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001 The previous chapter covered the phase NWS JetStream changes of water and the release or absorption References: of latent heat in connection with them. These processes are key factors in determining (i) 003 NWS JetStream Homepage whether the atmosphere will be stable or unstable, (ii) whether clouds will form, and if 00 http://www.srh.noaa.gov/jetstream they do, what kinds they will be, and (iii) 004 Clouds whether it will rain or snow—all subjects dealt http://www.srh.noaa.gov/jetstream/ with in this chapter. synoptic/clouds.htm 002 Upon completion of this chapter the stu- 005 Precipitation dent should: http://www.srh.noaa.gov/jetstream/ • Be familiar with the concept of stability synoptic/precip.htm • Understand the four lifting processes 006 Weather Maps http://www.srh.noaa.gov/jetstream/ • Know the different lapse rates and types of synoptic/wxmaps.htm stability 007 Flash Floods • Recognize common clouds and what they http://www.srh.noaa.gov/jetstream/ indicate tstorms/flood.htm • Know how to estimate cloud base height • Be familiar with the various forms of winter precipitation Stability • Be aware of cold weather boating hazards 008 Moving air is normally associated with the wind—horizontal airflow. But the vertical movement of the air—particularly with respect

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to clouds and precipitation—is also a critical 013 When the air is unstable (i) clouds tend to determinant of the weather. The atmosphere is be puffy with high vertical development (some- said to be “stable” when there is relatively lit- times towering), (ii) any precipitation is usually tle vertical movement and “unstable” when heavy but varying in intensity and more local- there is a lot. ized (scattered), and (iii) the winds are gusty 009 An air parcel (just like a balloon) rises when and shift direction frequently. it is warmer (less dense) than the surrounding air and sinks when it is cooler (more dense). The analogy to a balloon is a good one because Lifting Processes an air parcel acts as if it had an imaginary elas- tic film around it with practically no heat 014 An air parcel does not rise unless something exchanged between it and the surrounding air. causes it to do so. The four basic lower level lifting mechanisms (processes) that can give an 010 Stable: When the atmosphere is stable an air air parcel an initial push upward are: parcel resists being pushed up or down. This • Orographic Lifting—wind blows an air par- happens when a rising air parcel becomes cel up an elevated terrain such as a moun- cooler (denser) and a sinking air parcel tainside (e.g., the western side of the Rocky becomes warmer (less dense) than the station- Mountains); ary surrounding air. 011 When the air is stable (i) clouds tend to be • Frontal Wedging—in a frontal weather in layers that are not very thick compared to system warm air slides up a wedge of how spread-out they are, (ii) any precipitation colder air or a wedge of cold air pushes up is usually light to moderate, steady, and wide- warmer air (e.g., the typical mid-latitude spread, and (iii) the wind is light to moderate frontal storms); and steady. • Surface Convergence—whenever air con- verges at the surface (such as in a low) air is 012 Unstable: When the atmosphere is unstable forced up because it cannot go down; and an air parcel has a tendency to continue to rise • Localized Convective Lifting—uneven heat- when pushed up or continues to sink when ing on the Earth’s surface can cause a parcel pushed down. This happens when a rising air of air to become warmer than the surround- parcel remains warmer (less dense) and a sinking air (e.g., a parking lot versus a wooded ing air parcel remains cooler (more dense) park). The parcel then rises much like a hot than the surrounding air. air balloon.

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015 The first three conditions are external lifting feet increase in altitude, i.e., on average the mechanisms and the last is the result of the air is cooler the higher up it is. The rate is parcel’s own buoyancy. positive only when there is a temperature inversion—the most stable condition. The existing Environmental Lapse Rate at a par- Lapse Rates ticular time and place is based on actual observations of temperatures at various alti- 016 How the air parcel behaves once it has tudes taken mainly by radiosondes attached been initially lifted depends on the relation- to weather balloons and by satellites. ships among various rates of temperature • Dry Adiabatic Lapse Rate—is the rate at change with altitude (lapse rates). There are which a rising parcel of unsaturated air cools three different lapse rates that must be taken or a sinking parcel warms. It is 5.5°F per into account: 1000 feet. • Environmental Lapse Rate—is the rate that • Wet Adiabatic Lapse Rate—is the rate at applies to a stationary column of air (i.e., the which a rising parcel of saturated air cools or currently existing column of air at a particu- a sinking parcel warms. The rising parcel lar place). This rate (also sometimes cools at a lower rate than the dry rate referred to simply as the “lapse rate” ) because of the warming effect of the latent varies with atmospheric conditions. The heat released during condensation. Simi- global average is a negative 3.5°F per 1000 larly, a sinking parcel warms at a lower rate

