Old San Juan, a historic islet connected to the main island of Puerto Rico by bridges, experiences a tropical monsoon climate characterized by consistent warmth, high humidity, and localized weather patterns driven by its coastal geography.
Typical Weather Patterns
The weather in Old San Juan is defined by two primary seasons: the dry season (December through April) and the wet season (May through November). Despite these labels, rain is a year-round presence. In the dry season, precipitation is typically limited to brief, light showers fueled by the trade winds. During the wet season, rainfall becomes more intense and frequent, often appearing as heavy afternoon downpours that clear up quickly to reveal bright sunshine.
Daily Climate Progression
- Morning: Early mornings are often the most pleasant time of day. The air is relatively cool (low to mid-70s°F), and the skies are typically clear. Locals often use this window for outdoor exercise and errands before the heat intensifies.
- Midday to Afternoon: As the sun climbs, the humid air heats rapidly, often reaching highs in the mid-80s°F to low 90s°F. This is when the humidity "feels" heaviest. In the wet season, it is common for clouds to build up after 2:00 PM, leading to a sudden shower.
- Evening: Once the sun sets, the heat radiates off the blue cobblestone streets (adoquines). However, the coastal position of Old San Juan allows for a reliable sea breeze that provides significant cooling relief in the evenings.
Seasonal Trends
- Winter (December–February): This is the "coolest" period and the peak of the dry season. Humidity is at its annual low, and the trade winds are strongest, making it the most comfortable time for walking tours of the forts.
- Spring (March–May): Temperatures begin a steady climb. March is historically the driest month of the year. By May, the transition to the rainy season begins, marked by more frequent rain events.
- Summer (June–August): The hottest and muggiest months. The air can feel heavy and "oppressive" during the midday peak. This period also marks the start of the Atlantic hurricane season.
- Autumn (September–November): This is the peak of the wet season and the height of hurricane activity. September and October typically record the highest rainfall amounts. November often remains wet but begins to see the first hints of cooler winter breezes.
Microclimates and Variations
Old San Juan is a unique microclimate compared to the inland metropolitan area of San Juan. Being a small peninsula surrounded by the Atlantic Ocean and San Juan Bay, it benefits from constant maritime breezes.
- The Fort Breeze: Areas near the Castillo San Felipe del Morro are significantly windier and feel cooler than the interior narrow streets, where tall colonial buildings can trap heat and humidity.
- The "Wall Effect": The massive 400-year-old city walls and the dense masonry of the buildings act as thermal mass, absorbing heat during the day and releasing it slowly at night.
Weather Phenomena: Trade Winds and Hurricanes
- Alisios (Trade Winds): These steady winds from the northeast are a defining feature of life here. They provide natural ventilation for the historic homes, which were designed with high ceilings and interior courtyards to capture this airflow.
- Tropical Cyclones: Between June and November, the region is in the path of potential tropical storms and hurricanes. While direct hits are infrequent, the resulting "outer bands" can bring days of sustained rain and gusty winds.
- Vaguadas (Troughs): These low-pressure systems can cause several days of overcast, rainy weather even outside of the peak hurricane months.
The Local Experience
Locals have lived in harmony with the heat for centuries. You will notice that many daily rhythms are built around the sun; the streets are quietest in the early afternoon when the sun is harshest and come alive after sunset when the air temperature drops. Locals often refer to the brief afternoon showers as "liquid sunshine," as it is common to see people continue their walk through a light rain, knowing it will pass within minutes.