So. Iceland is having a very, very big volcanic eruption, shutting down Euro-airspace. Since big volcanic eruptions have caused cold weather in the past, will this eruption have an effect on the weather, & how much?
Well, the eruption of Tambora cooled things off the following year but Tambora was a lot bigger than this volcano is. As such I doubt this one will have much of an effect.
Weather effects depend in part on how long this eruption continues, as well as how intensely it erupts. Probably too early to say, though at this point I doubt it’s put out enough debris to have much effect.
I can say it hasn’t had any effect on the weather here in the UK so far, even though the cloud’s been directly overheard for a good 7-10 hours now. If you didn’t pay attention to the news you’d have literally no way of telling. The sun was just as bright earlier, the stars are just as clear now, the temperature’s the same as it’s been for a week. I know the ash is supposed to be in the upper atmosphere but I at least thought it was going to be noticeable when there were no clouds! Flights here are supposed to resume tomorrow morning and there’s been no mention of long-lasting weather effects so it seems like the answer is that there won’t be much effect on the weather at all.
From the New York Times:
Actually, it isn’t. This is a fairly normal eruption for an Icelandic volcano, which means it isn’t terribly large on a world scale. What makes it newsworthy is the ash cloud - it happens to be spewing a lot of ash, and the winds are carrying it into an area that significantly disrupts air travel. Oh, and is making the air downwind smell a bit odd. I’m not getting the rotten egg smell some people have complained about, more a major fire in a hot dog factory kind of smell, but it’s still weird and not exactly pleasant.
I think I could fit in a few more weasel words if you’d like.
As **flodnak **says, Eyjafjallajoekull is fairly small beer. According to some of the reports the worry is that this is a precursor to an eruption of the much larger Katla which is linked.
ps It’s great hearing the newsies trying to say “Eyjafjallajoekull” 
It annoys me that journalists are writing it this way. The original Icelandic term is Eyjafjallajökull. I can understand that when transliterating into English, you have to do something about that diaresis over the o. But Icelandic is not German, and ö is not optionally written as oe. I mean, when’s the last time you saw an English news report about the singer Bjoerk? So BBC, Evening Standard, et al., if you feel you must transliterate, then just leave the damn accent off. Quit pretending that Iceland is a German province.
OK I assumed it would be pronounced something along the lines of “ey-SHAAF-SHAAL-a-SHO-kull”. But I just listened to the pronunciation on Wikipedia. What the hell is that?! How are English newscasters attempting to pronounce that?
Sounds easy. Eva - lay - vuk
Why would you think that? In most European languages <j> is pronounced like the English <y>. (I don’t know of any in which it’s pronounced like English <sh>.) And with that in mind, the word is pronounced exactly as you would expect, with one exception: in Icelandic <ll> is usually pronounced as English <tl>, as in “hatless”. So the whole thing would come out as something like “EY-ah-FYAT-lah-YEUH-kutl”, or more precisely /ˈeɪaˌfjatlaˌjœkʏtl/.
The BBC news articles doesn’t do the Katla volcano justice. Katla has a history of volcanic eruptions including very, very large volcanic flows, floods that rival the annual output of the Amazon River, and ejecting dust and gases that can disrupt weather patterns for years and create poison clouds spreading across northern Europe.
How will the ash cloud affect the continental U.S. when it gets here?
Probably spectacular sunrises and sunsets at first. Potential short-term weather changes.
There is a BBC article about a previous eruption in Iceland and it’s effects on the UK.