The Atmosphere: an Introduction to Meteorology Lutgens Tarbuck Tasa Twelfth Edition

The Atmosphere Lutgens et al. Twelfth Edition

The Atmosphere: An Introduction to Meteorology Lutgens Tarbuck Tasa Twelfth Edition

ISBN 978-1-29204-229-9

9 781292 042299 ISBN 10: 1-292-04229-X ISBN 13: 978-1-292-04229-9

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ISBN 10: 1-292-04229-X ISBN 10: 1-269-37450-8 ISBN 13: 978-1-292-04229-9 ISBN 13: 978-1-269-37450-7

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Copyright_Pg_7_24.indd 1 7/29/13 11:28 AM Forms of Condensation and Precipitation

TABLE 4 Types of Precipitation

State of Type Approximate Size Water Description Mist 0.005–0.05 mm Liquid Droplets large enough to be felt on the face when air is moving 1 meter/second. Associated with stratus clouds. Drizzle < 0.5 mm Liquid Small, uniform droplets that fall from stratus clouds, generally for several hours. Rain 0.5–5 mm Liquid Generally produced by nimbostratus or cumulonimbus clouds. When heavy, size can be highly variable from one place to another. Sleet 0.5–5 mm Solid Small, spherical to lumpy ice particles that form when raindrops freeze while falling through a layer of subfreezing air. Because the ice particles are small, any damage is generally minor. Sleet can make travel hazardous. Freezing Layers 1 mm–2 cm thick Solid Produced when supercooled raindrops freeze on contact with solid objects. Glaze Rain can form a thick coating of ice that has sufficient weight to seriously damage trees (Glaze) and power lines. Rime Variable accumulations Solid Deposits usually consisting of ice feathers that point into the wind. These delicate frostlike accumulations form as supercooled cloud or fog droplets encounter objects and freeze on contact. Snow 1 mm–2 cm Solid The crystalline nature of snow allows it to assume many shapes, including six-sided crystals, plates, and needles. Produced in supercooled clouds where water vapor is deposited as ice crystals that remain frozen during their descent. Hail 5–10 cm or larger Solid Precipitation in the form of hard, rounded pellets or irregular lumps of ice. Produced in large convective, cumulonimbus clouds, where frozen ice particles and supercooled water coexist. Graupel 2–5 mm Solid “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.

Snow Recall that at very low temperatures, the moisture content of air is low. The result is the generation of very light Snow is precipitation in the form of ice crystals or, more and fluffy snow made up of individual six-sided ice crys- often, aggregates of ice crystals. The size, shape, and con- tals (Figure 18). This is the “powder” that downhill skiers centration of snowflakes depend to a great extent on the covet. By contrast, at temperatures warmer than about −5°C temperature at which they form. (23°F), the ice crystals join together into larger clumps consisting of tangled aggregates of crystals. Snowfalls consisting of these composite snowflakes are generally heavy and Figure 17 Virga, Latin for “streak.” In the arid west, rain have a high moisture content, which makes them ideal for frequently evaporates before reaching the ground. (Photo by making snowballs. James Steinberg/Photo Researchers, Inc.) Students Sometimes Ask… What is the snowiest city in the United States? According to National Weather Service records, Rochester, New York, which averages nearly 239 centimeters (94 inches) of snow annually, is the snowiest city in the United States. However, Buffalo, New York, is a close runner-up.

Sleet and Freezing Rain or Glaze Sleet is a wintertime phenomenon that involves the fall of clear to translucent particles of ice. Figure 19 illustrates how sleet is produced: An above-freezing air layer must overlie a subfreezing layer near the ground. When the raindrops, which 166 Forms of Condensation and Precipitation

In January 1998 an ice storm of historic proportions caused enormous damage in New England and southeastern Canada. Five days of freezing rain deposited a heavy layer of ice on exposed surfaces from eastern Ontario to the Atlan- tic coast. The 8 centimeters (3 inches) of precipitation caused trees, power lines, and high-voltage towers to collapse, leav- ing over 1 million households without power—many for nearly a month following the storm (Figure 20 ). At least 40 deaths were blamed on the storm, which caused damages in excess of $3 billion. Much of the damage was to the elec- trical grid, which one Canadian climatologist summed up this way: “What it took human beings a half-century to con- struct, took nature a matter of hours to knock down.”

