- To learn the standard procedures for observing and reporting sky condition and visibility
- To encode and decode METAR reports of sky condition and visibility
Aviation Weather, Peter Lester, Chapter 6, “Atmospheric Moisture” and Appendix B, “Standard Meteorological Codes and Graphics for Aviation.” Supplemental reading: AC 00-45H, Aviation Weather Services, Chapter 3 Observations.
We want to consider the results of condensation and deposition; that is, the formation of clouds and other obstructions to vision composed of water and ice particles.
The formation of clouds and fog results when air with adequate moisture and condensation nuclei is cooled to the dewpoint temperature. With continued cooling, excess water vapor changes state and forms liquid or ice particles. The resulting “visible moisture” has far-reaching effects on aviation including low ceilings, low visibilities, ice, and the decrease in stability caused by the release of latent heat. To help you understand these processes, several optional “kitchen” experiments related to cloud formation are included in the question section at the end of this exercise.
This exercise provides a practical look at the aspects of cloud and visibility observations including not only report decoding, but also how observations are made and how reports are encoded. With regard to the observed characteristics of clouds, sky condition refers to the state of the sky at the time of an observation (METAR) or terminal forecast (TAF). It is described by the five items listed in table 7-1.
|Cloud Type||Cloud category according to height and appearance.|
|Sky Cover||Amount of celestial dome hidden by clouds and/or|
|obscurations. Figure 7-2 illustrates the|
|Summation Layer Amount||A categorization of the amount of sky cover at and|
|below each reported layer.|
|Layer Height||The height (AGL) of the base of each reported|
|layer of clouds and/or obscurations above ground|
|level; the vertical visibility into an indefinite ceiling.|
|Ceiling||The height above ground level of the lowest layer|
|that is reported as broken or overcast; the vertical|
|visibility into an indefinite ceiling.|
Pilots will be reading, listening to, and interpreting METAR reports. While being able to decode the reports is very important, their correct interpretation requires knowledge of how the observation is made and the report is written. There are certain subtleties that cannot be learned from decoding alone. When you read a METAR report, you must “stand in the shoes of the observer” to “see” what the observer sees. This exercise gives you some opportunities to do that.
Finally, the effect of the release of latent heat on stability during cloud formation will be examined. The release of latent heat is critical in producing strong convection, especially thunderstorms.
Clouds are generally classified according to their height and appearance. In the following, you will review cloud classifications.
Exercise 1: Document the common height categories and height ranges of cloud bases in the following spaces. Also, list the names of the common cloud types in each height category.
|Height Category||Height Range of Cloud Base||Cloud Types|
|Low Clouds||< 6,500 feet|
Exercise 2: List the clouds that are known for vertical development:
Exercise 3: List examples of types of clouds that are formed in a stable environment and those formed in an unstable environment.
Exercise 4: Clouds are reported in layers where all cloud elements have bases at about the same height. The amount of sky cover is determined in eighths of the celestial dome. Each layer is measured and reported in categories that indicate how much of the sky is covered by the clouds and/or obscurations.
METAR weather observations are taken from the ground; and the Summation Principle is used, i.e., sky cover at any level is the sum of all lower layers including surface-based obscurations, such as fog, smoke, dust, and haze
You may sometimes here this referred to as the “at and below” rule, i.e., the cloud amount reported for any cloud layer is the sum of the amounts “at and below” that level.
- Define the following sky cover terms:
|Categories||METAR CODE||Sky dome covered|
- Write the definition for “Ceiling”
- Define “Indefinite Ceiling” VV
EXERCISE 6: Give the ceiling and visibility criteria for the following conditions:
EXERCISE 7: Prevailing Visibility is the greatest distance that can be seen over at least half of the horizon circle. It does not have to be continuous. Prevailing visibility and actual visibility in a particular sector may be different. Use the following examples of visibility observations below to determine prevailing visibility. The first three examples are shown as examples.
- __________________ e. __________________ f. __________________
- __________________ h. __________________ i. __________________
EXERCISE 8: The examples of the celestial dome shown below depicts cross sections and layer-by-layer cloud cover. They represent that portion of the sky that is seen by an observer standing on the ground. In each example, the observer stands at point “O.” Cloud layers at various altitudes are indicated by irregular bands. The “pie-shaped” segments correspond to eighths of the celestial dome. Cloud layers are coded sequentially from the ground up and coverage is cumulative. When multiple layers are present, no higher layer can be reported with less coverage than the layer below it.
- List the METAR code for the examples using the Summation Principle (at-and-below this altitude). See Examples in textbook, Chapter 6.
- Indicate the ceiling where applicable. (Example: Ceiling 2,000’)
- _____________________________ 2. _______________________________
- _______________________________ 4. ____________________________
- ______________________________ 6. _______________________________