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Parts of Wales were hit by heavy thunderstorms yesterday including Merthyr Tydfil where a young girl was hit by lightning. Erin Moran had a lucky escape but the chances of being struck by lightning are very low, about 1 in a half a million.

Some thunder and lightning facts:

• An estimated two thousand thunderstorms are going on in the world at any one time.

• About 30 to 60 people are struck by lightning each year in Britain

• The diameter of a lightning bolt is about a half-inch to an inch wide, but can be up to five inches wide.

• The average length of a lightning bolt from a cloud to the ground is three to four miles long.

• Lightning can occur not only in thunderstorms, but also in snow storms, sand storms, above erupting volcanoes and from nuclear explosions.

• A bolt of lightning can travel at up to 136,000mph and reach temperatures of 30,000 Celsius - hotter than the surface of the Sun!

• The sound of thunder is made by lightning heating the air around it. The air expands very rapidly and causes sound waves.

• What we see as a single flash of lightning may actually be three or four different strokes in exactly the same place, one right after another - that's why lightning seems to flicker.

• If you count the gap between the lighting and thunder, you can tell how far away the storm is. For every kilometre count 3 seconds, for every mile count 5 seconds. So if you count 15 seconds, the storm is 5 km or 3 miles away from where you are.

• There are many different types of lightning including sheet, fork, ribbon, bead and ball lightning.

Thunderstorms can happen at any time of year but they are most common during the summer months when there is more heat in the atmosphere.

Thunderstorms in this country can form in three ways - they can be home grown, imported from Spain or France or brought to us by a cold front or trough.

Yesterday had all the right ingredients for thunderstorms i.e very unstable air, moisture and wind shear, producing large, towering, cumulonimbus clouds.

John Goodger who runs a weather station at Velindre near Glasbury recorded almost an inch of rain, 24mm, in a few hours - the wettest day there since 2 October last year!

I think we've seen the worst of the thunderstorms in Wales for a while but showers over the next few days could be heavy in places with a risk of hail and thunder.

Sadly there's no sign of the warm and settled weather we had earlier this month, returning. It will turn cooler this week as the wind shifts more into the north west and ground frosts are still possible in rural parts of the country this weekend.

Derek