12-8 Conversely, a column of air in the atmosphere will become less stable when it ascends (rises). As it rises, the rapid decrease in air density aloft causes the column to stretch out vertically. As long as the layer remains unsaturated, the entire layer cools at the dry adiabatic lapse rate (see Table 11-1). However, due to the stretching effect, air at the top of the column cools more than the air at the bottom of the column. This process acts to increase the temperature lapse rate and decrease stability.
Figure 12-7. Vertical Motion Effects on Stability
A rising column of air will become less stable when air at the bottom has a higher relative humidity than air at the top. As the air moves upward, the bottom becomes saturated first and cools at the lesser moist adiabatic lapse rate (see paragraph 11.2). The net effect is to increase the lapse rate within the column and decrease stability. This process is called convective instability, and is associated with the development of thunderstorms.
12.4.3 Diurnal Temperature Variation Effects on Stability. Diurnal (day/night) temperature variations (see Figure 12-8) can have a significant impact on atmospheric stability.
Daytime heating of the surface increases temperature lapse rates and decreases stability.
Conversely, nighttime cooling of the surface decreases temperature lapse rates and increases stability. Diurnal temperature variations are most pronounced in the lower troposphere because air is a poor conductor of heat (see Table 2-3).
12-9 The magnitude of diurnal temperature (and stability) variation is primarily influenced by surface type, latitude, sky cover (e.g., clouds and pollutants), water vapor content of the air, and wind speed. Temperature variation is maximized over land, at low latitudes, with a clear sky, dry air, and light wind. Conversely, temperature variation is minimized over large bodies of water, at high latitudes, with a cloudy sky, moist air, and strong wind.
Figure 12-8. Diurnal Temperature Variation Effects on Stability
12.5 Measurements of Stability. Several stability indexes and other quantities exist that evaluate atmospheric stability and the potential for convective storms. The most common of these are Lifted Index (LI) and Convective Available Potential Energy (CAPE).
12.5.1 Lifted Index. The LI (see Figure 12-9) is the temperature difference between an air parcel (usually at the surface) lifted adiabatically (see Chapter 11) and the temperature of the environment at a given pressure (usually 500 millibars) in the atmosphere. A positive value indicates a stable column of air (at the respective pressure), a negative value indicates an unstable column of air, and a value of zero indicates a neutrally stable column of air. The larger the positive (negative) LI value, the more stable (unstable) the column of air.
12-10 LI is generally used in thunderstorm forecasting; however, CAPE is generally considered a superior measurement of instability. However, LI is easier to determine without using a computer.
Figure 12-9. Lifted Index Example
12.5.2 Convective Available Potential Energy. CAPE is the maximum amount of energy available to an ascending air parcel for convection. CAPE is represented on a sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel over the layer within which the latter is warmer than the former.
Units are joules per kilogram of air (J/kg). Any value greater than 0 joules per kilogram indicates instability and the possibility of thunderstorms.
CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate the potential for stronger updrafts. Observed values in thunderstorm environments often exceed 1,000 joules per kilogram, and in extreme cases may exceed 5,000 joules per kilogram.
12-11
12.6 Summary. Atmospheric stability influences weather by affecting the vertical motion of air. Stable air suppresses vertical motion while unstable air enhances it. Chapter 13 will discuss how stability determines cloud and precipitation type.
13-1 CHAPTER 13. CLOUDS
13.1 Introduction. A cloud is a visible aggregate of minute water droplets and/or ice particles in the atmosphere above the Earth’s surface. Fog differs from cloud only in that the base of fog is at the Earth’s surface while clouds are above the surface. Clouds are like signposts in the sky that provide information on air motion, stability, and moisture.
Clouds help pilots visualize weather conditions and potential weather hazards.
Clouds form in the atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest layer of fog. Rising currents of air are necessary for the formation of vertically deep clouds capable of producing precipitation heavier than light intensity, so we will focus on that process in this chapter.
13.2 Cloud Forms. There are four basic cloud forms (appearances) observed in the Earth’s atmosphere (see Table 13-1).
Table 13-1. Cloud Forms
Cirriform High-level clouds that form above 20,000 feet (6,000 meters) and are usually composed of ice crystals. High-level clouds are typically thin and white in appearance, but can create an array of colors when the sun is low on the horizon. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation.
Nimbus Nimbus comes from the Latin word meaning “rain.” These clouds typically form between 7,000 and 15,000 feet (2,100 to 4,600 meters) and bring steady precipitation. As the clouds thicken and precipitation begins to fall, the bases of the clouds tend to lower toward the ground.
Cumuliform Clouds that look like white, fluffy cotton balls or heaps and show the vertical motion or thermal uplift of air taking place in the atmosphere. The level at which condensation and cloud formation begins is indicated by a flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base. The tops of these clouds can reach over 60,000 feet (18,000 meters).
