Chapter 5 - Forms of Condensation and Precipitation

Question 1

Cloud

Answer 1

A visible aggregate of minute water droplets and/or ice crystals that are suspended in the atmosphere above Earth’s surface.

Question 2

What conditions must be met for condensation to occur?

Answer 2

1. The air must be saturated; and

2. There must be a surface on which water vapour can condense.

Question 3

Cloud Condensation Nuclei

Answer 3

Microscopic particles that serve as surfaces on which water vapour condenses.

Ex: Microscopic dust, smoke, and salt particles.

Question 4

Hygroscopic (Water-Seeking) Nuclei

Answer 4

Condensation nuclei having a high affinity for water, such as salt particles.

Question 5

Hydrophobic (Water-Repelling) Nuclei

Answer 5

Particles that are not efficient condensation nuclei. Small droplets will form on them whenever the relative humidity reaches 100%

Question 6

What is the classification of clouds based on?

Answer 6

Form and height.

Question 7

Cirrus (Ci)

Answer 7

Clouds that are high, white, and thin. They form delicate veil-like patches or wisplike strands and often have a feathery appearance (cirrus is latin for “curl” or “filament).

Composed of delicate, icy filaments.

Winds aloft often cause these fibrous ice trails to bend or curl. Cirrus clouds with hooked filaments are called “mares’ tails.”

Question 8

Cumulus (Cu)

Answer 8

Dense, billowy clouds often characterized by flat bases. May occur as isolated clouds or closely packed.

Clouds that consist of globular cloud masses that are often described as cotton like in appearance. Normally cumulus clouds exhibit a flat base and appear as rising domes or towers (cumulus means “heap” or “pile” in Latin) with tops that resemble a head of cauliflower.

Question 9

Stratus (St)

Answer 9

Clouds that are sheets or layers (strata) that cover much or all of the sky. Although there may be minor breaks, there are no distinct individual cloud units.

Resembles fog but does not rest on the ground.

May produce drizzle.

Question 10

High Clouds

Answer 10

Clouds with bases above 6000m.

Low temperatures and small quantities of water vapour present at high altitudes result in high clouds that are thin, and made up primarily of ice crystals.

Question 11

Middle Clouds

Answer 11

Clouds that occupy heights from 2000 to 6000m.

Question 12

Low Clouds

Answer 12

Clouds that form below 2000m.

Question 13

Clouds of Vertical Development

Answer 13

Clouds that extend upward to span more than one height range.

Question 14

What is the high cloud family?

Answer 14

Cirrus, cirrostratus, and cirrocumulus.

Question 15

Cirrostratus (Cs)

Answer 15

Thin sheet of white, ice-crystal clouds that may give the sky a milky look. Sometimes produce halos around the Sun and Moon.

Question 16

Cirrocumulus (Cc)

Answer 16

Thin, white, ice-crystal clouds in the form of ripples or waves, or globular masses all in a row. May produce a “mackerel sky.” Least common of high clouds.

Question 17

Are high clouds associated with precipitation?

Answer 17

Although high clouds are generally not precipitation makers, when cirrus clouds give way to cirrocumulus clouds they may warn of impending stormy weather. “Mackerel scales and mares’ tails make lofty ships carry low sails.”

Question 18

What is the middle cloud family?

Answer 18

Altocumulus and altostratus.

Question 19

Altocumulus (Ac)

Answer 19

White to gray clouds that tend to form in large patches composed of rounded masses or rolls that may or may not merge. Because they are generally composed of water droplets rather than ice crystals, the individual cells usually have a more distinct outline.

Question 20

Altostratus (As)

Answer 20

Stratified veil of clouds that is generally thin, covers large portions of the sky, and may produce very light precipitation. When thin, the Sun or Moon may be visible as a “bright spot,” but no halos are produced.

Question 21

What is the low cloud family?

Answer 21

Stratus, stratocumulus, and nimbostratus.

