Thunder is not always preceded by lightning but lightning is always followed by thunder. Why?

Thunder is not always preceded by lightning but lightning is always followed by thunder. Why?

Thunder is always generated only by a lightning bolt. The bolt, 1 to 5 cm in diameter, could cause the air temperature to rise up to 30,000 degrees C (Compare it with Sun's surface (Photosphere) temperature of 5,400 degrees C). Such a sudden rise of air temperature causes explosive expansion of the bolt area to produce compression (or shock) waves that are heard as thunder or thunder rumblings.

The time lapse between the two in seconds multiplied by 330 gives roughly the distance of the lightning in metres.When there is a series of thunderclouds and the most rear one some 20 to 30 km away produces a lightning, the thunder may not be heard; especially so when the prevailing surface wind is towards the thundercloud.

Also, during a bright day, an isolated thundercloud may produce a lightning that can hardly be seen; but the thunder could be heard. Also, in a maturing thunder cloud with updrafts of 100 kmph or more, a lightning in the middle area of the cloud could be seen as a bright flash illuminating the upper portions of the cloud; the thunder may not be heard as the updrafts will disperse the sound waves. (A MIG jet aircraft approaching overhead can be seen but the sound can be heard only after it passes over your head).

When thunderclouds merge as in the `eye wall' of a very severe cyclone wherein the sustained surface wind speed exceeds 150 kmph, rapid collision of positively charged areas with negatively charged areas could produce intermittent flashes of lightning without producing thunder. This, I have personally observed for hours from very close quarters to the `eye wall'. This was confirmed under similar situations by some highly experienced meteorologists by independent judgement.

Source: thehindu.com

Why do clouds turn dark when it is going to rain even though water is colourless?

Why do clouds turn dark when it is going to rain even though water is colourless?

Image Source: windingstream.wordpress.com

Clouds consist of fine droplets of water - droplets having a size typically of the order of about several microns. A micron is one millionth of a metre or one thousandth of a millimetre. Visible light has a wavelength of around 0.5 micron. So compared with the wavelength of light, the size of a droplet is thus quite large. These droplets scatter light in all directions almost uniformly. Since the clouds contain large number of droplets, a beam of light passing through such a cloud will be subjected to multiple scattering. We see a cloud by means of the light so scattered. Sunlight appears white due to the presence of all components of the well-known rainbow colours, and since all these components are scattered almost equally, the colour quality of the scattered light is practically unchanged. Hence a cloud very often appears white most of the time of the day. That is not the case during sunset and sunrise as the sunlight is reddish in colour; the clouds in turn assume that colour.But scattering has also the effect that when light passes through a mass of cloud, its intensity drops down continuously. The intensity of scattered light at the top of the cloud that faces the sun is large compared with the intensity of light from the bottom end of the cloud that is seen from the ground. Despite the droplets scattering all wavelengths of light uniformly, a drop in intensity of the light towards the bottom end of the cloud prevents it from appearing uniformly white. Some portions of it appear bright, whereas others appear darker and greyish. In the case of a thin mass of cloud with low density of the water droplets, the effect is not very significant. But when sun's light has to travel through very thick and dense clouds then light reaching some parts of the clouds falls down almost to nil, and those parts appear dark grey. Rain-bearing clouds belong to this category. Usually we see the lower side of the overhead cloud, while the sun shines above it on the other side. Interestingly, the same clouds would appear bright white, when they are seen from an airplane flying above the cloud.

Source: thehindu.com