The Sky Handbook

Snow ain usually begins as snow in clouds and drifts down before melting to become rain, but in colder conditions it will continue to fall in those delightful crystals which create the wonder of Christmas in the northern hemisphere! Most people love snow, especially children who get days off school to build snowmen and go sledding, but snow can be deadly in the form of avalanches and can cause havoc with our transport systems.

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NO TWO THE SAME?

no It’s a common saying that fact a two snowflakes are alike, discovered by the first s, photographer of snowflake Wilson Bentley (1865– 8, 1931) of Vermont. In 198 very t researchers discovered tha w sno small, hexagonal-prism But e. crystals could be the sam for the larger crystals, the ms traditional snowflakes, it see two no that Bentley was right; have ever been found to be identical.

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Right: A close-up of snow crystals.

Inset images, left, above, opposite: Microphotographs of a series of snowflakes, by Wilson Bentley.

He published photos of almost 2500 individual snowflakes. .

Ice crystals form into uniquely patterned aggregates called snowflakes, which, once they land and coalesce under pressure, form a snowpack. In extreme conditions, at our polar regions or in mountain valleys, this merges into thicker icepacks and glaciers.

Snowflake shapes are determined by temperatures. Between 32 and 26.6ºF (0 and -3ºC), water droplets freeze into planar or flattened ice crystals; between 26.6 and 17.6ºF (-3 and -8ºC) they add on needlelike prism arms; below 17.6ºF (-8ºC) they develop further plates and filigree crystals. The whole mixture can result in

a white kaleidoscope of repeating crystal patterns, the record for which was a snowflake of 15 inches in diameter (380mm) found in Fort Keogh, Montana in 1887!

It is something of a myth that the Inuit have 50 words for snow.

The various Inuit dialects are polysynthetic; that is, they string various words into another meaningful word, such as qinmiqtuqtuq (“to travel by dog team�). In reality the Inuit peoples have four groupings for snow: aput for lying snow, gana for falling snow, piqsirpoq for drifting snow, and qimuqsuq for a snowdrift. Combinations can then be made on these roots. In English, comparatively, we have the many distinct names for snow and its forms: snow, blizzard, flurry, hail, graupel, sleet, rime, hoar, frost, powder, firn, crust, crud, windslab, slush, snirt (snow mixed with dirt) and many more.

Opposite: Snow drifting against trees.

Below: Inuit building an igloo.

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Wind n Ireland there is an old expression that insists “pigs can see the wind,” and one wonders what fantastic shapes they must see.

We of course can’t see the movement of air, but we can certainly feel it and we know its effects all too well: inverted umbrellas, lost hats, dust devils, falling trees, shattered roof slates, tornadoes, hurricanes. Wind is the driving force behind our weather systems.

Wind is caused by pressure differentials, which themselves are caused by temperature differentials. Air moves from high-pressure gradients toward low-pressure systems, trying to balance itself and “spread out” into stillness. In the northern hemisphere, the coriolis effect of the earth’s curved surface means that the winds flow

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clockwise around high-pressure systems into low-pressure systems, where they change direction and swirl counterclockwise. This is reversed in the southern hemisphere, just like the similar reversing spirals of water flowing down a drain—clockwise south of the equator and counterclockwise north of the equator.

When two air systems come into close contact, high winds occur along the densest isobar lines; a bit like putting a baseball into an automatic pitcher, the spinning edges of the systems accelerate air masses into high winds. As weather systems pass over us, wind changes direction, either backing in a counterclockwise direction or veering in a clockwise direction. A low-pressure system to the west of you in the northern hemisphere brings more southerly winds, which will then turn into northerly winds as it passes eastward.

STORMY WEATHER

Above: A satellite image of a windstorm forming over Europe; while the continent is rarely affected by hurricanes, windstorms can cause severe damage.

Opposite: The familiar effects of a windy day.

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in Wind storms are most common e On . northern and central Europe hit of the most memorable to England in the last century was s the Great Storm of 1987. Thi n ther windstorm swept across sou 15 n tha England, uprooting more e million trees. Fortunately, ther rms were no fatalities. Windsto can be deadly; the Night of the in Big Wind (January 6, 1839) and Ireland killed between 250 s 300 people and ruined hundred of thousands of homes.

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Left: A cup anemometer, invented in 1846 by Irish physicist John Robinson.

Opposite: Stronger winds bring stronger waves.

Below: A windmill-type anemometer.

