UBC ATSC 113 - Cloud Coverage (2022)

Learning Goal 1c. Relate cloud coverage amounts to the visualappearance of the sky.

Pilots flying visually (VFR, see Learning Goal 1g) must stay outside of clouds.Even those flying in instruments (IFR)are concerned if there is a low overcast (i.e. a low ceiling) overtheir destination airport. For this reason, cloud coverage is importantto pilots.

The fraction of the sky (celestial dome) covered by cloud is called sky cover, cloud cover, or cloud amount. It ismeasured in eights (oktas) according tothe World Meteorological Organization (WMO).Table 9-10 gives the definitions for different cloud amounts, theassociated symbol for weather maps, and the abbreviation (Abbr.) foraviation weather reports (METARs =Meteorological Aviation Reports). For aviation, the altitude of cloudbase for the lowest cloud with coverage ≥ 5 oktas (i.e. lowest broken or overcastclouds) is considered the ceiling (see Learning Goal 1d).

Sometimes the sky is obscured,meaning that there might be clouds but the observer on the groundcannot see them (see Learning Goal1i). For obscurations, the verticalvisibility (VV) distance is reported as a ceiling instead (see Learning Goal 1e).

UBC ATSC 113 - Cloud Coverage (1)

From Stull, 2017: Practical Meteorology: An Algebra-based Survey of Atmospheric Science.

The photos below show different cloud coverages as viewed from theground. Note that all of these photos were taken at oblique angles, sowhen you look through the clear spaces between the clouds, you can seethe vertical sides of the next clouds behind the clear space. Hence,the cloudiness looks greater than it actually is.

Cloud Coverage

Examples

Sky Clear
(SKC, 0/8 coverage)

(except for a few clouds off in the distance)

UBC ATSC 113 - Cloud Coverage (2)
Few Clouds
(FEW, 1/8 coverage)
UBC ATSC 113 - Cloud Coverage (3)
Few Clouds
(FEW, 2/8 coverage)
UBC ATSC 113 - Cloud Coverage (4)
Scattered
(SCT, 3/8 coverage)
UBC ATSC 113 - Cloud Coverage (5)
Scattered
(SCT, 4/8 coverage)
UBC ATSC 113 - Cloud Coverage (6)
Broken
(BKN, 5/8 coverage)
UBC ATSC 113 - Cloud Coverage (7)
Broken
(BKN, 6/8 coverage)
UBC ATSC 113 - Cloud Coverage (8)
Broken — a few breaks betweenthe clouds
(BKN, 7/8 coverage)
UBC ATSC 113 - Cloud Coverage (9)
Overcast
(OVC, 8/8 coverage)
UBC ATSC 113 - Cloud Coverage (10)
Overcast (as reported by a humanobserver)
(OVC, 8/8 coverage)

Clear below 12,000 feet (as reported by an automated weather station).
(CLR)

These are high cirrostratus clouds (see Learning Goal 1a).So even though this is nice flying weather with bright sunlight shiningthrough the clouds, the sky is still completely covered with clouds.

UBC ATSC 113 - Cloud Coverage (11)

How observers on the ground try to estimate sky cover by eye

A tricky aspect of estimating cloud cover is that lower-altitudeclouds block the view of higher-altitude clouds that might or might notreally be there. To be conservative (i.e. extra safe), weatherobservers should always assume that if any clouds are visible at mid orhigher levels, then clouds at those same levels are assumed to existeven if they are hidden by lower-altitude clouds.

This procedure is sketched in Figure 3-4 (taken from an FAA AdvisoryCircular publication), which describes how to describe (i.e., code) thecloud cover based on your observations. It is somewhat confusingat first. Read the figure caption carefully, and compare the cloudcoverage amounts listed there (FEW, SCT, BKN) to the view in thesketch. The yellow lines in the sketch divide the sky into eightsectors, to enable cloud coverage to be reported in oktas. In reallife, weather observers would make a similar division to the wholehemisphere of sky (i.e. in 3-D), not just to the semicircle of sky assketched in 2-D.