Question 22

Stratocumulus (Sc)

Answer 22

Soft, gray clouds in globular patches or rolls. Rolls may join together to make a continuous cloud.

Question 23

Nimbostratus (Ns)

Answer 23

Layers of dark grey clouds with no definite forms. One of the primary precipitation-producing clouds.

Question 24

How do nimbostratus clouds form?

Answer 24

Nimbostratus clouds form under stable conditions when air is forced to rise, as along a front. Such forced ascent of stable air leads to the formation of a stratified cloud layer that is large horizontally compares to its thickness. Precipitation associated with nimbostratus clouds is generally light to moderate but long duration and widespread.

Question 25

Which clouds are clouds of vertical development?

Answer 25

Cumulus and cumulonimbus.

Question 26

How do cumulus clouds form?

Answer 26

Cumulus clouds most often form on clear days when unequal surface heating causes parcels of air to rise convectively above the lifting condensation level.

Question 27

Cumulonimbus (Cb)

Answer 27

Towering cloud, sometimes spreading out on top to form an “anvil head.” Associated with heavy rainfall, thunder, lightning, hail, and tornadoes.

Question 28

Unicus

Answer 28

Cloud adjective meaning “hook shaped” that is applied to streaks of cirrus clouds that are shaped like a comma resting on its side.

Question 29

Fractus

Answer 29

Cloud adjective used to describe when stratus or cumulus clouds appear broken (or fractured).

Question 30

Mammatus

Answer 30

Cloud adjective used when clouds have protuberances on their bottom surface, similar to a row welder.

Associated with stormy weather and cumulonimbus clouds.

Question 31

Lenticular

Answer 31

Lens-shaped clouds.

Common in rugged or mountainous topographies, where they are called lenticular altocumulus.

Can form whenever the airflow undulates sharply in the vertical direction, they most frequently form on the leeward side of mountains. As air passes over mountainous terrain, a wave pattern develops. Clouds form when air is ascending, whereas areas with descending air are cloud free.

Question 32

Fog

Answer 32

A cloud with its base at or very near the ground.

Question 33

What is the main difference between fog and cloud?

Answer 33

The essential difference is the method and place of formation. While clouds result when air rises and cools adiabatically, fog results from cooling or when air becomes saturated through the addition of water vapour (evaporation fog).

Question 34

How is fog formed by cooling and what types of fog are formed this way?

Answer 34

When the temperature of a layer of air in contact with the ground falls below its dew point, condensation produces fog. Depending on the prevailing conditions, fog formed by cooling are called either radiation fog, advection fog, or upslope fog.

Question 35

Radiation Fog

Answer 35

Fog that results from radiation cooling of the ground and adjacent air.

It is a nighttime phenomenon requiring clear skies and a high relative humidity. Under clear skies, the ground and the air immediately above cools rapidly. Because of the high relative humidity, a small amount of cooling will lower the temperature to the dew point.

If the air is calm, the fog is usually patchy and less than 1 meter deep. For radiation fog to be more extensive vertically, a light breeze of 3 to 5 km/h is necessary, to create enough turbulence to carry the fog upward 10-30m without dispersing it. High winds, on the other hand, mix the air with drier air above and disperse the fog.

Because air containing the fog is relatively cold and dense, it flows downslope in hilly terrain. As a result, radiation fog is thickest in valleys whereas the surrounding hills may remain clear.

Normally, radiation fog dissipates within one to three hours after sunrise and is often said to “lift.” However, the fog does not actually “lift.” Instead, as the Sun warms the ground, the lowest layer of air is heated first, and the fog evaporates from the bottom up.

Question 36

Advection Fog

Answer 36

Fog that forms when warm, moist air blows over a cold surface becoming chilled by contact with the cold surface below.

A certain amount of turbulence is needed for proper development of advection fog; typically winds between 10 and 30 km/h are required. Not only does the turbulence facilitate cooling through a thicker layer of air, but it also carries the fog to greater heights. Thus, advection fogs often extend 300-600m above the surface and persist longer than radiation fog.

