Cloud Base Right Now — Your Live Ceilometer
The instrument above is measuring the height of the cloud base over your location. Just as a real ceilometer works, a beam of light rises from the ground unit until it strikes the underside of the cloud, and the length of that beam represents the cloud ceiling — the height of the lowest significant cloud layer. When the sky is clear the beam reaches far up into empty air; when a low, damp overcast rolls in, the beam is short and the cloud presses close overhead. Below the display you will find the cloud base in both metres and feet, the current cloud cover, the ceiling classification and what kind of cloud level it represents.
Cloud base is one of those measurements you rarely think about until it matters enormously — to a pilot on approach, a mountain rescue team, or anyone judging whether the hills will be shrouded in cloud today.
What a Ceilometer Does
A ceilometer measures the height of the base of the cloud above the ground. The modern instrument is a laser device: it sends a short, intense pulse of light vertically upward and listens for the faint reflection scattered back from the underside of the cloud. Because light travels at a precisely known speed, the tiny delay between sending the pulse and receiving the echo translates directly into a distance — the height of the cloud base. A ceilometer can sample many times a second, track several cloud layers stacked above one another, and run unattended day and night, which is why they line the edges of airport runways around the world.
Earlier ceilometers used a rotating beam of light and a photocell, or a spotlight projected onto the cloud and measured by triangulation. The laser (lidar) method that replaced them is more accurate, works in daylight, and needs no moving searchlight — but the goal has never changed: to answer the question "how low is the cloud?"
Cloud Ceiling and Why It Matters
In aviation, the cloud ceiling has a precise definition: the height above ground of the lowest cloud layer that covers more than half the sky (reported as "broken" or "overcast"). It is one of the two numbers — the other being visibility — that decide whether an aircraft can operate under visual flight rules or must switch to instruments, and it sets the minimum heights for takeoff and landing at every airport. A ceiling that drops below an airport's minimums can close it entirely. For pilots, glider crews, drone operators and parachutists, the cloud base is not a curiosity; it is a hard operational limit.
Reading the Cloud Base Height
Cloud base height sorts weather into recognisable types. A base right at the surface is fog. A very low base of a few hundred metres, often with full overcast, usually means damp, grey, drizzly weather and is a serious constraint on flying. A moderate base of one to two kilometres is typical of changeable, partly cloudy days. A high base with plenty of clear air beneath it generally accompanies fair, settled weather. Watching the base rise through the morning is a classic sign of improving conditions as the day warms and low cloud lifts and breaks.
Low, Middle and High Clouds
Meteorologists divide clouds into three height families, and the ceilometer's reading tells you which one is overhead. Low clouds (base below about 2,000 m) include stratus, stratocumulus and the thick, rain-bearing nimbostratus — these are the clouds that most affect the ceiling. Middle clouds (roughly 2,000–6,000 m), the altostratus and altocumulus, often veil the sun and can signal an approaching front. High clouds (above about 6,000 m) are the wispy, ice-crystal cirrus, cirrostratus and cirrocumulus. A ceilometer is primarily concerned with the lowest layer, because that is the one that limits what you can see and do beneath it.
Cloud Cover and the Sky Condition
Alongside the base height, the amount of the sky covered by cloud matters. Weather reports describe this in eighths of the sky, called oktas, and translate it into words: "clear" or "sky clear" for none, "few" for one to two eighths, "scattered" for three to four, "broken" for five to seven, and "overcast" for a completely covered sky. The ceilometer display shows the current cover percentage and the matching sky condition, so you can see not just how high the cloud is but how much of it there is — a broken sky at 1,500 metres is a very different day from a solid overcast at 300.
How Cloud Base Forms
Cloud forms when rising air cools to its dew point — the temperature at which the moisture it carries begins to condense into droplets. Because air cools at a steady rate as it rises, the height at which condensation begins is remarkably predictable: it depends on how far apart the temperature and dew point are at the surface. Dry air (a large temperature–dew point spread) must rise a long way before it cools enough to form cloud, giving a high base; humid air (a small spread) forms cloud close to the ground. This relationship — roughly a rise in cloud base of about 125 metres for every degree Celsius of spread — is how forecasters, and this page, estimate the cloud base from surface conditions.
Ceilometers in Everyday Weather
Though born in aviation, the cloud base measurement reaches into daily life. Mountain walkers use it to judge whether summits will be "in cloud". Astronomers and stargazers watch it to know if the sky will clear. Wind-farm and solar operators track low cloud that affects output. Film crews and event planners weigh it for outdoor shoots. And for anyone who simply wants to understand the sky overhead, the height of the cloud base turns a vague grey ceiling into a real, measurable number that explains what kind of day it is going to be.
Cloud Terms Across Languages
Cloud is nuage in French, Wolke in German, nube in Spanish and Italian, and nuvem in Portuguese, while the cloud base or ceiling is the plafond, Wolkenuntergrenze, techo de nubes or base delle nubi. The instrument is a célomètre, Wolkenhöhenmesser or ceilómetro depending on the language. Wherever aircraft fly, the cloud ceiling is measured and reported the same way.