Hail and Lightning

Last weekend, in a moderate southerly upper flow regime, a lot of thunderstorms developed over Romania.
In the evening of Friday 6 august I had the opportunity to watch and photograph the first good night-time thunderstorms of 2010:























Saturday afternoon numerous storms developed in Transsylvania. Here is a satelite-image of 17:45 local time with many "small" anvils blowing north by the southerly upper flow.




In the early evening an interesting small storm developed just to the S-SE of my place.


Despite the moderate southerly flow (30-35 kt at 500 hPa) this developing storm was almost stationary for about 20-25 minutes.









Main cause for this was that surface winds where from a northwesterly direction, as can be seen in the GFS prognostic sounding for 15 UTC. However, surface winds were not 3 kt as indicated by the sounding but an estimated 10-15 kt. It is likely that this had impact on storm-motion, although it cannot explain the stationary character for 100%. Nonetheless, 0-6 km (vectorial) windshear was increased to 35-40 kt, which is sufficient for well-organized storms.




I observed thunder and when precipitation started, the storm started moving (slowly) north, because the storm grew to the tropopause, and was steered by the stronger mid-level and upper winds.
In the village where I live, rain turned into a rain/hail mix for a few minutes, with largest stones of 1,5 cm (0.6 inch). Not that big, but a few stones showed an interesting appearance: one stone with a double embryo and clear ice around it.


"Hailstones grow by collision with supercooled water drops. Supercooled drops are liquid drops surrounded by air that is below freezing. There are two methods by which the hailstone grows, wet growth and dry growth. In wet growth, the hailstone nucleus is in a region where the air temperature is below freezing, but not super cold. Upon colliding with a supercooled drop the water does not immediately freeze around the nucleus. Instead liquid water spreads across tumbling hailstones and slowly freezes. Since the process is slow, air bubbles can escape resulting in a layer of clear ice."



Later in the evening active thunderstorms in the north and northeast caused some nice CG's.











One day later (Sunday 8th), this was repeated, but then most lightning was in a southerly direction:


What you see here occured in the whimp of an eye: all the lightning flashes stroke within one second.