Cloud Identification and Classification
Introduction
Understanding cloud types and their characteristics is essential for pilots, particularly those pursuing the CASA Private Pilot License (PPL). This section explores cloud classification based on height, identifies the ten cloud genera, and explains standard abbreviation usage. Additionally, it outlines the methods used to report cloud amounts in meteorological observations.
Classification of Clouds
Clouds are classified based on the height of their base above the Earth’s surface. This classification helps in forecasting weather and understanding atmospheric conditions. They are mainly categorized into three groups: high-level, middle-level, and low-level clouds, along with clouds of vertical development.
High-Level Clouds (Above 6 km to ~13 km)
High-level clouds are composed primarily of ice crystals due to their altitude and cold temperatures. They typically appear white, thin, and wispy.
- Cirrus (Ci): Wispy, feathery clouds that indicate approaching warm fronts or jet streams.
- Cirrostratus (Cs): Whitish veil-like clouds that often produce halos around the sun or moon.
- Cirrocumulus (Cc): Layered clouds with small lumps, forming in patterns or “streets” Cloud Classifications and Characteristics - National Weather Service.
Middle-Level Clouds (2 - 7 km)
These clouds can consist of water droplets, ice crystals, or supercooled droplets and often cover much of the sky.
- Altostratus (As): Smooth, grey sheets, often precursors to storms, may thicken and cause light precipitation.
- Altocumulus (Ac): Convective, heap-like clouds indicating instability Cloud Classification - National Weather Service.
Low-Level Clouds (Surface - 2 km)
Primarily composed of water droplets, low-level clouds are often associated with overcast conditions.
- Stratus (St): Uniform, grey layers that may produce drizzle.
- Stratocumulus (Sc): Wavy grey clouds commonly seen near frontal systems.
- Nimbostratus (Ns): Dense, thick clouds causing continuous rain or snow Cloud Types - UCAR Center for Science Education.
Clouds with Vertical Growth (Surface to above 13 km)
These clouds grow vertically and are associated with unstable atmospheric conditions.
- Cumulus (Cu): Distinguished by flat bases and rounded tops; can evolve into cumulonimbus.
- Cumulonimbus (Cb): Towering storm clouds capable of producing heavy rain, lightning, and severe weather Cloud Classification - National Weather Service.
Cloud Genera and Abbreviations
The World Meteorological Organization (WMO) defines ten cloud genera, each with a standard abbreviation essential for aviation communication.
| Cloud Genera | Abbreviation | Characteristics |
|---|---|---|
| Cirrus | Ci | Wispy, white; indicates nearby storms |
| Cirrostratus | Cs | Thin veil, forms halos |
| Cirrocumulus | Cc | White ripples; limited convective instability |
| Altostratus | As | Grey sheet, often captures incoming precipitation |
| Altocumulus | Ac | Patches, grey or white; sign of possible precipitation |
| Stratus | St | Grey, uniform; may signal drizzle, appears as fog |
| Stratocumulus | Sc | Waves or rolled clouds; usually suggest light rain |
| Nimbostratus | Ns | Dark and thick with continuous precipitation |
| Cumulus | Cu | Dense, well-defined; indicative of fair weather |
| Cumulonimbus | Cb | Tall, thunderstorm clouds; result in severe weather |
References: Cloud abbreviations and symbols - International Cloud Atlas
Reporting Cloud Amount
Cloud amounts are reported using the okta scale, a standard measurement that divides the sky into eighths.
Okta Scale
- 0 oktas: Sky completely clear
- 1-2 oktas: Few clouds (FEW)
- 3-4 oktas: Scattered clouds (SCT)
- 5-7 oktas: Broken clouds (BKN)
- 8 oktas: Overcast (OVC)
- Indicator ‘9’: Sky totally obscured by phenomena like fog or snow Okta - Wikipedia.
Methods for Reporting Cloud Cover
- Separate Layers Method: Reports each cloud layer independently, focusing on layers more substantial than the previous.
- Summation Method: Provides a cumulative view of cloud coverage at or below a given altitude.
- Summation Including Surface-Based Obscurations: Includes phenomena such as fog or dust in estimations.
For practical aviation use, the summation method offers a comprehensive overview, essential for METAR, SPECI, and TAF reports.
References: AERODROME METEOROLOGICAL OBSERVATION - ICAO
Conclusion
This section provided a detailed overview of cloud classifications, including the ten genera’s characteristics and abbreviations. Additionally, it explained the standard methods for reporting cloud amounts, emphasizing their relevance in aviation meteorology. Understanding these elements is critical for pilots to make informed decisions regarding weather conditions.