About Humidity....

Relative Humidity....Relative to What?

The Dew Point Temperature...a better approach

by Steve Horstmeyer, Meteorologist, Cincinnati, Ohio, USA


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A. THE CONCEPT

Humidity is a complicated concept. So complicated that most TV meteorologists I know cannot and do not explain it correctly - More on this in part "B".

Humidity refers to the amount of water vapor in the air (not liquid water drops or ice crystals or snowflakes).

For years you have seen RELATIVE HUMIDITY during TV weathercasts, and it seems to make sense but as I mentioned it is complicated and even the complications about humidity have complications.

But because the essence is very simple it is amazing There is SOOOO much confusion about humidity.

First when thinking about humidity - ALWAYS THINK ENERGY!


WHAT IS TEMPERATURE? - A definition you will need when reading this.

Temperature - The amount of thermal energy. In a gas like the atmosphere temperature is a measure of the average speed of the molecules. Scientists would say temperature is a measure of the average KINETIC ENERGY of the molecules. The faster air molecules move, the more kinetic energy and the higher the temperature.

B. HUMIDIMYTHS (Myths About Humidity)


Humidimyth #1. Warm air can hold more water vapor.

NO WAY! Since 1802 when John Dalton demonstrated that air is a mechanical mixture of gasses we have known that air in NO WAY "holds" water vapor. Being a mechanical mixture of gasses means the molecules (nitrogen, oxygen, carbon dioxide, water vapor and others) merely occupy the same space. In a large jar containing marbles of 5 different colors, the blue marbles do not hold the red marbles and the red the yellow, the marbles of different colors just coexist in the jar.

This is a bit misleading because in the jar the marbles are stacked on top of each other, in the atmosphere there is plenty of space between molecules SO MUCH SPACE THAT ait is mostly EMPTY SPACE. So if anyone asks you what the atmosphere is mostly - it is mostly empty space.

Oxygen and nitrogen alternately do not have "hook & loop" areas or little hands to grab and hold water vapor molecules.

It comes down to energy.

For the meteorologist the only factors that determine how much water vapor will be in the air are the availability of water and the amount of thermal energy to do the work of evaporation.

A liquid water molecule evaporates from a body of water or from the sweat on your skin if it gains enough energy to break free of the attractive forces holding the molecule to neighboring water molecules. The energy comes from collisions with neighbors and if a molecule gains enough energy it can rocket free of the water. The liberated molecule is then water vapor zipping around with zillions of other molecules in the air.

It has nothing to do with a magical power of the air molecules above. In science there is no magic, nearly everything when investigated at a fundamental level is elegantly simple, rational and quite amazing.

CORRECT TERMINOLOGY:
ON A WARM DAY MORE WATER CAN EVAPORATE BECAUSE
THERE IS MORE THERMAL ENERGY AVAILABLE TO DO THE WORK OF EVAPORATION.


Humidimyth #2. Warm air expands so there is more room for water molecules.

NO WAY AGAIN! There is always plenty of space between air molecules for water vapor molecules to fit. If we imagine oxygen and nitrogen molecules enlarged to about the size of two joined tennis balls at sea level and about 50°F (10°C) there is an average distance of about 4.75 feet (145cm) between air molecules.

It is important to realize that the molecules are zipping around in all directions at hundreds of miles per hour and many collisions are taking place all the time. If a "snap shot" is taken at an instant in time and all the distances measured the average inter-molecular distance between tennis ball size molecules at sea level will be 4.75 feet. (145cm).

Even at 50°F (10°C) there is plenty of room for more tennis balls to fit. When the air is heated to 80°F (27°C) the average distance between the tennis-ball size molecules increases to about 5.5' (168 cm), not a great increase in average inter-molecular distance.

Conclusion: The amount of water vapor in the air has nothing to do with distance between molecules.

CORRECT TERMINOLOGY:
ON A WARM DAY MORE WATER CAN EVAPORATE BECAUSE
THERE IS MORE THERMAL ENERGY AVAILABLE TO DO THE WORK OF EVAPORATION.

C. THINK ENERGY

When thinking of humidity always think in terms of energy and there is only so much energy to go around and not all of it is available to do the work of evaporation. The remainder goes to the other molecules in the air - each gas in the mechanical mixture gets its fair share.

There is a connection between humidity and air temperature, but the connection has nothing to do with warm air "holding" more water vapor. Think of air as a kinetic energy delivery system. Warmer air moving into a region has more thermal energy than the air it is replacing. At the molecular level we say the average kinetic energy of the molecules is greater in the warmer air and the thermal energy of the warmer air is transferred to water molecules as the faster moving air molecules collide with the slower molecules in the water. The faster moving air molecules lose energy and the slower moving water molecules gain energy and begin to move faster and collide with each other more violently. Some of the water molecules will gain enough kinetic energy ( or speed or thermal energy - all three are the same) to escape the liquid and become a free moving gas molecule. If the newly arriving air is colder the opposite occurs.


Myth #3. Evaporation is precisely the opposite of condensation.

FOR A THIRD TIME - NO WAY! A liquid molecule of water is closely surrounded by many others, all moving about, twirling, swirling and gliding around each other in an incessant dance. Almost all the kinetic energy a liquid molecule needs to evaporate is gained from collisions with its surrounding liquid neighbors.

Air does deliver some thermal energy to the liquid, but because the number of molecules (molecular number density) in the air above the interface is about 1000 times less than the number of molecules in the liquid, by far most of energy for evaporation will come from the liquid.

A molecule evaporates when sufficient kinetic energy is gained through collisions with its neighbors for it to overcome the attractive forces between the liquid molecules. These forces include hydrogen bonds and Van der Waals force. As the molecule escapes it takes with it kinetic energy, leaving the water surface with a diminished total kinetic energy.

