It takes time gentleman. The longer it's here the better. It's a cyclic thing that when there's action abroad that's where the traffic will be, but be patient. The site is looking good and you've got some great things happening.
Everyone appreciates the efforts of you two giving birth to this forum and the weather in the UK doesn't always help either! But I'll add some suggestions as I go.
(1) Try and get some sponsors to show their stuff on the site from the local paper or some other business that can see an audience they have not reached.
(2) Contact your local radio station and work out some sort of partnership whereby they can give you access to 'live broadcasts' through the site, that way people can listen in whilst they on their computers doing stuff. I would go and book a slot with them on their morning show and let them know you are chasers and what you offer through the mediums you are using.
(3) Through other weather websites, add banners and get those websites to link your forum on their pages.
(4) Photos, photos, photos! People like seeing lots of photos. even if they are not from the UK in the quiet periods, just ask the storm chasing mates on FB, and other sites to use their images, just choose a few and get their permission to show them - and that includes VIDEOS! Many chasers will gladly share videos and stuff when each party links each others' sites.
(5) Newspaper - for sure - hammer them! As chasers you have access to a lot of never before seen footage they may otherwise not even know about, so if they want photos you can get them.
(6) Do you have craft fares or markets? Set up a table, get some videos from people and set up a TV - make a banner up with the MetweatherUK logo and simply meet people and tell them what you do and in reality everyone is interested in weather.
(7) Every time members have a conversation and weather is brought up, then hammer this forum - I try to let everyone I know about the forum and at least it gets the forum out there.
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Weather forums are 'protective' beasts. They tend not to have other forums as a link on theirs for fears of diverting traffic, but, one thing is for certain, if the topic threads are of interest and show a varied range of conversation then they will more than likely join and share what they do or do not know.
(9) Even approach your local UKMet office and get some stuff linked from them, if they don't there's plenty of ther sites than can provide something that can be instantly seen on the home page or on the side bars.
All websites start out a little slow, but in time once more information, videos, photos etc are added then people will notice and be curious.
The weather plays a huge role in most cases and even my site has its down time during the dry season. But if the forum is kept active and refreshed it won't go stale - and us, the members, need to keep that going also.
Keep up the great work guys and it will happen!