Rain and storm for Cpt

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StrontiumDog said:
What's the generally accepted method of measuring rain? Is it in a 1 metre squared box where 1mm = 1 litre in that box?
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Its called a rain gauge. It can be any shape, as long as you can calculate the volume from the depth.

The simplest ones have a uniform diameter, so the depth equals the amount of rain. The disadvantage is that 1mm is hard to measure accurately.

The most common are tapered cylindrical, with a calibrated scale that allows easier reading of both small and large amounts of rain.

More advanced models measure continuously, and can be connected to the internet...

Edit: The principles are simple, but the technology can be as basic, or complicated as you wish. Getting accurate and consistent results is not simple, due to interfrence from buildings and trees, etc.

There's a whole Facebook group dedicated to rainfall (if you have lots of time and patience, mostly in Afrikaans). See: https://www.facebook.com/groups/reenvalsa/
 
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StrontiumDog said:
What's the generally accepted method of measuring rain? Is it in a 1 metre squared box where 1mm = 1 litre in that box?
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Absolutely mind-boggling to realize that 1mm yields 1 liter for every square meter. We were just speechless last night. We have a 400 to 440m^2 roof. We raised our roughly 18m^2 pool with more than 250mm last night. Still backwashed twice and lots of water went to waste as you simply couldn't keep everything.
 
Krypt0n1te said:
Absolutely mind-boggling to realize that 1mm yields 1 liter for every square meter. We were just speechless last night. We have a 400 to 440m^2 roof. We raised our roughly 18m^2 pool with more than 250mm last night. Still backwashed twice and lots of water went to waste as you simply couldn't keep everything.
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Its all about depth, collection area and volume ratios (see the water restriction thread for discussion about dam levels). I'm still way behind, should have done a FAQ about this long ago.

Our block collects only from a small part of the roof (100m^2) and has a big garden (400m^2), so rainwater alone is hopeless, with a 1:4 collection ratio.

A house with a big roof (400m^2) and a small swimming pool (20m^2), can easily be topped up, due to the 20:1 collection ratio. P.S. My cousin uses a tank to store the surplus for the pool.

If you want to fill a large reservoir such as Theewaterskloof, you need a huge catchment area (or high rainfall mountains), otherwise it will never fill up. Annual average rainfall 500mm, and depth of reservoir is 20m, so the catchment area needs to be 40 times the reservoir area. Wikipedia says the catchment area is in fact only 10 times!? Link: https://en.wikipedia.org/wiki/Theewaterskloof_Dam#/media/File:Theewaterskloof_catchment_area.png

The Berg River Dam is the opposite (medium reservoir and large catchment and rainfall intensity), and fills up even in below average years. The surplus is piped to Theewaterkloof via a tunnel, as part of the WCWSS.

To supply an typical city requires a volume of fresh water equivalent to the annual rainfall measured over the urban area itself. Cape Town is rather sprawling, but Wikipedia puts the area at 1500km^2. The catchment area of Theewaterskloof is a mere 500km^2, but if you add all the other dams, and the high rainfall in the mountain catchments, the numbers just about add up.

Useless factoid: You are shipwrecked on a desert island, and it starts pouring with rain. You open your mouth hoping to get something to drink. You will be able to wet your tongue, but not be able to survive this way. You need a sheet of plastic to collect enough water to live on!
 
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Heavy rain parts of the karoo. It is just possible that the band of moisture reaches CT in the next two days.
 
Geoff.D said:
Heavy rain parts of the karoo. It is just possible that the band of moisture reaches CT in the next two days.
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There will be a significant inflow of upper-level tropical moisture over much of the western half of SA during the next two days (Mon 12th and Tues 13th). At a minimum this will bring a dense layer of high-level altocumulus clouds, with scattered thundershowers starting later on Mon 12th.

Some recent model runs are showing the possibility of significant rainfall (10-20mm) for Cape Town, others have shown zero. It is going to be one of those systems, where the distribution of moisture is impossible to predict in advance.

The CT weather radar (Constantiaberg) hasn't been working for several months, so it will be hard to give real-time updates.

Edit: The CT radar started working this morning (somebody must have woken up and fixed it!) Unfortunately I am unable to post a public link (it is password protected), but I will be monitoring developments.

The EUMetsat satellite animation should give an overview of developments: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/SOUTHERNAFRICA/
 
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44 mm steady light rain in the Hanover district overnight. Very little thunder. But enough to strike a tree a low it to pieces set it alight.
 
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The thunder has just arrived and I can hear it raining. Oh please let it chuck it down!
 
akescpt said:
Heard the thunder. Where are it raining?
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The cloud base is still very high, so you can get thunder and lightning, without the rain ever reaching the surface...

Edit: The technical term for this is dry lightning: https://en.wikipedia.org/wiki/Dry_thunderstorm

Dry thunderstorms are notable for two reasons: they are the most common natural origin of wildland fires, and they can produce strong gusty surface winds that can fan flames.
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