What's the difference between dew/rain and fog/cloud?
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Maybe this question looks very stupid, but is not, because all things happend at 100% relative humidity. Why once is dew/rain and second one fog/cloud?
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@vujacicm
It's all about how, when and where water vapour changes states.Simply put:
Air temperature is the measurement of how hot or cold something is.
Dewpoint is the temperature air needs to cool to in order to reach saturation. (without the addition of moisture) and therefore be able to condense.When air temperature and dewpoint temperature are equal, the air is saturated (100% RH). When the air is saturated water droplets or ice crystals can form.
If this occurs in the atmosphere, meaning a parcel of air from the surface rises and cools to the point where air temp and dew point are equal, the water vapour in that air can change states onto microscopic particles called condensation nuclei (dust, smoke etc). This forms water droplets or ice crystals and that's what clouds are made of.
You ever look outside and see a layer of clouds and the bases of those clouds are all at the same level? That level is where air temperature and dewpoint are equal, also referred to as the Lifting Condensation Level (LCL).
Alternatively, at the surface, the air can cool to the point where air temperature and dewpoint are equal and the water vapour at the surface will form water droplets or ice crystals on surfaces. This is "dew". "Frost" is the same process but the temperature is below freezing. This happens most often in the morning because the air has cooled overnight.
Rain is when water droplets in a cloud grow enough to fall to the surface.
Fog is a cloud but at or very close to the surface.Hope that makes sense!
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@Wheats I will give you one example. In my town we always have dew in the morning. I am on 460m. 30 km away from my town is town on 201m high and there is always fog every morning. Why we always have dew and there is always fog?
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@vujacicm sounds like a question for someone local. I don't know where you are and what is affecting the terrain and atmosphere in your area.
There are different types of fog. Temperature, moisture content, wind, stability, terrain/topography, moisture sources and more all have significant impacts on fog development....too much to consider in a forum setting while drinking my morning coffee before I dive into my work for the day haha.
All I know for sure from what you've said is there is plenty of moisture in the air and the temperature/dew point spread is minimal.
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@Wheats Both towns are next to the river in the valley next to the mountains. Town with a fog has lower elevation and higher dew point. Question is, why fog is not higher next to the river.
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@vujacicm , hi to you...can you tell a little about windpatterns where you are? most of all it would be great to know which river also, and some E,W,N,S positioning had been great to know for everyone else who chews on this topic.
it sounds like you are above a local ( river runs the rule )inversion, just dry enough or hot enough for fog to not develop where you are. do the river city have the same amount of dew as your own???
im not a meteorologist by all means, but its an interesting complex beehive youve stuck your hand into...
b.r.
ørjan -
@Ørjan-P-Stien This is a map. Bovec is dew town, Tolmin is foggy town. Both are near Soča river
This is Bovec meteo
This is Tolmin meteo
You will also see town Kobarid, where is fogg too. But not in Bovec.
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@vujacicm , hi, thanks for super info, i just have to collect myself from asleep before i write my complete thesis, but i see your terrain features, and thats not coastal nor mississipi climate...
si yo leiter
ørjan -
@vujacicm , hi again, well you live in an alpine valley area, by a river, with steep mountains in north and south, i guess you are also living in bovec on a plateu 300 feet above the soca river, and topographically youre at the start of the valley system that follows the soca river to tolmin, that is 600 feet lower at the river basis. the valley also narrows in towards peak 498, the hump where the juliana trail runs. this peak is like a cork in that valley, if you look at air as an atmospheric river flow, the air may thicken around this bottle neck. if there is a lot of airpolluting industry in that valley, that will also make fog creation more probable due to water clinging to particles...
but most of all its the height difference, with a dominant fallwind sloping down from the mountains during the night hours, making a basis of a local cold inversion that differs your startvalley that also runs east-west, and the valley of tolmin that runs northwest toward sse, making fog a tolmin problem, and you in bovec with a clear sky , dew on the ground problem.
can you point out where the fog starts usually (city name, road juctions etc )? is it when you enter the valley around Kobarid???
air tends to flow along valleys and the lower it runs the thicker it becomes, just watch cloud videos from valleys at high speed and youll see. its a seldom phenomenon that fog raises upslope when weather is stabile, it evaporates( and may cumulate into higher clouds that may give you tough afternoon showers of rain, till it clears up and the whole process restarts with fallwinds and a new local inversion begins) due to warming by the sun.
read yourself up on adiabatic processes, the føhn, and note especially pressure gradients (+ or - 0,8hpa pr 100 meters and the difference between ground surface temp(5 cm above grass level) and 2 meters temp, that may show many deg.celsius differance... underground temp at 1 cm to 100cms below grass level may also convey/store heat that will create dew on your bovic lawn as a local nanoclimate system
but between bovic and tolmin i will say elevation is a clue, and of course river valley topography.
and remember im not a meteorologist, dew and fog is far too complex systems for an amateur, so if you feel nebulized by the theoretics i wont disagree with you.
let our fellow windy meteorologist have the last word on this, it gave me a slovenian geography lesson for free anyway, maybe we meet next march for the planiza skiflying events
have a sunny day
best regards
ørjan -
@Ørjan-P-Stien Bovec is not near Soča river. Village Čezsoča is next to Soča river. But Tolmin is next to river. Also Čezsoča has a lot of fog. Valley between Kobarid and Tolmin, where fog is, is very wide. When you go from Kobarid north, river is like a canyon. But between Bovec and village Žaga is also flat part, but not fog. Fog always appear on clear skies in Tolmin. In Bovec opposite happend. There is fog on rainy days, but not in Tolmin. And if you go from Tolmin to Nova Gorica, there is no fog again.
So, fog is only on flat part between Kobarid and Tolmin, but not anymore, if you go north and higher and not anymore if you go south and lower. Only there is trapped.
