The home page of http://www.ukweatherworld.co.uk/
states that on 20th March 2009, Altnaharra ranged from -4.5 deg. C. to +18.5 deg. C., a range of 23 deg. C. (although http://www.ukweather...w.asp?tid=29...
gives a minimum at Altnaharra on 20th of -3.0 deg. C., and -4.2 deg. C. at Aboyne, Aberdeenshire (no -4.5 deg. C. is given). Which is right I cannot say, though clearly there was a big range of temperature at Altnaharra of at least 21.5 deg. C.
In the last 15 years or so, I have very rarely recorded daily ranges of more than 20 deg. C. either at my place of work or in my garden. I think the highest was in late May 1994 when (in my garden) it ranged from 0.6 deg. C. to about 23.2 deg. C.; at my place of work I cannot remember the exact figures but I think a range of around 19 deg. C. was recorded in August 1995 and 1 or 2 of 20 deg. C. or more in July (perhaps 1999) and September (latter possibly 1998). A few years ago in 2003, Altnaharra had a range of 28.2 deg. C. on http://www.personal....er_in_april.htm
, falling from 24.5 deg. C. on Good Friday, 18th April, to -3.7 deg. C. the following night. On 16th February 2003 a minor example of a large daily range of 18 deg. C. occurred at Altnaharra, which had both the lowest (-9.0 deg. C.) and the highest temperature (9.0 deg. C.) in the UK on the same day, presumably a rare event in itself (see http://www.met.rdg.a...y2003.html#0302
Clearly high daily ranges can occur for a variety of reasons and at all seasons (perhaps more frequently in spring or very late summer) and both in the English lowlands and Northern Scotland.