Dew Point And Fog: Relationship And Detailed Facts

Dew point occurs due to the atmospheric phenomenon of fog and hence dew point and fog are related to each other.

Fog consists of small tiny water vapors suspended in the atmosphere and dew is formed due to the condensation of these water vapors if the temperature falls below the dew point forming droplets of water.

How Does Dew Point Affect Fog?

At a dew point, the air is fully saturated and condenses to form a water droplet which is seen as fog.

If the saturation of the water vapor reaches the dew point then vapors will condense into the water drop and accelerate down thus forming fog.

During the daytime, the incident radiation energy from the sun is absorbed by the water bodies and plants. The excess radiation energy received is thus given out in the form of evaporation and transpiration. The radiation energy is converted into kinetic energy thus allowing the water vapors to evaporate in the air at greater heights and gradually converting kinetic energy to the potential energy. These evaporated vapors are trapped by the plants and mountains and remain saturated in the atmosphere or even rise further and form clouds.

The saturation of water vapor increases its density in the air and the vapor pressure also increases. These vapors start condensing at constant mass per unit volume and pressure. The temperature of the air goes down and reaches a saturation point. The condensation process is at the same rate at which water evaporates forming vapors.

As the temperature gets cooler at night, the dew point occurs due to the condensation of the water vapors, you will see this effect in the early morning. But as the temperature goes slightly above the dew point, the water vapor condensed into the tiny droplets remains suspended in the atmosphere, this phenomenon is called fog.

Fog in the forest area; Image Credit: Pixabay

At What Dew Point Does Fog Form?

Fog is formed when the temperature of the air is 2.50 C above the dew point.

If the temperature of the air is 2.5 degrees higher than the dew point temperature then the water vapor condensed will remain suspended in the air in the form of tiny droplets of water forming a fog.

If the air temperature is below the dew point, then the condensed water vapor will condense further to form small drops of water and form dew. These condensed water will flow down due to the gravity of the Earth. Hence we find it similar like it was raining in the early morning during the cold winter season.

Does Dew Point Predict Fog?

At dew point, the air is completely saturated with water vapors and condenses to form dew.

The dew is converted into fog if the temperature differs from the temperature of the air at a dew point of about 2.50 C, hence by knowing the temperature of the dew point we can predict the time of fog.

Dewdrops on leaves; Image Credit: Pixabay

If the temperature of the air goes below the dew point, the tiny water vapors particles will not condense further to condense back into the water and remain suspended in the air forming fog. From a far distance, the dew is also appears as fog.

Is Dew and Fog the Same?

The dew and fog are not the same, but both phenomena occur due to the condensation of the evaporated water vapors.

Basically, the fog is formed when the water vapors mix with the dry air and the dew is due to the saturation of the water vapors at constant pressure conditions and condenses to form water.

The dew is formed if the temperature of the air fall below the dew point and the fog is formed when the atmospheric temperature is slightly above the dew point temperature. The condensation of the water vapors at dew points converts the vapors into the water at the same rate as the water converts into the vapors state during evaporation, while the condensation of water into tiny droplets of water vapors forming fog remains suspended in the air and rises further forming clouds.

Why Does Dew Point Decrease After Fog?

A dew point is a temperature at which the water vapor condenses into the water drops.

On attaining the atmospheric temperature slightly higher than the dew point temperature the condensation of the tiny water droplets does not occur thus decreasing the dew point after the fog.

Dew is formed on the condensation of the water particles when the temperature difference between the atmospheric temperature and the temperature at the dew point is less than at least 2.50 C. The fog is formed when the temperature goes above the temperature required for the condensation process to occur. Moreover, the density of the water vapor in the atmosphere also decreases after the formation of fog as the saturation of the water vapors present at the dew point decreases.

Density of Water Vapour v/s Temperature Graph Explaining the Dew point and Fog

Here is a graph of density v/s temperature showing the relation between the dew point and fog.

dew point and fog
Graph of the density of water vapor v/s temperature

This graph clearly indicates that as the increases the dew point falls and the fog is formed. The temperature above the fog point does not indicate the present fog but the aerosol particles that remain suspended in the air during the high temperature too.

As the temperature rises further the fog disappears as these water vapors sway at greater heights. The temperature difference between the temperature at a dew point and fog is denoted in the above graph. TD indicates the dew point temperature at which the dew is formed. The dew temperature can be minus too at certain pressure conditions, and TF denotes the temperature of a system at the formation of fog.

Frequently Asked Questions

How does aerosol affects fog?

The presence of aerosols in the atmosphere will raise the atmospheric temperature.

The evaporated water vapors will mix with the aerosol to form fog, thus increasing the saturation of the water vapor and sustains for a longer duration in the air.

Does humidity affect the dew point?

Humidity indicates the amount of water vapors present in the air columns.

If the humidity is higher, then the occurrence of the dew point is seen more frequently during the winter weather as the concentration of the water vapors in the atmosphere will be more and will soon reach the saturation point.

What are the factors affecting the dew point?

There are many factors affecting the dew point and saturation of the water vapors in the atmosphere.

The factors affecting dew point are the temperature, pressure, humidity, aerosol concentration in air, the intensity of radiations received during the daytime, etc.

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