Stratiform Stratus is Latin for “layer” or “blanket.” The clouds consist of a featureless low layer that can cover the entire sky like a blanket, bringing generally gray and dull weather. The cloud bases are usually only a few hundred feet above the ground. Over hills and mountains, they can reach ground level when they may be called fog. Also, as fog lifts off the ground due to daytime heating, the fog forms a layer of low stratus clouds.
From National Weather Service (NWS) Jetstream – Online School for Weather
13-2
13.3 Cloud Levels. By convention, the part of the atmosphere in which clouds are usually present has been divided into three levels: high, middle, and low (see Table 13-2). Each level is defined by the range of heights at which the cloud of a certain type occurs most frequently. The levels overlap and their limits vary with latitude. The approximate heights of the limits are as follows:
Table 13-2. Approximate Height of Cloud Bases above the Surface
LEVEL POLAR REGIONS TEMPERATE REGIONS TROPICAL REGIONS High Clouds 10,000-25,000 ft (3-8 km) 16,500-40,000 ft (5-13 km) 20,000-60,000 ft (6-18 km) Middle Clouds 6,500-13,000 ft (2-4 km) 6,500-23,000 ft (2-7 km) 6,500-25,000 ft (2-8 km) Low Clouds Surface-6,500 ft (0-2 km) Surface-6,500 ft (0-2 km) Surface-6,500 ft (0-2 km)
13.4 Cloud Types. In each level, the clouds may be divided by type (genera). The definitions of the cloud types (given below) do not cover all possible aspects, but are limited to a description of the main types and of the essential characteristics necessary to distinguish a given type from another type having a somewhat similar appearance.
Clouds are identified based upon one’s observation point at a particular elevation. From sea level, one might observe stratus clouds enveloping the top of a mountain. However, on the mountaintop, one would observe and report that same cloud as fog.
The exception to this is cumulus or cumulonimbus clouds over a mountain. Even though these low-level clouds typically have bases under 6,500 feet (2,000 meters), if one were to observe them over a mountaintop, their base might be 12,000 feet (3,600 meters) or higher relative to a particular location closer to sea level. However, due to their location over the mountain, one would still call them cumulus or cumulonimbus clouds, as appropriate.
13.4.1 High Clouds. Cirrus (Ci), Cirrocumulus (Cc), and Cirrostratus (Cs) are high-level clouds.
They are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon. They are composed almost entirely of ice crystals.
13.4.1.1 Cirrus (Ci). Cirrus (Ci) (see Figure 13-1) is a cloud type composed of detached cirriform elements in the form of white, delicate filaments of white (or mostly white) patches, or of narrow bands. These clouds have a fibrous (hair-like) appearance and/or a silky sheen. Many of the ice crystal particles of cirrus are sufficiently large to acquire an appreciable speed of fall;
13-3 therefore, the cloud elements often trail downward in well-defined wisps called mares’ tails. Cirrus clouds in themselves have little effect on aircraft and contain no significant icing or turbulence.
Figure 13-1. Cirrus (Ci)
13.4.1.2 Cirrocumulus (Cc). Cirrocumulus (Cc) is a cirriform cloud type appearing as a thin, white patch, sheet, or layer of cloud without shading, and is composed of very small elements in the form of grains, ripples, etc.
The elements may be merged or separate, and more or less regularly arranged;
they subtend an angle of less than 1° when observed at an angle of more than 30° above the horizon.
Cirrocumulus (see Figure 13-2) may be composed of highly supercooled4 water droplets, as well as small ice crystals, or a mixture of both; usually, the droplets are rapidly replaced by ice crystals. Pilots can expect some turbulence and icing.
4 Supercooled water droplets are below freezing. Even though their temperature is below the freezing point, they have not turned into ice. See paragraph 18.2 and paragraph 18.3.2.1.
13-4 Figure 13-2. Cirrocumulus (Cc)
Cirrocumulus is not very common. It is composed of very small elements, which never show shading. The cloud is frequently associated with cirrus or cirrostratus.
13.4.1.3 Cirrostratus (Cs). Cirrostratus (Cs) (see Figure 13-3) is a cloud type appearing as a whitish veil, usually fibrous (hair-like) but sometimes smooth, that may totally cover the sky, and that often produces halo phenomena, either partial or complete. Cirrostratus occasionally may be so thin and transparent as to render it nearly indiscernible, especially through haze or at night.
At such times, the existence of a halo around the sun or moon may be the only revealing feature.
The angle of incidence of illumination upon a cirrostratus layer is an important consideration in evaluating the identifying characteristics.
When the sun is high (generally above 50° elevation), cirrostratus never prevents the casting of shadows by terrestrial objects, and a halo might be completely circular. At progressively lower angles of the sun, halos become fragmentary, and light intensity noticeably decreases. When near the horizon, cirrostratus may be impossible to distinguish from cirrus.
Cirrostratus clouds are composed primarily of ice crystals and contain little, if any, icing and no turbulence.