Also, the white arc in Figure 3-4 represents altitudes (feet), andthis assumes that the weather observer can correctly measure orestimate the altitudes. While vertically pointing ceilometers can giveaccurate cloud base directly over the airport, there are no accuratemeasures off-vertical, unless pilots coming in to land or taking offradio in pilot reports (PIREPs) of cloud base heights. As discussed in Learning Goal 1d, unfortunately, cloud heightestimates by eye are rarely accurate.

Caution interpreting that figure. Notice that the sketch seems toshow two clouds touching the surface. But that is NOT really the case.The "surface" is at only one point — the point where all the yellowlines converge. Any clouds not at that point are actually at someheight above the surface. For example, the cloud in the sketchto the right of the word "SURFACE" is a cloud that the observer see bylooking to the right, but for which this cloud has a cloud base (bottomof the cloud) at 1000 feet AGL (above groundlevel).


UBC ATSC 113 - Cloud Coverage (12)


With this conservative method, weather observers always neverunderestimate the cloud coverage, but often overestimate the coverage.So when cloud "observations" are reported, such as in a MeteorologicalAviation Report (METAR), the actual cloud coverage might not be as badas reported. But you cannot count on this always being the case.

My personal approach when planning a flight is to "go look and see",but always with a safety net. Namely, I might try to fly VFR to alocation with an "observed" low ceiling, in the hopes that it is notreally that bad. While flying toward that location, I believe what Isee out the windscreen, not what was "observed" or forecast. But Ialways have an alternate plan in case the ceiling is as bad asforecast. The alternative could be turning back or landing at analternate airport where weather conditions are forecast to be very goodfor VFR flight. Or, in my case, as an instrument pilot and flightinstructor, getting the appropriate IFR clearance to fly through theclouds if needed.

Cloud coverage as viewed from an aircraft

The advantage while flying over or under clouds is that you can keeptrack of the clouds and clear spaces as you fly. So by lookingvertically, you can create a fairly accurate estimate of cloud cover.Also, if the sun is fairly high in the sky, you can look toward theground to see what fraction of the ground is covered by shadows ofclouds, as shown in the figure below.

UBC ATSC 113 - Cloud Coverage (13)

Caution: If you are flying at an altitude just above or just belowthe clouds, then in your slant or oblique view, the sky will look muchmore overcast than it really is. The reason is that with an obliqueview, the vertical body of the cloud blocks your view of clear spaceshorizontally hidden behind the cloud. Most of the photos below looklike greater cloud coverage than they actually are.

Below are photos I took of different cloud coverages. All photos areoblique views, so the actual coverage is less than it looks.

Cloud coverage

Example 1

Example 2

Few Clouds
(FEW, 1/8 coverage)
UBC ATSC 113 - Cloud Coverage (14) UBC ATSC 113 - Cloud Coverage (15)
Few Clouds
(FEW, 2/8 coverage)
UBC ATSC 113 - Cloud Coverage (16) (no photo)
Scattered
(SCT, 3/8 coverage)
UBC ATSC 113 - Cloud Coverage (17) UBC ATSC 113 - Cloud Coverage (18)
Scattered
(SCT, 4/8 coverage)
UBC ATSC 113 - Cloud Coverage (19) (no photo)
Broken
(BKN, 5/8 coverage, or perhaps SCT 4/8)
UBC ATSC 113 - Cloud Coverage (20) (no photo)
Broken
(BKN, 6/8 coverage)
UBC ATSC 113 - Cloud Coverage (21) UBC ATSC 113 - Cloud Coverage (22)
Broken — a few breaks betweenthe clouds
(BKN, 7/8 coverage)
UBC ATSC 113 - Cloud Coverage (23) UBC ATSC 113 - Cloud Coverage (24)
Overcast
(OVC, 8/8 coverage)
UBC ATSC 113 - Cloud Coverage (25) UBC ATSC 113 - Cloud Coverage (26)

Key words: above ground level (AGL), broken,ceiling, cloud amount, cloud cover, IFR, METARs, obscured, oktas,overcast, sky cover, vertical visibility, VFR, WMO, cirrostratus

Extra info for Experts; Not Needed for thisCourse.

