Not all clouds are easily classified! Cloud Classification schemes. Clouds by level 9/23/15

Transcription

1 Cloud Classification schemes 1) classified by where they occur (for example: high, middle, low) 2) classified by amount of water content and vertical extent (thick, thin, shallow, deep) 3) classified by structure/appearance (for example: stratus, cirrus, cumulus, etc.) 4) classified by the physical processes that make them occur. {Your book combines 1) and 3)} Not all clouds are easily classified! As a pilot, you need to be familiar with the typical properties of basic cloud types and the risks/threats associated with them. Just what are those threats...? Visibility, turbulence/updrafts/downdrafts, lightning, icing, precipitation, others? Clouds by level As we discuss this, keep in mind that temperature generally (but not ALWAYS) decreases as you go higher in the atmosphere. So... Hi clouds (~6000 m (20,000 ft.) and up) usually contain ice rather than liquid water (always below freezing up high) Hi clouds tend to be thin (small cloud-water mixing ratio) since cold air can hold little water Low clouds (surface 2000m (6600 ft) )can be liquid, mixed-phase (both liquid and ice) or ice, depending on the temperature Low clouds can be thick (lots of cloud water) especially if they are warm. Mid-level clouds have properties between high and low clouds (NO SURPRISE) Lets have a few more definitions, so that our discussion is more clear: 1

2 DEFINE: Thick Cloud deep, dark base CONDENSATE mixing ratio: the ratio of water/ ice to air. (g of condensate)/(kg dry air) THICK clouds have a high water and/or ice content, i.e., a high condensate mixing ratio (lots of grams of water per kg of air). Thick clouds tend to look dark grey from underneath since they have a lot of water to absorb/scatter sunlight. Thick clouds often look bright white from the side since they have lots of water to scatter (reflect) the sunlight. DEFINE: THIN clouds have a low amount of cloud water or ice, i.e., a low condensate mixing ratio Thin clouds often look light grey or white from underneath, or may even be translucent (can kind of see through them) DEFINE: DEEP clouds have a large vertical extent maybe several thousand meters, and may or may not be thick as well. shallow clouds cirrus Also thin (low condensate MR) SHALLOW clouds have a small vertical extent (1 500 meters) and may be either thick or thin. 2

3 Summary of Cloud Types Cirrus Clouds shallow cloud Figure 6.20 deep cloud Cirrus Clouds thin and shallow, often almost transparent, clouds composed of ice crystals. Name comes from fact they are filamented (wispy) like a horse s tail. Cirrus are almost exclusively high clouds. Subtypes of Cirrus cirrostratus- (cross btwn. cirrus and stratus) more continuous that cirrus, i.e., a more-or-less continuous cirrus layer (still high, thin and shallow) cirrocumulus- (cross btwn. cirrus and cumulus) a little thicker that cirrostratus, often seen as organized (periodic) puffs with associated wisps. Mackerel sky (looks like fish scales) most common form of cirrocumulus. Cirrus Clouds Cirrocumulus Clouds Cirrus are high clouds (above 6000 m in middle latitudes) that are thin and wispy and comprised mostly of ice crystals. High clouds that are rounded puffs, possibly in rows, are less common than cirrus. 3

4 High clouds that thinly cover the entire sky with ice crystals. Light passing through these crystals may form a halo. Cirrostratus Clouds Sun dogs an atmospheric optics phenomena, typically occur in conditions where cirrus are just starting to form. Remember, cirrus are high, thin, and wispy. Also, usually associated with very weak updrafts (or none at all) Cumulus Clouds Cumulus or cumuliform clouds are the result of local convective instability. They are puffy looking, and tend to have a great extent vertically often quite deep and thick. Since they result from localized forcing they do not tend to be continuous horizontally, but rather are more patchy. Virtually any puffy or fluffy cloud is cumuliform. Cumulus Clouds (cont.) Thunderstorms (cumulonimbus) are composed of an ensemble (group) of precipitating cumulus clouds typically with great vertical extent. Since cumulus are associated with convection (convective instability) they are fundamentally different from other cloud types and often have: strong vertical velocities, heavy precip, large liquid water content, lightning, hail, and large supercooled drops. Cumulus are in many ways the most dangerous clouds from the aviation perspective. 4

5 Cumulus Humilis Clouds Cumulus Congestus Clouds Figure 6.17 Clouds with vertical development that take a variety of shapes, separated by sinking air and blue sky. Shredded sections are called cumulus fractus. Clouds with vertical development that become larger in height, with tops taking a ragged shape similar to cauliflower. Cumulus congestus, Mt. Shasta Cumulonimbus Cloud Clouds with vertical development that have grown into a towering thunderstorm cloud with a variety of key features, including the anvil top. 5

6 Altocumulus Clouds Middle clouds (between 2000 and 7000m in middle latitudes) that are puffy masses of white with gray edges. With your hand overhead, they are about the size of your fingernail. Mature Cumulonimbus, with precipitation and anvil Altostratus Clouds Stratus Clouds Stratus clouds appear in layers, at any (and every) level of the atmosphere. They typically result from lifting of a layer of air (for example along a front) rather than instability due to localized heating. Stratus clouds can be thick or thin, shallow or deep, and often occur as several overlying layers. Most clouds that are neither cumulus or cirrus are stratus clouds. Middle clouds that cover the entire sky and may create a dimly visible or watery sun and diminish formation of shadows. 6

7 Stratus Clouds Stratus Clouds (cont.) Altostratus clouds (midlevel, i.e., 6K 23 K ft.) are the most common cloud form in Anchorage much of the year. They are often quite uniform and gray. Stratocumulus is an almost continuous layer of shallow clouds composed of individual convective elements. They are a hybrid between stratus and cumulus clouds and often occur in a stable maritime environment. Figure 6.16 Low clouds that resembles a fog, but does not reach the ground, and can generate a light mist or drizzle. Altostratus Nimbostratus Low clouds (below 2000m) with precipitation that reaches the ground. ( nimbus implies precipitating) Low stratus Note diffuse cloud base 7

8 Nimbostratus Cloud Stratocumulus Shredded parts of these nimbus clouds are called stratus fractus or scud clouds. Stratocumulus (SC) Clouds Marine Stratocumulus... are a hybrid between stratus and cumulus clouds and often occur in a stable maritime environment with clear air (good flying) above. Marine Sc decks (layers) often reach ground forming a thick fog and can extend horizontally for hundreds of km. Figure 6.15 Low clouds with rounded patches that range in color from light to dark gray. With your hand extended overhead, they are about the size of your palm. 8

9 Drizzling SC Sc usually do not precipitate more than a drizzle, but can have supercooled liquid water in the right environment. Sc can form nearly continuous layers and can have enough liquid water content to create poor visibility Timescale of formation: minutes to hours. This cloud deck formed in about 15 min! Other Stratocumulus (Sc) issues for pilots: Summary of Cloud Types Sc decks can form (and dissipate) rapidly over vast expanses, especially over water. Sc can be rather hard to forecast, since they ofter result from calm, quiet (not stormy) conditions (but of course in moist environments) Figure