Figure 12-3. Absolute Instability Example Environmental Parcel Temperature Parcel Air Temperature/ Difference Stability Temperature Dewpoint °C 5 0°C 717 -7°C Unstable 4 4°C 9/9 -5°C Unstable 1000's OF FEET 3 Lifting Condensation Level 8°C 11/11 -3°C Unstable (LCL) 2 12°C (14/11.5 -2°C Unstable 1 16°C 17/12 - 1°C Unstable 20°C 20/12.5
12.3.4 Conditional Instability. Conditional instability (see Figure 12-4) is the state of a column of unsaturated air in the atmosphere when its lapse rate of temperature is less than the dry adiabatic lapse rate, but greater than the moist adiabatic lapse rate. An air parcel lifted upward would be initially stable, but at some point above its Lifted Condensation Level (LCL) it would become unstable. The term conditional means the parcel must be lifted to a particular level (altitude) before it becomes unstable and rises because of its own buoyancy. The Level of Free Convection (LFC) is the level at which a parcel of air lifted dry adiabatically until saturated and moist adiabatically thereafter would first become warmer than the surrounding environmental air (i.e., unstable) in a conditionally unstable column of air in the atmosphere. The LFC is a defining feature of a conditionally unstable column of air.
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Figure 12-4. Conditional Instability Example Environmental Parcel Air Temperature/ Temperature Dewpoint °C 5 6°C 717 - 1°C 4 Level of Free Convection 9°C 9/9 0°C (LFC) 1000's OF FEET Lifting Condensation Level 12°C 11/11 JoLt Stable (LCL) 2 15°C (14/11.5 JoLt Stable 1 18°C 17/12 +1°C 20°C 20/12.5 12-5
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12.3.5 Summary of Stability Types. Figure 12-5 below summarizes the possible atmospheric stability types.
Figure 12-5. Stability Types
Temperature Sounding Unsaturated Parcel Saturated Parcel Stability Type A Unstable Unstable Absolute Instability B Neutral Unstable C Stable Unstable Conditional Instability D Stable Neutral E Stable Stable Lapse Absolute Stability F Stable Stable Isothermal G Stable Stable Inversion
12.4 Processes that Change Atmospheric Stability. Changes in atmospheric stability are inversely related to temperature (density) changes with height (see Figure 12-6).
If temperature lapse rates increase, then stability decreases. Conversely, if temperature lapse rates decrease, then stability increases. Most of these changes occur as a result of the movement of air, but diurnal (day/night) temperature variations can play a significant role.
Figure 12-6. Temperature Lapse Rate Effects on Stability Environmental Parcel Temperature Environmental Parcel Temperature Air Temperature/ Difference Air Temperature/ Difference Temperature Dewpoint°C Temperature Dewpoint °C 4 11°C 3/1 +8°C 7°C 8/1 - 1°C 3 12°C 6/1.5 +6°C 10°C 11/1.5 - 1°C 1000's OF FEET 2 13°C 9/2 +4°C 13°C 14/2 -1°C 14ºC 12/2.5 +2°C 16°C 17/2.5 -1°C 15°C 15/3 20°C 20/3 More Stable Less Stable The column of air on the right is less stable because its temperature lapse rate is higher.
12.4.1 Wind Effects on Stability. Wind can act to change the stability of a column of air in the atmosphere by changing the temperature lapse rate. Stability increases when wind blows colder air into the bottom of the air column (cold air advection) and/or warmer air at the top (warm air advection). Conversely, stability decreases when wind blows warmer air into the bottom of the air column and/or colder air at the top.
12.4.2 Vertical Air Motion Effects on Stability. A column of air in the atmosphere will become more stable when it descends (subsides) (see Figure 12-7). As it subsides, it becomes compressed by the weight of the atmosphere and shrinks vertically. The entire layer warms due to adiabatic compression. However, the upper part of the column sinks farther and, thus, warms more than the bottom part. This process acts to decrease the temperature lapse rate and increase stability.
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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.
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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
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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.