May 2009 Weather Report for Potrerillos Arriba
Here in Potrerillos, April 2009 marked an unusually dry ending to the dry season, and May marked an unusually wet beginning to the early rainy season (May-July). In this area (see the Rainfall Records page), the rainy season typically does start with a “bang” - average rainfall dramatically increases from an April average of 7 inches to a May average of 22 inches. After that, the monthly rainfall drops off for a couple of months and then increases again for the late rainy season (August-November).
Our "bang" this year, though, was louder than usual. We had 0.9 inches of rain in April (6 inches below average) and 33 inches of rain in May (11 inches above average). That's a 17 inch difference in rainfall between the two months. In the 16 years that Ricardo Espinosa has been keeping rainfall records, in only four of them did May rainfall exceed 30 inches and of those four, two were in the past two years. So 2009 marks the third consecutive year of unusually high May rainfall.
(Note that the numbers above the bars are rounded to the nearest inch, hence the bar for the year 2000 falls slightly below the 30-inch level.)
Other averages
The fluctuations in average temperature are really too small to make much note of. However, as we move into the rainy season, we can see that the average relative humidity is on the rise, and that the solar input and wind speeds are decreasing.
Daily patterns
The seasonal weather patterns in Panama are related to the movement of the Intertropical Convergenze Zone, which in turn is related to the relative position of the sun with respect to Earth’s equator. In a nutshell, the ITCZ (a rainy zone also known as the Doldrums) is south of Panama during the dry season and is near or practically within Panama during the rainy season. Superimposed on seasonal patterns are the daily patterns.
For much of the rainy season, the mornings and early afternoon are sunny and beautiful. The rains arrive in the late afternoon and then end by or before nightfall. Later in the rainy season, this pattern breaks down, but for now, it’s worth taking a look at why this pattern exists at all.
To illustrate what happens here on the Pacific Slope of Panama’s tallest mountain, Volcan Barú, I am using the public domain images from a neat website called espere, “Environmental Science Published for Everyone Round the Earth”.
Two factors influence the local movement of air here: 1) land and sea breezes, and 2) mountain and valley breezes.
Land and Sea Breezes
During the day, land warms up from the sun’s rays more quickly than ocean water does, so that air rises and is replaced by the air off the ocean, the sea breeze.
At night, land cools off faster than ocean water does, so air over the ocean rises and that air is replaced by air from land, giving the land breeze.
Mountain and valley breezes
In our area, this pattern is enhanced by the movement of air due to the presence of Volcan Barú. During the day time, the warmer air of a valley rises up a mountain.
At night, the cooler air from the mountain top sinks down into the valley.
Here’s how these patterns play out in our area. Remember that Panama basically runs west-east so that the Atlantic Ocean (the Caribbean Sea, really) is north of us and the Pacific Ocean is south of us. The Provence of Chiriqui and the town of Potrerillos Arriba are on the Pacific side of Volcan Barú. When wind comes from the north, it is coming at us from higher up the mountain. When winds comes from the south, it is coming at us from the Pacific.
A Typical May Day
So, here’s a typical day in May - May 19, 2009, to be exact. First, we should look at the solar radiation so we can tell how the sun is heating the air. On May 19, the sunrise was at 6:10 AM and the sunset at 6:42 PM. And here’s the solar radiation graph for that day.
Sunlight streamed down upon us from sunrise until about 9:30 AM when some clouds began to roll in. It’s clear from this graph that the sun disappeared from our view sometime after 2:00 PM (1400 hours nautical time) even though the sun was still up.
Wind direction is really instructive. On this graph, a circle has been straightened out so that the very bottom of the graph represents North. As you move up, you’re moving clockwise, first to the Northeast, then to the East, then Southeast, South, Southwest, West, Northwest, and finally at the top you’re back at North. It takes a bit of getting used to, but after awhile it makes sense.
So from midnight until just about sunrise on that day, the air was swinging back and forth between Northeast and Northwest with an occasional gust from nearer East or West. Right at sunrise, the wind was coming down the mountain from a West-Northwest direction - the side of the mountain away from the rising sun.
About an hour later, it shifted to an East-Northeast direction. I believe the rising sun had begun to warm that side of the mountain and the air had begun to flow up the mountain. By about 8:00 AM the wind was coming from the Southeast. The southerly breeze continued, following the sun from a more southeasterly to a more southwesterly orientation, until about 2:00 PM, when the wind direction began to vary pretty wildly.
What happened at 2:00 PM?
It had begun to rain, and it got serious about it between 3:00 and 4:00 PM, dwindling to very little after the sun went down.
So what does this tell us about our daily weather pattern? During the night, the cool air from the mountain sinks down at least as far as Potrerillos and no doubt farther toward David. During the day, as the sun heats the land, the “valley” air rises up toward the mountain, but also the “sea breeze” moves from the south up, perhaps as far as Potrerillos or higher.
And where did the rain come from - south or north? Here’s where observations beyond the charts from the weather station data are necessary.
Early mornings, before and just after sunrise, Volcan Barú often stands clearly outlined against the sky, with few clouds in sight. But as the sun climbs, and the warm, moisture-laden “valley” air rises to meet the colder, drier mountain air, clouds begin to form. The cool air cannot hold as much moisture as warm air, so it condenses out.
From where we live, as the day progresses we often see a “shelf” of clouds appearing to sink down over our heads from the mountain. This shelf extends like a ceiling over us and continues for some kilometers at just above our altitude. Beneath this shelf we can see unclouded lowlands below us.
I believe that what is happening is that as the southerly sea breeze continues through the day, more and more moisture-laden air accumulates against the mountain, the clouds thicken and the bottom layer of the clouds sinks. Eventually, the air cools enough that the moisture cannot even be held in cloud form and it must precipitate out.
So at this time of year, it seems that the water for the rain is delivered to us by the southerly breeze and that the rain itself begins to fall when it meets air so cool that the moisture cannot be contained.
In the case of May 19, 2009, that collision of warm southerly air and cool mountain air lead to very heavy rainfall for a couple of hours.
Later in the rainy season, things will change, and the rain will march straight from the sea right up the mountain, but that’s a different story.
To wrap up:
The rain we had in May more than made up, in inches, the rain we lacked in April. We had been about 6 inches short of average in April and were 11 inches higher than average in May. Temperature ranges showed no interesting changes from the earlier months of the year, but average humidity increased as the rainy season began in earnest and both solar radiation and average wind speeds decreased.
The daily weather pattern we saw in May helps us understand a little about local air circulation patterns on the Pacific slope of western Panama. During the night we have northerly breezes from higher on the mountain. During the day we have southerly breezes from lower on the mountain if not from the ocean itself. When the warm, moisture-laden southerly wind meets the cool, dry northerly wind, we can get rain.
Addendum: See Lloyd Cripe's report on Boquete weather in May at his Boquete Weather site. It's fascinating that whereas we had far more rain than usual this May, he had far less. Our stations, both using WeatherHawk systems, are a mere six miles apart as the crow flies. His station is roughly due north of ours.