[September 2009 Cet]

Can anyone enter this now, or do I need to wait for January?

If I can enter... 13.5C

[Asperatus?]

Nice photos! I've been wondering what these clouds were since I took these pics back in April 2008 - figured at the time they were some kind of altostratus, so it's good to see some similar images. I noticed in the discussion forum that some people associate them with thunderstorms, however when I took the photos below it was after a day of heavy frontal rain without any convective activity (that I know of)...

[Lightning]

thanks very much for this informative reply John. I thought there probably wasn't a definitive answer, but it's good to see some rough figures for UK setups.

[Lightning]

In fact I read somewhere that the dominant parameter determining electrical activity is the thundercloud height. T-storm cloud tops in UK rarely get much above 35,000ft but in US can grow to 65,000ft more especially Spring-Summer, that's why hail gets so large and lightning so intense.

How high do cloud tops need to get in order for a storm to turn electrical? I've noticed a few times people have posted a Skew-T diagram and commented that the cloud tops don't look high enough for lightning, so I'm just curious as to what height is needed and whether this varies at different times of the year?

Also am I correct in assuming that it is the depth of the cloud that is important rather than just the height of the cloud tops?

[Post A Chart From The Day You Were Born]

Nothing spectacular - my parents told me it was raining (living in Wales now but was born in London). I remember my 10th birthday in 1988 when the temperature gauge in my dad's car reached 32 degrees though - and in 2007 I was woken at 2am by a thunderstorm

[Unusual High Pressure Systems]

Thanks for this informative reply J07. I did wonder why even in the summer the high pressure never seemed to be as intense as in the winter, so that's answered another question for me too.

I guess that's why sometimes people refer to high pressure over Greenland as not being a 'real' Greenland high then, if it is just a surface pressure rise caused by the cold air and not supported by the upper atmosphere?

Funnily enough those tightly packed isobars I mentioned have shown up nicely on the 12Z GFS today, I see what you mean about the grading point going here from 996 - 1056MB!

[Unusual High Pressure Systems]

Hi there,

New-ish member here, been lurking for a while in a futile attempt to try and learn a bit before posting!

I've got two beginner's questions relating to the legendary Greenland high. Firstly, I noticed this chart posted earlier this evening on the Model Output Discussion which I thought looked quite unusual. As I understand it, the "T" stands for Trough and signals an area of low pressure, yet there are several "T"s marked over Greenland and one over Svalbard in areas showing some of the highest pressure on the chart, over 1035MB. If I'm right, and these are troughs, what weather would you expect to find there at the time? Normally I would associate a trough with precipitation, however surely pressure at this level would inhibit convection so you might just get a bit of drizzle (in snow grains)?

This leads me to my second question! Quite often I notice that when the charts show Greenland highs, the isobars seem really tight in comparison with the type of high you would normally see over the UK, Azores or Europe. At a glance and without the figures the 1048MB high in this GFS F.I. chart would look to me more like a low pressure system (which may sound silly, but bear in mind I'm a complete beginner). Is there a reason why these type of highs tend to form over Greenland and not the UK? I appreciate this isn't a great example as I'm sure I've seen charts in the past with much tighter isobars, but hopefully you get the point.

So... putting these questions together, my basic knowledge leads me to the contradiction that in Greenland you could still have snowy and windy weather with the pressure at 1040MB?

I'm probably the only person on here wondering what is happening in Greenland and not the UK when looking at these charts! And now, having shown my ignorance, I'm going to hide in the corner again!

John