Stability, Clouds, and Precipitation 67

because of the cooling effect of the latent pushed up a mountain and sinks on the other heat absorbed during evaporation. The wet side. It is possible that the rising and sinking adiabatic lapse rate varies with the parcel’s adiabatic rates can differ on each side of the moisture content from 2.2°F(a lot of mois- mountain if the rising air loses moisture ture—i.e., water vapor) to a little less than through precipitation when the water vapor in the dry rate (little moisture). The average the rising air condenses. For example, on the rate is 3.2°F per thousand feet. windward side of the mountain the rising air cools at the dry adiabatic lapse rate (5.5°F/ 017 The latter two rates are called “adiabatic” 1000ft) until its temperature drops to the dew because no heat is added or removed in the point (the condensation level). Thereafter the temperature change. The temperature change air continues to cool but at the lower wet adia- occurs only because (i) a rising air parcel batic lapse rate (3.2°F/1000ft). expands and a sinking air parcel compresses 022 When the air sinks on the leeward side of as the surrounding air pressure changes with the mountain it warms at the higher dry adia- altitude, and (ii) expanding air cools and com- batic lapse rate (5.5°F/1000ft) all the way down pressing air warms. because the air having lost moisture never 018 A tire hand pump illustrates this adiabatic becomes saturated. The result is the air is process. When you pump up the tire the hand pump gets warm from the compression of the air and resulting increased motion of the gas molecules. When you let air out of the tire it is cold because the air expands as it comes out of the tire valve. 019 The critically important distinction to remember is: the environmental lapse rate applies only to air that is not moving up or down; and the two adiabatic lapse rates apply only to parcels of air that are moving up or down. 020 The simplest illustration of the adiabatic process is a balloon (an air parcel) going up and down. As the balloon rises it gets a little bigger and the air inside expands and cools. When the balloon sinks it gets a little smaller and the air inside compresses and warms. 021 A more complicated Adiabatic Cooling/Warming process can occur when air is © 1992, USA Today. Reprinted with permission Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 68

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warmer and drier on the leeward side of the examples of inversions are: radiation cooling of mountain. The Chinook (foehn) winds the ground on clear nights creating a cooler described in Chapter 2 are good examples of lower layer; and the advection of warm moist this adiabatic process. air from the Gulf of Mexico on top of a cold air layer above the mid-continent in winter. Types of Stability Absolute Instability

023 Once an air parcel has been initially 026 Absolute instability exists when a rising air pushed up by one of the four lifting mecha- parcel always remains warmer (less dense) nisms (orographic lifting, frontal wedging, sur- than the surrounding air and so it will continue face convergence, or localized convective lift- to rise. Technically “absolute instability” is ing), there are three different combinations of defined as the atmospheric condition when lapse rates that determine whether the atmos- both adiabatic rates are less than the environ- phere will be stable or unstable. mental lapse rate. 027 The intense solar heating that occurs on Absolute Stability clear summer days often results in temperature dropping very quickly with altitude. 024 Absolute stability exists when rising air Absolute instability usually occurs in the lower always becomes cooler (denser) than the sur- layer near the ground with the environmental rounding air. Air lifted from any level will sink lapse slowing at higher altitudes. Thermals, back when the lifting mechanism stops and the for example, are the result of absolute insta- air parcel is “on its own”. Technically “absolute bility over a localized area. stability” is defined as the atmospheric condition when both adiabatic lapse rates are Conditional Stability greater than the environmental lapse rate. 025 The slower the stationary surrounding air 028 Conditional stability exists when the cools with altitude (i.e., the lower the negative atmosphere is stable with respect to an unsatu- environmental lapse rate), the more likely it is rated air parcel but is unstable with respect to that the atmosphere will be stable. The most a saturated one. Technically “conditional stabil- stable condition is when the air actually warms ity” is defined as the atmospheric condition with altitude (a positive environmental lapse when the environmental lapse rate is between rate)—a temperature inversion. Two typical the dry and the wet adiabatic lapse rates.

