The terms dew point and pressure are related to one another as both of them largely influence the humidity of nature.
We are familiar with the term pressure, defined as the amount of perpendicular force exerted per unit area. At the same time, the dew point is related to the temperature and pressure associated with water vapor. In this post, we will give a brief note on dew point and pressure in various aspects.
Firstly, let us learn the definition of dew point. It can be defined as “the temperature at which air is cooled with the water vapour at constant air pressure and water content to achieve saturation so that water droplets (or commonly known as dew) are created on the solid surface.” It is the general definition of dew point. At the dew point, the moisture in the air condenses slowly to form the droplets.
How to find dew point from different pressure?
The dew point and pressure are interrelated. The dew value of the dew point varies for different temperatures. Both dew point and pressure are associated with relative humidity; we can find dew point from different temperatures.
The pressure associated with the dew point is both vapor pressure and atmospheric pressure. A small variation in the partial pressure changes the vapor pressure to influence the humidity, and thus dew point varies.
For better understanding, consider the example of water. The vapor pressure of the water is about 31.8 torr at 30°C. But the partial pressure of water will be zero so that there will be no dew point. When the water changes its form from gas to a liquid, condensation will occur, which is widely helpful to increase the partial pressure; when the water evaporates from liquid to gas, the temperature increases with the rise in the vapor pressure.
The humidity is due to both the process of condensation and evaporation. If the rate of evaporation is less than the condensation, then there will be no saturation taking the plane; if the rate of evaporation is equal to the rate of condensation, then there will be complete relative humidity, i.e., 100% relative humidity. The dew point is the temperature at which complete relative humidity is achieved.
So, to find the dew point from different pressure, first, we must know whether saturation has occurred or not. Then finding the relative humidity by using the formula;
We can easily find out the dew point.
How does pressure effect the dew point?
The dew point and pressure depend on one another. The effect of pressure on the dew point depends on which pressure influences the dew point.
- The atmospheric pressure does not have much effect on the dew point. If the relative humidity and air temperature remain the same, then the dew point will be the same, whether the atmospheric pressure is high or low.
- Suppose if the air gets compressed, then the vapor pressure of water molecules in the air increases, which is intended to increase the dew point.
The above statement generally means that at higher barometric pressure, there are more chances for the water vapors to be pressed and bound together closely so that condensation will be increased to form more dew, thus, increasing the dew point.
Dew point calculation formula
The dew point can be calculated using the given formula below;
Where; Td is the dew point, T is the observed temperature, and RH is the relative humidity.
The formula can give the relative humidity in terms of pressure is
Other than this, the relative humidity can also be given by using the Clausius-Clapeyron approximation equation as
The above dew point formula is the simplified approximation for the calculation. The original expression for dew point is given by
Where; T is observed air temperature and RH is the relative humididty
Where; b and c are the constants vary from 17-18 and 237-257 respectively depending on the air temperature and relative humidity.
Significance of dew point and pressure
- The dew point plays a vital role in the process of drying.
- The dew point will be greater if the air contains more moisture.
- Dew point is measured by an instrument called “hygrometer.”
- Pilots use the data of dew point regularly for proper and secured aviation to calculate the icing condition, formation of fog and cloud.
Frequently Asked Questions
What is meant by relative humidity?
Relative humidity describes the amount of moisture content in the atmosphere. The mixture of air and water describes the water content in atmospheric air.
More precisely, relative humidity is the maximum amount of moisture the atmosphere holds at a given temperature and pressure.
Why is the dew point measured by keeping the air pressure constant?
The dew point is the term largely influenced by the vapor pressure rather than the air pressure, and a small variation in the air pressure may largely affect the vapour pressure.
The vapor pressure is nothing but the product of total air pressure and mole fraction of the water vapor. The mole fraction is the quantity that remains the same even though there is heating or cooling until there is a change in the atmospheric pressure. If air pressure is not kept constant, then there will be a drastic change in vapor pressure, and evaporation of water molecules will take place; thus, cooling and condensing will be difficult.
Can we observe the dew point and when it is highest?
Yes, the dew formation on the ground during winter that we have all observed is nothing but the dew point.
We can observe the dew point when there will be relatively high humidity and where water vapor can condense in the air at a given temperature. The dew point in the atmosphere is highest in the morning time because the air temperature is relatively cool and humid.
Does a temperature change affect the dew point?
There is a misconception that the dew point is largely affected by a change in temperature. But in fact, the temperature change is nothing to do with the dew point.
Dew point largely depends on the relative humidity and pressure to always stay below the air temperature. The dew point is always independent of ambient temperature. The increase or decrease in the temperature will not affect the condensation of water vapor, and hence dew point remains unaffected by the change in temperature. But off-course temperature largely affects the relative humidity.