Costa Rica weather is truly amazing. Some scientists claim Costa Rica has over 1,000 micro-climates. Where you plan to visit will determine what types of clothing you should bring.
Generally speaking, Costa Rica has two major seasons: wet and dry. The wet season is sometimes referred to as the "green" season, for obvious reasons - everything is much greener! The wet season typically begins in mid to late April and lasts until around the end of November. Usually there is a "mini-summer" around July, a couple of weeks of sunshine. It's hard to predict exactly when the mini-summer will arrive, and some years it doesn't arrive at all.
Wet season: mid to late April until the end of November
Dry season: Early December through mid to late April
What to Bring (suggestions):
Each area of Costa Rica has it's own distinct variations on the weather, however they all have the wet and dry seasons in common. Another important thing to remember: the UV rays of the sun are many times more powerful than in North America. If it takes you 30 minutes to get a sunburn in California, it will likely take you less than half that time in Costa Rica to look like a cooked lobster.
Central Valley The Central Valley is actually a "Meseta" (sometimes called a Mesa) or high flat plain, located in the center of several mountain ranges. The higher elevation (1,161m [3,795.58 feet]) makes San Jose and the other cities within the Central Valley cooler and less humid than the lowlands. Temperatures in San Jose and it's surrounds tend to run from 75° to 85°F (25-32°C) year-round. December through February can be very windy. The rain tends to be the heaviest in October-November.
Northern Pacific: Located in the Guanacaste region, the Norther Pacific area is dry and hot in the dry season and wet and hot in the rainy season. The rains simply raise the humidity until 80°F feels like 95°F. Near the coast you will enjoy the evening breezes developed when the sea and land temperatures adjust to the sunset at different rates. The Northern Pacific area is a favorite for beach-lovers.
Central and South Pacific: From the southern tip of Nicoya Peninsula down through Jaco, Quepos, Manuel Antonio, Dominical and Golfito the humidity rises. Humidity actually rises the further south you venture. Light cotton clothing and bug repellent are the essentials for this region filled with tropical forests and beautiful beaches. In some areas (such as Parrita) you may find some beach areas swarming with "no-seeums", tiny little bugs that live on the ground and leave lots of little red bumps wherever they bite. You won't know you've been bitten until the red bumps appear and it's usually quite a few.
Dominical is becoming known as a major investment place for foreigners. It's been popular with surfers for quite some time because it's like the old surf towns in California were before population growth made them too expensive to live in, and too commercial to enjoy.
The Osa Peninsula near Golfito is one of the unspoiled natural wonders of the world. Quite a large foreign community has sprung up in the south and it is definitely the place to go for those seeking unspoiled beauty and a wildlife adventure they will never forget. Good hiking shoes and bug repellent are a must. It will be very humid so lightweight clothing is recommended. It wouldn't hurt to bring a lightweight waterproof windbreaker if arriving in the wet season. Whale watching in Drake Bay may require long pants and a windbreaker depending on the weather.
San Carlos/Arenal Area: Driving through San Carlos on the way to Arenal Volcano is like driving through a post card. The weather is beautiful in the dry season and, during the wet season, most mornings are picture perfect until just after lunch when you cam expect rain. Most of the TCR staff prefer to visit this region during the wet season, even though you may get wet, the area is lush, filled with green vegetation.
The Arenal area is it's own world. Near the volcano you have beautiful Lake Arenal. If you go fishing on the lake, there will be lots of spray from the fishing guides who love to race across the lake at full speed no matter how choppy the water (e.g. take waterproof clothing). In the dry season you should experience lots of sunshine so think about a good hat and some sunblock. In the wet season it will likely be sunny in the morning and rainy in the afternoon. Rarely is it cold, except for the evenings during the rainy season. Sometimes winds can blow in and make it a bit cool.
Atlantic Coast: If you look up the word "humid" in the dictionary, they may have a photo of the Costa Rica Carribean coast. The humidity can be very high but the quaint little towns along the coast and the beautiful beaches are worth it. The Atlantic region definitely has a bit of Afro-Caribbean flavor in the music, food and people. Many of the residents are descendants of former slaves and the older generation speak English. Obviously you will want to take light clothing and, as always, keep your bug repellent handy!
Important: Be careful about when you plan any trip to the Atlantic region. Many areas have significant flooding during the rainy season and you can become trapped in an area where the only way out is wait till the water level drops, or call in a helicopter.
Special Destinations Some areas of Costa Rica are unusual and require a separate weather advisory section:
Chirripo is the highest mountain in Costa Rica and the second highest in Central America. If you plan to hike to the top, not only are you a brave soul, you will also experience very cold conditions. Take a warm jacket, good hiking shoes and some band-aids for your blisters.
Volcanos are also cold places (sounds odd doesn't it?). The peaks of Poas and Irazu, the two most visited volcanos, are at high elevations and in many cases the wind can be strong. Most of the TCR staff, as a minimum, take a very warm sweater. Some prefer a jacket.
Monteverde Cloud Forest: The cloud forest is typically a wet experience all year-round. There are also a number of biting bugs in the forest (not dangerous, just a nuisance). Good hiking shoes and a water bottle would wise choices. Lots of water = some humidity. We have seen many naturalists with broad rimmed hats and khaki jungle type clothing wandering around the park.