How is humidity related to air pressure? You know humidity refers to the amount of water vapor or moisture in the air. But, how humidity is related to air pressure?
Well, there certainly is a connection, and an important one too, as it plays a big role in creating weather conditions around the world.
In fact, barometric pressure and humidity also have a third partner, and that is temperature.
So, how do these three work together, and why is the connection important?
Humidity is not directly related to air pressure but it can affect the air pressure by altering the density of the air and affecting the formation of precipitation.
Getting Familiar with Humidity and Air Pressure
Humidity is a unit of measurement and is used to refer to the moisture level present in the air at a given temperature.
These water vapors are water molecules that have evaporated from bodies of water such as the oceans, seas, lakes and rivers after being heated by the sun and changed into a gaseous state.
Water vapor is included as one of the gases in the atmosphere. The amount of evaporated water in the air is measured using an instrument called a hygrometer.
The amount of water vapor or the humidity level, fluctuates according to the local conditions of the air mass that contains it.
It rains when humidity levels reach 100%.
Fact: The highest amount of water that the air may hold at a given temperature is 100% relative humidity.
Are Humidity and Air Pressure the Same Thing?
Although they are related, humidity and air pressure are not the same thing.
Air pressure is also called atmospheric pressure and is a measurement of the weight of the air in a given location.
One atmosphere (atm) equals the average air pressure at sea level at 59 degrees Fahrenheit.
Air pressure is measured using a barometer that has a tube of mercury inside.
As the weight of the atmosphere rises and falls, the mercury rises or falls in the tube.
How Is Humidity Related To Air Pressure?
Humidity is related to air pressure and also to temperature. The three feed into one another to create weather conditions all over the planet.
Cool air forms where there is low air pressure whereas warm air forms in areas where there are high air pressures.
Variations in pressure generate winds which influence the weather temperature and its humidity.
Keeping Temperature in Perspective
In the nineteenth century, John Dalton put forward a sound argument against air pressure, low or high, having any significant influence on humidity.
He demonstrated how the water molecule did not interact with the oxygen or nitrogen when it was gas in the air.
As to what causes humidity, air temperature seems to play much more of a role. Cold air forms areas of low pressure but warm air forms high.
Warmer air has a store of energy spend on vaporizing water and enough to hold on to it as a vapor forming clouds.
Including the Altitude in the Equation
As a rule, as the altitude increases, the air pressure decreases, because there is less air available the higher up you go.
Extra space between air molecules makes it thin and difficult for some humans to breathe in enough oxygen at high altitudes.
The weight of the air is a good indicator of what to expect from the weather. A low-pressure system usually means clouds and the possibility of rain.
And high-pressure systems mean calm weather conditions and bright sunny days.
Fact: The optimal relative humidity range is 40-60%, and over 60% is likely that you are in a situation that requires immediate attention due to health risks.
Does High Humidity Mean High Air Pressure?
No, high humidity does not mean high air pressure although areas of high pressure do form warmer air.
As humidity levels rise, the air pressure actually decreases.
This is because each molecule of water vapor has less mass and is lighter than the oxygen and nitrogen gases already present in the dry air.
This is true even though both the water vapor and the air may be at the same temperature.
The water vapor is forced up the air column where it meets cooler air, condenses and forms clouds.
As the amount of water vapor in the air increases, its presence decreases the air’s overall density and it is this reduction in weight that lowers the air pressure.
Several factors can have an effect on air pressure but the main factors are:
- The temperature since the air heats up and expands adding to its mass.
- The humidity is because moist air is lighter than dry air so it rises.
- The terrain, because at altitude there is less weight in the falling air column to create the air pressure.
Fact: High humidity levels in your home can cause all sorts of problems, including bugs, dust mites, mold, and bacteria, as they favor low temperatures.
Does Low or High Pressure Cause Humidity?
In areas where the air is warm, the air rises.
The air pressure in these areas is low because most of the air mass is moving up and not pressing down.
Where the area is cold, the air pressure is high because more of the mass of air falling.
Although low and high pressure systems produce distinctive types of weather, to what extent air pressure influences humidity levels is uncertain.
What Happens to Air Pressure When Humidity Decreases?
A fall in humidity is an indication that more of the local air mass is falling from high up in the atmosphere rather than rising.
The falling air becomes compressed as its molecules move faster and faster toward the ground creating heat in the process that dries out the air.
As it warms the air molecules are excited and eventually move fast.
It makes it difficult for the water vapor molecules around them to connect with each other so the humidity is lower.
When the process is reversed and the air is rising and the molecules in the air around the molecules of water vapor move less, the water molecules form clouds.
It is mainly because they can link by their poles, positive to negative easily and clump together.
After a while, the process adds enough vapor to create rain.
The Effect of Temperature and Pressure on Relative Humidity
Relative humidity (RH) is a term used in much the same way as wind chill factor.
It is often used to describe local conditions and how the humidity feels.
It tells you more about the moisture level in the air in relation to the total amount air could hold, which is why it is measured as a percentage.
It is true that warm air holds more water.
But, in relative humidity any water in the air regardless of the air’s temperature is recorded as a fraction of what the air could hold if it was saturated.
Understanding the Link between RH and Temperature
Temperature and relative humidity are different things but related as being inversely proportional.
As temperature increases, relative humidity decreases.
When the temperature falls, the humidity increases because the air has more moisture.
High pressure is formed as an air mass sinks and typically has a low relative humidity, which leaves the skies clear and free from clouds.
Low pressure is rising air and the relative humidity is higher.
It is mainly because the low pressure enables the air mass to hold more water at high altitudes forming clouds and increasing the likelihood of precipitation.
Fact: South and Southeast Asia have some of the world's most humid cities, and in Saudi Arabia in 2003, the dew point was 95F, which is the highest humidity ever recorded.
Conclusion
How is humidity related to air pressure? Despite their lack of a causal relationship, it is possible to utilize both barometric pressure and humidity to foretell the weather.
The approach of a low-pressure system, for instance, might raise local humidity levels by sucking in moisture from the atmosphere.
The same is true of how high humidity can aid in the development of meteorological phenomena like thunderstorms.
Hence, while they do not have a one-to-one relationship, shifts in one can frequently foretell shifts in the other, especially when you also include temperature and altitude in the equation.
