The Met Office said the reading at Tulloch Bridge in Inverness-shire was the coldest April measurement since 2013 and followed on from the Minus 8.3C (17F) at the same location the previous day.
The mercury also dropped to minus 7.7C (18.1F) in Shap, Cumbria, minus 3C (26.6F) in Altnahinch, Northern Ireland, and minus 2.9C (26.8F) in Sennybridge, Wales.
Forecaster Nicky Maxey said: ”This is caused when you have very sunny days at this time of year. You don’t have any cloud cover to hold in the heat.
“The start of April has been pretty cold in Scotland with the average day time temperature 2.7C below what we would expect.
“As for the ‘mean’ temperature, which measures day and night, Scotland is 3.5C cooler than average.
“Day time temperatures will start to rise this week and by Friday we may see up to 16C (61F). That would be a good deal higher than the April average, of 10C (50F).”
The icy weather across the country has coincided with the opening of outdoor hospitality in England, as determined shoppers and drinkers rushed to the high street undeterred.
Hundreds of people dressed in hats and winter coats queued outside the world’s biggest Primark store in Birmingham, which reopened its doors at 6.30am yesterday.
Customers at the Royal Victoria Pavilion in Kent – believed to be the biggest pub in the UK – cheered as the first chilly pints were served.
Outdoor hospitality will open in Scotland on April 26 according to Scottish Government guidelines.
Another widespread frost was predicted for Monday evening.
The deep freeze comes after an erratic weather pattern saw sun, snow and hail combined briefly for a twist on an April shower at the weekend.
A number of social media users shared pictures and video of the perplexing precipitation, with both humans and dogs caught unawares.
One Glasgow resident said it lasted for around five minutes before becoming warm and sunny again, leaving both her and her pet baffled.
"My mum and I were enjoying the sun, about to take our picture and then the hail-snow descended, then disappeared as quickly as it arrived."
They added: "The texture of the snow was strange, very soft but took a while to melt, if that makes sense?
"Max is a puppy so I think he was slightly more confused than I was, but given how quickly it started and ended I was definitely close behind him!
"It wasn't hail or snow, it was somewhere between the two."
Geoff Barton, 60, professor of bioinformatics and head of computational biology at the University of Dundee, was similarly shocked.
"Ok so we've had fabulous sunshine, then snow, then sunshine again. And now? Both together!" he tweeted.
"Until about 12.20 this morning the weather was bright sunshine with blue skies," he said of the Dundee weather.
"Then we had a snow flurry and then more sunshine until we had this mix of snow and sun."
Mr Barton said "it is pretty unusual here to get snow and sun”.