What is the central focus of this passage?

We spread the blankets inside for a carpet, and eat

our dinner in there. We put all the other things handy

at the back of the cavern. Pretty soon it darkened up,

and begun to thunder and lighten; so the birds was right

about it. Directly it begun to rain, and it rained like all

fury, too, and I never see the wind blow so. It was one

of these regular summer storms. It would get so dark

that it looked all blue-black outside, and lovely; and the

rain would thrash along by so thick that the trees off a

little ways looked dim and spider-webby; and here would

come a blast of wind that would bend the trees down

and turn up the pale underside of the leaves; and then

a perfect ripper of a gust would follow along and set

the branches to tossing their arms as if they was just

wild; and next, when it was just about the bluest and

blackest--FST! it was as bright as glory, and you'd have

a little glimpse of tree-tops a-plunging about away off

yonder in the storm, hundreds of yards further than you

could see before; dark as sin again in a second, and now

you'd hear the thunder let go with an awful crash, and

then go rumbling, grumbling, tumbling, down the sky

towards the underside of the world, like rolling empty

barrels down stairs--where it's long stairs and they

bounce a good deal, you know.