Here is a rough guideline to different weather types and the warnings associated with them.
There are various types of fog that can affect the island and all of these fog types can bring hazardous driving/travelling conditions with them.
1: Radiation Fog
This is the type of fog that you often see on a cold clear night or early morning when there is little wind and clear skies, this type of fog can be very localised but it can also be extremely thick in prone locations too. Most of the island has experienced radiation fog at some point, even some of the larger towns have experienced it due to the fog drifting to these locations from surrounding fields etc. This is one of the most problematic fog types due to its density and sometimes very localised nature.
2: Valley Fog
This type of fog is rather elusive across most of Isle of Wight but occasionally it can develop where cold air moving down from hills “pool” in a valley or an area of low ground with hills surrounding it. Upper Ventnor through to Wroxall occasionally experience valley fog, Ashey and Nunwell sometimes see it developing too although this is often partly radiation fog too. Most commonly it is seen in the valley between Yarmouth and Freshwater Bay and also along Brading Valley. It can occasionally be dense but is usually very localised to specific locations and doesn’t often cause any issues.
3: Advection Fog and/or Sea Fog
The island is no stranger to sea fog, especially on warm spring and summer days when the wind is blowing in off the sea. This type of fog is formed due to warm air moving across a cold surface (usually just above a cold sea) which cools the layer of air closest to the surface causing condensation to form (fog). Occasionally this type of fog can develop when warm air moves across snow cover although this is rather rare on the island. Sea fog can often be fairly dense and can cause travel disruptions over land and sea.
4: Evaporation Fog
This type of fog often develops around the islands coasts during very cold nights and can be partly to blame for the dense fogs we sometimes see in the Solent. Quite often we see this fog type combine with radiation fog too, this is because when the air cools over land it can sometimes trigger radiation fog development and when this cold (sometimes foggy) air moves out over a warm sea it causes the sea to “steam” which in turn increases the density of the fog that may already be present. Of course we dont always need pr-existing fog to trigger evaporation fog, sometimes just cold air moving out over the warm water can be enough to to allow this type of fog to form albeit very locally if no other pre-existing fog is near by. This type of fog often causes travel delays in the Solent due to extremely poor visibility over the water and can affect coastal areas up to 3 or 4 miles from land.
5: Orographic Fog/Upslope Fog
This is another type of fog we quite often see on the island. It is often seen in spring and summer over Culver Down, St Catherine’s Down and Tennyson Down. It forms when fairly moist air is forced to rise which usually happens because the wind is pushing the air up and over a hill, and as the air rises it cools and its water vapor condenses into fog. It usually remains local to hillsides/cliff tops so doesn’t cause too many issues although it can sometimes be rather dense, potentially causing a few issues to hill walkers.
6: Hill Fog
Hill fog is nothing more than cloud which is much lower than normal. It often develops during mild/warm and humid airmasses from the south or southwest and the cloud base can easily cover some of the higher hills on the island such as Ventnor Down or St Catherine’s Down but can sometimes dip even further to cover the downs from Brading through to Nunwell and can cause some rather dangerous driving conditions at times.
7: Freezing Fog
Freezing fog can come in almost all of the above types with the exception of advection fog.
Freezing fog is when fog forms with the temperature below 0’c, this causes the fog to freeze on contact with surfaces creating a thick crust of white ice that can sometimes look like snow. Occasionally the ice can grow outwards from single objects too and will always grow towards the wind giving some unusual ice formations on trees, telegraph poles and even vehicles. Not only is the fog dangerous due to the density and poor visibility but the rather obvious danger is ice although unlike black ice or clear ice the ice from freezing fog is usually very visible and accidents can be avoided if caution is taken whilst out driving or walking.