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tively shallow temperature inversion induced by radiational cooling during the night keeps upper air from mixing with the surface layer. During the day, however, the environmental lapse rate becomes more strongly negative (the air cools more rapidly with altitude). The result is afternoon instability—a vertical mixing of lower and upper air layers with stronger winds from aloft reaching the surface. 031 Any factors that make the surface air warmer (or the air aloft colder) promote insta- Conditional Stability bility and any factors that cause the opposite promote stability. Similarly, smaller dew point 029 The following is an example of conditional spreads (i.e., the difference between air tem- stability: an unsaturated parcel of air is pushed perature and the dew point) favor instability up by one of the lifting mechanisms and as long because the chances that rising air will reach as it is unsaturated would sink back if the lift- the condensation level and then cool at the ing stopped; the parcel, however, is lifted high slower wet adiabatic rate are greater. The enough so that it reaches the lifting condensa- opposite (larger dew point spreads) make station level (i.e., the rising parcel’s air cools to its bility more likely. A big part of weather fore- dew point); the lifting process continues to casting depends on knowing and being able to push the now saturated parcel cooling at the predict the environmental lapse rates and dew lower wet adiabatic lapse rate up to the buoy- points. ancy level (i.e., the parcel is now as warm as the surrounding air); and thereafter the satu- Cloud Base rated parcel remains warmer than the surrounding air and continues to rise. Conditional 032 The height of the base of a cloud is at what Stability is a common state of the atmosphere. meteorologists call the lifting condensation 030 Atmospheric stability changes primarily level—the altitude at which a rising unsatu- with changes in the environmental lapse rate. rated parcel of air is cooled to its dew point. This principle is illustrated by a fairly typical The cloud base’s height can be estimated from daily occurrence. In the early morning there is the dew point temperature of the air at the stability as evidenced by little wind. A rela- surface. The calculation involves combining

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two different lapse rates—the dry adiabatic aloft. For example, flat layered clouds indicate lapse rate and the dew point lapse rate. The stable atmospheric conditions. In contrast, latter is the rate at which the dew point tem- lumpy piled up clouds are evidence of instabil- perature of a rising air parcel decreases with ity. Particular cloud types tend to be indica- altitude. The average dew point lapse rate is tors of particular weather patterns. Conse- -1°F per 1000 feet. quently, it is no wonder that a fairly 033 Below a cloud, rising air cools at the dry discriminating system for classifying different adiabatic lapse rate (5.5°F/1000ft) and its dew types of clouds has developed. point lowers at the dew point lapse rate (1°F/1000ft). In other words the temperature of a rising air parcel is approaching the lifting condensation level at the rate of 4.5°F/1000ft. Cloud Classification To estimate the cloud base height, divide the surface dew point spread by 4.5°F and multi- 037 An English scientist, Luke Howard, ply by 1000 feet. The following calculation devised the essentials of the present cloud illustrates the method: classification system in 1803. While a wide assortment of descriptive Latin prefixes and Temp 67°F suffixes are used to name cloud types, the Dew Pt 40°F basic criteria are only two: height and shape. DP Spd 27°F Cl Base: 27°F / 4.5°F = 6 Cloud Height 6000ft 038 Textbooks vary on the altitude criteria for classifying low, middle and high clouds. NOAA/ Clouds NASA’s Sky Watcher Chart, for example, uses broad ranges for high and middle cloud bases. 034 A cloud is an aggregation of billions of vis- For height classification purposes this manual ible minute water droplets or ice crystals (or a uses 6,500 and 20,000 feet as defining limits. mixture of both) that are small enough to stay They serve as rough approximations. suspended in air. Tiny particles called cloud • High: clouds with bases above 20,000 ft. in condensation nuclei (e.g., dust, smoke, salt the mid-latitudes (“cirro” prefix). All high particles) serve as surfaces on which the clouds are thin and white and made up of ice droplets form. When these microscopic parti- crystals. They generally do not produce pre- cles are absent the relative humidity can cipitation; exceed 100%. 035 Clouds are white because water droplets • Middle: clouds with bases between 6,500 and nd and ice crystals scatter all visible colors of sun- 20,000 ft. (“alto” prefix—2 highest voice in light (white) as compared to the smaller mole- the choir). They are generally composed of cules that scatter mostly blue light making the supercooled water droplets or a mixture of sky blue. Thicker clouds made up only of water water droplets and ice crystals, likely to be droplets can partially block sunlight giving thicker than high clouds and often associ- them a darker more opaque appearance. ated with light precipitation; 036 In the absence of instruments, clouds are • Low: clouds with bases under 6,500 ft. the most useful aid to forecasting because (“strato” or no prefix) are generally com- they are visible clues as to what is going on posed of water droplets; and Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 71

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• Vertical: clouds with bases as low as 1500 ft. and tops that can be above 50,000 ft. They are associated with unstable air.