  • An excellent USA user's guide: Aviation Weather Services, Advisory Circular, AC 00-45H —available for free as a PDFfrom the FAA, 427 pages. It is updated every several years or so. The Nov 2016 edition isvalid at the time of this writing (Sep 2018). See section 3.1.5.9 for cloudcoverage.
    Every US pilot should get a copy
    ,because it explains how to interpret text products (METARs, PIREPs,TAFs, FAs, etc.), graphics products (including analysis and forecastcharts, radar and satellite images), aviation hazards products(SIGMETs, AIRMETs, tropical cyclone and volcanic ash advisories,SIGWXs, icing and turbulence potential), and much more. You can alsobuy printed copies of this advisory from online retailers.
  • FederalMeteorological Handbook No. 1 (2017), Chapter 9 on clouds.
  • Canadian Aviation Weather Services Guide from NavCanada.

Image credits. All figures by Roland Stull, exceptthe following: Figure 3-4. METAR/SPECI Sky Condition Coding. Aviation Weather Services, Advisory Circular00-45H, (Nov 2016), page 3-16 (PDF page 44).

FAQs

How much cloud coverage is scattered? ›

"Scattered" clouds (3/8 to 4/8 cloud coverage) correspond to a partly cloudy or partly sunny sky, with "broken" clouds (5/8 to 7/8 cloud coverage) describing a partly cloudy or partly sunny (5/8 coverage) to mostly cloudy (6/8 to 7/8 coverage) sky.

What amount of cloud coverage constitutes a ceiling? ›

According to the International Civil Aviation Organization (ICAO), ceiling is the height above ground level (AGL) of the lowest cloud base (bottom of the cloud) that is below 20,000 ft (6,000 m) that is covering MORE than half the sky (i.e. cloud coverage amount is 5/8 or greater; namely, is either "broken" or " ...

How do you interpret cloud cover? ›

In meteorology, cloud cover is measured in oktas, or eighths of the sky. If you look up at the sky, and mentally divide it into eight boxes, then imagine all the cloud you can see squashed into these boxes. How many boxes does the cloud fill? This is how many oktas of cloud there are.

What is considered low cloud ceiling? ›

For aviation, the altitude of cloud base for the lowest cloud with coverage ≥ 5 oktas (i.e. lowest broken or overcast clouds) is considered the ceiling (see Learning Goal 1d).

What is good cloud cover? ›

About 30-60% cloud cover is ideal. Any more/less than that and your photo could be a flop. You tend to see more fiery colored skies as the sun is setting. This is because the sun is at its lowest angle and is about to slip below the horizon.

Does cloud cover make it warmer? ›

Clouds can also act like a blanket, trapping heat on Earth by absorbing the heat released by the surface of the planet. They radiate this heat back toward Earth, warming the lower regions of the atmosphere.

What's the difference between cloud cover and cloud ceiling? ›

They are not the same thing. Cloud base is simply the bottom "surface" level of the a cloud regardless of the sky coverage. Where cloud coverage constitutes a ceiling, then the cloud base would be the same as the ceiling, but regardless, the bottom margin of any cloud is its base.

How do you check a cloud ceiling? ›

1) Calculate the 'spread,' where you're taking the current temperature and subtracting the dew point. 2) Divide the spread by 4.4 (if temperatures are in °F) or 2.5 (if temperatures are in °C), then multiply by 1000. This will give you cloud base in feet above ground level.

Can you fly VFR in Broken clouds? ›

Cloud layers, of whatever type, are always given with altitude of the bottom of the layer, measured in feet AGL. The last two cases, broken and overcast, describe a cloud layer that may not be penetrated in VFR flight. The bottom of such a layer constitutes a ceiling for VFR flight.

Does cloud cover make it cooler? ›

Clouds can block light and heat from the Sun, making Earth's temperature cooler.

How do you predict cloud covers? ›

Daily averages of downward solar radiation can be used as a proxy for average cloud cover. To assess the skill of such radiation forecasts, we use daily averages of downward solar radiation at the surface provided by the Climate Monitoring Satellite Application Facility (CM SAF).