Question 37

Upslope Fog

Answer 37

Fog created when relatively humid air moves up a gradually slowing landform or, in some cases, up the steep slopes of a mountain. Because of the upward movement, air expands and cools adiabatically. If the dew point is reached, an extensive layer of fog will form.

Question 38

What are the types of evaporation fog?

Answer 38

Steam fog and frontal (precipitation) fog.

Question 39

Steam Fog

Answer 39

When cool air moves over warm water, enough moisture may evaporate from the water surface to saturate the air immediately above. As the rising water vapour meets the cold air, it condenses and rises with the air that is being warmed from below. Because the rising foggy air looks like steam, the phenomenon is called steam fog.

Common over lakes and rivers or clear, crisp morning in the autumn when the water is still relatively warm but the air is rather cold.

Steam fog is usually shallow because as it rises, the water droplets mix with the unsaturated air above and evaporate.

In a few settings, steam fogs can be dense, especially during the winter, as cold arctic air pours off the continents and ice shelves toward the comparatively warm open ocean. The temperature contrast between the warm ocean and cold air has been known to exceed 30*C. The result is an intense steam fog produced as the rising water vapour saturates a large volume of air. Because of its source and appearance, this type of fog is given the name arctic sea smoke.

Question 40

Frontal (Precipitation) Fog

Answer 40

Fog that occurs when raindrops falling from relatively warm air above a frontal surface evaporate into the cooler air below and cause it to become saturated.

Most common on cool days during extended periods of light rainfall.

Question 41

What two processes are responsible for the formation of precipitation?

Answer 41

The Bergeron process and the collision-coalescence process.

Question 42

Explain the Bergeron Process.

Answer 42

In the upper troposphere (where commercial aircraft cruise) the temperature typically approaches -50*C or lower, providing an ideal environment to initiate precipitation.

The process that generates much of the precipitation in the middle latitudes is named the Bergeron process and it depends on the coexistence of water vapour, liquid cloud droplets, and ice crystals.

Cloud droplets do not freeze at 0C. Pure water suspended in air does not freeze until it reaches ~-40C and is supercooled until that point.

Supercooled water readily freeze if it impacts an object. In the atmosphere, supercooled droplets freeze on contact with solid particles that have a shape that closely resembles that of ice (freezing nuclei).

Freezing nuclei are sparse in the atmosphere and do not generally become active until the temperature reaches -10C or below. Thus, at temperatures between 0 and -10C, clouds consist mainly of supercooled water droplets. Between -10 and -20C, liquid droplets coexist with ice crystals, and below -20C, clouds are generally composed entirely of ice crystals.

The saturation vapour pressure above ice crystals is slightly lower than above supercooled liquid droplets. This occurs because ice crystals are solid, so the individual water molecules are held together more tightly than those forming a liquid droplet. As a result, it is easier for water molecules to escape (evaporate) from the supercooled liquid droplets. Consequently, when air is saturated (100% relative humidity) with respect to liquid droplets, it is supersaturated with respect to ice crystals.

With all these facts in mind, the Bergeron process can be explained. Visualize a cloud at a temperature of -10*C, where each ice crystal is surrounded by many thousands of liquid droplets. Because the air is saturated with respect to liquid water, it will be supersaturated with respect to the newly formed ice crystals. As a result of this supersaturated condition, the ice crystals collect water molecules, lowering the relative humidity of the air. In response, water droplets shrink (evaporate) to replenish the lost water vapour. Thus the growth of ice crystals is fed to the continued evaporation of liquid droplets.

When ice crystals become large enough, they begin to fall. During their descent, the ice crystals grow as they intercept cloud droplets that freeze o them. Air movement will sometimes break up these delicate crystals, and the fragments will serve as freezing nuclei for other liquid droplets. A chain reaction ensues and produces many snow crystals, which, by accretion, form into larger masses called snowflakes which may consist of many individual ice crystals.