Cloud Shape

• Cirrus: thin wispy fibers or veil- like patches. • Stratus: layered or stratified— sheets; and • Cumulus: piled up or lumpy—cauliflower-like structure with a flat base.

039 In addition, a cloud producing precipitation has “nimbo” added at the beginning or “nimbus” added at the end. When the wind tears clouds into fragments the descriptive prefix “fracto” or the suffix “fractus” is often added. The naming system is not precise and there are odd combinations (e.g., stratocumulus). Cloud Shapes

Principal HIGH Cloud Types

040 There are ten principal cloud types. They MIDDLE are listed here with representative photo- graphs and the identifying letter codes used by meteorologists. LOW High

Cloud Height Chart Cirrus (Ci) 041 Cirrus clouds are thin and wispy with a fil- ament appearance. When the filaments are hooked they are called “mares’ tails”. Cirrus clouds usually indicate immediately stable fair weather but can precede a warm front bring- ing a weather change within 24 hours.

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Cirrostratus Cirrocumulus © Robert J. Sweet. Reprinted with permission © Robert J. Sweet. Reprinted with permission

Contrails Altostratus ©Marcus Milling American Geological Institute. © Marcus Milling, American Geological Institute. Reprinted with permission Reprinted with permission.

Cirrostratus (Cs) pattern, however, cirrocumulus clouds warn of 042 Cirrostratus clouds are flat with a veil-like deteriorating weather. When seen together appearance. They do not blur the outline of the with hooked-shaped cirrus clouds sailors pre- moon or sun and are easy to recognize when pared for stormy conditions according to a they produce halos. When cirrus clouds mariners’ adage. thicken into mid-level altostratus clouds they 044 Mares’ tails and mackerel scales are an indication of a warm front approaching make tall ships trim their sails within 12 to 24 hours. Contrails Cirrocumulus (Cc) 045 Aircraft contrails are streams of ice crys- 043 Cirrocumulus clouds appear as thin and tals formed in the upper troposphere where lumpy small white flakes or globules. They are hot moist exhaust from jet engines mixes with often referred to as a mackerel sky because of nearly saturated very cold air. Long contrails their likeness to bands of fish scales. Cirrocu- indicate high moisture content at the aircraft’s mulus clouds can send mixed messages. In the flying level and short or no contrails indicate winter they often indicate fair cold weather. If dry air. They are useful benchmarks for esti- they are part of a lowering thickening cloud mating the height of clouds. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 73

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Altocumulus Stratocumulus Courtesy NOAA © Robert J. Sweet. Reprinted with permission

Middle

Altostratus (As) 046 Altostratus clouds are flat and sheet-like with a blue-gray color. The sun is usually (and the moon may be) visible through them but with no edge and no halo. Altostratus clouds, however, can produce coronas. They are at the upper altitude range for middle clouds and are thicker and lower than cirrostratus. While light precipitation may accompany them, when Nimbostratus they are associated with a warm front and Courtesy NOAA thicken into darker nimbostratus, precipita- wool. When the edges of rolls join they can tion is likely to be long-lasting. give the sky a wavy appearance. Stratocumu- Altocumulus (Ac) lus clouds often foretell rain preceding a front. 047 Altocumulus clouds are fluffy and lumpy Nimbostratus (Ns) appearing as a layer of flattened globules. They 049 Nimbostratus clouds are a thick, dark, are generally composed of water droplets and gray and shapeless layer without a well are not associated with precipitation. They are defined base. They form in stable conditions easy to confuse with smaller less dense cirro- and are a main producer of precipitation. The cumulus and thicker denser stratocumulus. precipitation is usually light to moderate but Low widespread and long-lasting. Stratus (St) Stratocumulus (Sc) 050 Stratus clouds are uniformly gray with an 048 Stratocumulus clouds are low, lumpy, gray even base. The lowest of the low clouds they and quite thick. Their bottoms can have dis- occasionally produce light precipitation, but tinct gray or whitish patches with fairly well their limited depth prevents any significant defined rounded appearances. The clouds are amount. The outline of the sun may be visible arranged in groups, lines, or waves like twisted through stratus clouds. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 74

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Stratus Cumulus © Robert J. Sweet. Reprinted with permission © Robert J. Sweet. Reprinted with permission

storms and composed of ice crystals in the higher levels and water droplets in the lower levels. The upper parts have a fibrous texture and often have an anvil shape.

