Joan Kenworthy's letter (2010) brings back a West African memory. In February 1955, I was serving as touring Assistant District Officer in the southern Katsina Province of the then British Protectorate of Northern Nigeria. On the 15th, the usual seasonal change took place from the cool dry season to the hot dry season. The Harmattan ceased to blow dust southwards from the Sahara, the sky cleared and the temperature shot up 15 degC or more, all in one day. However, the usual symptoms were supplemented in mid-morning by a short-lived high-level thunderstorm. What might have caused this; where did the moisture come from?
A response from Jim Galvin
It seems that in February 1955 the Harmattan rather suddenly brought warmer air, heated strongly under sunny skies. This is likely to have generated some very high-based convective cloud that grew to become cumulonimbus with a base possibly around 4000m and top near 12 000m. It is relatively common to see lines of high-based deep convection over the Sahel (see e.g. http://rapidfire.sci.gsfc.nasa.gov/-realtime/single.php?T100461030).
Although the Harmattan has very low humidity, it is not moisture-free. When air in the upper troposphere is sufficiently cold, deep convection may produce precipitation. If the precipitation began as hail, it could have reached the ground (some of it as rain). In addition, above the Harmattan tropospheric winds are usually westerly or southwesterly over northern Nigeria in winter, north-west of the upper-tropospheric high centred over the northern Indian Ocean. These winds may bring moist air from the humid tropics, allowing the cloud to grow.