The Bergeron process can produce precipitation any time of year in the middle latitudes, provided that at least the upper portions of clouds are cold enough to generate ice crystals. The type of precipitation (snow, sleet, rain, or freezing rain) that reaches the ground depends on the temperature profile in the lower few kilometres of the atmosphere.

Question 43

Supercooled

Answer 43

The condition of water droplets that remain in the liquid state at temperatures well below 0*C.

Question 44

Freezing Nuclei

Answer 44

Solid particles that have a crystal form resembling that of ice; they serve as cores for the formation of ice crystals.

Question 45

Explain the collision-coalescence process.

Answer 45

Clouds made entirely of liquid droplets often contain some droplets larger than 0.02mm. These large droplets form when “giant” condensation nuclei are present or when hygroscopic particles exist (such as sea salt). Hygroscopic particles begin to remove water vapour from the air at relative humidities under 100%. Because the rate at which drops falls is dependent on their size, these “giant” droplets fall most rapidly.

As the larger droplets fall through a cloud, they collide with smaller, slower droplets and coalesce. They become larger in the process, and fall even more rapidly (or, in an updraft, they rise more slowly) increasing their chances of collision and rate of growth. After a million or so cloud droplets coalesce, a rain drop is large enough to fall to the surface without evaporating.

Because of the huge number of collisions required for growth to raindrop size, clouds that have great vertical thickness and contain cloud droplets have the best chance of producing precipitation. Updrafts also aid this process because the droplets can traverse the cloud repeatedly, which results in more collisions.

As raindrops grow in size, their fall velocity increases. This in turn increases the frictional resistance of the air, which causes the drop’s “bottom” to flatten out. As a drop approaches 4mm in diameter, it develops a depression. Raindrops can grow to a maximum of 5m when they fall at a rate of 33km/h. At this size, the water’s surface tension, which holds the drop together, is surpassed by frictional drag of the air. The depression grows almost explosively, forming a donutlike ring that immediately breaks apart. The resulting breakup of a large raindrop produces numberous smaller drops that begin anew the task of sweeping up cloud droplets.

The collision-coalescence process is not that simple, however. First, as the larger droplets descend, they produce an airstream around them that repels objects, especially small ones. Further, collision does not guarantee coalescence. If a droplet with a negative charge collides with a positively charge droplet, their electrical attraction may bind them together.

The collision-coalescence process is most efficient in environments where large cloud droplets are plentiful. The air over the tropics, particularly the tropical oceans, is an ideal setting: very humid and relatively clear air results in fewer condensation nuclei compared to the air over more populated regions. With fewer condensation nuclei to compete for available water vapour (which is plentiful), condensation is fast paced and produces comparatively few large cloud droplets. Within developing cumulus clouds, the largest drops quickly gather smaller droplets to generate the warm afternoon showers associated with tropical climates.

Question 46

Rain

Answer 46

Drops of water that fall from a cloud and have a diameter of at least 0.5 to 5 mm.

Most rain originates in either nimbostratus clouds or in cumulonimbus clouds.

Question 47

Cloudbursts

Answer 47

Unusually heavy rainfalls that originate in towering cumulonimbus clouds.

Question 48

Drizzle

Answer 48

Fine, uniform droplets of water having a diameter less than 0.5mm.

Generally produced in stratus or nimbostratus clouds, where precipitation may be continuous for several hours or for days on rare occasions.

Question 49

Vigra

Answer 49

Streaks of precipitation falling from a cloud that extend toward Earth’s surface without reaching it.

Occurs when rain evaporates before reaching the ground, which depends on the humidity of the air and the size of the drops.

Question 50

Fallstreaks

Answer 50

Wisps of ice particles falling from a cloud but not reaching Earth’s surface.