13-5 Figure 13-3. Cirrostratus (Cs)
13.4.2 Middle Clouds. Altocumulus (Ac), Altostratus (As), and Nimbostratus (Ns) are mid-level clouds. They are composed primarily of water droplets; however, they can also be composed of supercooled liquid water droplets and/or ice crystals when temperatures are below freezing. Altostratus is usually found in the middle level, but it often extends higher. Nimbostratus is almost invariably found in the middle level, but it usually extends into the other levels.
13.4.2.1 Altocumulus (Ac). Altocumulus (Ac) (see Figure 13-4) is a cloud type, white and/or gray in color, that occurs as a layer or patch with a waved aspect, the elements of which appear as laminae, rounded masses, rolls, etc. These elements usually are sharply outlined, but they may become partly fibrous or diffuse; they may or may not be merged.
Small liquid water droplets invariably compose the major part of the composition of altocumulus. This results in a sharp outline and small internal visibility. At very low temperatures, however, ice crystals may form. Pilots flying through altocumulus can expect some turbulence and small amounts of icing.
13-6 Figure 13-4. Altocumulus (Ac)
This altocumulus is a single level, the greater part of which is sufficiently transparent to reveal the position of the sun and moon. The clouds do not progressively invade the sky, and the individual elements change very little.
These thin altocumulus clouds usually produce a corona.
13.4.2.1.1 Altocumulus Lenticularis. Altocumulus Lenticularis, commonly known as Altocumulus Standing Lenticular (ACSL) (see Figure 13-5), are an orographic type of cloud. They often appear to be dissolving in some places and forming in others. They also often form in patches in the shape of almonds or wave clouds. These formations are caused by wave motions in the atmosphere and are frequently seen in mountainous or hilly areas. They may be triggered off by hills only a few thousand feet high and may extend downwind for more than 60 miles (100 kilometers). The cloud elements form at the windward edge of the cloud and are carried to the downwind edge where they evaporate. The cloud as a whole is usually stationary or slow moving. These clouds often have very smooth outlines and show definite shading.
The ACSL clouds indicate the position of the wave crests, but they do not necessarily give an indication on the intensity of turbulence or strength of updrafts and downdrafts. This is because the clouds depend on both lifting and moisture. A well-defined wave may be visible (i.e., ACSL cloud) in weak updrafts where there is an adequate supply of moisture, but may not be visible when the environment is very dry, even if the wave is intense.
13-7 Figure 13-5. Altocumulus Standing Lenticular (ACSL)
13.4.2.2 Altostratus (As). Altostratus (As) (see Figure 13-6) is a cloud type in the form of a gray or bluish (never white) sheet or layer of striated, fibrous, or uniform appearance. Altostratus very often totally covers the sky and may, in fact, cover an area of several thousand square miles. The layer has parts thin enough to reveal the position of the sun, and if gaps and rifts appear, they are irregularly shaped and spaced.
Within the rather large vertical extent of altostratus (from several hundred to thousands of feet), a very heterogeneous particulate composition may exist.
In this most complete case, there may be distinguished: 1) an upper part, mostly or entirely ice crystals; 2) a middle part, a mixture of ice crystals and/or snowflakes and supercooled water droplets; and 3) a lower part, mostly or entirely supercooled or ordinary water droplets. A number of partial combinations of these composition types may occur, but never an entire cloud like 3) above. The particles are widely dispersed enough so as not to obscure the sun except by its thickest parts, but rather to impose a ground-glass effect upon the sun’s image, and to prevent sharply outlined shadows from being cast by terrestrial objects. Halo phenomena do not occur. Pilots can expect little or no turbulence, but light to moderate icing in the supercooled water regions.
13-8 Figure 13-6. Thin Altostratus (As)
Thin altostratus usually evolves from the gradual thickening of a veil of cirrostratus. This means that unlike most clouds which increase in height as they grow, altostratus (and nimbostratus) grow as the base of the cloud lowers.
Altostratus is grayish or bluish color, never white, and the greater part is always translucent enough to reveal the sun (or moon) as through ground glass. Objects on the ground do not cast shadows and halo phenomena are never seen.
13.4.2.3 Nimbostratus (Ns). Nimbostratus (Ns) is a gray cloud layer, often dark, rendered diffuse by more or less continuously falling rain, snow, ice pellets, etc., which in most cases reaches the ground. It is not accompanied by lightning, thunder, or hail.
Nimbostratus is composed of suspended water droplets, sometimes supercooled, and of falling raindrops and/or snow crystals or snowflakes.
It occupies a layer of large horizontal and vertical extent. The great density and thickness (usually many thousands of feet) of this cloud prevent observation of the sun. This, plus the absence of small droplets in its lower portion, gives nimbostratus the appearance of dim and uniform lighting from within. It also follows that nimbostratus has no well-defined base, but rather a deep zone of visibility attenuation. Frequently a false base may appear at the level where snow melts into rain. It is officially classified as a middle cloud although it may merge into very low stratus or stratocumulus. Other cloud classification systems may identify it as a low-level cloud. Nimbostratus produces very little turbulence, but can pose a serious icing problem if temperatures are near or below freezing.