If you’ve spent time in Costa Rica, you’ve likely experienced the rhythm of sunny mornings followed by afternoon downpours during the rainy season. Maybe you’ve wondered what causes this pattern or why certain months have way more rain than others. In this post, we’ll explain what the Intertropical Convergence Zone is, and how it affects rainfall in Costa Rica. We’ll also offer some practical advice for planning around the rain.
Around our planet, there’s a wide weather belt near the equator. We think of it as the tropics. But within the tropics, something else is happening. Air currents are converging (colliding), resulting in instability and clouds. Meteorologists call this specific area, the Intertropical Convergence Zone (ITCZ).
What’s important is that the Intertropical Convergence Zone moves north and south of the equator throughout the year. As it moves, it creates the rhythm for Costa Rica’s wet and dry seasons.
What this means for you: Understanding how and when the ITCZ moves throughout the year helps us predict when the rainy season will start, intensify, and end in different parts of Costa Rica. This can help with everything from travel planning to scheduling home maintenance.
The Intertropical Convergence Zone is a hot and humid area. This is because the sun is almost directly overhead and there’s a lot of water (oceans).
Now add breezes, called trade winds, which flow toward the ITCZ from both the northern and southern hemispheres. When these winds collide, they have nowhere to go but up.
As the wind/air rises, it carries all that hot and humid air up too. At higher altitudes the air then cools, the water vapor condenses into clouds, and it ultimately falls as rain—often lots of it.
In Costa Rica, these tropical rainstorms can dump 2-4 inches (50-100 mm) or more in a single afternoon.
The intensity of rain depends on how much moisture is in the air and how quickly it rises. During the peak of rainy season, we can see impressive cumulonimbus (thunderstorm) clouds towering over 40,000 feet high.
What this means for you: Those predictable afternoon thunderstorms aren’t random. They’re the result of this atmospheric process that becomes stronger as the day heats up. This is why mornings are often clear, and afternoons are stormy during the rainy season.
Like we mentioned, the Intertropical Convergence Zone isn’t stationary. It moves throughout the year, following the sun’s most direct rays as the earth tilts. This movement creates Costa Rica’s distinct seasons.
During these months, the ITCZ is positioned south of Costa Rica, centered over countries like Colombia and Ecuador. With the rain-producing system gone, Costa Rica (especially the Pacific side) experiences its dry season.
As the Northern Hemisphere tilts toward the sun in spring, the Intertropical Convergence Zone begins its northward journey. By May, it arrives in Costa Rica, bringing the start of the rainy season with afternoon showers that gradually increase in frequency and intensity through June.
What this means for you: May is a transition month – a good time to check your roof for leaks with the first rains and clear your drainage systems before the heavier rains come. If you’re traveling, it can be a nice time when things green up after months without much rain.
One surprising part of Costa Rica’s rainy season is the Veranillo de San Juan (Little Summer of St. John), which typically occurs in late June and sometimes into early July. This happens when the ITCZ temporarily moves north, beyond Costa Rica’s borders. The result is a short dry spell lasting anywhere from 3-10 days.
This mini dry season is mostly noticeable in Guanacaste and the Central Valley but can extend to the central Pacific coast as well.
What this means for you: The brief break in the rain is the perfect opportunity to travel. Many locals (on school vacation at the same time) head to the beach.
After the Veranillo, the Intertropical Convergence Zone shifts south again. Costa Rica returns to its normal rainy season pattern of morning sun followed by afternoon downpours.
By September and October, Costa Rica experiences its rainiest period due to a powerful combination of weather factors:
These combined forces create Costa Rica’s wettest months, particularly on the Pacific slope, with rainfall amounts often reaching 15-20 inches (380-500 mm) per month (or more!) in the wettest regions.
Surprisingly, the Caribbean coastline can remain quite dry at this time.
What this means for you: September and October are when flooding risks are highest, landslides are most common, and having backup plans for indoor activities is essential. Many residents use these months for travel abroad if possible. If you are traveling to Costa Rica, consider the Caribbean side for the best chance of good weather.
As the year progresses toward November, the Intertropical Convergence Zone begins its journey southward again. But this final phase of the rainy season can be pretty dramatic some years.
During this month, cold fronts from North America can sometimes push down into the Caribbean basin, bringing bursts of rain to the Caribbean coast and Northern Zones of Costa Rica.
At the same time, late-season tropical systems (like hurricanes or tropical storms) traveling close to Costa Rica can directly or indirectly affect the country with rainfall, especially in Guanacaste and the Northern Zone.
This combination of weather patterns can sometimes create significant rainfall events. We’ve seen this in recent years, including 2016 (Hurricane Otto), 2017 (Tropical Storm Nate), 2020 (Hurricane Eta), 2022 (Tropical Storm Julia), and most recently in 2024 (Hurricanes Rafael and Sara).
What this means for you: November is a transition month with decreasing but still significant rain. It’s important to be prepared for occasional heavy rain, particularly if you’re in areas prone to flooding.
Costa Rica’s complex topography, with its mountain ranges, valleys, and two different coastlines, also affects the ITCZ’s influence.
What this means for you: The specific location in Costa Rica greatly influences how a certain town or region will experience the rainy season.
Knowing how the Intertropical Convergence Zone works really helps us make sense of Costa Rica’s seasons and weather patterns. Locals use these somewhat predictable patterns for everything from farming and scheduling roadwork to planning their beach getaways. For travelers coming to Costa Rica, you can never know the exact weather for your dates, but with this knowledge you can at least have a general idea and know what to expect when you are here.
