Hi:

I remember when I was jogging early every morning for years, I can't
recall ever getting wet. Are there any stats anywhere for precipitation
against time of day? I guess the afternoon is wetter than the morning.

(Too old to jog nowadays!)

Hugh

--

Hugh Newbury

Running Linux Suse 10.1 in deepest Dorset

On 9 Jan, 07:59, Hugh Newbury wrote:
Hi:

I remember when I was jogging early every morning for years, I can't
recall ever getting wet. Are there any stats anywhere for precipitation
against time of day? I guess the afternoon is wetter than the morning.

(Too old to jog nowadays!)

Hugh

--

Hugh Newbury

Running Linux Suse 10.1 in deepest Dorset

If you'll confirm your e-mail address to me, I can let you have a
little analysis I wrote up for COL a couple of years back on this
subject. It's a bit meaningless without the graphs, so can't post it
in the ng - unless someone kindly offers to post it somewhere on my
behalf.

Stephen Burt
Stratfield Mortimer, Berkshire

On Wed, 9 Jan 2008 at 07:59:37, Hugh Newbury
wrote in uk.sci.weather :

Hi:

I remember when I was jogging early every morning for years, I can't
recall ever getting wet. Are there any stats anywhere for precipitation
against time of day? I guess the afternoon is wetter than the morning.

That would be my guess too.

Frontal rain can happen at any time of day or night, but convectional
rain is less likely early in the morning.
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)

Here goes with just the text. If anyone can offer to post the doc
somewhere, it makes much more sense when you can see the graphs. But
you'll get the sense. This note prepared for COL some 3 years back.


The diurnal variation of rainfall frequency

From my AWS observations over the 10 years 1994-2003, I extracted a
count of the number of hours with rainfall 0.2 mm or more by hour of
the day and by month to assess the diurnal variation of rainfall
frequency (Fig. 1, columns).

Over the year as a whole there is a very pronounced bi-diurnal
variation in rainfall *frequency*, with minima at 00h and 10h GMT and
maxima at 04h and 16h. These coincide exactly with the daily peaks and
troughs of the bi-diurnal curve in barometric pressure (continuous
line on Fig 1 - reversed scale on right axis). The afternoon peak in
rainfall frequency is larger and broader than the overnight peak, and
is clearly the result of an increase in convective rainfall: there is
a particularly sharp increase in the frequency of rainfall between 12
and 13 GMT. The variation is significant - in the hour commencing 00h
it rained on 315 hours in 10 years, whereas at 16h it rained on 372
occasions, almost 20 per cent more frequently.

Looking at the figures by month of the year (Table 1), the annual
variation is immediately more apparent than the diurnal variation.
This table shows the percentage of hours where 0.2 mm or more was
recorded during the 10 year period, and is colour-coded, from green
cells (rainfall recorded less than 6 per cent of occasions) to red
(rainfall recorded on more than 12 per cent of occasions). The actual
percentages are shown to aid legibility in monochrome, and range from
below 4 per cent for many hours in July (a little over once per month
on average) to 19 per cent around dawn in November (about six days in
an average month).

When it is considered that these figures relate to the chance of any
part of the hour receiving 0.2 mm of rainfall, and that more often
than not any rain that falls will not be continuous for the whole
hour, it can be seen that in southern England at least the chance of
intercepting measurable rain in a short period at any particular time
of day such as 'the school run' is quite small - about 3 per cent or
once per month, which is about Mr Buttress's observed frequency. It is
also true that the Editor's perception of the morning school run being
less likely to be affected by rain than the afternoon is borne out by
the observations, at least at my site over the past 10 years.

When considering the diurnal variation in rainfall *quantity*,
however, there is a single daily broad afternoon peak (Fig. 2, average
hourly rainfall amounts per year derived from the same 10 year hourly
dataset). This is what would be expected from an increase in
convective rainfall at the time of peak heating; the hour commencing
1500 GMT has, over this 10 year period, been considerably wetter than
any other hour, and the rainfall totals for the hours 1300 to 1900 GMT
are higher than for any other hour in the day. There is a very sharp
fall-off after 2000, and little variation overnight - in fact, the
overnight peak in rainfall frequency is not evident at all in the
rainfall amounts.

We can combine the two sets of data into Fig. 3, which shows the
average intensity of rainfall (mm/hr) for all those hours with
rainfall over the 10 year period (total rainfall divided by the
numbers of hours with 0.2 mm or more). Now we can see that, although
rainfall is almost as frequent at 04h as it is at 16h, it is lighter -
typically 0.8 mm/h compared with over 1.1 mm/h at the afternoon peak,
an increase of almost 50 per cent.


In terms of 'the school run', therefore, 10 years observations show:

1. Rain is less frequent during the morning 'school run', and when it
does rain, it is normally lighter and thus more likely to be dynamic
(frontal or cyclonic) in origin.

2. Rain is about 20 per cent more frequent during the afternoon
'school run' than the morning, and when it does rain, it is typically
almost 50 per cent heavier because it is much more likely to be
convective in origin.


Stephen Burt
Stratfield Mortimer, Berkshire

21 December 2004

This is an interesting subject as where I live in South Essex, often when
rain is forecast overnight and into the early hours, it rarely seems to
happen until about mid morning of the next day.
Almost as though the front stalls and becomes less active during the hours
of darkness, and then is re-activated after several hours of daylight.
Regards.
Len.
"Paul Hyett" wrote in message
...
On Wed, 9 Jan 2008 at 07:59:37, Hugh Newbury
wrote in uk.sci.weather :

Hi:

I remember when I was jogging early every morning for years, I can't
recall ever getting wet. Are there any stats anywhere for precipitation
against time of day? I guess the afternoon is wetter than the morning.

That would be my guess too.

Frontal rain can happen at any time of day or night, but convectional rain
is less likely early in the morning.
--
Paul Hyett, Cheltenham (change 'invalid83261' to 'blueyonder' to email me)

On Jan 9, 6:49 pm, Paul Hyett wrote:
On Wed, 9 Jan 2008 at 07:59:37, Hugh Newbury
wrote in uk.sci.weather :

Hi:

I remember when I was jogging early every morning for years, I can't
recall ever getting wet. Are there any stats anywhere for precipitation
against time of day? I guess the afternoon is wetter than the morning.

That would be my guess too.

Frontal rain can happen at any time of day or night, but convectional
rain is less likely early in the morning.

Explain.

I remember finding that when I lived in Abergele, right on the sea
front, I would often find the hight of a storm or that a passing
squall would be very close to the tidal table.

wrote:
If you'll confirm your e-mail address to me, I can let you have a
little analysis I wrote up for COL a couple of years back on this
subject. It's a bit meaningless without the graphs, so can't post it
in the ng - unless someone kindly offers to post it somewhere on my
behalf.

Thank you, Stephen, that would be very kind. My email address is:



(I've stopped caring about spam!)

Look forward to seeing the graphics.

Hugh

--

Hugh Newbury

Running Linux Suse 10.1 in deepest Dorset