Precipitation

053 Even though all clouds are made up of water droplets or ice crystals, most clouds do not produce rain or snow because the droplets Cumulonimbus and crystals are so small they are practically Courtesy NOAA suspended in the air. Air currents can keep them from descending. But even without sup- Vertically Developed port from air currents they still do not reach the surface. The cloud droplets and crystals are so light they descend at extremely low Cumulus (Cu) speeds and evaporate long before reaching the 051 Cumulus clouds are individual masses that ground. Only when the droplets or ice crystals have a cauliflower like appearance with flat are large enough does gravity cause them to bases and domed tops. They are fair weather fall fast enough for them to make it to the sur- clouds that are convectively generated and can face. How they grow larger is a process that is reach heights of 16,000 feet. Cumulus clouds complicated and has baffled meteorologists for typically appear in the morning, grow through- a long time. out the day and dissolve at night. 054 Although this manual generally uses the English unit system, it is quite inconvenient Cumulonimbus (Cb) when dealing with small sizes. For the topic of 052 Cumulonimbus clouds are billowy moun- precipitation fractions of an inch have been tainous looking heaps that can extend to the largely abandoned in favor of the metric sys- upper edges of the troposphere (50,000 feet) or tem’s millimeter (1mm = 1 millimeter = .001 higher. They are dramatic examples of atmos- meters = 0.04 inch). pheric instability, associated with thunder- 055 Differences in size and mass matter a lot Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 75

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when it comes to precipitation and its causes. 14°F or below, water droplets exist at temper- Some typical diameters are: atures below 32°F. 058 Observations made from aircraft show that ° ° cloud droplet 0.02 mm in temperatures between 32 F and 14 F, clouds usually consist almost entirely of super- human hair 0.08 mm* cooled water droplets. In the range of +14°F drizzle drop 0.50 mm to -4°F, clouds are a mixture of supercooled typical rain drop 2.00 mm water droplets and ice crystals. At still lower ° large rain drop 5.00 mm temperatures, below -4 F, water droplets are never found. snow 1-20 mm hail 5-100 mm The Bergeron Process *for comparative purposes 059 When a cloud consists of water vapor, 056 The above numbers can be misleading. If water droplets and ice crystals, the solid ice you compare volume rather than diameter, it crystals grow at the expense of the liquid actually takes about 1 million cloud droplets to droplets in a process called the Bergeron make up a typical raindrop. process after the Swedish meteorologist who 057 Clouds that form in saturated air when the discovered it. The process depends on the temperature is below 32°F are not necessarily existence in cold clouds of liquid water composed of ice crystals. Often they consist of droplets at below freezing temperatures supercooled liquid droplets. Because there is a (supercooled). scarcity of freezing nuclei (the low tempera- 060 The key factors in the process are these: ture counterparts of condensation nuclei) in the atmosphere and freezing nuclei do not gen- • initially supercooled water droplets far out- erally become active until temperatures of number ice crystals in cold clouds because condensation nuclei are more abundant than active freezing nuclei; • at subfreezing temperatures aloft conditions can be such that supercooled water droplets change phase to water vapor (evaporation) while at the same time water vapor molecules change phase to ice crystals (deposition); • the removal of water vapor through deposition means that the air does not become saturated with water vapor as soon as it other- wise would and so evaporation continues; • the continuing evaporation provides more water vapor that can then become ice crystals; • consequently, the number and size of the ice crystals grow at the expense of water droplets.

061 As the ice crystals grow larger and heavier they collide and coalesce with supercooled Cloud & Rain Drops water droplets and other ice crystals. Eventu- © 1992, USA Today. Reprinted with permission ally the ice crystals become heavy enough to Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 76

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fall out of the cloud base as ice crystals. collision, moreover, coalescence may not occur. Depending upon the temperature and relative Meteorologists believe that coalescence may humidity of the air below the cloud, the ice be promoted when drops have opposite electri- crystals may not reach the ground or may cal charges. reach the ground as snow, rain or sleet. 064 To summarize, there are two processes that initiate precipitation—the Bergeron The Collision- process and the collision-coalescence process. Coalescence Process In the mid-latitudes where cold clouds (or their cold tops) dominate, the Bergeron process is 062 In warm clouds such as those in the tropics the main generator. In the tropics where warm cloud droplets can grow into raindrops by the clouds and special condensation nuclei exist in collision-coalescence process. Collisions are the right amount, the collision-coalescence infrequent when droplets are the same size process is an initiator. After the initial phase, because their velocities are about the same. however, in both the mid-latitudes and the When their sizes vary, however, the larger tropics, the additional growth necessary to heavier droplets fall faster than the smaller produce precipitation is the result of collision- ones and the frequency of collisions and hence, coalescence . coalescence increases. 063 The collision-coalescence process is quite complicated. The initial formation of large Forms of drops depends on the existence of special con- Precipitation densation nuclei. The “giant” sea-salt nuclei found in the tropics are ideal. Even with large 065 In the mid-latitudes precipitation mostly drops, however, collisions are inhibited by the begins as ice crystals, but the different forms air-streams around falling drops. Even after a and amounts of precipitation that reach the

