When is the best time to visit Iceland - especially its waterfalls?
When it comes to waterfalling, you really don't have a whole lot to worry about because most of them (with relatively few exceptions like Systrafoss) flow year-round. With Global Warming, hiking conditions may be attainable as early as April, but the uninhabited highland interior is generally not accessible until July when enough snow has melted to make roads less muddy and unbridged rivers crossable (but do so with extreme caution or else just take a tour!).
Even during the summer tourist season, high temperatures can be below freezing (we've had multiple days where highs not counting wind chill factor were 2 degrees Celsius!). However, the weather is quite fickle and the high can quickly rise to midge-friendly levels around the mid 20s to low 30s (Celsius) just a day or two later. And speaking of winds, it does indeed get blustery around the country, which can quickly bring the body core temperature down to hypothermic levels if you don't bring enough clothing to counteract that heat loss.
So why is the rest of the year off-peak? That's because the country experiences polar winter nights (i.e. nearly 24 hours of night time!), violently cold weather is more common, and services are neither available nor reliable due to lack of visitor demand. The flip side is you can see the Aurora Borealis (Northern Lights), there are no crowds, and prices are cheaper (though Iceland is still ridiculously expensive).
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