Now, measuring cloud tops is a different story — Page 199, Lesson 168

I want to walk you through how we measure and classify clouds — starting with the most accurate tool for finding cloud base height. Let's look at the laser ceilometer. A laser ceilometer consists of a vertically pointing laser and a receiver, both housed in the same instrument assembly. It determines the height of the cloud base by measuring the time required for a pulse of light to be scattered back from the cloud base. Because it's more accurate and more reliable than other types of recorders, the laser ceilometer is the main type of recorder currently in use. Now, measuring cloud tops is a different story. The height of cloud tops cannot be easily measured. Forecasters can deduce the height from the ELR and SALR — that's the Environmental Lapse Rate and the Saturated Adiabatic Lapse Rate — but this is not very accurate. So forecasters rely on pilot reports of cloud top height to assist with the forecasting. Next, cloud movement. Meteorological stations measure the movement of clouds by means of a nephoscope. A nephoscope measures the angular speed of movement of cloud. If the base height is known, the speed of movement may be calculated. One example is the Besson nephoscope. Now let's move into cloud classification. Classification of cloud type is based primarily on the shape, or form, of the cloud. The basic forms of cloud are stratiform, cumuliform, and cirriform. Stratiform cloud is a layered type of cloud of considerable horizontal extent, but little vertical extent. Think of a blanket spread across the sky. Cumuliform cloud is heaped cloud, displaying a marked vertical extent, of greater or lesser degree. These are the puffy, towering clouds. Cirriform cloud is a cloud which is fibrous, wispy, or hair-like in appearance. This type of cloud is found only at high levels in the troposphere. Clouds are also identified by reference to the height at which they occur. There are three distinct cloud levels within the troposphere: low, middle, and high. The height bands vary by latitude. In polar regions: low-level clouds go from the Earth's surface to 2 km, or 6,500 feet. Middle-level clouds are between 2 and 4 km, or 6,500 to 13,000 feet. High-level clouds are between 3 and 8 km, or 10,000 to 25,000 feet. In temperate regions: low-level clouds are from the surface to 2 km, or 6,500 feet. Middle-level clouds are between 2 and 7 km, or 6,500 to 23,000 feet. High-level clouds are between 5 and 13 km, or 16,500 to 45,000 feet. In tropical regions: low-level clouds are from the surface to 2 km, or 6,500 feet. Middle-level clouds are between 2 and 8 km, or 6,500 to 25,000 feet. High-level clouds are between 6 and 18 km, or 20,000 to 60,000 feet. Now, let's look at the specific cloud types in each level. Low-level clouds may be stratus, stratocumulus, cumulus, and cumulonimbus. The prefix "nimbo" and the suffix "nimbus" imply "rain bearing." However, cumulus and cumulonimbus will have significant vertical development and will extend from low-level to higher levels. Cumulonimbus clouds may even extend into the lower stratosphere. Medium-level clouds are found between 6,500 feet and 23,000 feet. The names of medium-level clouds are characterized by the prefix "alto-": such as altostratus and altocumulus. Nimbostratus is also classified as a medium-level cloud. High-level clouds have names prefixed by "cirro-": cirrostratus, cirrocumulus, and cirrus. The Latin "cirrus" means curl.