Stability, Clouds, and Precipitation 77

ground vary depending on the temperature and Sleet (Ice Pellets) humidity of the air layer(s) below the clouds. 068 Sleet (also know as ice pellets) begins as Rain (Drizzle and Mist) snow that melts in a warmer layer, then freezes into solid drops in a colder lower air 066 Rain much of the time begins as snow that layer and finally reaches the ground as solid melts as it falls through a warmer air layer ice pellets. The pellets are usually around the and finally reaches the ground. The only dif- size of the original raindrops. ference among rain, drizzle and mist is the size of the liquid drops: raindrops are more Hail than 0.5mm (0.02 inches) in diameter; Drizzle drops are smaller; and mist drops are less 069 Hail consists of round pellets or irregu- than 0.05mm. Most rain originates in either larly formed lumps formed by the accretion of nimbostratus clouds or cumulonimbus clouds. successive shells of freezing water. The layers Mist generally originates in stratus or nimbo- build-up as the hailstones rise and sink in stratus clouds. cumulonimbus clouds with strong convective currents. When the hailstones get too heavy to Freezing Rain be supported by the convective currents, they fall out of the cloud base. Hail ranges in size 067 Freezing rain begins as snow that melts from that of a pea to a golf ball. Hailstones into rain in a warmer air layer, and then falls bigger than peas, however, are rare in most through a colder air layer where it becomes parts of the United States. Hail normally falls supercooled but does not freeze. The super- for a shorter period of time (e.g., a few min- cooled drops finally reach the ground where utes) than rain because a smaller part of the they instantly freeze upon contact with a solid storm has the right conditions to make hail- surface forming an ice glaze. Weather reports stones. often refer to freezing rain as an ice storm. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 78

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Snow 074 Glaze: is a coating of ice, generally clear, which forms on exposed objects by the freez- 070 Snow consists of ice crystals (snow flakes) ing of supercooled water drops deposited by that fall from clouds through only a cold air freezing rain. On boats, glaze forms into a coat- layer never melting before reaching the ing of solid ice that is much more difficult to ground. If the flakes originate in very cold break up and remove than rime. This, plus its (and hence relatively dry) air the snow is “pow- heavier weight for corresponding thickness, der” snow (light and fluffy). When they form makes it a greater marine hazard. The build- at warmer temperatures closer to freezing the up of either of them, however, can seriously flakes are larger and moister. While the mois- reduce a vessel’s stability. ture content of snow varies widely, a popular rule-of-thumb is that 10 inches of snow is the equivalent of one inch of rain. Winter Marine Concerns 071 Blizzard: Most stormy conditions are described in terms of wind speed and/or inches of rain or 075 The colder air temperatures in the winter snow. A blizzard, however is defined by the result in an increase in air density. Conse- extent to which wind driven snow, whether quently, for a given wind speed there is greater falling or blowing, limits visibility. The NWS physical discomfort and higher more violent defines blizzard conditions as winds blowing waves. The winds are also much stronger (the 35 mph (30 knots) or greater and visibility temperature contrasts are greater and hence reduced by snow in the air to less than a 1/4 the pressure gradients are more severe) and mile. Earlier definitions included a low temper- squalls and storms are more frequent. ature requirement, but it has been eliminated. 076 On inland waters there are special winter hazards: a cold spell can reduce the flow of a river (snow does not produce a run-off until a Winter Deposits thaw); thaws can quickly increase river flows and floating hazards; lakes may be drawn down 072 In the previous chapter both the formation in anticipation of thaw flooding; and detritus- of dew and frost were covered—dew being the laden water can deposit its load as rushing water condensation of water vapor and frost being enters into a still lake forming new gravel bars. the deposition of water vapor on a cold surface. 077 Safety considerations loom larger in win- Most commonly, the surface is chilled through ter. As discussed above riming and ice glaze radiational cooling of the ground at night. can jeopardize a boat’s stability. Ice can cause Because of the unique marine hazard they rep- unnoticed hull damage. Snow and ice on decks resent, two additional deposits are listed below. increase the risk of falling. Snow restricts visi- They are formed from supercooled water bility more than rain and makes obstacles droplets or drops—not from water vapor. appear farther away. 078 Winter conditions, moreover, directly 073 Rime: is a white, granular, frosty deposit affect our bodies and senses. People who are formed by the freezing of supercooled water cold are less vigilant (the senses are dulled); droplets on a surface. It results from the con- survival time in the water is reduced; exposure tact of the water droplets with objects such as in cold air below 24°F leads more quickly to trees, bushes, sheds or ship’s rigging when dehydration; and a combination of wind and the air temperature is below the freezing cold (the “wind chill” factor) can result in frost- point. bite and hypothermia from heat loss. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 79

