The Beirut explosion shockwave

That was clearly a huge explosion.

Just after the explosion, we see a cloud of ‘fog’ moving outwards at high speed. This is a shockwave, rather similar to that which causes a sonic boom. The ‘fog’ is caused by water condensing from the atmosphere in areas of intense low pressure and temperature, and parallels the sonic boom cloud, or vapour cone, such as in this photo.

The shockwave consists of a front of extremely high pressure, caused by compression (squashing) of the air, which is followed by a region of very low pressure where the air has been stretched. When air is rapidly expanded, it cools. Feel the nozzle of a compressed aerosol spray next time you use one. It will be cold. The resulting cold, low pressure air cannot hold as much water vapour and it condenses out as water droplets. Formation of clouds (as in normal weather) follows a similar but less dramatic process. Rising air (e.g. due to it flowing over a mountain range, or due to strong heating of the ground) reduces in pressure, expands and cools. Water vapour that is contained in the air then condenses out as droplets.

The existence of a visible shockwave over several hundred metres from the source shows the intensity of the explosion. This was really big.  I am amazed that the death toll is being counted in hundreds, not thousands.


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