As you asked for a “a human description” I have kept my answer anecdotal without reference to stats that you probably have already read anyway. I lived in Glasgow for 28 years so know the weather personally. Though bear in mind that the west coast gets significantly wetter weather than the east. The West Highlands get significantly more rain than the West lowlands.
As you experienced in your holiday before, Scotland’s weather is quite erratic. Spring is no exception to this. Any season can have good or bad days. I’ve seen a days with the temp above 22C in February and I have seen it snow and hail in summer so any statistic that says the average is something something should be taken with a big pinch of salt. Average weather is not the dominant norm in Scotland. You get a wide spread to either side of the average.
Spring usually sees a period of heavy prolonged rain at the tail end of winter then heavy showery weather before moving into summer. The weather you described of 15 mins rain and 30 mins sun sounds more like spring than summer to me. Summer usually has rain at night but dry in the day. So if you like showery then you could be OK in April.
The exact timing of the weather described above can be problematic. Winters can last a little longer and summer can come early. I have seen it rain constantly from March to May without a single days break. I have also seen the converse. I saw decent snow fall twice at Easter but also seen it warm enough to go outside without a jacket.
I think what I am saying is that nobody really knows what the weather will be like in April in Scotland. Anything is possible. You could get lovely weather all through your holiday but you could also get gale force winds and storms too. This is one of the joys of visiting Scotland and will be sure to surprise you every time you come.
One advice I would close on: the weather is often localised. Jumping in the car and driving 50 miles north or south can completely change your weather. I have seen it sunny at Crianlarich but blizzards at Bridge of Orchy. It can be pelting cats and dogs in Paisley but bone dry in north Glasgow. If you keep your plans flexible and have easy transport, then you can often dodge the worst of the weather.
April is actually the driest month in Edinburgh, as you’ve found. However, while Edinburgh is considered the ‘sunny’ side of Scotland, it still gets 186 days of rain a year on average. It’s WET. So bear that in mind, even if you are going in the driest month.
Interestingly, May is like the ‘early’ summer – more sunny days than any other month, and hours, despite not being the middle of summer in terms of the earth.
I think the Average temperature is misleading – the average high in April is 11, the low being 3, so I suppose the average is in the middle, but you can certainly expect higher during the day.
Now, description. I’ve been in summer, winter, and autumn – and the autumn (October) is meant to be almost identical to April, so that works out well. It’s darker than London, being further north the sun sets earlier. You can and likely will get strong winds – fierce winds even. And yes, you’ll likely get rain.
HOWEVER, when I was there in October, we had 1 day in 6 of which was sunny. It was still chilly, but outdoors most days, even in the rain – we had torrential rain and storm the one night. This was spent mostly in Edinburgh, the highlands, Isle of Skye, and Fort William, but from what we heard, it was dismal in most places – but it doesn’t matter if you go in expecting it. It didn’t stop us hiking, exploring ruins, lochs and cities. The only thing it prevented was a view of Ben Nevis as we drove by – the cloud was too low.
So go prepared for bad weather, bring good wind-proof jackets, but hey, you could get lucky, and then it’ll seem like a bonus!
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