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Wind Chill Index also contain data about clouds and precipitation. The back of the 1 Oct daily weather map 079 The Wind Chill Index is a way of indicating contains a detailed explanation of the symbol in temperature degrees the rate of heat loss codes and where they are placed on the model. from exposed skin in cold weather. It takes into account both wind and air temperature. To illustrate, if the air temperature is 0°F and Summary the wind speed is 20 mph, the Wind Chill Index would be -22°F—meaning body heat would be 082 The following facts and concepts in this lost as if the air temperature were -22°F with section should be familiar and understood by no wind. The index gives a sense of “how cold it all weather course students: feels”. It does not apply to inanimate objects. • The weather characteristics of atmospheric 080 Just as it does for other severe weather stability and instability conditions, the National Weather Service • The orographic, frontal wedging, surface issues advisories and warnings when wind chill convergence, and localized convective lifting temperatures reach critical thresholds. In processes addition to the equivalent heat-loss-tempera- • The differences among environmental, and tures, the NWS Windchill Chart also indicates wet and dry adiabatic lapse rates frostbite times. • Absolute stability, absolute instability, and conditional stability in the atmosphere Station Model • The 10 principal cloud types • The estimation of cloud base height 081 The previous chapters presented the sta- • The different types of precipitation and how tion model in stages covering temperature, they are produced pressure and pressure tendency, wind speed and direction, and dew point. Station models • Cold weather boating hazards

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Homework Chapter 4: Stability, Clouds and Precipitation

Stability Lifting Processes 1. Stable air is indicated by: 6. A converging flow of winds at the surface tends to create: a) good visibility. b) piled up clouds. a) instability, clouds, and precipitation. c) flat clouds. b) inversions and stability. d) strong winds. c) a change from warmer to colder weather. 2. A gusty, changeable wind is an indicator d) dry and warm conditions. of: 7. The following are lifting processes: a) stability. b) rain. a) orographic. c) instability. b) frontal wedging. d) warmer weather. c) surface convergence. d) all of the above. 3. Just offshore, a skipper notes a low, smooth layer of cloud and a gentle, Lapse Rates steady breeze. This indicates: 8. The environmental lapse rate applies to: a) squalls. a) a rising parcel of air. b) rain. b) a sinking parcel of air. c) instability c) a stationary column of air. d) stability. d) a parcel of air moving horizontally. 4. Atmospheric stability or instability can be determined: 9. An adiabatic lapse rate applies to: a) only by plotting radiosonde reports. a) a parcel of air moving horizontally. b) only by professional meteorologists. b) only a rising parcel of air. c) often just by visual observation. c) a rising or sinking parcel of air. d) often by visual observation, but only d) a stationary column of air. with pressure, temperature, and humidity measurements at the sur- 10. The average environmental lapse rate is: face. a) 5.5°C per thousand feet. 5. Turbulent updrafts and downdrafts are a b) 5.5°F per thousand feet. result of: c) 3.5°C per thousand feet. a) unstable atmospheric conditions. d) 3.5°F per thousand feet. b) stable atmospheric conditions. c) change of latitude. d) the hydrological cycle. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 81

