21-9 Figure 21-4. A TUTT Moves Eastward Across the Hawaiian Islands. Extensive Cloudiness Develops East of the Trough
TUTTs and lows aloft produce considerable amounts of rainfall in the Tropics, especially over land areas where mountains and surface heating lift air to saturation. Low pressure systems aloft contribute significantly to the 300+ inches of annual rainfall over the higher terrain of Maui and the big island of Hawaii. Other mountainous areas of the Tropics are also among the wettest spots on Earth.
21.2.3 Tropical Wave. Tropical waves (also called easterly waves) are common tropical weather disturbances, normally occurring in the trade wind belt. In the Northern Hemisphere, they usually develop in the southeastern perimeter of the subtropical high pressure systems.
They travel from east to west around the southern fringes of these highs in the prevailing easterly circulation of the Tropics. Surface winds in advance of a wave are somewhat more northerly than the usual trade wind direction. As shown in Figure 21-5, as the wave approaches, pressure falls; as it passes, surface wind shifts to the east-southeast or southeast. The typical wave is preceded by very good weather, but followed by extensive cloudiness (see Figure 21-6), and often by rain and thunderstorms.
The weather activity is roughly in a north-south line.
21-10 Figure 21-5. A Northern Hemisphere Easterly Wave Progressing from A–B
Note that winds shift generally from northeasterly to southeasterly. The wave moves toward the west, and is often preceded by good weather and followed by extensive cloudiness and precipitation.
Figure 21-6. Vertical Cross Section along Line A–B in Figure 21-5
Tropical waves occur in all seasons, but are more frequent and stronger during summer and early fall. Pacific waves frequently affect Hawaii; Atlantic waves occasionally move into the Gulf of Mexico, reaching the U.S. coast.
21.2.4 West African Disturbance Line (WADL). On occasion, a line of convection similar to a squall line moves westward off the continent at tropical latitudes into the oceanic trade winds. In the North Atlantic this is known as the West African Disturbance Line (WADL). A WADL can move faster than easterly waves at 20-40 mph. Some WADLs eventually develop into tropical storms or hurricanes.
21-11
21.2.5 Tropical Cyclones. Tropical cyclone is a general term for any low that originates over tropical oceans. Tropical cyclones are classified according to their intensity based on the average 10-minute wind speeds. Wind gusts in these storms may be as much as 50 percent higher than the average 10-minute wind speeds. Over the north Atlantic and northeast Pacific Oceans, tropical cyclone classifications are:
1. Tropical Depression—sustained winds up to 34 knots (64 kilometers per hour).
2. Tropical Storm—sustained winds of 35 to 64 knots (65 to 119 kilometers per hour).
3. Hurricane—sustained winds of at least 65 knots (120 kilometers per hour), or more.
In other regions of the world, different terms are used for tropical cyclones meeting hurricane strength, such as typhoon in the northwest Pacific, severe tropical cyclone in the southwest Pacific and southeast Indian Oceans (e.g., near Australia), severe cyclonic storm in the north Indian Ocean, and just tropical cyclone in the southwest Indian Ocean. The term super typhoon is used if the maximum sustained winds are at least 130 knots (241 kilometers per hour).
21.2.5.1 Development. The prerequisites to tropical cyclone development are optimum sea surface temperature under low-level convergence and cyclonic wind shear.
Favored breeding grounds are: shear lines, TUTTs, tropical waves, and lines of convection in low latitudes moving from the continent to the tropical ocean
(e.g., WADL).
The low-level convergence associated with these systems by itself will not support development of a tropical cyclone. The system must also have horizontal outflow (divergence) at high tropospheric levels. This combination creates a chimney, in which air is forced upward causing clouds and precipitation. Condensation releases large quantities of latent heat which raises the temperature of the system and accelerates the upward motion.
The rise in temperature lowers the surface pressure, which increases low-level convergence. This draws more moisture-laden air into the system. When these chain-reaction events continue, a huge vortex is generated which may culminate in hurricane force winds.
Figure 21-7 shows regions of the world where tropical cyclones frequently develop. Notice that they usually originate between latitudes 5° and 20°.
Tropical cyclones are unlikely within 5° of the Equator because the Coriolis force is so small near the Equator that it will not turn the winds enough for them to flow around a low pressure area. Winds flow directly into an equatorial low and rapidly fill it.
21-12 Figure 21-7. The Tracks of Nearly 150 Years of Tropical Cyclones and their Strength Weave Across the Globe
The map is based on all storm tracks available from the National Hurricane Center (NHC) and the Joint Typhoon Warning Center (JTWC) through September 2006. The accumulation of tracks reveals several details of hurricane climatology, such as where the most severe storms form and the large-scale atmospheric patterns that influence the track of hurricanes (from National Aeronautics and Space Administration (NASA)).
