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Notes on February Wind Patterns

The following analyses was made to help with the date selection of the launch. All of the figures are based on data from February 2000 through September 2011. For all but plot E, included is the average of quality-controlled measurements made between 9 AM and 1 PM for each day of the year, averaged together and then calculated a 5-day running average to smooth out high-frequency variability. Plot E displays the average daily change in glacier surface height, accumulated from a datum of 0 on January 1st.

Each of the 5 plots have semi-arbitrarily drawn vertical lines (in pink), suggesting that the time between these lines looks best from the perspective of that particular variable.

A. Solar Irradiance is a measure of the sun’s power. Since this changes seasonally, reference the smooth curve above the wiggles, which shows a calculated 75% of the power outside our atmosphere, at the location of Kilimanjaro. From mid-January through to mid-March, irradiance increases due to both the earth-sun geometry and relatively less cloudiness (on average).

B. Downwelling Longwave is essentially sky temperature, which is higher (warmer) when there is cloud cover. Note the increase after mid March, which remains relatively high through May.

C. Vapor Pressure is a measure of how much humidity is in the air at summit level, and on an annual basis, vapor pressure clearly shows the seasonal precipitation pattern up there (and is associated with cloudiness). The mini-dry season is apparent in this plot, which is not as reliably and continuously dry as during the longer dry season (i.e., July-September).

D. Albedo is a ratio between incoming and reflected solar radiation, with higher numbers indicating a brighter surface. Snow is brighter than bare ice, and the albedo of snow decreases as it ages. The time between my pink lines is quite distinct.

E. Surface Height only increases with snowfall, yet can decrease due to melting, sublimation or wind. Note the increase after mid March; although the magnitude seems small, Kilimanjaro is a very dry place where 5 cm of snowfall might be considered a storm!

Alternatively, the first 2 weeks of July might be the ‘nicest’ weather on top, or at least the driest and within the least windy period. But, this is also the coldest season of the year and, more importantly, the weather down on the plains – where pilots will be landing – may be less suitable for flying. The low-elevation weather during February have conditions which are often clearer.

Source: Dr Douglas Hardy’s Kilimanjaro Climate blog – http://kiboice.blogspot.com.au/