Frequency of fog - a global outlook

Welcome to take advantage of the following .KMZ files to gain an insight into spatial distribution of both annual (ANDF) and seasonal (SNDF) number of days with fog on a global scale. KMZ is a file extension for a placemark file used by Google Earth.

Annual average
Max average in spring
Max average in summer
Max average in fall
Max average in winter

KMZ files list:

  1. Annual average – annual average fog frequency
  2. Max average in spring – meteorological stations with the highest fog frequency during spring. Description consists of two numbers: the first number means - annual average fog frequency, and the second one - seasonal fog frequency.
  3. Max average in summer (as described above);
  4. Max average in fall (as described above);
  5. Max average in winter (as described above).

The northern hemisphere seasons are defined as follows: March, April and May as spring; June, July and August as summer; September, October and November as fall; December, January and February as winter.
The southern hemisphere seasons are defined as follows: September, October and November as spring; December, January and February as summer; March, April and May as fall; June, July and August as winter.;

Instructions to view .KMZ files from Google Earth

  1. Make sure you have Google Earth downloaded on your computer. If you don’t, please visit this link to download Google Earth http://www.google.com/earth/download/ge.
  2. Import a .KMZ file or a set of .KMZ files on your computer.
  3. Open Google Earth on your computer, go to File, then Open and select all the .KMZ files.
  4. The list of the files will appear under the Places panel.
  5. Stations with fog frequency information will be automatically added to the map and available in a list in the Place panel.
  6. If you have several .KMZ files (e.g. seasonal fog frequency), you can merge them opening altogether.
  7. To change the color and opacity, right-click on the file; select Properties. A new box will open; select the Style, Color tab.

Materials and methods

The Global Daily Summary database ran by the US NOAA with synoptic information from above 8000 stations all over the world makes a useful basis for worldwide comparison. After selection of a subset of stations, the database has been used to focus on average annual as well as seasonal fog frequency. The presence of fog, at least in one of 8 synoptic hours (00, 03, 06, 09, 12, 15, 18, 21 UTC), is one of many information categories stored. To get well established climatic information a 20 years long period between 1991 and 2010 was taken into consideration. Only stations with gaps below 5% of the total time span were selected for further analysis.

Introduction

Both annual number of days with fog (NDF) and fog annual cycle are widely varied depending on the type of climate and local conditions present. Generally, fog frequencies tend to be high where water vapor is abundant like in oceanic, lake, river, coastal and other humid locations as well as cooling processes take place e.g. due to the negative radiation balance, atmospheric mixing or sensible heat flux. Furthermore, synoptic-scale settings of pressure systems and atmospheric circulation, mesoscale factors (for example distance to the coast, exposure to advection of humid air masses) and local conditions (like altitude, type of landform, landuse) affect the occurrence and duration of fog. The aim of this study is to characterize fog worldwide by its frequency as well as annual and diurnal cycles.

Fog frequency worldwide

In equatorial and subequatorial zone due to ample moisture and nocturnal radiative cooling, fog occurs quite often inland (Iquitos/Peru, NDF=102). In montane tropical environment fog is formed even more frequently (Quito/Equador, NDF=208). Interestingly fog is a rare phenomenon at coastal locations of islands spread over warm waters of the tropical oceans (Kaneohe Bay/Hawaii/USA, NDF=11).

In coastal zones of tropical and subtropical latitudes fog tends to persist in regions influenced by cold oceanic currents like in Chile and Namibia). In such conditions advection fog predominates but in places may be completed by slope fog. As the result significant increase of NDF is observed e.g. at a Chilean coastal station (189), Swakopmund/Namibia (128), Cape Columbine/Republic of South Africa (108). Also in moderate latitudes regions where cold and warm oceanic currents meet are notorious for high fog frequency with St. John’s/New Foundland, Canada (NDF=119) as an example.

Fog is also quite frequent in polar zones, particularly in coastal locations where advection fog is typical (Nuuk/Greenland, NDF=81; Base Marambio/Antarctic NDF=138) having the highest frequency in summer but at sites influenced by dry katabatic winds from glaciers fog formation is relatively rare (Mittarfic Narsarsuaq/Greenland NDF=25; Casey/Antarctic, NDF=9).

Role of altitude and landuse for fog frequency

Altitude is one of the predominant factors which controls NDF. As a rule NDF increases with altitude, however above a given level (e.g. 2000 to 3000 m a.s.l. in the Alps) a moderate decrease of fog frequency is observed. It is particularly characteristic for colder half of a year in moderate climate zones when condensation level goes down because of seasonal decrease of solar radiation and predominant atmospheric stability with Stratus and Stratocumulus clouds usually below 2000 m a.s.l. This is the reason why the highest NDF worldwide is observed in middle size mountains 1000-2000 meters high: Mt. Washington/USA (NDF=311), Śnieżka/Poland (298), Harz/Germany (284) where slope fog formation is typical. In case of such stations, there are the following important factors influencing extremely high NDF: exposure to advection of humid air masses, proximity to the coast as well as lower topography on a windward side favourable for the air ascent along the slope.

Stations situated in the bottom of valley-basins show relatively high NDF poorly correlated with altitude. Predominant type in this group is radiation fog which is formed at night in low situated locations but dissipates during the following day as a rule. That is why fog frequency at the lower part of slope is significantly smaller than in the valley bottom.

 

Mieczysław Sobik, Marek Błaś
Department of Climatology and Atmospheric Protection, University of Wrocław, Poland