Thunderstorms

Very often in most years it's true.

The warming of the landmass can trigger in unstable climatic situations some heavy home brewed storms, at other times they develop over France, moving northwards on a south/south-easterly air flow, reaching the Worcestershire area late afternoon or through the eventide.

They are in this country one of the most dramatic of phenomenon, especially if they occur through the darkness of the night, when the electrical discharge can be witnessed at it's best.

It is a fact that at any time of the day or night, about 2000 such electrical storms are going on around the globe and the energy they release is phenomenal. Also most of the worlds rainfall comes from these deep vertical clouds we in the trade call cumulonimbus.

Due to the fact these huge columns of rising warm moist air cause extreme up and down drafts of great force, aircraft are warned of these climatic conditions and steer clear of these turbulent events.

Due to the fact that the warmer air through late Spring/Summer and early Autumn can hold larger volumes of moisture, when conditions are right for those deep thunder heads to build and break, rainfall is often torrential and monsoon-like, cause flash flooding. Often the sewage and drainage systems cannot cop with the sudden increase in water, lifting heavy iron drain tops off giving spectacular fountain effects.

When the air is unstable and warm moist air rises at great speed, water vapour condenses. This releases latent heat which helps to maintain the warmth of the air, so the air continues to rise until a towering storm cloud develops (cumulonimbus Cb). Inside the thunder cloud warm air rises rapidly, cools at the top of the cloud and starts to descend, then rises again. This forms convective cells making the cloud formation look like turrets around a castle, these clouds are called Altocumulus castellatus.

Once this motion is in play, down drafts from one storm triggers off more cells within the near neighbourhood, ending up with a mass of fully blown thunderstorms, which can go on for hours once formed.

The turbulent motion causes the top of the cloud to acquire a positive electrical charge and the base of the cloud , a negative charge. The collisions between small ice particles as they move upward and downward inside the cloud builds up static electricity charges. Small positive charged ice crystals accumulates at high level, falling as hailstones which acquire a negative charge. This static electricity builds to very high levels in the cloud until eventually it sparks-the spark is lightning.

Electrical discharge may occur within the cloud, from one cloud to another or between cloud and ground.

A leader stroke is a channel of ionised air, formed when electrons are accelerated by the intense electrical field. The stroke darts downwards from the cloud base in progressive steps of about 150-300 ft (50-100m) until it establishes an open conductive channel to the ground. The first leader strokes are often invisible but the final stepped leader which reaches the ground is visible, and looks like an illuminated river with tributaries. Speed of the leader stroke varies from as little as 100 per second to 1000 miles per second.

The return stroke streaks up the channel from ground to cloud, and is a faster flash usually referred to as lightning. The speed of the return stroke may approach 87,000 miles per second, which is nearly half the speed of light. The flash , therefore, last a mere millionth of a second, but persistence of the image on the retina of the human eye makes it appear to last longer. Temperature of the return stroke may reach 53 to 54,000f (30,000c), more than five times hotter than the sun.

One flash consists of three or four leaders each followed by a return stroke, the width of the channel is seldom more than 1inch or 25 mm.

Sound travels at 1 mile per 5 seconds, if you count from the flash to the sound of thunder which is caused by a huge heating of the air, expanding and causing the sound wave thunder and divide by 5....that's the distance of the storm from the observer in miles...therefore if you counted ten seconds...divide by 5 and that tells you the storm is 2 miles away from you.

Next Week.....Whirlwinds, Tornadoes, Water Spouts and Dust Devils.

The answer to last week's question was 1962/63

This week's question is:If you count 20 seconds from the flash to the thunder how many miles it it from you:

A/ 2 miles

B/ 4 miles

Best of luck.

Glossary of Common Weather Terms.

Clouds: Water vapour which has condensed.

Cloud type of the week:

Cumulonimbus (Cb): Heavy dense vertical clouds with anvil shaped tops. The anvil top is smooth, fibrous or striated and nearly always flat. Beneath the base the cloud is black or very dark. Hail, thunder, squalls and rain storms accompany these storm clouds. Height of base 1,500 ft or lower to 6,500 ft. This cloud brings heavy storms.

Weather for week Monday 9th April-Friday 13th April General Weather Situation High pressure will fend off any bad weather during this period.

Weather Prediction Long sunny spells bringing warmer than usual temperatures and light winds often variable. Risk of mist and slight ground frost in rural locations night and early morning-tide.

Maximum temperatures 18-21c 64-70f.

Minimum temperature 2-4c 36-39f rising later.

Last Week's Observations for Worcester (Monday-Friday) Highest day temperature..........20.2c 68f on Thursday 5th April.

Lowest night temperature........minus 0.2c 32f on Wednesday 4th April.

Wettest day.......0.2 mm on Tuesday 3rd April.