Dew Point and Altitude: Relationship and Detailed Facts

by

We encounter cooler air surroundings when we travel at altitudes. The article discusses the relationship and detailed facts about the air’s dew point and altitude.

The dew is the water state of air as it condenses, and the temperature at which it condenses is named the dew point. Due to the descending temperature or dew point at altitude or at height, the air expands to become saturated with the water vapor and condenses.  

We have followed that the air temperature is dropping as we are at altitude or above the earth’s surface. When the air temperatures reach the dew point at altitude, the air particles function as condensation nuclei to condense the water vapor into the liquid water. 

The condensed water will either become cloud or fog, relying on the dew point at altitude where it creates. It will form a cloud at maximum altitude from the earth’s surface. While at minimum altitude, it becomes the dew when in contact with the colder solid surface. Whereas at the layer becomes minimum and maximum altitude, it becomes the fog.

How does Altitude affect Dew Point?

The altitude affects the dew point dramatically based on the relative humidity. 

The relative humidity is an essential factor that reveals how much percentage of the water vapor the air contains. At altitude, the relative humidity becomes 100%, so the air condenses to become clouds only when its dew point and temperature become identical. 

Many of us mistake the distinction between relative humidity and humidity. The air’s humidity indicates how much mixture of water and other elements the air holds. Since we have to study the dew point and altitude, we only view the relative humidity that only describes the amount of water vapor. The relation between dew point and altitude represents the relative humidity concept. 

The moisture in the air is low due to atmospheric pressure, which describes its dew point at altitude. The 100% relative humidity indicates that the air temperature and dew point at altitude are the same. That means the air holds the whole water vapor it can hold. Hence, the air is saturated with water to the highest level at altitude to create clouds. 

While the air at the sea level comprises high moisture, which reduces the relative humidity due to an increase in the air’s temperature, and the air’s dew remains the same at sea level. It shows how the relative humidity relies on the present air temperature or altitude of air. 

Suppose Washington city is at 30 ft and Denver city is at 5000 feet. Since Denver is at a higher altitude than Washington, it will have to lower atmospheric pressure. So if the air temperature and dew point are exact in both cities, the amount of water vapor owned by air is greater in Denver due to its higher altitude. 

Dew Point and Altitude
Cloud Formation at Altitude
(credit: shutterstock)

The air utilizes the heat energy to expand at altitude, resulting in the losing its heat quickly. Therefore, the cooler air cannot carry the moisture like warmer air, and hence, it is saturated with water vapor.

There is no precise value of altitude at which air temperature equals its dew point. The unsaturated air loses heat energy of 3°C for every 1000 feet of altitude gain. So no matter what triggers the air to condense, the saturated air consistently forms clouds at altitude. 

For air at the sea level, its temperature T = 30°C, its dew point Td = 15°C, and relative humidity RH = 55%. 

The atmospheric pressure becomes low if we go vertical in the same air at about 3000 feet. It cools continuously at the rate of 3°C / 1000 feet. 

Finally, for air at the altitude of 3000 feet, T = 15°C, Td = 15°C, and relative humidity RH = 100%.

That is how the air is saturated at 3000 feet to create clouds when its temperature equals its dew point at altitude. 

Does Dew Point change with Altitude?

The dew point does not change with altitude due to its water vapor content.

If the dew point is higher, the air holds a higher moisture. When we cool such air by moving into altitude, the moisture decreases because the air temperature gets lowered due to altitude. It displays that it is not the dew point that changes with altitude, but the air temperature is.

Dew Point and Altitude
Dew Point and Altitude
(credit: shutterstock)

The dew point and altitude is a critical air parameter that depicts its temperature and relative humidity. The dew point does not vary with altitude due to low air pressure – if air temperature equals the dew point or relative humidity becomes 100%. Usually, at sea level, when warm air is in touch with the cold solid surface, its temperature decreases and reaches the dew point. The air then condenses to create dew on the solid surface in such cases.

Hence, the air temperature of 20°C and dew point of 10°C feel more humid or moist than the air temperature of 20°C and dew point of 5°C; even the air temperature is identical.

The cloud is formed when the difference or spread between air temperature and dew point at altitude becomes zero. Let’s calculate the altitude at which the cloud forms at the dew point. 

The altitude at which the air temperature is reduced to the dew point is the Lifted Condensation Level (LCL). At the LCL point, the air cooling rate shifted to the Saturated Adiabatic Lapse rate from the Dry Adiabatic Lapse Rate. Meteorologists say that the non-saturated air displays a temperature drop of 5.5°C per 1000 ft, termed the ‘dry lapse rate’, while the saturated air displays a slower temperature drop of 3°C per 1000 ft altitude which is termed ‘moise lapse rate’. The difference between the two terms is 4.4°C, attributed to how moisture level changes the air temperature as air starts to condense at the dew point.

So once we comprehend the altitude, air temperature, and dew point of the air, the formula for cloud altitude is given by,

Cloud altitude = (air temperature – dew point)/ 4.4 x 1000 + altitude

Also Read:

     

Leave a Comment