Clouds are a visible indicator of the atmosphere’s condition, and their distance from the ground, known as altitude, is far from uniform. This altitude can vary from a few hundred feet to over fifty miles, depending on where the cloud forms. Most familiar weather clouds are confined to the troposphere, the lowest layer of the atmosphere, which extends from the surface up to an average altitude of about six to twelve miles. This vertical space is organized into distinct layers, allowing meteorologists to classify clouds based on their approximate height.
The Standard System of Cloud Classification
Meteorologists classify clouds using a universally recognized system based on both appearance and altitude. Established in the early 1800s, this system uses Latin roots to describe the cloud’s form and vertical placement. The four main types describing appearance are:
- Stratus for layers
- Cumulus for heaps
- Cirrus for high, wispy formations
- Nimbus for rain-producing clouds
To organize clouds by height, the troposphere is divided into three general altitude categories. Low-level clouds have no prefix, mid-level clouds use alto-, and high-level clouds use cirro-. This framework organizes the ten main cloud genera, where the base height determines the primary classification. Towering cumulonimbus clouds can span all three layers, but their base is categorized by its starting height in the low-level range.
Specific Altitude Ranges for Cloud Groups
The three main altitude groups provide distinct ranges for cloud bases, though these ranges are approximate and vary geographically.
Low-Level Clouds
Low-level clouds are found from the surface up to about 6,500 feet (2,000 meters) and are primarily composed of liquid water droplets. This group includes stratus clouds, which appear as featureless gray sheets, and the common, puffy cumulus clouds. When stratus clouds descend to touch the ground, they are known as fog, meaning their base altitude is zero.
Mid-Level Clouds
Mid-level clouds, prefixed with alto-, occupy the space between 6,500 feet and approximately 20,000 feet (2,000 to 6,000 meters). Formations like altocumulus and altostratus can be composed of supercooled water droplets, ice crystals, or a mixture of both. The base height of these clouds is often higher in tropical regions and lower toward the poles, reflecting the varying depth of the troposphere.
High-Level Clouds
High-level clouds, identified by the cirro- prefix, form at altitudes above 20,000 feet (6,000 meters) and can extend up to 40,000 feet (12,000 meters) or more. Due to the extremely cold temperatures at this height, clouds like cirrus and cirrostratus are composed almost entirely of ice crystals. These wispy, thin clouds are often the first sign of an approaching weather system.
Clouds Beyond the Troposphere
While most clouds exist within the troposphere, some clouds form in the layers above. These formations require specific, frigid conditions to condense.
Nacreous Clouds
Nacreous clouds, also known as Polar Stratospheric Clouds, form in the stratosphere at heights between 9 and 16 miles (15 to 25 kilometers).
Noctilucent Clouds
Noctilucent clouds are the highest clouds in Earth’s atmosphere, forming in the mesosphere at altitudes around 50 to 53 miles (80 to 85 kilometers). They are composed of tiny ice crystals that become visible only when sunlight reflects off them after the sun has dipped below the horizon.
Determining Cloud Altitude
Scientific organizations use specialized equipment to precisely measure cloud base height.
Ground-Based Measurement
A common ground-based tool is the ceilometer, which sends a pulse of laser light vertically into the sky. By measuring the time it takes for the light to scatter back from the cloud base, the instrument calculates the altitude.
Atmospheric Profiling
Weather balloons, or radiosondes, are also used to gather data on cloud height as they ascend through the atmosphere. Instruments attached to the balloon record changes in temperature and relative humidity, pinpointing the altitude where moisture condenses. For large-scale measurements, satellites employ active sensing instruments, such as lidar, to profile the atmosphere and determine cloud heights from orbit.