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11. The rate at which ascending dry air cools 16. Clouds are useful in the forecasting of adiabatically: weather: a) is 5.5°C per thousand feet. a) only to the professional meteorolo- b) is 5.5°F per thousand feet. gist. c) is 3.5°C per thousand feet. b) only when complete data are avail- able concerning their height, temper- d) is 3.5°F per thousand feet. ature, and movement. 12. The measurements of temperature, pres- c) only when added to measurements of sure, and moisture made by a rising bal- pressure, temperature, and humid- loon-carried radiosonde may be used to ity. find the: d) even in the absence of instrumental a) declination rate. data. b) environmental lapse rate. 17. “ Nimbo” added to a cloud name c) condensation rate. means: d) fall rate. a) high clouds. 13. The difference between the dry adia- b) precipitation. batic rate of cooling and the moist adia- c) middle clouds. batic rate of cooling is: d) storm clouds. a) due to the latent heat released by 18. An example of clouds that have a thick condensation. horizontal layered appearance would b) called the normal lapse rate. be: c) called the condensation spread. a) cumulus type. d) caused by instability. b) cirrus type. Dew Point Rate/Cloud Base c) stratus type. d) nimbus type. 14. For every thousand feet of ascent, the dew point of rising unsaturated air: 19. Clouds that have a piled-up or lumpy a) increases 1°F. appearance would usually be called: b) increases 1°C. a) cumulus type. c) decreases 1°C. b) cirrus type. d) decreases 1°F. c) stratus type. d) nimbus type. Clouds/Cloud Classification 20. Clouds that have a thin wispy appear- 15. Most clouds and weather are concen- ance would usually be: trated in the: a) cumulus type. a) thermosphere. b) cirrus type. b) mesosphere. c) nimbus type. c) troposphere. d) none of the above. d) stratosphere. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 82

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21. Middle-level cloud bases range in height: 27. A thick, shapeless cloud sheet, dark gray in all directions, from which a steady rain a) below 6,500 feet. falls is named: b) between 6,500 feet and 20,000 feet. a) altostratus. c) above 20,000 feet. b) fractostratus. d) above 30,000 feet. c) nimbostratus. Principal Cloud Types d) cumulonimbus. 22. With what type of cloud would you nor- 28. Clouds of vertical development are: mally associate mares’ tails? a) cirroform clouds. a) cumulonimbus. b) stratiform clouds. b) thick altostratus. c) cumuliform clouds. c) cirrus. d) cirrus. d) fractostratus. 29. What atmospheric clue can one get from 23. Cirrus clouds consist of: observing long contrails? a) super cooled water vapor. a) the air at the aircraft’s flight altitude b) water droplets. is moist. c) fine mist. b) it depends upon how fast the airplane d) ice crystals. is moving. c) one can get no atmospheric clue from 24. Nimbostratus are low clouds that often observing long contrails. produce: d) an indication of wind flow at the a) halos. Earth’s surface. b) rainbows . Precipitation c) precipitation. d) beautiful sunsets. 30. In order for snow or rain to fall from a cloud: 25. In the summertime, mid-latitude, non- a) air must rise adiabatically. frontal cumulus clouds normally form over land: b) small water droplets or ice crystals must coalesce. a) around sunset c) air must sink adiabatically. b) during the night d) the wind velocity must be greater c) around sunrise than 10 kts. d) during the day 31. The controlling factor determining the 26. Halos around the Sun or Moon are seen in kind of snow flakes is: connection with: a) temperature. a) cirrostratus. b) intensity of rain. b) stratocumulus. c) barometric pressure. c) altostratus. d) wind. d) cumulus. Chapter 04.2011 Rev2:Layout 1 12/21/11 10:12 PM Page 83

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32. Hailstones: 35. Rime is a deposit formed when______comes in contact with a surface. a) are always smaller than 0.5 inch in diameter. a) freezing rain. b) freeze instantly upon contact. b) snow. c) begin as frozen raindrops that are c) supercooled water droplets. kept from falling by a thunder- d) sleet. storm’s updraft. d) is the term also used for “rime.” Winter Marine Concerns/ Wind Chill 33. According to a popular rule-of-thumb, 36. Waves are higher in the winter because: ten inches of wet snow will melt down to a) cold air makes the water thicker. the equivalent of ______inch(es) of rain. b) the fetch is greater. a) 1. c) the winds are stronger and the air is b) 3. denser. c) 5. d) the air is less dense. d) 7. 37. The wind chill factor: Winter Deposits a) is a concern only for those participat- ing in winter sports. 34. A winter deposit that can severely jeopardize the stability of a vessel is: b) can bring on frostbite and/or hypothermia more quickly. a) snow. c) affects a vessel’s hull as much as peo- b) glaze. ple. c) hail. d) is only a concern when it is below d) sleet. zero.