Change in amount of rainfall from a given type of cloud?

[chrisbell-nottheweatherman]

Recently, both of my parents have noticed something that I'd also been wondering about, without being able to produce any meteorologically-logical explanation.  Over the course of the last few months, roughly coinciding with the dry spell we've had, a certain pattern seems to have been prevalent here in South Norfolk (with the exception of the recent hot spell, of course).  It has been characterised by thick, stratiform and cumuloform clouds in the mid-to-low levels, often quite dark and shower-like with low bases, against a backdrop of higher, thinner cloud, often accompanied by brisk winds, but almost never producing rain.  Each of us has independently noticed that the low base and dark appearance of these clouds has led us to expect imminent and, sometimes, quite heavy rain, yet they've moved through without producing anything at ground level and, often, nothing further-up indicated on NW radar either.  As I said, I can think of no logical explanation for this, but it has been quite marked and unusual; indeed, as I type, it's a similar situation; dark, lowering clouds but no rainfall.

Does anyone have any idea as to whether we're paying attention to something that we've not noticed as frequently in the past, or is there something in the synoptic pattern that might be causing this?

[Terminal Moraine]

I can only suggest that you've experienced a lot of days with Stratocumulus, or Cumulus which has built to Mediocris or Congestus size at most, large enough to look dark and threatening and cast a deep shadow but not quite large enough to produce rainfall.

A thick layer of Stratocumulus, particularly Stratocumulus cumulogenitus ( formed by the spreading out of Cumulus) can look very dark and menacing at times but produces no rainfall as there is a mid level inversion which has caused the Cumulus to stop building and spread out into a sheet of cloud.

As to the synoptics causing this, both are common under relatively quiet conditions where an anticyclone is near enough to suppress convection but not near enough to give mainly clear skies. I suppose given the relatively dry nature of the last few months, particularly where you are, this synoptic situation will have been more frequent than in wetter years but whether or not it's more frequent than in any other drier than average year I've no idea.

[chrisbell-nottheweatherman]

2 hours ago, Terminal Moraine said:

I can only suggest that you've experienced a lot of days with Stratocumulus, or Cumulus which has built to Mediocris or Congestus size at most, large enough to look dark and threatening and cast a deep shadow but not quite large enough to produce rainfall.

A thick layer of Stratocumulus, particularly Stratocumulus cumulogenitus ( formed by the spreading out of Cumulus) can look very dark and menacing at times but produces no rainfall as there is a mid level inversion which has caused the Cumulus to stop building and spread out into a sheet of cloud.

As to the synoptics causing this, both are common under relatively quiet conditions where an anticyclone is near enough to suppress convection but not near enough to give mainly clear skies. I suppose given the relatively dry nature of the last few months, particularly where you are, this synoptic situation will have been more frequent than in wetter years but whether or not it's more frequent than in any other drier than average year I've no idea.

Thanks for your detailed reply.  Having just checked some photis of Stratocumulus cumulogenitus, I can confirm that's indeed typical of what I was describing.  We've been having quite a lot of that and normal stratocumulus against a backdrop of altostratus and cirrostratus, giving quite a gloomy, overcast setup with the dark bases of the cumulogenitus which has lent it quite an oppressive, lowering appearance without generating much preipitation.  We could certainly do with some more ppn. for the garden and the farmers around here.