Geology and Climate

Geology
Puerto Rico is composed of volcanic and plutonic rocks of the Cretaceous and Eocene periods.  There are also sedimentary rocks of the Oligocene period, along with those of more recent times.  Most of the caves in the karst area in the north are made up of the rocks of Oligocene and later periods.  The oldest rocks on the island are from about 190 million years ago and are located in Sierra Bermeja, in the southwestern portion of the island.  These rocks represent part of the oceanic crust, which could have been moved from the area of the Pacific Ocean to its current place in the Caribbean.

Puerto Rico lies at the boundary between the tectonic plates of the Caribbean and North America.  This means that the effects created by such a boundary are currently distorting the island.  These effects may cause earthquakes and tsunamis.  These seismic events, along with landslides, represent some of the most dangerous geological disasters in the island in the northeast Caribbean.

Climate
As a whole, Puerto Rico has a tropical climate with an average temperature of 76ºF (24°C) and a high total rainfall.  However, despite its being an island, temperatures and rainfall vary greatly throughout the different areas of Puerto Rico.

In the east, the National Forest of El Yunque is a rainforest that receives higher rainfall than the rest of the island.

With a subtropical climate, the tropical rainforest to the north is generally rainy throughout the year.  The northern portion of the island tends to be cooler, with temperatures between 55°F (13°C) and 80°F (27°C) in winter, and between 70°F (13°C) and 90°F (32°C) in summer.

The southern portion of the island contains the area known as the Guanica Dry Forest.  This area is drier, with a semi-desert climate almost the whole year.  Temperatures range between 75°F (24°C) to 100°F (38°C) in summer, and between 55°F (13°C) and 90°F (32°C) in winter. Places like San German and Mona Island have recorded 105°F (40°C) in the summer.  The area receives very little rainfall in summer; it is very hot, with thermal sensations that can reach 115°F (46°C).

In the west it is rainy in the winter instead of in the summer.  The temperatures are warm in the summer, but quite cool in the winter, where they can go down to 50°F (10°C).

The central portion of Puerto Rico is mountainous, rainy, and rather cold with temperatures that can drop to below 40°F (4°C), especially in winter.  The climate here ranges from subtropical to temperate in areas where the elevation is over 3,000 feet high.  The average rainfall is over 100 inches (2540 mm), and temperatures range between 40°F (4°C) and 90º F (32°C).
The waters of the north are cold in the winter, but the southern waters are over 80°F (27°C) throughout the year.
Storms and hurricanes pass over or near Puerto Rico every summer, and often cause flooding.  In Puerto Rico you can see tornadoes, hail, and severe waterspouts, with hail and waterspouts being the most common.  Tornadoes are not very common in Puerto Rico, and those that do occur usually do not develop winds over 70mph (110 km/h).

Other weather events are cold fronts, troughs, tropical waves and high and low pressure. The high-pressure air mass brings heat and sun, while the low pressure brings heavy rains and strong winds.  These events abound throughout the year, though less frequently in summer. The summer rains are mainly due to storms and troughs.

In terms of weather conditions, the best time to visit Puerto Rico is during April, June and July.  The most recent hurricanes that have directly affected Puerto Rico are Georges in 1998 and Jeanne in 2004.

Time zone
Puerto Rico uses Atlantic Standard Time all year round, i.e. GMT (-4:00).  Daylight Saving Time (known as DST in the United States) does not apply in the archipelago because there is not much difference between the rising and setting of the sun throughout the year.  Sunset varies between 17:40 in winter and 19:10 in summer, while the sunrise varies between 7:00 in winter and 5:30 in summer.