why, when you have a sky full of white clouds , a black one appears in the middle of it.what causes this?

Question

Please explain low barometric pressure

Answer 1

BBQ....Hahahaha...good one!

Well, that black cloud is a thunderstorm.

Severe thunderstorms are often preceded by what's sometimes called a field of cumulus (puffy white clouds). The cumulus is there because the air near the ground (the Planetary Boundaty Layer, or PBL) is warm and humid, which is just what thunderstorms "like". The cumulus usually develops in the afternoon after the sun has warmed things up, and the PBL starts trying to rise. At that level, it has risen just enough to expand and cool to where the temperature drops to the dew point, and condensation occurs. Thinderstorms may not occur immediately because there is an inversion of warmer air, called a cap, at about 3000 to 7000 feet, so the rising PBL stops rising any higher.

As the unstable PBL continues to warm, eventually the cap can no longer hold, and in some spot (usually where there is some sort of boundary, like a cold front, an outflow boundary from an earlier--even previous night's--thunderstorm, the dryline, or where the PBL is forced up higher terrain) the warm, humid, rising air punches a hole through the cap.

If there is instability (cold air at altitudes above the cap), PBL air explodes through the hole in the cap, and rockets upward. If the instability is extreme, that updraft can be greater than 100mph, and you've got yourself quite a thunderstorm.

Barometric pressure is just a measure of air pressure.

Answer 2

Clouds may appear black for two reasons.

1)
The cloud is big and thick, not letting much sunlight through it.

2)
The cloud consists of rather large water drops.
(All clouds consist of small water droplets or small ice crystals that float in the air. Small water droplets and ice crystals have the perfect size for scattering all wavelengths of the visible light equally, which is why clouds usually look white. If the water drops are relatively big, they are not as efficient in scattering the light. Instead they absorb some of the light, making the cloud look more grey.)

Answer 3

low barometric pressure

Answer 4

Busing?

Answer 5

A heavy cloud.... i.e Rain

Answer 6

its either in shadow from the other clouds, or the neighbour has just lit the b-b-q...

Answer 7

There's a black sheep in every family.

Answer 8

PMT.