The Road to the Horizon: storm

The Guatemala City "sinkhole", about 60 feet (18 meters) wide and 300 feet (100 meters) deep, swallowed a three story building in an almost perfect cylindrical hole. A burst sewer pipe or storm drain probably hollowed out the underground cavity that allowed an underground cavity to form.

The hole appears to have been caused by the deluge from tropical storm Agatha blocking sewerage pipes. Spill water gushed away the underground. Guatemala city is built in a region where the first few hundred meters of ground are mostly made up of a material called pumice fill, deposited during past volcanic eruptions. And just like loose gravel can be washed away by water, so was the underground in this spot. (Full)

With thanks to Juan for the tip.
Picture courtesy Paulo Raquec, National Geographic.

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Mageed, a friend and colleague now working in Afghanistan, sent me these pictures taken in Khartoum (Sudan)...

Nope, it is not the fall-out from a nuclear bomb. It is a haboob, an intense sand storm, regularly observed in the Sahara desert, Sudan, the Arabian Peninsula all the way up to Iraq.

Haboobs are frequently created when a thunderstorm collapses and begins to release precipitation. Strong winds will blow down in the storm centre, and then stream outwards, away from the storm, generally gusting the strongest in the direction of the storm's travel.
When this downdraft reaches the ground, dry, loose sand from the desert settings is blown up creating a wall of sediment preceding the storm cloud. And a haboob is born...

This wall of sand can be up to 100 km wide and several kilometers in elevation. At their strongest, haboob winds can travel at 35-50 km/hr, and may approach with little to no warning.
Often no rain is seen at the ground level as it evaporates in the hot, dry air. When the rain *does* persist, it contains a considerable quantity of dust. This will change the sand storm into a mud storm.

The haboobs in Sudan are famous. So famous actually that even Jan Pronk, the ex-UN Special Representative to Sudan, featured some pictures of haboobs in his -otherwise mainly serious- political blog. In the pictures above (a full resolution version, you can find on my Flickr set), you can clearly see the size of this thing, as it approaches Khartoum over the Nile.

Once the haboob hits town, everything will turn dark yellowish grey, like a bad case of smog. Visibility is often reduced to almost 'nil'. And the dust creeps into everything. It blocks air filters, and sabotages all kinds of mechanical stuff. It gets into the house even through the smallest cracks in the walls or woodwork.

The dust is a phenomena by itself. When sailing in the Caribbean last summer, we once woke up with a fine yellow layer all over our boat. The news told us this was Sahara dust, blown up by a storm, thousands of miles away. The dust is lifted up to an altitude of 15,000 feet (5 kilometers), and comes down as far as Florida, or North-West Europe.

We had regular haboobs in Dubai, too. No surprise, as we were sitting in the midst of the desert. Once a sandstorm hit us, we could clean as much as we wanted, there was no way to keep the dust and the mud out of the office.
One evening, a few years ago, we were all working late, and decided to go and have dinner at the Irish Village, near the airport in Dubai. The "Village" is a nice outside place, where we could enjoy the last evenings of 'bare-able heat' before summer would have everyone lock themselves up in AC'd houses for four months.

As we left the office, the skies got dark, and the wind started to blow a bit, so we agreed to stay in touch via the cell phones as we all drove in separate cars to the restaurant. Not more than ten minutes later, we were all stuck in a massive traffic jam, as a haboob had hit Dubai.
Inches of sand started to build up on the highways, and the dense foggy air reduced the visibility to just a hundred meters. As we all continued driving, we changed our plans several times, still generally heading towards the Dubai airport. By the time we got there, the storm got that strong that lamp posts were blown over the streets, corrugated iron sheets were lifted a hundred feet up as if they were corn flakes and came swirling down. Debris from the construction - Dubai is one big construction zone - was blown off the multi-storey buildings. Traffic lights collapsed over the cross roads, and cars got hit by just about everything possible. On top of that, it started raining like there was no tomorrow.

When we finally got to the airport area, we decided to take shelter in an underground garage of one of the hotels. Bad decision, as the garage was flooded by ten inches of water in no time, so we had to wade our way up.
Anyway, we had a good dinner at the Al Bostan Rotana, had excellent company, so a bit of haboob-ing could not destroy the atmosphere...

Pictures courtesy: Erich Ball (2007)