Saharan Sandstorms Sloughing Seawards

I was talking to a friend who, like me, has a background in Geology. I'm more into the palaeogeography and climate change, he's more into rocks from the structural and engineering side. However, we were talking about the sudden spells of stormy weather we're getting just now. My thought was my day would only be complete when someone from Fiends of The Earth or some other eco-nasty group was on tv, spluttering 'global warming' or 'climate change' as is their wont. He absolutely threw me when he said "Bet he" - it's nearly always a he, isn't it - "never even mentions the effects of dust storms from the Sahara."

At that point I stopped in my tracks. I realised a long time ago that desert sand gets everywhere. Usually it's into places you don't want it. Inside cameras, clothes, eyes and ears (look at how camels have evolved to keep it out) plus others too delicate to mention as well ! That fine, silky stuff is in the air around you all the time. Often you don't realise how much there is until the sun starts to set. Then that golden orb suddenly changes to a hazy reddish ball which becomes ever more indistinct as it settles through the last 15 degrees of arc down to the horizon, its rays passing through progressively more and more dust-laden air and their angle becomes more oblique. I'm almost willing to bet that's why camel are so tall - keeps their heads above the worst of the air-borne sand. That and keeping their bodies above the really hot air immediately above the day-time sand would be my best bets.

I found out quite a bit more about its movement 30 years ago from general curiosity. In 1976, that lovely hot dry summer, my car was rained on by a heavy convectional shower (near Pratt's Bottom, of all places. No rude comments please). Drying rapidly, it was covered by fine red blotches. On taking a few samples and testing them I found it was Saharan sand. (Red is the colour of the desert, more or less - oxidised iron, once again). Intrigued by this I actually spent some time with my nose in a book on weather and found out about air streams. It seems that we were being affected by 'cT air' (Tropical continental) from the Sahara. Why they reverse the name compared to the letters is lost on me I'm afraid. It seems that large blobs of air 'sit' over source areas such as the Sahara and take on the temperature and moisture conditions of the source area. After a while they start to stream out. In this case, over us. They take their characteristics with them and modify the weather we would normally get. In the case of 1976, we had that really long spell of hot dry weather. So that fine dust can travel all the way from northern Africa to Britain.

From my studies into the effects of volcanoes I knew a fair amount about the various cooling effects of dust from eruptions. Tambora in 1815 put so much of the stuff into the air that there was no summer and it was known as 'Eighteen Hundred and Frozen To Death'. Turner's paintings with their spectacular skies are about the only real evidence we have of the global spread of that dust since photography didn't exist. Krakatoa in 1883 wasn't quite as devastating but it still caused significant cooling and incredible sunsets. Did you know that the background in 'The Scream' by Munch is that of a sunset affected by volcanic dust, not long after Krakatoa blew its cone away ? I was surprised to find that out. It's one of those stupid little factoids I'm liable to use from time to time. For what it's worth, my favourite factoid of the moment is that Pietro Mascagnia was so proud of his Intermezzo in "Cavaliere Rusticana" that he used to rush up to people whenever he heard it being played and tell them he wrote it. Mind you, I'd be proud to have written anything even half as good at that: it's really stood the test of time. It must be on the radio several times a week.

Dust in the air is also a contributor to the variation in sun's energy reaching the surface of the Earth. At the Equator the sun's rays are close to vertical all the time. They reach the surface and are concentrated into a small area, giving strong heating. At higher latitudes the sun's rays are quite oblique so they have to pass through a lot of atmosphere. On the way through the insolation is reduced significantly by the dust. That plus the diffusion of the sun's rays over larger areas make higher latitudes quite a lot cooler.

Anyway, once I'd thought it through, I wasn't so surprised. On reading up various articles while I've been unwell, I've found that there is an amazingly strong correlation between hurricanes in the Atlantic and the absence of sand blown from the Sahara. When there is no dust there are hurricanes and vice versa. It looks as though these sand storms can blow up in as little as five days and they smother hurricanes by depriving them of the heat and moisture they need to grow. Some years it seems to happen, others not.

Now there's something constructive to build into the analysis and forecasting of hurricanes.