Calc Keys To The Kingdom - Cloud parting in the void-stained abyss

Every now and then, Grim Tuesday sends out a Sun Burst to clear up parts of the Pit. He does so to the top layers of the pit.

ARTHUR HAD BARELY hit the ground, with Suzy and Japeth on either side, when there was a sudden flash of light so intense that he had to shut his eyes even though he was looking at the ground and his hood was pulled over his face.

Strangely, there was no heat or shock wave, though Arthur had flinched in expectation. There was only the initial flash, then a slowly lessening but still brilliant light.

A few seconds later, a faint but piercing whistle echoed down through the Pit, like the cry of a distant bird. The all-clear whistle, Arthur presumed. Scrunching up his left eye and keeping the right completely closed, Arthur risked a look.

What he saw astounded him. A giant glowing ball the size of a hundred hot-air balloons hung in the air about a mile up and eight or nine miles away, like a small, comfortably bright sun. It had banished all the rain clouds, the rain and the smog, and its slowly fading light illuminated the upper reaches of the Pit in all its vastness, a hole so big the far side was just a blurry smudge at least twenty miles distant and so deep that even the sunburst’s light could not penetrate its depths.

‘So that’s a sunburst,’ said Suzy, with a sniff. ‘Thought it’d be better than that. More like a big firework, you know, knock the dust out of yer ears with a bang.’

‘It’s bigger than I thought . . . could have thought,’ whispered Arthur. He’d been to the Grand Canyon and was thinking of the Pit on the same sort of scale. But it was much, much wider than the Grand Canyon and much, much deeper. ‘The Pit, I mean.’ - Grim Tuesday

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We're given a time length and a distance. As it's far further down than across but to an unknown degree, I'll give a low end of at least twice as far as the diameter of the pit, as it's only said to affect the Upper levels of the Pit (Up Station, Way Station 1 & Way Station 2).

How cloudy is it in the Pit?

It was also completely dark, without even the soft glow of the distant ceiling of the Atrium, and certainly no elevator shafts illuminating the scene. Even worse, there was smoke everywhere – thick, cloying smoke that instantly made Arthur’s lungs tighten and constrict. - Grim Tuesday

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She looked up at the sunburst, which was very faint now. The clouds were practically solid again all around it, and she and Arthur were in twilight that was rapidly turning into darkest night. - Grim Tuesday

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Pretty damn cloudy (as in thick smog & clouds all the way down), so let's do this! The Pit's basically a giant cylinder, so that makes our cloud easier to work with.

20 miles = 32186.88m
10 miles = 16093.44m
40 miles = 64373.76m

V = πr^2h
= π X 16093.44^2 X 64373.76
= 52378917600000

Density of a cloud is 1.003m^3.

52378917600000 X 1.003 = 52536054400000 kg

As for the timeframe, "a few seconds" is more than 2, but still not many, so I'll go with something between 3 and 10.

Low end
64373.76 m/10 s = 6437.376 m/s (mach 18.77)

KE = (0.5) X 52536054400000 X 6437.376^2
= 1088542050000000000000 joules
= 260.167793977 gigatons

Mid end
64373.76 m/6 s = 12874.752 m/s (mach 37.83)

KE = (0.5) X 52536054400000 X 12874.752^2
= 4354168199999999970000 joules
= 1.04067117591 teratons

High end
64373.76 m/3 s = 21457.92 m/s (mach 62.56)

KE = (0.5) X 52536054400000 X 32186.88^2
= 27213551300000000000000
= 6.50419486138 teratons

Results
Sunbursts energy (low end) = 260.167793977 gigatons
Sunbursts energy (mid end) = 1.04067117591 teratons
Sunbursts energy (high end) = 6.50419486138 teratons
Sunbursts speed (low end) = Mach 18.77
Sunbursts speed (mid end) = Mach 37.83
Sunbursts speed (high end) = Mach 62.56