Hail Hail is precipitation in the form of hard, rounded pellets or irregular lumps of ice. Hail is produced only in large cumulonimbus clouds where updrafts can sometimes reach speeds approaching 160 kilometers (100 miles) per hour and where there is an abundant supply of supercooled water. Figure 21a illustrates this process. Hailstones begin as small embryonic ice pellets (graupel) that grow by collect- ing supercooled droplets as they fall through the cloud. If Figure 18 Snow crystals. All snow crystals are six sided, but they encounter a strong updraft, they may be carried up- they come in an inﬁ nite variety of forms. (Photo by Ted Kinsman/ ward again and begin the return downward journey. Each Photo Researchers, Inc.) 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 are often melted snow, leave the warmer air and encounter continues until the hailstone grows too heavy to remain the colder air below, they freeze and reach the ground as suspended by the thunderstorm’s updraft or encounters a small pellets of ice roughly the size of the raindrops from downdraft. which they formed. Hailstones may contain several layers that alternate be- Occasionally, the distribution of temperatures in a col- tween clear and milky ice (Figure 21b ). High in the clouds, umn of air is such that freezing rain, or glaze, results (Fig- rapid freezing of small supercooled water droplets traps air ure 16d ). In these situations, the raindrops become super- bubbles, which cause the milky appearance. By contrast, cooled because the subfreezing air near the ground is not clear ice is produced in the lower and warmer regions of thick enough to cause them to freeze. Upon striking objects the clouds, where colliding droplets wet the surface of the on Earth’s surface, these supercooled raindrops instantly hailstones. As these droplets slowly freeze, they produce turn to ice. The result can be a thick coating of glaze that has relatively bubble-free clear ice. sufficient weight to break tree limbs, down power lines, and Most hailstones have diameters between 1 centimeter make walking and driving extremely hazardous. (pea size) and 5 centimeters (golf ball size), although some can be as big as an orange or larger. Oc- casionally, hailstones weighing a pound or more have been reported; most of these are composites of several stones frozen together. Cold air The record for the largest hailstone Temperature less than ever found in the United States was set on 0°C (32°F) July 23, 2010, in Vivian, South Dakota. The Snow stone was over 20 centimeters (8 inches)

Rain Cold air Figure 19 Sleet forms when rain passes Warm air Temperature less than through a cold layer of air and freezes into Temperature greater than Sleet 0°C (32°F) ice pellets. This occurs most often in the winter, 0°C (32°F) (ice pellets) when warm air is forced over a layer of subfreezing air. 167 Forms of Condensation and Precipitation

Students Sometimes Ask… What is the difference between a winter storm warning and a blizzard warning? A winter storm warning is usually issued when heavy snow exceeding 6 inches in 12 hours or possible icing conditions are likely. It is interesting to note that in Upper Michigan and mountainous areas where snowfall is abundant, winter storm warnings are issued only if 8 or more inches of snow is expected in 12 hours. By contrast, blizzard warnings are issued for periods in which considerable falling and/or blowing snow will be accompanied by winds of 35 or more miles per hour. Thus, a blizzard is a type of winter storm in which winds are the determining factor, not the amount of snowfall.

Ice nucleus (graupel) Path of hailstones

0°C (32°F) Downdrafts

Updrafts

Hail shower

(a)

Figure 20 Glaze forms when supercooled raindrops freeze on contact with objects. In January 1998 an ice storm of historic proportions caused enormous damage in New England and southeastern Canada. Nearly ﬁ ve days of freezing rain (glaze) caused 40 deaths and more than $3 billion in damages, and it left millions of people without electricity—some for as long as a month. ( Photo by Syracuse Newspapers/The Image Works )

(b) in diameter and weighed nearly 900 grams (2 pounds). The stone that held the previous record of 766 grams (1.69 Figure 21 Hailstones. (a) Hailstones begin as small ice pellets pounds) fell in Coffeyville, Kansas, in 1970 ( Figure 21b ). that grow through the addition of supercooled water droplets as The diameter of the stone found in South Dakota also sur- they move through a cloud. Strong updrafts may carry stones upward in several cycles, increasing the size of the hail by adding passed the previous record of a 17.8-centimeter (7-inch) a new layer with each cycle. Eventually, the hailstones encounter stone that fell in Aurora, Nebraska, in 2003. Even larger hail- a downdraft or grow too large to be supported by the updraft. stones have reportedly been recorded in Bangladesh, where (b) This cut hailstone, which fell over Coffeyville, Kansas, in a 1987 hailstorm killed more than 90 people. It is estimated 1970, weighed 0.75 kilogram (1.67 pounds). (Photo courtesy of that large hailstones hit the ground at speeds exceeding 160 University Corporation for Atmospheric Research/National Science kilometers (100 miles) per hour. Foundation/Visual Communications NCAR) 168 Forms of Condensation and Precipitation