Wind speeds are measured by an anemometer and recorded on the Beaufort scale. This was created by Francis Beaufort (1774– 1857), a British Army admiral who in 1805 felt that sailors needed a scale to categorize wind speed at sea in order to set the appropriate number of sails. The scale runs from 0 to 12: at force 12, all sails would have to be stowed away; at Force 6, half of the sails could be set. The scale was originally measured in knots and was later adapted for land wind speeds as well. In 1946 it was extended from 12 to 17 to reflect the magnitude of wind speeds in tropical cyclones and typhoons. Force 6 corresponds to a wind speed of about 25 mph, and is described as producing spraycrested waves; Force 8 is an official gale of 45mph, when trees lose branches; Force 10 is a storm with 60 mph winds; Force 12 is simply 266

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known as hurricane force, with wind speeds of more than 70 mph.

Winds can also form on calm summer days due to convective thermal currents. We are most familiar with these as the frustrating sea breezes that can be strong enough to blow your hat off or knock the ice-cream off your cone. The reason for this is that on a summer’s day, the land heats up more quickly than water, thus creating low pressure. Air moves from over the water toward the low pressure to fill the void, building in the afternoons to a strong and familiar sea breeze. As night approaches, the convection cell dies off as land and sea equalize temperatures (creating the calm conditions around sunset), then during the night the land cools quicker than the sea and the reverse happens—we get a distinct land breeze moving toward the body of water.

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Thunder & Lightning umulonimbus clouds are our tallest atmospheric clouds and contain a concentration of energy released by the condensation that forms their turbulent, billowing clouds. When several cells of cumulus cloud build together, they can form a giant supercell of rapid convective thermals. This updraught turns into a 1.8 to 6-mile (3 to 10 km) rotational mesocyclone, with temperatures as low as -4ยบF (-20ยบC). The warm, moist air rises through this cyclonic core and condenses into water droplets and ice crystals. The cloud mass also begins to drag lighter, positively charged particles upward, leaving the base of the cloud as a negatively charged base of heavier particles. The massive polarities caused by the supercell release energy as the characteristic cracking or booming sound of thunder and the jagged electrical discharges of lightning.

Lightning can be seen sparking across the gap between the negatively charged cloud base and the positively charged earth below, accumulating to the point at which it discharges through the broken electrical resistance of the air in between. Lightning also travels upward from the top of the cloud to the atmosphere, though we can only see this

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Above: Cumulonimbus clouds, more commonly known as thunderclouds or stormclouds.

Left: A fireworks display (left in Perth, Australia.

On the right is a lightning display; faintly visible in the center is Comet McNaught.

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from aircraft or space photography. The heat generated by the lightning makes the air vibrate like a snare drum, and this after-effect we know as thunder. It is a common rule of thumb that every second that elapses between lightning and thunder is equivalent to one mile (1.6 km) distance from the source. All lightning forms fork or bolt formations, but is called sheet lightning when clouds obscure the forks and only a simple flash is seen. Lightning will strike the tallest available object to earth itself electrically, so in a thunderstorm it is wise not to carry any conducting objects like umbrellas, fishing rods or golf clubs. It is also unwise to shelter under trees or in cave mouths, as the lightning will still use you as a spark-plug to bridge the gap on its way to ground. Sitting in a car seat without touching the metal frame is perfectly safe, if a little scary.

Lightning can also form into unusual and rare ball lightning, which has been reported by witnesses as relatively slow-moving, footballsized globes that are sometimes stationery, or else track slowly through the air, almost as if conscious. They are suspected to be the phenomenon that gives rise to most presumed UFO sightings! As lightning is extremely hot, up to 54,000ยบF (30,000ยบC), it often blows the bark off trees as the sap inside is turned instantaneously to steam. When lightning hits sand, it fuses the silica into beautiful filigree shapes known as fulgurites, or petrified lightning.

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HOLY SMOKE

Above: Forked lightning strikes at night.

Left: Daytime lightning is often equally dramatic, especially against dark, moody skies.

One of the deadliest lightning strikes ever recorded was in Brescia, Italy, in 1769. Lightning struck the Church of San Nazaro; unfortunately a large quantity (99 tons, or 90,000 kg) of gunpowder was being stored in the church. The lightning strike started a fire, which ignited the gunpowder. The explosion killed more than 3,000 people and destroyed one-sixth of the city.

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Hurricanes urricanes, also known as tropical cyclones or typhoons, are enormous weather formations that we can see readily in photographs from space, appearing on our blue marble planet as great, spinning white wheels, each with a circular eye. They begin as cyclonic depressions over warm oceans and feed on moist air, which drives their convection engines. As warm air rises over an extremely low pressure point, the moist air condenses, spreads out and sinks, forming thunderclouds and fueling a positive feedback loop, which reinforces the storm system and builds the depression into an eventual hurricane. As warm air rises, it condenses and gives out energy, which drives the cyclone. The inner eye is conversely calm and clear, with high-pressure air forming the narrow column around which the storms gather and spin. The eyewall is a vortex of high, spinning winds, producing a dense area of storms. When these make landfall, they cause severe flooding, high winds and damage to buildings, like Hurricane Katrina, which devastated New Orleans and the Mississippi Delta in 2005.