Note: See Table 21-1 for wind strength associated with each scale on the Saffir-Simpson Scale).
21.2.5.2 Movement. Tropical cyclones in the Northern Hemisphere usually move in a direction between west and northwest while in low latitudes. As these storms move toward the mid-latitudes, they come under the influence of the prevailing westerlies. At this time the storms are under the influence of two wind systems: the trade winds at low levels and prevailing westerlies aloft. Thus, a storm may move very erratically, and may even reverse course or circle. Finally, the prevailing westerlies gain control, and the storm recurves toward the north, then to the northeast, and finally to the east-northeast. By this time, the storm is well into mid-latitudes.
21.2.5.3 Decay. As the storm curves toward the north or east (Northern Hemisphere), it usually begins to lose its tropical characteristics and acquires characteristics of lows in middle latitudes. Cooler air flowing into the storm gradually weakens it. If the storm tracks along a coastline or over the open sea, it gives up slowly, carrying its fury to areas far removed from the Tropics. However, if the storm moves well inland, it loses its moisture source and weakens from
21-13 starvation and increased surface friction, usually after leaving a trail of destruction and flooding.
When a storm takes on middle latitude characteristics, it is said to be extratropical, meaning “outside the Tropics.” Tropical cyclones produce weather conditions that differ somewhat from those produced by their higher latitude cousins and invite our investigation.
21.2.5.4 Weather in a Tropical Depression. While in its initial developing stage, the cyclone is characterized by a circular area of broken to overcast clouds in multiple layers. Embedded in these clouds are numerous showers and thunderstorms. Rain shower and thunderstorm coverage varies from scattered to almost solid. Diameter of the cloud pattern varies from less than 100 miles in small systems to well over 200 miles in large ones.
21.2.5.5 Weather in Tropical Storms and Hurricanes. As cyclonic flow increases, the thunderstorms and rain showers form into broken or solid lines, paralleling the wind flow that is spiraling into the center of the storm. These lines are the spiral rain bands frequently seen on radar. These rain bands continually change as they rotate around the storm. Rainfall in the rain bands is very heavy, reducing ceiling and visibility to near zero. Winds are usually very strong and gusty and, consequently, generate violent turbulence. Between the rain bands, ceilings and visibilities are somewhat better, and turbulence generally is less intense.
Most tropical cyclones that form eyes do so within 48 hours of the cyclone reaching tropical storm strength. In the eye, skies are free of turbulent cloudiness, and wind is comparatively light. The average diameter of the eye is between 15 and 20 miles, but sometimes is as small as 7 miles and rarely is more than 30 miles in diameter. Surrounding the eye is a wall of cloud that may extend above 50,000 feet. This wall cloud contains deluging rain and the strongest winds of the storm. Maximum wind speeds of 175 knots have been recorded in some storms. See Figure 21-8 and Figure 21-9 below, which contain a radar display and satellite photograph of a mature hurricane, respectively. Note the spiral rain bands and the circular eye. Notice the similarity between these two figures.
Table 21-1 identifies the wind speed and characteristic house damage for each level on the Saffir-Simpson Hurricane Wind Scale.
Figure 21-8. Radar Image of Hurricane Katrina Observed at New Orleans, Louisiana, on August 29, 2005
BASE REFLECTIVITY
KLIX - NEW ORLEANS, LA 08/29/2005 13:54:29 GMT LAT: 30/20/13 N LON: 89/49/30 W ELEV: 138.0 FT Mississippi MODE/VCP: A / 121 ELEV ANGLE: 0.50 ° MAX: 59 dBZ RANGE 248 NM Legend: (Category) dBZ
(15) 75
(14) 70
(13) 65
(12) 60
(11) 55
(10) 50
(9) 45
(8) 40
(7) 35
(6) 30
(9) 20
(3) 15 ПОАА (2) 10
(1) 5 Figure 21-9. Hurricane Andrew Observed by Satellite in 1992 Hurricane Andrew NORA NOAA-12 AVHRR HRPT Multi-spectral False Color Image August 23, 1992 @ 12:31 UTC 21-14
21-15 Table 21-1. Wind Speed and Characteristic House Damage for the Saffir-Simpson Hurricane Wind Scale Saffir-Simpson Hurricane Wind Scale Wind speed Characteristic House Damage 5 ≥157 mph ≥137 knots ≥252 km/h Almost complete destruction of all mobile homes will occur, regardless of age or construction A high percentage of frame homes will be destroyed, with total roof failure and wall collapse.
