is there a specific height a hill(s) should
be before a rain shadow occurs?

Mag,

In two articles in 'Weather' in 1983, John Stone, then a climatologist in
London concluded that hills of a little of 50m. relative elevation can
influence rainfall patterns in eastern England, in terms of (slight) orographic
enhancement and rain shadow effects. So the Yorkshire Wolds are certainly high
enough, though it all depends on low level wind direction of course. I would
imagine that the Lincs Wolds may be important with the frequency of S/SW
winds. Others will have more detailed local knowledge but the rainfall averages
do follow elevation closely in even 'lowland' eastern England. Of course, if
there is strong cyclonic uplift or convection, this relationship may be
obliterated.

yours,

Julian Mayes,
Univ of Surrey Roehampton & West Molesey,
Surrey
- often in the rain shadow of the North Downs.