Phi Phi, where they are

The climate of the Phi Phi islands, an archipelago in the Andaman Sea belonging to the province of Krabi, is tropical, hot all year round, with a rainy season from May to October due to the southwest monsoon, and a dry season from December to March. In April, before the monsoon, there is a certain increase in rain showers. In November, there can still be abundant rainfall, especially in the first part, either for any delay in the retreat of the monsoon, or because of a tropical cyclone (see below). However, there may be some showers or downpours also in the dry season, especially in December, when occasionally, one among them can be particularly intense.
The archipelago is made up of the main islands, Phi Phi Don and Phi Phi Leh, plus some smaller islands such as Ko Mai Phai (also called Bamboo Island), Bida Nok, and Bida Nai. They are coral islands, where tourists scuba dive. The islands have been hard-hit by the tsunami in December 2004, and the infrastructures have been rebuilt in the following years.

Map of Phi Phi islands and surrounding areas

Temperatures are high throughout the year, in fact, average highs never drop below 30 °C (86 °F). However, from December to March, the humidity is slightly lower. Night temperatures very rarely drop below 20 °C (68 °F), although this may sometimes happen between December and March. From February to April, before the arrival of the monsoon, there is a slight increase in temperature, so, March and April (along with May, the first monsoon month) are usually the hottest months of the year, when highs can quite often reach 35/36 °C (95/97 °F).
Here are the average temperatures of the main island, Phi Phi Don.
Phi Phi Don - Average temperatures
Min (°C)222223242424242424232323
Max (°C)313333333332313131313031
Min (°F)727273757575757575737373
Max (°F)889191919190888888888688

On the Phi Phi islands, an average of approximately 2,400 millimeters (94.5 inches) of rain falls each year. Since the south-western winds affect them directly, on these islands and on the Andaman Sea coast, during the period of the southwest monsoon, the rains are more abundant than on the other coast of peninsular Thailand, that is, that of the Gulf of Siam. However, there are two peaks in the rainy season, in the first part (May) and the last (September-October). During some years, however, there can be torrential rains also in July and August.
Here is the average precipitation.
Phi Phi Don - Average precipitation

The amount of sunshine on the Phi Phi islands is good in the driest period, from late December to March, while during the rainy season, the sun is not missing, but it alternates with cloudiness, which can sometimes give rise to rain showers.
Here are the average daily sunshine hours in nearby Krabi.
Krabi - Sunshine

Sometimes, the Phi Phi islands, like the rest of Thailand, may be affected by tropical cyclones. This area on the Andaman Sea is relatively protected, since cyclones can be formed nearby, but then they will head towards the Bay of Bengal, and this happens preferably between April and December, with two peaks at the beginning and at the end of the period (in April-June and October-December), while typhoons can arrive from the east, but they generally pass more to the north, from the South China Sea to mainland Thailand; however, from time to time, they can also move to these latitudes, which has happened in the last few decades from mid-October to about December 20.

The Beach, Maya Bay, Phi Phi Leh island

The sea on the Phi Phi islands is warm enough to swim in all year round. It should be said that in summer, the sea can be rough because of the monsoon, and there can be dangerous currents. Here are the average sea temperatures.
Phi Phi Don - Sea temperature
Temp (°C)282829303030292929292928
Temp (°F)828284868686848484848482

Best Time

The best time to go to the Phi Phi islands and enjoy their beaches is the dry season, from late December to March. As mentioned, in December and early January, at times, there may still be some intense showers, while in February and March, it can be hot.