What is a normal cloud ceiling? ›

Cloud Ceiling Definition

High clouds have base heights of 3,000 to 7,600 meters (10,000 to 25,000 ft) in polar regions, 5,000 to 12,200 meters (16,500 to 40,000 ft) in temperate regions, and 6,100 to 18,300 meters (20,000 to 60,000 ft) in the tropical region.

Are low clouds the same as fog? ›

Clouds form at many different altitudes and can be several miles above the surface. So, the only real difference between clouds and fog is the altitude. Typically, if it's lower than 50 feet, it is considered fog. If it's higher than 50 feet, it is considered a cloud.

How high is fog in the sky? ›

Fog forms only at low altitudes.

They can be as high as 12 miles above sea level or as low as the ground. Fog is a kind of cloud that touches the ground. Fog forms when the air near the ground cools enough to turn its water vapor into liquid water or ice.

Is cloud cover warm or cool? ›

This relationship creates a complicated system of climate feedbacks, in which clouds modulate Earth's radiation and water balances. Clouds cool Earth's surface by reflecting incoming sunlight. Clouds warm Earth's surface by absorbing heat emitted from the surface and re-radiating it back down toward the surface.

Is cloud cover good for photos? ›

Thick cloud cover is also a great time for landscape photography—for many of the same reasons as portrait photography. The light is diffused and shining softly. This keeps the contrast down so you don't have dark shadows with no detail. Plus the highlights don't look overexposed.

What color is Benjamin Moore cloud cover? ›

Benjamin Moore Cloud Cover (OC-25, 855) is a soft light greige color with barely there violet undertones. It has an LRV of 82, which is on the light side for a greige, but it will look like a beautiful soft off-white on your walls.

Is it colder when there are no clouds? ›

When forecasting daytime temperatures, if cloudy skies are expected, forecast lower temperatures than you would predict if clear skies were expected. At night cloud cover has the opposite effect. If skies are clear, heat emitted from the earth's surface freely escapes into space, resulting in colder temperatures.

Why do you feel cold in a clear night? ›

On clear nights, there are no clouds in the sky to trap heat. As a result, heat is able to escape Earth's atmosphere. The temperature drops. That's why clear nights can be quite a bit colder than cloudy nights.

How does cloud cover affect humidity? ›

The researchers concluded that cloud cover is directly proportional to humidity and inversely proportional to temperature. Meaning as cloud cover increases so its humidity and as cloud cover increases temperature decreases.

What is broken cloud cover? ›

So how is broken and overcast measured? By something called "octals", which are 8 equal segments of the sky. If the sky is covered between 5/8 and 7/8 with clouds, it's reported as broken. And if it's covered 8/8ths with clouds, it's overcast.

Do scattered clouds count as ceiling? ›

Cloud types that do not count as a ceiling: SCT – scattered clouds.

What is the minimum VFR ceiling? ›

IFR means a ceiling less than 1,000 feet AGL and/or visibility less than three miles. Low IFR (LIFR) is a sub-category of IFR. VFR means a ceiling greater than 3,000 feet AGL and visibility greater than five miles.

What does the height of the cloud base tell us? ›

How is the height of a cloud base determined? - YouTube

How high the clouds are? ›

Low level clouds are generally based at or below 6,500 feet above ground, mid-level clouds are generally based between 6,500 feet and 15,000 feet above ground, and high level clouds are generally based more than 15,000 feet above ground.

Are cloud heights MSL or AGL? ›

Cloud Height:

Remember, METAR and TAF report the height of clouds AGL, not MSL.

Is it legal to fly VFR-on-top? ›

VFR over the top - flying VFR above a layer of clouds, even a solid layer, and it's perfectly legal.

Can a VFR pilot fly MVFR? ›

Can a Private Pilot Fly MVFR? Yes, private pilots are authorized to fly MVFR, though should probably avoid doing so until they have plenty of experience flying in MVFR conditions with an instructor.

Can student pilots go above clouds? ›

The only regulatory restriction is that student pilots are not allowed to fly above a cloud layer without ground reference. VFR over-the-top can be a very useful tool to get above a cloud layer instead of flying in haze or restricted visibility below the clouds.