Occurs when ice crystals sublimate when they enter the dry air below.

Question 51

Mist

Answer 51

Precipitation containing the very smallest droplets able to reach the ground.

0.005 to 0.05mm

Associated with stratus clouds.

Question 52

Snow

Answer 52

Precipitation in the form of ice crystals to, more often, aggregates of ice crystals.

1mm- 2cm

Low temperatures have air with low moisture content which produces snow made up of individual six-sided ice crystals or “powder.” Warmer, moister air causes ice crystals to clump together making moist snow for snowballs.

Question 53

Sleet

Answer 53

Aka ice pellets.

A wintertime phenomenon that involves the fall of clear to translucent particles of ice.

Produced when an above-freezing air layer overlies a subfreezing layer near the ground. When the raindrops, which are often melted snow, leave the warmer air and encounter the colder air below, they freeze and reach the ground as small pellets of ice roughly the size of raindrops from which they formed.

0.5 - 5mm.

An accumulation of sleet sometimes has the consistency of dry sand.

Question 54

Freezing Rain (Glaze)

Answer 54

Occurs when raindrops become supercooled because the subfreezing air near the ground is not thick enough to cause them to freeze. Upon striking objects on Earth’s surface, these supercooled raindrops instantly turn to ice. The result can be a thick coating of glaze that can be quite heavy.

Layer 1mm to 2cm thick.

Question 55

Hail

Answer 55

Precipitation in the form of hard, rounded pellets or irregular lumps of ice.

5 - 10cm or larger.

Produced only in large cumulonimbus clouds where updrafts can sometimes reach speeds approaching 160 km/h and where there is an abundant supply of supercooled water.

Question 56

How is hail formed?

Answer 56

Hailstones begin as grapple that grow by collecting supercooled droplets as they fall through the cloud. If they encounter a strong updraft, they may be carried upward again and begin the return downward journey. Each trip through the supercooled portion of the cloud results in an additional layer of ice. Hailstones can also form from a single descent through an updraft. Either way, the process continues until the hailstone grows too heavy to remain suspended by the thunderstorm’s updraft or encounters a downdraft.

Question 57

Graupel

Answer 57

“Soft hail” that forms as rime collects on snow crystals to produce irregular masses of “soft” ice. Because these particles are softer than hailstones, they normally flatten out upon impact.

Question 58

Rime

Answer 58

A deposit of delicate crystals that point into the wind formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing.

Question 59

Snow Flurries

Answer 59

Snow falling for short durations at intermittent periods and resulting in generally little or no accumulation.

Question 60

Blowing Snow

Answer 60

Snow lifted from the surface by the wind and blown about to such a degree that horizontal visibility is reduced.

Question 61

Drifting Snow

Answer 61

Significant accumulations of falling or loose snow caused by strong wind.

Question 62

Blizzard

Answer 62

A winter storm characterized by winds of at least 56 km/h for at least 3 hours. The storm must also be accompanied by low temperatures and considerable falling and/or blowing snow that reduces visibility to one-quarter mile or less.

Question 63

Severe Blizzard

Answer 63

A storm with winds of at least 72 km/h, a great amount of falling or drifting snow, and temperatures 12*C or lower.

Question 64

Heavy Snow Warning

Answer 64

A snowfall in which at least 4 inches in 12 hours or 6 inches in 24 hours is expected.

Question 65

Travelers’ Advisory

Answer 65

An alert issued to inform the public of hazardous driving conditions cause by snow, sleet, freezing precipitation, fog, wind, or dust.

Question 66

Cold Wave

Answer 66

A rapid fall of temperature in a 24-hour period, usually signifying the beginning of a spell of very cold weather.

Question 67

Standard Rain Gauge

Answer 67

A gauge that has a diameter of about 20cm and funnels rain into a cylinder that magnifies precipitation amounts by a factor of 10, allowing for accurate measurement of small amounts.

