The main characteristic of a Mediterranean climate is rain in the winter season. This means that option 3 is the correct answer.

Option 1, which states that a Mediterranean climate has high temperatures throughout the year, is incorrect. While Mediterranean areas do experience warm temperatures, they also have distinct seasons with cooler temperatures in the winter months.

Option 2, which suggests that a Mediterranean climate has rainfall throughout the year, is incorrect. Mediterranean climates typically have dry summers with most of the rainfall occurring in the winter. This precipitation pattern is influenced by the movement of weather systems and the proximity to bodies of water.

Option 4, which mentions convectional rain, is not directly related to Mediterranean climates. Convectional rain refers to a process where the sun heats the ground, causing air to rise and form clouds. This type of rainfall is more commonly associated with tropical regions.

In summary, the main characteristic of a Mediterranean climate is the occurrence of rain in the winter season, making option 3 the correct answer.