You can dissolve salt in hot water in a glass to yield a saturated solution. If you allow the saturated solution to cool down, some of the dissolved salt will be deposited down at the bottom of the glass. Similarly, water vapour can mix in air and upon reduction of temperature some water vapour should deposit down as dew.
Dew point is the temperature a parcel of humid air can be cooled, keeping pressure constant, for the water vapour in the humid air to condense into liquid water. The dew point is thus the saturation temperature. The condensed water is called dew. The amount of water vapor in air is called relative humidity. This means that the higher the relative humidity, the higher the dew point. When the relative humidity is close to 100%, the dew point is close to the current air temperature. When barometric pressure increases, the air is able to hold more water vapor. This means that the higher the barometric pressure, the lower the dew point (temperature), and the lower the barometric pressure, the higher the dew point (temperature). In any given day of 24 hours, the lowest barometric pressure occurs at around 03.00 hours and highest barometric pressure occurs at around 15.00 hours.
Formation of Dew
From the above, we can deduce the following:
- When the air is very moist (high relative humidity), there is a higher chance of dew forming as the dew point is close to current air temperature.
- When the sky is clear at night it gets colder and there is a high chance of dew forming as the low temperature is easy to drop to dew point temperature.
- Dew has a high chance of forming after 00.00 hours as this is the time when barometric pressure is lowest and dew point is highest relative to current air temperature (current air temperature gets closer to dew point).
You must have seen deposition of dew on grass in early mornings.
Clear Sky at Night
When the sky is clear at night it gets colder as most heat on the earth’s surface escape to space leaving the earth’s surface colder. When the sky is cloudy, not much heat is able to escape to space and this will have the earth’s surface relatively warmer. When the sky is cloudy it is more likely that the air above the earth’s surface is moister. When the earth’s surface get very cold perhaps as a result of clear sky, the temperature will drop to reach dew point and dew will form. At that point when the dew is forming, you will need just a light wind to keep the condensed water droplets (dew) mixed up with the air above. Air mixed up with water droplets is fog. It is called fog when the visibility (maximum distance you can see with healthy eyes and identify an object) is less than or equal to 1000. It’s called mist when visibility is between 1000 metres 2000 metres. It is called haze if the visibility is between 2000 metres and 5000 metres. Haze is just dust, smoke and other dry particles obscuring the clarity of the sky, and haze may or may not contain water droplets.
Formation of Fog
From the above we can say the following regarding formation of fog:
1) Fog can form by cooling the surface to dew point temperature by radiation of heat, and just having a light wind to keep the condensed water droplets (dew) mixed up with the air above.
2) Fog can form by advection of moist air over an already cool surface to trigger formation of dew, and just having a light wind to keep the condensed water droplets (dew) mixed up with the air above.
Fog - Visibility Hazard
Fog is a visibility hazard that affects the transport industry. In aviation, fog is one of the most feared weather hazards. Fog will cover runways paralyzing take-off and landing of aircraft which is very expensive cost for the industry. And no matter how high this cost is, fog is just better if it’s avoided. Fog is a weather feature that will have a deceptive appearance or impression making you think you can see properly when in the real sense you are only clearly seeing just a few hundred metres.
Formation of Frost
Unlike dew where deposition of water is formed on surfaces when the current air temperature falls to dew point temperature, frost is deposition of small translucent ice crystals on surfaces when the dew point temperature falls below freezing point of water. Frost forms on surfaces directly from the water vapor state, without condensing as dew. Sublimation is to change state from solid to gas or, from gas to solid, without passing through the liquid state. So, frost is formed by sublimation where water vapor moves directly to a solid state of ice.
Unlike fog that will need just a light wind to keep the condensed water droplets (dew) mixed up with the air above, frost will need just a light wind to keep spicules of ice mixed up with the air above.
The ideal conditions for the formation of frost is a night with clear skies, light winds, and a temperature forecast to be near or a little below freezing.
Types of Frost:
1. Hoar frost, also known as radiation frost
2. Advection frost
3. Frost flowers
4. Window frost
5. White frost
Crops' Damage by Frost
Cold air is denser than warm air meaning that cold air will tend to flow down and replace warm air into valleys thus making frost more likely to be found in valleys.
When water freezes into ice its volume increases. When frost deposit on plants’ leaves, stems and shoots the moisture in the cells of the leaves, stems and shoots will freeze into ice. When the temperatures start improving there will be a sudden melting of the frozen moisture that will damage plant tissues. The damaged leaves, stems and shoots may look brown or black resembling burning.
Frost damage to crops can be substantial. You just need one morning with frost and thousands upon thousands of acres with crop is all damaged. If you are a farmer living in an area prone to frost, you may consider using some greenhouse farming which is frost free.
I hope this article will help you understand the difference between fog and frost, difference between fog and mist, and difference between mist and haze.
If you have liked this article, and you would want this page to keep up and improved, you can help in any way you can. A free way to help would be to link back to this webpage from your web page, blog, or discussion forums.
The Author’s page is designed to help beginners and average readers make some money as an extra income to supplement what they may be earning elsewhere - details of which you can find in My Page, if you will.
kabbalraj on February 19, 2015:
Great, while reading and understanding a concept we usually get confused from the next terminology ex. frost and mist etc but here a wonderfully work is that a clear explanation to the subject matter really a great work
Joan Whetzel on January 06, 2012:
I love weather articles like this one. You've discussed a lot of related weather phenomena in a way that is understandable and interesting. I like the way you broke the subject matter down into smaler segments. It makes it easier to read.