Hurricanes vary considerably in size, with a core from 2 to 230 miles across and a radius from 100 miles to over 500 miles, spanning several degrees of latitude in their largest incarnations. In the northern hemisphere they are counterclockwise spirals, and in the southern hemisphere, clockwise. If you check the direction your water drains from your bathtub, and you will see confirmation of which hemisphere you live in!.

Hurricane season in the Atlantic runs from June to September, when ocean temperatures are at their highest. A hurricane’s track is determined initially by the coriolis effect, but is then compounded by higher steering winds such as jet streams and trade winds. Hurricanes tend to dissipate upon landfall, as the moist air they rely on for their convective engines is scarcer over dry land.

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THE MIGHTY WRATH OF ANGRY GODS

Above: An image of a hurricane storm system, taken from space.

Right: An example of the devastation that can be caused by a hurricane

In the northwestern Pacific , tropical cyclones are commo nly called typhoons. The nam e ultimately derives from the Greek god of the winds, Typhon, who caused hot winds. The wor d “hurricane” has a similar origin —it comes from Huracan, a storm god of the indigenou s peoples of the Caribbean.

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Storm Chasers & Tornadoes he modern fascination with storm-chasing has become, for some, a near-obsessive desire to witness and record the most powerful weather on Earth. To experience the raw power of nature first-hand is a dangerous, highly risky activity, but the study of such notoriously unpredictable events as tornadoes may lead to a safer, more predictable world in which perhaps we can avoid the sudden, seemingly vengeful destruction of these elemental forces.

Tornadoes are among our most spectacular and extreme weather phenomena. Twisters are chaotic nozzles of spinning air that rip through the lowlands of the USA in a band known as Tornado Alley.

They do occur globally (especially in Bangladesh and in northern Europe) and over oceans as waterspouts, but it is the central USA that

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Left: A tornado and rainbow, an unusual combination portending different fates.

Below: A map showing Tornado Alley, in the Heartland states.

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Above: A water spout is a tornado that forms over water.

Right and far right: Distinctive funnel clouds of tornadoes.

witnesses the most destructive and powerful examples. Tornadoes are measured on the Fujita scale, which runs from F0 to F5, with an F5 producing wind speeds of over 300 mph, capable of tossing cars the length of a football field. They can be whip-thin or broad, wedge tornadoes, and the reasons for their sudden dissipation are as mysterious and as unpredictable as those behind their formation.

The spinning air column of a tornado forms underneath cumulonimbus thunderstorm clouds. As high wind speeds build around a central low-pressure vortex, a condensation funnel forms and forces the updrafting vortex to spin faster. The supercell creates a mesocyclone, which as it advances pulls down a trailing sink of cold air known as a “rear flank downdraft.” When this pulls down the supercell’s rotating conewall and touches ground, it picks up debris, dust and, yes, even cows and trucks! It whips around with the iconic twister SOMETHING IN THE AIR

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motion, destroying anything in its path. Typically, a tornado will be up to 500 feet (150m) across and touch down across a length of up to 5 miles. The inside, or eye, of the tornado is a dark column of relatively calm, clear air, like the eye of a hurricane. Light will be obscured by the debris wall, and only lightning illuminates the mysterious interior of this monster. Tornadoes are always accompanied by supercell thunderstorms, lightning and cloudy blackout conditions, in which often only the onset of heavy hail suggests the tornado’s arrival.

The USA records the most tornadoes of any country, due to its topographical combinations. Tornadoes are formed when moist warm air meets cooler, drier air and this collision forms rapid thunderstorms and supercells. As the skies over the desert states contain dry air from the rain shadow of the Rockies, and the Gulf feeds in moist, tropical air, the Southwest is the perfect mixing bowl for tornadoes. The largest tornado ever recorded was the awesome F5 Tri-State Tornado of March 18, 1925, which lasted for over three hours, plowed its way unstoppably through Missouri, Illinois and Indiana and killed 695 people in its wake.

Storm-chasers are the most likely candidates for alleviating the murderous tracks of tornadoes. As they gather more and more data from their “Doppler on Wheels” mobile weather stations, these nationally funded radar vehicles might just be able to dissect the patterns and structures of tornadoes and provide ready warning of their paths.

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Opposite and below: Tornado funnels—and the devastation they leave in their wake.

SAFETY FIRST There are more than a few misconceptions about tornados and tornado safety. One of the most common is that opening the windows will lessen the damage caused by pressure changes that accompany tornadoes. However, the pressure change is not severe enough to cause a house to explode, and experts advise that rather than wasting time opening windows, it is better to seek shelter in an interior or underground room.

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