When I first came to Laikipia, I did not understand the weather at all. After a couple of years, I began to get a pretty good idea. After ten years, I had it pretty much figured out. Now, 25 years on, I do not understand the weather at all. That admission notwithstanding, I will tell you a little about what I think I do (not) know about Laikipia weather.
There is a common misunderstanding that rainfall in Laikipia can be described as long rains in March-May and short rains in October-November, separated by two dry seasons. People come in the summer and wonder where all the rain is coming from. Although much of the area to the south and east of Laikipia does have a simple bimodal rainfall pattern, Laikipia (and many areas to the west) also get a set of rains in July-August. These are sometimes called the lake rains or the continental rains. These blur into the long rains that preceded them, and the short rains that follow. The net result is a fairly predictable dry season from December to mid-March, and a weakly trimodal distribution of rain thereafter, with a distinct peak in April-May, and perhaps minor dips in June and September. The long-term records for Mpala suggest that August is the third wettest month at Mpala.
There is a rainfall gradient of south to north in central Laikipia. Average yearly rainfall averages 700-800mm at Ol Pejeta and southern Segera, 600-700mm at central Segera, 500-600mm at southern Mpala, and 400-500mm at northern Mpala.
Of course, these are merely long-term means. In any given year things differ, sometimes considerably. In the last few years, we have had one of the wettest years on record (1997/8), and two of the driest (2000/1 and 2005/6). In 1998, there even heavy rain in January, the most reliably dry month. In 1999-2001, northern Segera got more rain than southern Segera.
There is an apparent rhythm to the rain on a more local level. Often, periods of a few days of rain alternate with a few days of dry. A particularly heavy rain is often followed the next day by more rain. I call this recycled rain, because of my (totally unsupported) belief that it draws from the moisture evaporating from the previous day's rainfall. Sometimes, a period of dry weather is accompanied by strong winds that can blow all day and night, especially in July and August. It seems to me that the cessation of these winds (at night) often signals the onset of rain.
Rainfall is spatially more local in the tropics, and Laikipia is no exception. On a given day, the rain may fall over an area of a few square kilometers to hundreds of square kilometers. A storm that dropped rain on even half of Laikipia would be unusual. There are no fronts here, just local patterning. A rainfall "hot spot" may be consistent over a several day cycle. It is not uncommon for the rain at the Centre to stop at the escarpment, leaving the black cotton dry. Although much of Laikipia's weather comes from the east (typical in the tropics), it can boil in from the north or even west, especially in July-August.
Storms can arise fairly quickly. A few billowy clouds can turn into a serious rainstorm in a matter of a few minutes. Many are the victims who have played chicken with the clouds, and lost. Wet black cotton soil is an unforgiving judge of the audacious.
One of the exhilarating experiences in Laikipia is to see a storm approaching like a squall line across the thirsty land. First the far horizon disappears into the storm, then the wall of white-gray moves steadily closer. You can smell the rain before it gets to you, brought by the gusty wind that precedes the rain. It is a dusty wet smell, combining the two driving forces of life in Laikipia, drought and rain. A few sprinkles precede the wall as a warning to those who have been inattentive. But it is a brief warning, as moments later the deluge arrives. An afternoon shower may come with tremendous force, but often does not last long. Individual showers of more than an a couple of centimeters are unusual. Evening/night rain is usually more gentle, and often longer.
Responses of nature to interannual variation
Life responds to these interannual patterns and others as yet unknown to us. In wet years, the differences among my exlosure plots is muted, perhaps because the grass growth keeps up with herbivory. After the El Nino rains, there was a great increase in Hibiscus, especially in and near the black cotton glades.
We have seen two complete population cycles of jackals. In the early 1990s they were abundant. By mid 1990s, they had all but disappeared. They returned in force after the El Nino year (1997-8), but have recently declined again. By 2003/4, they appeared to be making a comeback. The El Nino year had myriad cascading effects. First the grass grew tall and thick, raising the specter of uncontrollable bush fires. The rodents responded, and these usually shy creatures were seen daily. On their heels were the predators, the jackals and the snakes.
1999 was an big puff adder year. We saw one along the road at least once a week. In previous years (Lynne Isbell's) sightings had been limited to the snakes the patas monkeys had pointed out on Segera. I am guessing that in 1999 we were seeing dispersing animals from a very good recruitment year. One memorable afternoon, the KLEE and ant crews stopped to take a look at one of these roadside snakes. We all piled out of the vehicles and watched and photographed from a safe distance. The puff adder had no time for us, and headed for the nearest cover. It took a moment or two to realize its intention. "No!", I shouted, as it headed for my Land Rover. I picked up some dirt clods and started throwing them, but this only hastened his pursuit of shelter. Sure enough, he did not simply crawl under my car, but crawled up into one of the wheel wells.
We tried everything to get him out. We banged on the wheel well with a lug wrench so loudly that it would have deafened an animal that actually had ears. We poked it with sticks that were uncomfortably short, but it just retreated deeper. We even poured petrol on it. Nothing worked. I figured I had a couple of choices. I could leave the vehicle there and hope that when I came back in a few hours, the snake had left. But that seemed to give the snake too much credit. Instead, I decided to drive off. Perhaps the snake would drop off or be shredded by the axle. (Its amazing how your concern for animals declines when there is a puff adder in your wheel well.) I had two unsettling images. The first was of taking the snake all the way home and inadvertently delivering it to my five-year-old son's backyard. I had decided that I would park a kilometer away, and walk the rest. My other image was even more disturbing. Old Land Rovers are miracles of low-tech engineering, and I could not be sure that the puff adder could not find a way from the wheel well into the cab. I imagined driving along and feeling the dry coarse rasp of its scales against my leg just before the lightning strike. I searched the car, and found no obvious route, but was little reassured. I finally did just drive off, with another vehicle spotting behind. Sure enough, the puff adder was flung off within the first couple of hundred meters, and slithered off into the long grass.
The year 2001 will be remembered in our Segera house as the year of the hairy caterpillars (see "Nuisance bugs of Laikipia").
Birds, being highly mobile, respond to wet and dry years by moving down and up the rainfall gradient. In dry years, you can find many Yellow-rumped Seedeaters at Mpala, and in wet years, the Widowbirds come down from Nanyuki.