, he might require only 100 m MOR.
On the other hand, if the pilot is about to land in foggy conditions, he would need much more.
What Is A Weather Visibility Scale?
Visibility scales are used to measure how well you can see something. The scale goes from 0 to 10. Zero represents no visibility, while ten represents perfect visibility.
- 0 – No Visible Horizon
- 1 – Poor Visibility
- 2 – Moderate Visibility
- 3 – Good Visibility
- 4 – Very Good Visibility
- 5 – Excellent Visibility
- 6 – Clear Sky
- 7 – Partly Cloudy
- 8 – Overcast
- 9 – Mostly Sunny
- 10 – Perfect Visibility
As we can see, visibility varies greatly based on a number of factors including the weather, the time of day, and even the location.
As a pilot, knowing these factors and being aware of what constitutes “good” visibility can help you avoid potential issues when planning your next flight.