I am tired of hearing about the dry season in Costa Rica. The concept is confusing for tourists, for gardeners and for anyone planning recreation. The confusion arises because most of the country does not have any season that is dry. To be sure, there is a dry season along the Pacific coast, especially in Guanacaste. The precipitation goes to zero for several months in most years, the grasses turn brown, and things can look a bit dusty. Anywhere else, it makes more sense to talk about the less-wet season.

Here are a couple of graphs from Costa Rican Ecosystems [Herrera, W. (2016) Chapter 2: Climate of Costa Rica. In M. Kappelle (Ed.). Costa Rican Ecosystems (pp. 19-29). Chicago and London: Univeristy of Chicago Press.] that demonstrate the problem.

Assessing these two charts, displaying rainfall in some representative areas of Costa Rica, you will discover that there is no time of the year when areas other than the Pacific coast have truly dry weather. The weather is only less-wet. Even this statement is perilous, due to the large number and often small size of climatic areas in Costa Rica. Costa Rican microclimates can be close together and wildly disparate. For most of the country, dry season means the rains are mostly in the afternoon or evening and the mornings are mostly sunny. Even the number and type of major climate zones in the country is debatable, varying from the traditional seven to as many as twelve. Pity the poor gardener in Perez-Zeledon obtaining gardening advice online from someone in Bagaces or Limón.

The precipitation regime for the Lake Arenal area, taken as a whole, is somewhere between that of coastal Atlantic and mountain Atlantic regimes, lines with triangles and squares on the first graph. It is rare to have more than a few sequential days without rain. Even speaking of an average climate for the lake area is somewhat misleading, as anyone comparing relatively dry/windy Tronadora to relatively windless/rainy Nuevo Arenal can easily observe. The weather in any area of Costa Rica depends much more upon altitude and geography than it does upon the tropical location.

The seven traditional climatic zones of Costa Rica are North Pacific, Central Pacific, South Pacific, North, Central, Caribbean North and Caribbean South. Lake Arenal is in the North climatic zone. The country is divided northwest to southeast by volcanic mountains. In Nuevo Arenal, these divide into Pacific foothills to the southwest and Caribbean foothills to the northeast, not truly becoming mountains until you get south of the lake. The lake runs about 30 degrees off of an east-west line. The relatively high mountains of the Tilarán Cordillero are due south of the lake and Arenal Volcano is due east of the southern end of the lake.

The winds are almost entirely from the east due to the Coriolis effect. The earth moves eastward and the overlying air does not move as rapidly as the earth?s rotation, so the wind moves westward. Turbulence makes the pattern much more complex as you move away from the equator, but as a first approximation we can assume a steady easterly wind (for some reason that means toward the west) when talking about Costa Rican climate. Warm, moist Atlantic air is pushed over the Caribbean foothills. Temperature falls with increasing altitude, so that the moist air becomes saturated and it can no longer hold the water. Thus, the eastern edge of the lake gets rain. Air must climb higher to get over higher foothills to the southeast; therefore, rain is heavier in and around Arenal National Park. Southern mountains drop in height to foothills toward the northwest. Air is funneled toward Tilaràn, where it again climbs. Since it is moving faster and is no longer as moist, there is less rain on the western lake margin. The funneling effect explains the placement of wind turbines toward the northwest margin of the lake. But if high pressure on the Pacific side pushes moist Pacific air over the Pacific foothills, the cooler Atlantic air causes rain as it encounters and cools the moist Pacific air. For this reason, one can often see rain advancing from Tronadora toward Nuevo Arenal against the prevailing winds. When air moisture is lower, the altitude difference as air climbs the foothills is not adequate to create rains. The air becomes saturated and rains begin only when the temperature falls in the afternoon and evening. That is the ?dry? season for Lake Arenal.

Rain storm moving across Lake Arenal, in this case toward Nuevo Arenal.

This brings us to the effect of climate change on our area. For the purposes of this discussion, it is irrelevant whether or not you believe that climate change is man-made. It is sufficient to recognize that climate change is observable. A higher surface temperature means more heat stored in seawater. Since we are sandwiched between Atlantic Ocean on the east and Pacific Ocean on the west, the higher water temperatures mean that our terrestrial temperatures will also increase on average. The higher air temperature over the oceans means the air will hold more water. Unless temperature increases on both Atlantic and Pacific sides are exactly equal, average rainfall will increase. Greater pressure differences will increase the average wind strength. Cooling effects of both wind and rain will tend to counteract the effect of overall warming. The lake will continue to serve as a heat sink, absorbing more heat than land during the day and releasing more heat than land at night (called the lake effect). The lake effect will also increase with increasing temperature.

There is almost always a difference in temperature between the two oceans. When one is hotter it is an ?El Niño? year and when the other is hotter it is a ?La Niña? year. One increases the rainfall on the Caribbean side and increases drought on the Pacific side. The other increases the rainfall on the Pacific side; so far, drought on the Caribbean side is not a thing. The larger the temperature difference, the more severe the weather. With higher average temperatures, the statistical likelihood of a larger temperature difference between the two oceans is increased. Since what can happen will happen, given sufficient time, our weather will likely be more severe on average. This paradoxically means both more floods and more severe droughts. Severe winds should be more common, but so should cooling breezes due to an increased lake effect. How much, how soon, and how often are questions that remain unanswered.

I have left out many other factors, such as clouds blocking sunshine. Since I am not an atmospheric scientist, my analysis probably lacks sophistication. Nevertheless, you can at least understand some of the factors that affect our local climate. The number of factors that need to be included and the granularity of the weather data employed can strongly affect the outcome of climate modeling with computers. The trend is clear, but the magnitude of the changes is not clear.

What does all this mean for an ecologist or a gardener, for a bird or a plant? It all depends upon the speed with which climate change occurs. It is already abundantly clear that we do not intend to do anything significant to prevent climate change. It follows that we need to think about how we are going to adapt. Currently, ecologists are working to restore native species. One must recognize that our records of native plants go back only a couple of hundred years. In this context, native just means the things that were growing there before our current industrial civilization began to seriously alter the environment. The focus is on native plants because they are better adapted to the current environment, but there is also an element of trying to return to some mythical environmental ?golden age?. It is clearly true that saving a native species which has coevolved with a native pollinator will save both. It is also clear that introduced species that lack appropriate pollinators will fare poorly. Concerning questions remains. What happens when the current environment changes very rapidly? Will we end up removing native species and seeking invasive species that can survive in a new climate? Will climate change occur slowly enough that species without legs can migrate to appropriate climates?

I hope it does not get that bad. But climate change is a purely practical reason to learn about plants and pollinators in climates different than your own, if practicality is what floats your boat. In the meanwhile, what should you tell someone asking about the dry season in Costa Rica? Bring an umbrella and Pura Vida.