Extensive damage to roof covers, windows, and doors will occur.
Large amounts of windborne debris will be lofted into the air.
Windborne debris damage will occur to nearly all unprotected windows and many protected windows.
4 130-156 mph 113-136 knots 209-251 km/h Nearly all older (pre-1994) mobile homes will be destroyed. A high percentage of newer mobile homes also will be destroyed. Poorly constructed frame homes can sustain complete collapse of all walls as well as the loss of the roof structure. Well-built homes also can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Extensive damage to roof coverings, windows, and doors will occur. Large amounts of windborne debris will be lofted into the air. Windborne debris damage will break most unprotected windows and penetrate some protected windows.
3 111-129 mph 96-112 knots 178-208 km/h Nearly all older (pre-1994) mobile homes will be destroyed. Most newer mobile homes will sustain severe damage with potential for complete roof failure and wall collapse. Poorly constructed frame homes can be destroyed by the removal of the roof and exterior walls. Unprotected windows will be broken by flying debris.
Well-built frame homes can experience major damage involving the removal of roof decking and gable ends.
2 96-110 mph 83-95 knots 154-177 km/h Older (mainly pre-1994 construction) mobile homes have a very high chance of being destroyed, and the flying debris generated can shred nearby mobile homes. Newer mobile homes can also be destroyed. Poorly constructed frame homes have a high chance of having their roof structures removed, especially if they are not anchored properly. Unprotected windows will have a high probability of being broken by flying debris. Well-constructed frame homes could sustain major roof and siding damage. Failure of aluminum, screened-in, swimming pool enclosures will be common.
1 74-95 mph 64-82 knots 119-153 km/h Older (mainly pre-1994 construction) mobile homes could be destroyed, especially if they are not anchored properly as they tend to shift or roll off their foundations. Newer mobile homes that are anchored properly can sustain damage involving the removal of shingle or metal roof coverings, and loss of vinyl siding, as well as damage to carports, sunrooms, or lanais. Some poorly constructed frame homes can experience major damage, involving loss of the roof covering and damage to gable ends, as well as the removal of porch coverings and awnings. Unprotected windows may break if struck by flying debris. Masonry chimneys can be toppled. Well- -constructed frame homes could have damage to roof shingles, vinyl siding, soffit panels, and gutters. Failure of aluminum, screened-in, swimming pool enclosures can occur.
22-1 CHAPTER 22. ARCTIC WEATHER
22.1 Introduction. Strictly speaking, the Arctic is the region shown below in Figure 22-1, which lies within the Arctic Circle (66.5° N latitude). However, it is loosely defined as the northern regions, in general. This chapter includes Alaskan weather, even though much of Alaska lies south of the Arctic Circle.
As an introduction to Arctic weather, this chapter surveys climate, air masses, and fronts of the Arctic, introduces you to some Arctic weather peculiarities, and discusses weather hazards in the Arctic.
Figure 22-1. The Arctic Circle
22.2 Climate, Air Masses, and Fronts. There are a number of factors that influence Arctic climate. Climate of any region, especially in the Arctic, is largely determined by the amount of energy received from the sun; however, local characteristics of the area, such as mountains and ice cover, also influence climate.
22-2
22.2.1 Long Days and Nights. A profound seasonal change in length of day and night occurs in the Arctic because of the Earth’s tilt and its revolution around the sun. Any point north of the Arctic Circle has fall and winter days when the sun stays below the horizon all day and has spring and summer days with 24 hours of sunshine when the sun stays above the horizon. The number of these days increases toward the North Pole; there the sun stays below the horizon for 6 months and shines continuously during the other 6 months.
Twilight in the Arctic is prolonged because of the shallow angle of the sun below the horizon. In more northern latitudes, it persists for days when the sun remains just below the horizon. This abundance of twilight often makes visual reference possible at night.
22.2.2 Land and Water. Figure 22-1 shows the water and the land distribution in the Arctic.
Arctic mountain ranges are effective barriers to air movement. Large masses of air stagnate over inland continental areas; thus, Arctic continental areas are air mass source regions.
A large portion of the Arctic Ocean is covered throughout the year by a deep layer of ice, known as the permanent ice pack. This ice pack goes through a seasonal cycle where ice melts in the spring and summer and increases in the fall and winter. Even though a large portion of the Arctic Ocean is ice-covered through much of the year, the ice and the water below contain more heat than the surrounding cold land, thus moderating the climate.
Oceanic and coastal areas have a milder climate during winter and a cool climate in summer. As opposed to large water bodies, large land areas show a more significant seasonal temperature variation.