Worst Winter Weather

xtremes, whether they be the Blizzard A winter storm char- tallest building or the record acterized by winds of at least 56 low temperature for a loca- kilometers (35 miles) per hour for tion, fascinate us. When it at least three hours. The storm must Ecomes to weather, some places take also be accompanied by low tem- pride in claiming to have the worst peratures and considerable falling winters on record. In fact, Fraser, and/or blowing snow that reduces Colorado, and International Falls, visibility to one-quarter mile or less. Minnesota, have both proclaimed themselves the “ice box of the Severe blizzard A storm with nation.” Although Fraser recorded winds of at least 72 kilometers (45 the lowest temperature for the 48 miles) per hour, a great amount of contiguous states 23 times in 1989, falling or drifting snow, and tem- its neighbor, Gunnison, Colorado, peratures –12°C (10°F) or lower. recorded the lowest temperature 62 times, far more than any other Heavy snow warning location. A snowfall in which at least 4 Such facts do not impress the inches (10 centimeters) in 12 hours residents of Hibbing, Minnesota, or 6 inches (15 centimeters) in 24 where the temperature dropped hours is expected. to –38°C (–37°F) during the first Freezing rain Rain falling in week of March 1989. But this a liquid form through a shallow is mild stuff, say the old-timers in subfreezing layer of air near the Parshall, North Dakota, where the ground. The rain (or drizzle) freezes temperature fell to –51°C (–60°F) on impact with the ground or other on February 15, 1936. Not to be objects, resulting in a clear coating left out, Browning, Montana, holds of ice known as glaze. the record for the most dramatic 24-hour temperature drop. Here Sleet Also called ice pellets. the temperature plummeted 56°C Sleet is formed when raindrops or (100°F), from a cool 7°C (44°F) FIGURE D A winter blizzard of historic proportions struck melted snowflakes freeze as they to a frosty –49°C (–56°F) during a Chicago Illinois, on February 2, 2011. (AP Photo/Kiichiro Sato). pass through a subfreezing layer of January evening in 1916. air near Earth’s surface. Sleet does Although impressive, the tem- So, determining which location has the not stick to trees and wires, and it usually perature extremes cited here represent only worst winter weather depends on how you bounces when it hits the ground. An accumu- one aspect of winter weather. What about measure it. Most snowfall in a season? Lon- lation of sleet sometimes has the consistency snowfall (Figure D )? Cooke City holds the gest cold spell? Coldest temperature? Most of dry sand. seasonal snowfall record for Montana, with disruptive storm? 1062 centimeters (418.1 inches) during the Travelers’ advisory An alert issued winter of 1977–1978. But what about cities Winter Weather Events to inform the public of hazardous driving like Sault Ste. Marie, Michigan, and Buffalo, Here are the meanings of some common conditions caused by snow, sleet, freezing New York? The winter snowfalls associated terms that the National Weather Service uses precipitation, fog, wind, or dust. with the Great Lakes are legendary. Even for winter weather events. larger snowfalls occur in many sparsely Cold wave A rapid fall of temperature inhabited mountainous areas. Snow flurries Snow falling for short dura- in a 24-hour period, usually signifying the Try telling residents of the eastern United tions at intermittent periods and resulting in beginning of a spell of very cold weather. States that heavy snowfall alone makes for generally little or no accumulation. the worst weather. A blizzard in March Wind chill A measure of apparent 1993 produced heavy snowfall along Blowing snow Snow lifted from the sur- temperature that uses the effects of wind with hurricane-force winds and record low face by the wind and blown about to such a and temperature on the human body by temperatures that immobilized much of the degree that horizontal visibility is reduced. translating the cooling power of wind to a region from Alabama to the Maritime Prov- temperature under calm conditions. It is an inces of eastern Canada. This event quickly Drifting snow Significant accumulations approximation only for the human body earned the well-deserved title Storm of the of falling or loose snow caused by strong and has no meaning for cars, buildings, Century. wind. or other objects.

169 Forms of Condensation and Precipitation

The destructive effects of large hailstones are well known, especially to farm- ers whose crops have been devastated in a few minutes and to people whose win- dows, roofs, and cars have been dam- aged (Figure 22 ). In the United States, hail damage each year can run into the hundreds of millions of dollars. One of the costliest hailstorms to occur in North America took place June 11, 1990, in Den- ver, Colorado, with total damage estimated to exceed $625 million.

Rime Rime is a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing. When rime forms on trees, it adorns them with its characteristic ice feathers, which can be spectacular to observe (Figure 23 ). In these situations, objects such as pine needles act as freezing Figure 22 Hailstorm damage to a greenhouse. (Photo by nuclei, causing the supercooled droplets to McPHOTO/KPA/agefotostock) freeze on contact. On occasions when the wind is blowing, only the windward surfaces of objects will accumulate the layer of rime.

Concept Check 5

1 Compare and contrast rain, drizzle, and mist. 2 Describe sleet and freezing rain and the circumstances under which they form. Why does freezing rain result on some occasions and sleet on others? 3 How does hail form? What factors govern the ultimate size of hailstones?

Precipitation Measurement The most common form of precipitation, rain, is probably the easiest to measure. Any open container that has a con- sistent cross section throughout can be a rain gauge (Figure 24a). In general practice, however, more sophisticated de- vices are used to measure small amounts of rainfall more accurately and to reduce loss from evaporation.

Standard Instruments The standard rain gauge (Figure 24b ) has a diameter of about 20 centimeters (8 inches) at the top. Once the water is caught, a funnel conducts the rain through a narrow open- ing into a cylindrical measuring tube that has a cross-sec- Figure 23 Rime consists of delicate ice crystals that form when tional area only one-tenth as large as the receiver. Conse- supercooled fog or cloud droplets freeze on contact with objects. quently, rainfall depth is magnified 10 times, which allows (Photo by Siepman/Photolibrary) for accurate measurements to the nearest 0.025 centimeter 170