![]() If the difference is too great, night-time temperatures won't cool down enough for condensation to occur. This means the difference between the daytime temperature and dew-point temperature also plays a part in how likely fog is to form. If this occurs-and the winds are 'just right'-then the chance of a fog forming are very high. The key requirement for fog formation is for the air temperature and dew-point temperature to be very close together. This is measured by the dew-point temperature, which represents how much moisture is in the air-the higher the dew-point temperature, the greater the atmospheric moisture content. The other critical factor in fog formation is the amount of moisture in the atmosphere. If there is too little or no wind, dew forms on the grass instead. If there is too much wind, low clouds form rather than fog. It only forms when conditions are 'just right'. Not all high-pressure systems produce fog though. The greatest rate of radiation occurs when skies are clear and winds are very light, which happens when a high pressure system is controlling the weather. The most common way the air cools is overnight as the ground radiates (loses) the heat it absorbed during the day. Once this happens, the invisible water vapour that surrounds us condenses into tiny water droplets and it is these that form fog. For both fog and clouds to form, the air temperature needs to cool to the dew-point temperature-the temperature at which condensation occurs. ![]() Credit: RenSmart Photography How does fog form?įog forms in the same way as cloud, except the process takes place at the surface rather than higher in the atmosphere. Image: Fog on the outskirts of Dookie, Victoria, 2 August, 2018. The World Meteorological Organization defines fog as a suspension of very small water droplets in the air, that reduce visibility at ground level to less than one kilometre. So what causes fog, are there different types of fog and how will you know if one is coming to blanket your world? (4.Thick fog can make a stroll in the backyard feel like you're in a spooky film and throw a spanner in the works for your travel plans by disrupting flights. (a) To what temperature must this layer be cooled to form radiation or advection fog? (b) To what altitude must this layer be lifted to form upslope fog? (c) How much water must be evaporated into each kilogram of dry air from falling rain drops to form frontal fog? (d) How much evaporation (mm of lake water depth) from the lake is necessary to form steam fog throughout a 100 m thick layer? ![]() Alternately, frontal passage or change of wind direction might blow out the advection fog, and replace the formerly-foggy air with cold dry air that might not be further cooled by the underlying surface.įog formation: A layer of air adjacent to the surface (where P = 100 kPa) is initially at temperature 20☌ and relative humidity 68%. At that point, dissipation depends on the same factors that dissipate radiation fog. If the surface becomes warmer (e.g., all the snow melts, or there is significant solar heating), or if the wind changes direction, then the conditions that originally created the advection fog might disappear. ![]() Such cooling makes the fog more dense and longer lasting as it can evolve into a well-mixed radiation fog, described in the next subsection.ĭissipation of advection fog is usually controlled by the synoptic and mesoscale weather patterns. Surprisingly, neither the temperature evolution nor the distance to fog formation depends on wind speed.įor example, advection fog can exist along the California coast where warm humid air from the west blows over the cooler “Alaska current” coming from further north in the Pacific Ocean.Īdvection fog, once formed, experiences radiative cooling from fog top. ![]()
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