Question 68

Trace of Precipitation

Answer 68

An amount of precipitation less than 0.025cm.

Question 69

Tipping-Bucket Gauge

Answer 69

A recording rain gauge consisting of two compartments (“buckets”) each capable of holding 0.025cm of water. When one compartment fills, it tips, and the other compartment takes it place.

Question 70

Weighing Gauge

Answer 70

A recording precipitation gauge consisting of a cylinder that rests on a spring balance. As the cylinder fills, the movement is transmitted to open that records the data.

Question 71

How is snowfall measured?

Answer 71

Depth and water equivalent are measured.

Depth is measured by taking several depth measurements in an open place, away from trees and obstructions, and then averaging them. To obtain the water equivalent, samples may be melted and then weighed or measured as rain.

Question 72

How does weather radar work?

Answer 72

A radar unit has a transmitter that sends out short pulses of radio waves. The specific wavelengths that are used depend on the objects the user wants to detect. These radio waves are able to penetrate small cloud droplets but are reflected by larger raindrops, ice crystals, and hailstones. The reflected signal, called an echo, is received and displayed on a TV monitor. Because the echo is “brighter” when the precipitation is more intense, modern radar is able to depict both the regional extent and rate or precipitation.

Question 73

Intentional Weather Modification

Answer 73

Deliberate human intervention to influence atmospheric processes that constitute the weather - that is, to alter the weather for human purposes.

Question 74

What are the three broad categories of weather modification?

Answer 74

1. The employment of energy to forcefully alter the weather i.e. mixing of air to disperse fog.
2. Modifying land and water surfaces to change their natural interaction with the lower atmosphere.
3. Triggering, intensifying, or redirecting atmospheric processes ie cloud seeding.

Question 75

Cloud Seeding

Answer 75

The introduction into clouds of particles (most commonly dry ice or silver iodide) for the purpose of altering clouds’ natural development.

Question 76

What is the most common method for fog/cloud dispersal? How does it work?

Answer 76

Spreading dry ice (solid carbon dioxide) into layers of supercooled fog or stratus clouds to disperse them and thereby improve visibility. Such applications trigger a transformation in cloud composition from supercooled water droplets to ice crystals. The ice crystals then settle out, leaving an opening in the cloud or fog.

Question 77

Frost

Answer 77

Ice crystals that occur when the temperature falls to 0*C or below.

Question 78

White Frost

Answer 78

Ice crystals that form on surfaces instead of dew when the dew point is below freezing.

Question 79

How is a frost hazard generated?

Answer 79

A frost or freeze hazard can be generated in two ways: when a cold air mass moves into a region or when sufficient radiation cooling occurs on a clear night. Frost associated with an invasion of cold air is characterized by low daytime temperatures and long periods of freezing conditions that inflict widespread crop damage. By contrast, frost induced by radiation cooling is strictly a nighttime phenomenon that tends to be confined to low-lying areas.

Question 80

In order for cloud seeding to work, what atmospheric condition must exist?

Answer 80

Supercooled water droplets.

Question 81

Describe one technique used to disperse “warm fog” to improve visibility.

Answer 81

Mixing drier air from above into the fog. When the layer of fog is very shallow, helicopters have been used. By flying just above the fog, the helicopter creates a strong downdraft that forces drier air toward the surface, where it mixes with the saturated foggy air.

Question 82

What are some methods of frost prevention?

Answer 82

Methods of frost prevention either conserve heat (reduce the heat lost at night) or add heat to warm the lowermost layer of air.

Heat-conservation methods include covering plants with insulating material, and generating particles that, when suspended in air, reduce the rate of radiation cooling.

Warming methods employ water sprinklers, air-mixing techniques, and/or orchard heaters. Sprinklers add heat in two ways: first, from the warmth of the water and, more importantly, from the latent heat of fusion released when the water freezes. As long as an ice-water mixture remains on the plants, the latent heat released will keep the temperature from dropping below 0*C.