A molecule condenses when it is moving slowly enough and is pulled back to the water surface by the attractive forces, i.e. its velocity is insufficient to resist the pull of the various forces of attraction. The molecule plunges into the water, transferring energy to the molecules near where it hit the surface and is once again liquid.

It is easy to see why evaporation cools a surface and condensation warms it - when you THINK ENERGY

So.....

CORRECT CONCEPT:
EVAPORATION IS PRIMARILY DEPENDENT ON THE TEMPERATURE OF THE WATER, AND CONDENSATION IS PRIMARILY DEPENDENT ON THE TEMPERATURE OF THE AIR. THE NET EXCHANGE OF ENERGY IS THE SAME BUT STRICTLY SPEAKING THE PROCESSES ARE FUNDAMENTALLY DIFFERENT.

Relative humidity expresses how much of the energy available for evaporation has been used to "free" liquid water molecules from hteir neighbors. A relative humidity of 50% means half the available energy has been used to evaporate water from the ground, streams, lakes anywhere else it is and 50% is still available to do more evaporation.

Relative to What?

On a sticky summer morning the temperature may be 75°F and the relative humidity 90%, a very sticky morning indeed.

As the temperature rises during the day the amount of available energy increases. If by early afternoon the amount of available energy doubles (and it does in summer very often) Without changing the number of water vapor molelcules in the air the relative humidity drops to 45% BECAUSE THERE IS TWICE THE AMOUNT OF ENERGY AVAILABLE - REMEMBER RELATIVE HUMIDITY IS WHAT PERCENT OF AVAILABLE ENERGY HAS BEEN USED AND BECAUSE IT DOUBLED DURING THE DAY THE PERCENT USED IS HALF THE ORIGINAL! RELATIVE HUMIDITY IS RELATIVE TO WHAT .... RELATIVE TO THE AMOUNT OF ENERGY AVAILABLE TO DO THE WORK OF EVAPORATION. Because the amount of energy increased as the sun warmed the atmosphere the percentage of the energy available that was used decreased, i.e. the relative humidity, all the while there was no change in the amount of vapor in the air.

So when you hear someone say its feels worse than 52% relative humidity today, they do not understand the concept of RELATIVE HUMIDITY because 92°F and 52% is a very humid afternoon. Because the concept is confusing (even many TV meteorologists do not understand it) a better measuer is the dew point temperature.

The Dew Point Temperature

Dew point temperature is a measure of humidity. If air is cooled eventually enough energy will be removed for water vapor to begin to condense. When we say condense it just means some of the molecules slow enough so that the attractive forces between liquid molecules are strong enough to make the molecules stick together.Remember the water vapor was originally liquid water and to get it to evaporate you had to add energy. As long as a molecule is moving fast enough (faster = warmer) it will remain vapor, but as a molecule cools (cooler = slower) at some point it will slow enough so it will stick to other water molecules. When that happens scientists say the molelcule condensed.

The temperature where condensation first begins is the dew point temperature.

In terms of relative humidity, as the parcel of air is cooled, the relative humidity increases, when the relative humidity reaches 100%, the air parcel has cooled to the dew point temperature. At a relative humidity of 100% the dew point temperature ALWAYS equals the temperature. The greater the difference between temperature and dew point the lower the relative humidity.

Dew Point vs. R.H.

Unlike relative humidity if dew point increases, it is only because the amount of moisture in the air increases. If relative humidity changes it can be because of temperature changes or moisture changes, or both. REMEMBER - THINK ENERGY - if the air cools less thermal energy is available so the proportion utilized for evaporation is greater. For example if the relative humidity is 45% and half the thermal energy is removed because the air cools at night the relative humidity will rise to 90%, without changing the amount of moisture in the air. When using dew point temperature as a measure of humidity any change is strictly due to moisture change.

Dew point can never be higher than the temperature. At saturation, i.e. 100% relative humidity the temperature and dew point are the same.

So How Does Dew Point Feel?

On a typical summer day the following apply:

Dew Point
Temp. °F
Human Perception Relative Humidity
Air Temp 90°F
75°+ Extremely uncomfortable, oppressive 62%
70° - 74° Very Humid, quite uncomfortable 52% - 60%
65° - 69° Somewhat uncomfortable for most people
at upper limit
44% - 52%
60° - 64° OK for most, but everyone perceives the humidity
at upper limit
37% - 46%
55° - 59° Comfortable 31% - 41%
50° - 54° Very comfortable 31% - 37%
49°
or lower
Feels like the western US
a bit dry to some
30%


CLICK HERE for a Table of Morning and Afternoon Relative Humidities for Every State

A Common Misconception

One last thing if you ever hear someone say it was 90°F and the humidity was 90%, that has never happened in Cincinnati, (and unless the greenhouse effect goes into overdrive never will). 90°F/90% requires a dew point of 85.5°F. In Cincinnati the highest ever dew point was 81°F. for just a few minutes.

In August 1995 we had four hours of 78°F,79°F,78°F,77°F dew points, the highest persistent dew points I have seen in Cincinnati since working here as a meteorologist. For one hour I did see a dew point of 81°F, just after a thunderstorm.

World Record Dew Points

However many veterans of the Persian Gulf War know what 90°F/90% feels like. The Persian Gulf and Red Sea both attain sea water temperatures in the mid 90's when that happens there is plenty of energy available, along with the 115°F air temperatures, to evaporate water.

The dew point has been measured on the shore of Ethiopia, the area is now part of Eritrea, at 94°F. The highest known dew point temperature in the world. The relative humidity with a temperature of 115°F and a dew point of 94°F is 54% this doesn't tell you as much as the dew point when you consider the table above.

© 2008 Steven L. Horstmeyer, all rights reserved



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