'Heat' haze is a bit of a misnomer. It's all to do with changes in temperature between different bits of air in between you and the subject... so as you've seen you can get it on even the coldest of days, because the difference between "friggin' cold" and "bluddy cold" can be enough to produce the effect.
Nice Alpha Jet pic. So where's the multi Herc pics from Lyneham then??