22.2.3 Temperature. As one would expect, the Arctic is very cold in winter, but due to local terrain and the movement of pressure systems, occasionally some areas are surprisingly warm. During winter, coastal areas are warmer than the interior. During summer, interior areas are pleasantly warm due to many hours of sunshine, while coastal areas have relatively short, cool summers due to their proximity to water.
22.2.4 Clouds and Precipitation. Cloudiness over the Arctic is at a minimum during winter due to the ice pack being more widespread. Spring brings many cloudy days, with cloudiness reaching a maximum in summer and fall when a portion of the sea ice melts and exposes additional water in the Arctic Ocean.
During summer afternoons, scattered cumulus clouds forming over the interior occasionally grow into thundershowers. These thundershowers move generally from northeast to southwest in the polar easterlies, which is opposite the general movement in the mid-latitudes.
During the winter, polar lows can form over the open ocean. Polar lows are small, intense low-pressure systems that can develop rapidly when cold air flows over warm water.
They produce severe weather, strong surface winds, and heavy precipitation. Polar lows dissipate quickly once they move over land.
Precipitation in the Arctic can vary drastically per region, but is generally light. Some areas are known as polar deserts. In winter, the only precipitation received in the Arctic
22-3 is snow, while in summer precipitation falls mostly in the form of snow over ice caps and oceanic areas and rain over interior areas. Annual amounts of precipitation over the ice pack and along the coastal areas tend to be less than the interior areas.
22.2.5 Wind. Typically, Arctic winds are light. Strong winds occur more often along the coastal areas in fall and winter. Wind speeds are generally light in the continental interior throughout the entire year, but are normally at their strongest during summer and fall.
22.2.6 Air Masses—Winter. In winter, air masses form over the expanded ice pack and adjoining snow-covered land areas. These air masses are characterized by very cold surface air, very low humidity, and strong low-level temperature inversions.
Occasionally, air from unfrozen ocean areas flows northward over the Arctic. These intrusions of moist, cold air account for most of the infrequent wintertime cloudiness and precipitation in the Arctic.
22.2.7 Air Masses—Summer. During the summer, the top layer of the Arctic permafrost layer melts, leaving very moist ground, and the open water areas of the Polar Basin increase markedly. Thus, the entire area becomes more humid, relatively mild, and semi-maritime in character. The largest amounts of cloudiness and precipitation occur inland during the summer months.
22.2.8 Fronts. Occluded fronts are the rule. Weather conditions with occluded fronts are much the same in the Arctic as elsewhere: low clouds, precipitation, poor visibility, and sudden fog formation. Fronts are much more frequent over coastal areas than over the interior.
22.3 Arctic Peculiarities. Several Arctic phenomena are peculiar to the region.
22.3.1 Effects of Temperature Inversion. The Arctic experiences frequent low-level temperature inversions, which occur when temperature increases with height (i.e., cold air settled near the ground with warm air directly above). Inversions can slow down surface winds and trap pollutants, creating smoggy and hazy conditions that persist until the inversion ends.
In addition, light rays are bent as they pass at low angles through the inversion, creating an effect known as looming, which is a form of mirage that causes objects beyond the horizon to appear above the horizon. These low-level inversion mirages distort the shape of the sun, moon, and other objects.
22.3.2 Light Reflection by Snow-Covered Surfaces. Much more light is reflected by snow-covered surfaces than by darker surfaces. Snow often reflects Arctic sunlight sufficiently to blot out shadows, thus decreasing the contrast between objects. Dark, distant mountains may be easily recognized, but a crevasse normally directly in view may be undetected due to lack of contrasts.
22.3.3 Light from Celestial Bodies. Illumination from the moon and stars is much more intense in the Arctic than in lower latitudes. Even illumination from the stars creates visibility far beyond that found elsewhere. Only under heavy overcast skies does the night darkness in the Arctic begin to approach the degree of darkness in lower latitudes.
22-4
22.4 Weather Hazards. Weather hazards include visibility restricting phenomena, blowing snow, icing, frost, and lack of contrast (whiteout).
22.4.1 Fog and Ice Fog. Fog occurs when water droplets or ice particles are suspended in the air at the Earth’s surface. Water-droplet fog occurs in coastal areas during the summer.
Ice fog (also called ice-crystal fog, frozen fog, frost fog, frost flakes, air hoar, rime fog, and pogonip) is a type of fog formed by direct freezing of supercooled water droplets.
