Here is a view of Arenal volcano with a few clouds. The weather and cloud cover can change frequently, from cloudy to clear and vice versa. You can see that the summit is above some of the clouds. Some of the clouds appear to be the vapor from the volcanic eruptions.
The volcanic activity occurs at several different places in the volcano, not merely at the top. There are several craters, and there are fumaroles that release volcanic activity about two-thirds of the way up the volcanic mountain.
The volcano is active most of the time, and at times the eruptions are especially intense. In 1993 one wall of the crater collapsed and there were four fiery avalanches of molten lava. In 1998 there were 23 rivers of lava flowing from one of the 5 main craters.