Are cloudy days warmer than clear days? ›

During the day, sunlight heats the Earth. If skies are clear, more heat reaches the ground leading to warmer temperatures. On cloudy days, clouds reflect the sun's light into space, keeping much of the energy away from the surface and leading to cooler temperatures.

Do clouds block infrared? ›

Cloud cover reduces the intensity of UV light, visible light, and infrared radiation (IR). IR produces the sense of warmth from the sun. Clouds reduce IR transmission more than UV transmission.

How does cloud cover affect wind? ›

'We often think clouds are propelled by the wind but clouds themselves also influence the wind. Clouds are related to strong upward and downward motions, which transport air from the surface to higher levels in the atmosphere and the other way around.

Is there an app that shows cloud coverage? ›

Astrospheric is another awesome weather app that shows cloud cover by the hour and includes a radar that shows the clouds moving over your location. You can watch the radar or click on a particular hour to see the cloud cover prediction and location of the clouds.

What is total cloud cover? ›

Total cloud cover is the fraction of the sky covered by all the visible clouds. Cloud amount refers to the fraction of the sky covered by clouds of a particular type or combination. It can refer to a genus, species, variety, layer, or a certain combination of clouds.

Can you forecast clouds? ›

Here are some hints for predicting weather by reading clouds. Isolated, wispy, or very high clouds are an indication of fair weather. Crowded, dense, dark, and towering clouds indicate changing or worsening weather. The sharper the edge of a thundercloud and the darker its color, the more violence it may contain.

What is the distance between cloud and earth? ›

At the upper reaches of the troposphere you'll find high clouds, which, depending on geographic location, occur between roughly 10,000 and 60,000 feet. Below that is the home of mid-level clouds, which generally occur between 6,000 and 25,000 feet.

Can clouds touch the ground? ›

Some clouds are high up in the sky. Low clouds form closer to Earth's surface. In fact, low clouds can even touch the ground. These clouds are called fog.

What does a cloud feel like? ›

Clouds are composed of vaporized water, and they feel like mist. If you put your hands through one, you probably won't feel anything because the water present in it is in gaseous form. As a result of this, clouds can move with the weather.

Which type of cloud gives rain? ›

The prefix "nimbo-" or the suffix "-nimbus" are low-level clouds that have their bases below 2,000 meters (6,500 feet) above the Earth. Clouds that produce rain and snow fall into this category. ("Nimbus" comes from the Latin word for "rain.") Two examples are the nimbostratus or cumulonimbus clouds.

What are the 4 types of fog? ›

There are several different types of fog, including radiation fog, advection fog, valley fog, and freezing fog. Radiation fog forms in the evening when heat absorbed by the Earth's surface during the day is radiated into the air.

How long does it take to clear fog? ›

It is the early morning sunlight hours that fog tends to dissipate. Under certain meteorological circumstances, fog can persists all day long and can develop at times besides the overnight hours. In general though, fog develops overnight and dissipates (mixes out) in the early morning sunlight hours.

Can you have rain and fog at the same time? ›

Precipitation Fog: This is fog that forms when rain is falling through cold air. This is common with a warm fronts but it can occur with cold fronts as well only if it's not moving too fast. Cold air, dry at the surface while rain is falling through it evaporates and causes the dew point to rise.

How much of Earth do clouds cover? ›

One study based on nearly a decade of satellite data estimated that about 67 percent of Earth's surface is typically covered by clouds. This is especially the case over the oceans, where other research shows less than 10 percent of the sky is completely clear of clouds at any one time.

Is scattered considered a cloud layer? ›

3. Cloud cover and measurement of ceiling. Different types of cloud cover conditions include SKC (sky clear), FEW (trace), SCT (scattered), BKN (broken) and OVC (overcast).

What are scattered clouds? ›

: clouds covering one tenth to one half of the sky.

Is cloud cover higher over oceans or land? ›

Oceans are significantly cloudier than continents. Slightly more than 70 percent of the sky over oceans is cloudy, but a little less than 60% of the total land area is usually covered with clouds.

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