Ice fog is composed of suspended particles of ice, partly ice crystals 20-100 micron in diameter, but chiefly (especially when dense) ice particles about 12-20 micron in diameter. It occurs at very low temperatures, and usually in clear, calm weather in high latitudes. The sun may cause the appearance of a halo. Effective visibility is reduced considerably more when looking toward the sun. Ice fog is rare at temperatures warmer than -30 °C, and increases in frequency with decreasing temperature until it is almost always present at air temperatures of -45 °C in the vicinity of a source of water vapor.
Such sources of water vapor are the open water of fast-flowing streams or of the sea, herds of animals, volcanoes, and especially products of combustion for heating, automobiles, and aircraft. At temperatures warmer than -30 °C, these sources can cause steam fog of liquid water droplets, which may turn into ice fog when cooled.
22.4.2 Blowing and Drifting Snow. Over the frozen Arctic Ocean and along the coastal areas, blowing snow, drifting snow, and strong winds are common hazards during fall and winter. Blowing snow is wind-driven falling or accumulated snow that reduces surface visibility. Drifting snow is an uneven distribution of snowfall or snow depth caused by strong surface winds, which may occur during or after a snowfall. Drifting snow is usually associated with blowing snow. In the Arctic, because the snow is dry and fine, it can be picked up easily by light winds that raise the snow several feet off the ground, obliterating objects. A sudden increase in surface wind may cause an unlimited visibility to drop to near zero in a few minutes. This sudden loss of visibility occurs frequently without warning in the Arctic.
22.4.3 Frost. Frost is the formation of thin ice crystals on the ground or other surfaces on solid objects below the freezing point of water. It develops in Arctic coastal areas during spring, fall, and winter.
22.4.4 Whiteout. Whiteout is a visibility-restricting phenomenon that occurs most often in the spring and fall in the Arctic when the sun is near the horizon. It occurs when a layer of overcast clouds overlies a snow- or ice-covered surface. Parallel rays of the sun are broken up and diffused when passing through the cloud layer so that they strike the snow surface from many angles. The diffused light then reflects back and forth countless times between the snow and the cloud, eliminating all shadows. The result is a loss of depth perception. Buildings, people, and dark-colored objects appear to float in the air, and the horizon disappears.
23-1 CHAPTER 23. SPACE WEATHER
23.1 The Sun—Prime Source of Space Weather. The sun is the dominant source of the conditions commonly described as space weather. The term space weather is used to designate processes occurring on the sun and in Earth’s magnetosphere, ionosphere, and thermosphere, which have the potential to affect the near-Earth environment.
Emissions from the sun are both continuous (e.g., solar luminescence and solar wind) and eruptive (e.g., coronal mass ejections (CME) and flares). These solar eruptions may cause radio blackouts, magnetic storms, ionospheric storms, and radiation storms at Earth.
Similar to the charged particles that come from the sun, Galactic Cosmic Rays (GCR) are charged particles that originate in more distant supernovae and contribute to the space weather conditions near Earth. Essentially, these charged particles comprise a steady drizzle of radiation at Earth.
The sum of the solar and nonsolar components equal the full extent of the potential radiation dose received. The size of the GCR flux varies inversely with the sunspot cycle (sunspots are described in later paragraphs); that is, during sunspot minimums when the interplanetary environment near Earth is laminar and steady, the GCR component is large due to its easier access to the near-Earth environment. At sunspot maximum, the turbulence and energetics associated with solar eruptions reduce GCR access to the vicinity of the Earth.
23.2 The Sun’s Energy Output and Variability. The sun is a variable star. That means the balance between the continuous emissions and the eruptive emissions changes with time.
One metric that is commonly used to track this variability is the occurrence of sunspots.
Astronomers have made sunspot observations continuously for hundreds, maybe even thousands, of years. Though the underlying physics is still not well understood, on average sunspots come and go in an 11-year period. The magnitude and duration of individual cycles varies, but typically more eruptive events occur near the solar maximum, while few are observed near solar minimum. All solar electromagnetic emissions, from radio to x rays, are also stronger during solar maximum and less intense near solar minimum.
23.3 Sunspots and the Solar Cycle. Because space weather activity varies with sunspot activity, they are often used as a proxy index for changing space weather conditions. This is because sunspots, by their very nature, exist due to strong local magnetic fields. When these fields erupt, severe space weather can occur. While sunspots are easily seen, other events such as GCR, CMEs, and increased solar wind are more difficult to observe from the ground and may not be related to long historical records of sunspots.
23.4 Solar Wind. The solar wind is the continuous flow away from the sun of charged particles and magnetic field, called plasma. Solar wind is a consequence of the very high temperature of the solar corona and the resultant expansion of the plasma into space.