How and where does Dew come from?

Answer 1

Dew is water condensed from the atmosphere onto objects on the Earth's surface. Dew forms when the air temperature falls low enough to cause water vapor in the air to condense. The temperature at which this occurs is called the "dewpoint". A little more technically, the dewpoint is the temperature at which the air becomes saturated (i.e. the temperature at which the relative humidity would be 100%)

For example, if a weather report shows a current temperature of 70F and dewpoint of 50F, and the overnight low is forecast to be 50F or lower, then dew will likely form. If however, the low is forecast to be greater than 50F, then dew will not form.

Dew forms most often on clear nights with no wind because temperatures fall more rapidly -- and therefore have a greater chance to reach the dewpoint -- under these conditions, as compared to cloudy or windy nights when temperatures fall less rapidly.

Answer 2

Dew comes from the moist air which is near the earth's surface and is in contact with it. Condensation of the water vapour in the above air, on a surface whose temperature is reduced by radiational cooling to below the dew point is the reason for the dew formation.
On clear nights,the earth surface will lose more heat than it received from the solar radiation during the day time.So, condensation of water vapour will be faster than the evaporation of moisture leading to the ideal condition for dew formation.
Sometimes dew forms in a different way.Water vapour from the soil diffuses upwards and condenses on the exposed cooling surface which is in contact with it.This happens when there is a calm wind and the condensation takes place over grass,leaves etc.
There is a difference between the fog formation(which also forms near the surface) and the dew formation.Fog forms when water vapour condenses on condensation nuclei whereas dew forms when the water vapor condenses staraight away on the exposed objects over the earth's surface and does not require the above mentioned nuclei.

Answer 3

It is a little more complex than it seems and is a version of a glass with a iced drink in it.
Just like air is not as hot as the hood of your car with the sun shinning on it, ground may be cooler at night than the air just above it.

All material that is above absolute zero gives off a long wave radiation that is in the infra-red (IR) wavelength. When the sky is clear, and the winds are calm or near calm, the surfaces will radiate IR radiation directly into space. Air (no clouds) is for all practical purposes transparent to IR radiation but being a gas does not give off IR radiation to the extent a solid object like the ground, cars, grass bushes...etc does. Thus the air cools more by direct contact with the colder, "skin" temperatures of the solid surfaces than by radiating into space. So the solid objects lose heat directly to space and can be as much as 15 to 20 degrees F colder than the air 5 to 6 feet above ground where air temperatures are measured.

Thus, water vapor will condense onto these objects that are actually cooler than the air just like water will condense on the outside of a glass of ice water. The air next to the cooler object will be cooled to below it's dew point and the excess water vapor will condensed out of the air onto the object.

That is also why you can get frost with air temperatures several degrees above freezing. The grass or windshield of the car is below freezing and the water vapor goes straight to a solid without the liquid phase.

A neat little gadget that measures IR temperature of objects can be bought at most good electronic stores. Sort of fun to play with to see what cools the fastest at night. You would be amazed at the differences in temperatures of various objects. Then take the temperature of the air with a regular thermometer and see the difference.

Answer 4

Dew comes from the air. When the air is warm or in motion it can hold more water than it does as it cools or when calm. As the night progresses eventually a temperature is reached the water will condense out of the air as due. If the temperature is never reached you get none. If sufficient water condenses you get fog.

Answer 5

dew is moisture in the air (dew point) once the air temp reaches the dew point, the moinsture collects on the surface and there you have it. hope this helps

Answer 6

Well, God drinks Mountain Dew in Heaven and when it drips off the can it makes "Earth" dew.

Mrs. God tries to tell him to use a cupholder but He doesn't listen. Sometimes I worry about those two!