The concept of farsightedness

Farsight is the ability to distinguish distant targets. It is essential to differentiate between day vision and night vision because different reasons cause the two.

Factors that affect the ability to distinguish targets in daylight include atmospheric transparency, target size, light contrast between target and background, and the brightness sensitivity of the observer’s eye.

By the way, determining the target at night mainly depends on the light intensity of the artificial flame.

Atmospheric transparency

  • Atmospheric transparency is the ability of air to let solar radiation through, which largely depends on the amount of water vapor, solid and liquid compounds, their sizes, and the vertical and horizontal velocities of the atmosphere wind.

    Penetration also depends on height. For the most part, transparency is lower horizontally than vertically, but sometimes due to convection in the upper atmosphere, which brings dust and other compounds, vertical transparency is worse than in size horizontal.
  • When the transparency is comprehensive, less water vapor is in the air, fewer suspended solid particles, and more solar radiation reaches the soil surface and other targets. The higher the apparent brightness of the target, the better the ability for a larger target.
  • On cloudless days, the illumination intensity increases from sunrise to noon and gradually decreases until sunset. The illumination depends on the sun’s height and constantly changes throughout the year.

    Cloud amount and cloud pattern significantly change the brightness of the light Diffusers. As the transparency decreases, the direct illumination decreases, while the clouds are dense, covering the sky, completely disappearing. In this case, a description with diffuse radiation prevails.
  • Natural lighting drops significantly after sunset until before sunrise. At night, targets are difficult to see due to low illumination. In this case, distinguishing the mark requires artificial light.

Target size

During daylight hours, the naked eye can distinguish a target whose angular size is not less than 1 (α ≥ 1′). The angle of view α = 1′ is called the eye’s resolving power.

A target angular size of not less than 20 is required to distinguish it from the surrounding background. The angular size is fixed, meaning the more extensive the target, the farther it can be seen.

Thus, the ratio of the actual size of the target (h) to the distance (l) will be its angular size. From the figure, we get the following:

Figure 1. The magnitude of the target is the angle of view α

The light contrast between the target and the background

Is the most essential factor in determining the ability to distinguish a target under the prevailing atmospheric transparency conditions during the day. So the concept of far vision is the concept of light contrast.

If the target’s visible illuminance is called B₁, and the background’s visible illuminance is B₂, the light contrast K is:

The naked eye can distinguish a target whose illumination is at least 2% different from the background. The amount of Knin = 2/100 is called the contrast sensitivity limit of the eye.

The greater the difference between B₁ and B₂, the greater the contrast, and the higher the ability to distinguish the target, the greater the distance.

Meteorological visibility

To characterize the ability to see far away, people use the quantity of meteorological foresight.

Meteorological visibility

Meteorological vision is the maximum distance at which the naked eye can still distinguish a black target in conditions of atmospheric transparency during the day, having a corner size of not less than 20′ in the background near the horizon.

The above definition shows that major or minor visibility ultimately depends primarily on atmospheric transparency when standardizing three other factors, such as target size α ≥ 20′; the contrast between the black target and the sky background – the most excellent contrast and a reasonable observer’s eye.

Mathematical visibility

Mathematical visibility is the distance between the observer’s eye and the actual horizon and depends on the eye’s height above sea level.

Over the ocean, in ideal weather conditions, visibility can reach its maximum value: the distance to the horizon. Below is the empirical formula for determining the length from sea level to eye height – the mathematical horizon.



Inside: h – eye height above sea level (m)
           R – Average radius of the earth (6,370,000m)

Suppose: h= 1.8m then d = 4789m
h= 40m then d=22574m.

Optical visibility

  • Optical visibility is the distance to the visible target, largely dependent on the atmosphere’s optical properties and the background’s properties and subjectivity of the eye of the observer.
  • In practice, visibility varies greatly, from a few meters to tens of meters in dense fog, and during heavy rainfall, completely transparent in the atmosphere to tens of kilometers.
  • Since optical forecasting cannot be strictly standardized, its concept in weather service is simplified and incorporated into meteorological forecasting, one of the characteristics of transparency ozone.

Meteorological sight observation.

At fixed meteorological stations

  1. At fixed meteorological stations, long-distance vision is observed horizontally – at the eye level of the observer. Other meteorological stations, such as aviation meteorology, sometimes specify vertical images and slopes…

  2. Horizontal vision is measured with an instrument or estimated by eye. Measurement with a more objective instrument, but the agent does not yet measure the required distance, so it has been prepared using the visual estimation method.

    Targets used to observe horizontal visibility are preselected. They should have good color contrast with the background, and the height in the horizontal range should not be more than 5-6°.

  3. When one of the targets is still visible in the background, the distance from that target to the station is the meteorological horizontal view range.

Meteorological sight observation on ships

Meteorological sight observation on ships is made with the naked eye. The assessment is based on an international 10-level predictive scale.

Table 1. International 10-level scale for horizontal visibility

Distance vision level Range Weather features Features of the vision convention
0 0-50m Dense blind Extremely bad vision
1 50-200m Thick blind Very bad vision
2 200-500m Moderately blind, very heavy snow Very bad vision
3 500-1000m Weak blind, heavy snow Bad vision
4 1-2km Moderate snow, heavy rain Bad vision
5 2-4km Soft snow, heavy rain Average visibility
6 4-10km moderate rain, very weak snow, fog light Average visibility
7 10-20km Light rain, changing clouds Good view
8 20-50km No precipitation, sunshine Very good view
9 Over 50 km Completely transparent atmosphere Extremely good view

Each level in the table is a distance within the limits of the current weather conditions at the time of observation; the target is still distinguishable from the naked eye.

When ships navigate close to shore, targets such as individual mountains, headlands, lighthouses, and buildings are used to determine visibility ranges that are in scope and mapped.

Such objects are visible from the pier at an angle not exceeding 5-6° from the horizon. Things visible below a grade of up to 11° are allowed when visibility is more than one nautical mile.

In the far sea, when there are no longer any coastal targets in range, the actual visibility of the sea surface must be evaluated against the sharpness of the apparent horizon.

Table 2. Horizontal visibility hierarchy according to visible horizon features

Eye height above sea level Estimated range of vision Horizon features
From 1-7 m 8 and 9 The horizon line is very delicate
7 The horizon line is obvious.
6 Clear horizon, easy to see
5 The horizon is not very clear (slightly blurred)
4 The horizon is entirely invisible
From 8-27 m 9 The horizon line is obvious, apparent
8 Clear horizon, easy to see
7 The horizon is not very clear (slightly blurred)
6 The horizon is entirely invisible.


Observation of long-range meteorological vision at night

Night vision is determined by atmospheric phenomena and their intensity observable by the naked eye or by the degree of daytime vision.

Before determining night vision, it is necessary to address the day vision, especially the vision before sunset. If there is no meteorological phenomenon that reduces night vision, night vision can be defined as afternoon visibility.

If there is a phenomenon that reduces the visibility, estimate the visibility according to the table indicating the intensity of the phenomenon from far sight, depending on the intensity of the phenomenon. The table indicating the intensity of the phenomenon was recorded in the book “Surface meteorological observation criteria”.

Weather 01852 - Lowell MA

moderate rain

37°

moderate rain

Feels like 30°
Sunrise/Sunset - 01852
Sunrise/Sunset

06:31/19:07

Temperature - 01852
High/Low

40°F

/

35°F

Humidity in 01852
Humidity

82%

Pressure
Pressure

1001 hPa

Visibility
Visibility

7 mi

Wind
Wind

10.36 mph