In this task, you are given a question and a context passage. You have to answer the question based on the given passage.

What prevents good visibility?, Context: Temperatures below 0 °C (32 °F) or above 25 °C (77 °F) are rare. Cork Airport records an average of 1,227.9 millimetres (4.029 ft) of precipitation annually, most of which is rain. The airport records an average of 7 days of hail and 11 days of snow or sleet a year; though it only records lying snow for 2 days of the year. The low altitude of the city, and moderating influences of the harbour, mean that lying snow very rarely occurs in the city itself. There are on average 204 "rainy" days a year (over 0.2 millimetres (0.0079 in) of rainfall), of which there are 73 days with "heavy rain" (over 5 millimetres (0.20 in)). Cork is also a generally foggy city, with an average of 97 days of fog a year, most common during mornings and during winter. Despite this, however, Cork is also one of Ireland's sunniest cities, with an average of 3.9 hours of sunshine every day and only having 67 days where there is no "recordable sunshine", mostly during and around winter.
fog