Predict The Weather (Part 1)

6/8/2012 11:21:30 AM

Getting spectacular shots of towering thunderclouds or churning seas is all about watching the elements

‘Professional weather photographers use hourly meteorological data and rainfall radar to predict and chase storms into the perfect position for a shot.’

Look at any landscape photography competition and you’ll see how the right weather can bring drama to your photos. Winners of the $15,000 Take A View 2011 landscape photography awards, for example, captured storm waves exploding in a Guernsey bay, brooding thunderclouds over a Scottish salmon fishery, and a motorbike ride in glorious sunshine. But how do you shooting fleeting cloud formations, rough seas and violent storms while minimising long waits and wet equipment? The trick is to understand and predict the weather.

Description: Autumn Landscapes

Autumn Landscapes

Knowing how to find interesting clouds is a great way to liven up the skies in your photographs, and clouds form by three main processes. The first is simple convection, which creates fair-weather cumulus clouds and heavy summer showers. Cumulus clouds look like large ball of floating cotton wool, and they can produce really attractive images because they have well-defined outlines and impressive clearly-visible structures.

Cumulus form as the sun warm the ground, which heats overlying air and makes it lighter than the surrounding cool atmosphere. A bubble of warm air rises, expands and cools – a process called convection – to from an invisible air column, or thermal. Cooler air holds less water than hot air. A cumulus cloud is born when the air column cools enough to drop, or condense, its excess water onto floating dust or ice crystals. A good time to spot cumulus is a few hours after dawn on clear days, especially in summer. They scatter as the sun’s heat wanes at dusk.

Description: Cumulonimbus are classic storm clouds that can produce rain, lightning and hail

A single cumulus billows upwards until be rising air loses heat, energy and buoyancy, and it disperses after about 15 minutes. Look for cumulus above strongly sunlit round, such as south-facing slopes in the northern hemisphere. Sometimes cumulus clouds also spawn above forest fires or power station cooling towers.

Heavy afternoon showers are likely if cumulus clouds grow fuzzy cauliflower-shaped turrets on a spring or summer morning. These clouds are swelling into enormous cumulonimbus thunderheads. Cumulonimbus are classic storm clouds that can produce rain, lightning and hail. They develop when there’s abundant hot, moist air and wind speeds increase with height. Air rising through the thermal is blown sideways, which sucks additional warm air upwards at wind speeds of up to 113km (70 miles) per hour. The taller the cloud, and stronger the updraught, the more water can condense and the heavier the rain. A cumulonimbus grows until the rising air cools to the temperature of the surrounding environment. If the cumulonimbus reaches the top of the lower atmosphere, the cloud spreads sideways into an anvil shape facing the wind direction.

Spectacular-looking mammatus clouds are occasionally found beneath cumulonimbus anvils. Air bubbles begin to sink, and warm, as the anvil spreads sideways, causing the moisture in the bubble to evaporate. Evaporation cools the air, making it descend faster. When the bubbles reach the cloud base, they drag it down into pouch shapes.

Air streaming from the anvil can also form feathery high clouds called cirrus. Falling ice crystals buffeted by fluctuating winds create the clouds’ wavy-brushstroke shapes, nicknamed mare’s tails. Cirrus can be spotted ahead of a cumulonimbus anvil.

Photograph cumulus and cumulonimbus a few miles to one side to capture their billowing vertical structure and the cumulonimbus anvil. Shooting beneath a cumulonimbus cloud can be dangerous because of the high winds, lightning and heavy rain. Look for mammatus towards the back of a storm. Use a super wide-angle lens for shots with a foreground or a zoom lens for close-ups of mammatus pouches, and face the west at sunset to shoot mammatus with an orange glow behind them.

Description: mountain sides and hillslopes

A second good place to spot clouds is on mountain sides and hillslopes. When moist air blows over a hill, it cools and water condenses into clouds. If condensation occurs below the peak, mist or hill fog forms. Lenticularis clouds have dramatic streamlined shapes, rather like UFOs, and sometimes form in fair weather on the downwind side of hills. The air continues oscillating up and down after passing over the hill. When it bounces up, it cools and forms clouds.

In temperate regions, such as the UK, huge weather systems called cyclones or depressions also cause clouds and rain. Depressions affecting the UK generally form over the North Atlantic and are steered from west to east by winds. At a depression, huge masses of similar temperature air rotate around an area of rising air. Cold air masses affecting the UK typically come from northern Canada and the Arctic Ocean. Warm air masses come from the tropics. After three to 10 days, the cold air pushes beneath the warm air and lifts it off the ground,

The depression’s warm front is where the warm-air mass slowly rises over cold air, producing wide sheets of cloud. Cirrus and cirrostratus clouds form more than 600km (373 miles) ahead of the warm front. Cirrostratus are high ice clouds that form a thin, translucent layer which veils the sun and can create stunning optical effects, such as mock suns or ice halos. As the warm front nears, the clouds lower and thicken from mid-level altostratus clouds into low grey stratus. These cloud sheets reduce visibility and produce steady rain that extends perhaps 240km (150 miles) before the front. As the warm front, passes, the rain stops, although the sky remains overcast.

At the cold front, cold air forces warn air upwards, forming a narrow band of cumulonimbus called a squall line. Cirrus and cirrostratus stream ahead of the storm clouds. Cumulus is often found behind the cold front. Wind direction changes and visibility improves once the cold front has passed.

Local weather reports on the internet, TV and radio provide clues about good clouds to shoot. You may find isolated cumulonimbus separated by patches of clear sky when the forecast is ‘bright spells with showery rain. Heavy thundery showers are possible’.

Description: BBC Weather website

You can predict country-wide weather from surface pressure charts on websites like BBC Weather. The curved lines on these charts are called isobars, and link locations with equal atmospheric pressure. Pressure is measured in units called millibars (mb) or hectopascals (hPa). One millibar equals one hectopascal. The UK’s average atmospheric pressure is around 1013hPa, and rarely rises above 1050hPa or falls below 950hPa.

Sinking air pushes downwards on the ground, increasing air pressure and creating high-pressure areas. These bring clear days because descending air stifles cloud formation. Rising air reduces air pressure on the ground. Low-pressure areas are associated with rainfall and depressions. The closer the isobars, the bigger the pressure change, and the faster air is rising. Tightly-packed isobars suggest strong winds and violent storms.

Some professional weather photographers use specialist meteorological data, updated hourly or less, to track cumulonimbus into the perfect position for a shot. They pre-select good vantage points in non-storm conditions, such as mountain or wide-open fields, and drive there when they forecast a storm.

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