The most important factor affecting eclipse viewing, particularly in Cornwall, is the weather. A total eclipse even with overcast skies ought to be impressive; but we all want to see the real thing!
Overview
The probability of seeing the eclipse at any given point can only be guessed, by looking at the average cloud cover on a typical mid-August day at that point. Still, such figures are useful to those planning an eclipse trip.
The chances are worst for anyone putting to sea off the Canadian coast; the chance of seeing the Sun at the start of the eclipse path at dawn is less than 20%. By the time the eclipse has crossed the Atlantic and reached Cornwall, this has improved to 45%.
The figure rises slightly as the eclipse crosses northern France, to about 46/47% at Paris, then climbs steadily to over 50% at Munich, then almost 55% at Vienna; it's in the high 50s at Szeged, and over 60% at the Black Sea coast, the best area in Europe to see the eclipse.
Crossing into Turkey, the chance of visibility approaches 70%, and climbs to almost 75% at Sivas, over 80% at Diyarbakir; it reaches 90% at Mosul, and peaks at over 95% at Esfahan, Iran.
From there, things get rapidly worse, due to the rainy season. By the time the eclipse reaches Pakistan, the chance of seeing it has dropped to 80%; but at Karachi, it's down to 65%, and drops from 30% to 5% as the track crosses India. The fact that the Sun is very low in the sky -- close to setting -- doesn't help chances of seeing the eclipse here.
Please remember that these numbers only represent statistical averages based on past weather observations -- on the day, there might be clear skies in Cornwall and thunderstorms in Iran! So keep your fingers crossed, but above all prepare to be mobile, if possible. Watching the weather forecasts 2-3 days in advance can help you to plan a trip to the area that offers the best chance of clear skies.
Cornwall
Unfortunately, detailed weather records for Cornwall are very scarce; and what little data is available mostly shows that the Cornish weather is very unpredictable. While the south-west may offer the best prospects for sunshine anywhere in the UK, there is still less than a good chance of clear skies.
The toe of Cornwall is a long, narrow strip of land with sea on three sides. This creates very localised weather systems; there may be cloud cover in one place with sunshine just a few miles away. The average total sunshine in August varies between 190 and 215 hours, depending on location, with the extreme south and west points generally being the better off. Cloud cover varies over the whole range from none to total; but completely clear skies are rare. Daily sunshine is again very unpredictable; it can vary from 0 to 13 hours between one day and the next!
The weather is mainly westerly; though often cold fronts can stretch into England from invading northerly low pressure areas, accompanied by clouds and rain. Following a cold front, coastal areas are likeliest to be clear; on the other hand, warm, moist air moving up from the south may make a northerly location, beyond any available hills, the best bet, to the point where moving well north of the center line may pay off.
So, that's all very unhelpful; what to do? Well, it's probably best to watch the weather in the days before, and figure out whether we have predominantly stable, westerly, or northerly weather, and then take a guess on whether the coast or inland will be clearer. My best advice is to pick a good "base camp" and get there early to avoid the (potentially horrible) traffic; but check out the weather on the day, and prepare to be mobile if possible!
