A Discussion of Water Vapor, Humidity, and Dewpoint, and Relationship to Precipitation

 

WATER VAPOR:

Water is a unique substance. It can exist as a liquid, solid (ice), and gas (water vapor). A primary way water vapor increases in the atmosphere is through evaporation. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain. A lot or a little water vapor can be present in the air. Winds in the atmosphere then transport the water vapor from one place to another. A major source of water vapor in Kentucky is the Gulf of Mexico. Most of the water vapor in the atmosphere is contained within the first 10,000 feet or so above the earth's surface. Water vapor also is called moisture.

ABSOLUTE HUMIDITY:

Absolute humidity (expressed as grams of water vapor per cubic meter volume of air) is a measure of the actual amount of water vapor (moisture) in the air, regardless of the air's temperature. The higher the amount of water vapor, the higher the absolute humidity. For example, a maximum of about 30 grams of water vapor can exist in a cubic meter volume of air with a temperature in the middle 80s. SPECIFIC HUMIDITY refers to the weight (amount) of water vapor contained in a unit weight (amount) of air (expressed as grams of water vapor per kilogram of air). Absolute and specific humidity are quite similar in concept.

RELATIVE HUMIDITY:

Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air.

DEWPOINT:

Meteorologists routinely consider the "dewpoint" temperature (instead of, but analogous to absolute humidity) to evaluate moisture, especially in the spring and summer. The dewpoint temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated. Although weather conditions affect people differently, in general in the spring and summer, surface dewpoint temperatures in the 50s usually are comfortable to most people, in the 60s are somewhat uncomfortable (humid), and in the 70s are quite uncomfortable (very humid). In the Ohio Valley (including Kentucky), common dewpoints during the summer range from the middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have been recorded, which is very oppressive but fortunately relatively rare. While dewpoint gives one a quick idea of moisture content in the air, relative humidity does not since the humidity is relative to the air temperature. In other words, relative humidity cannot be determined from knowing the dewpoint alone, the actual air temperature must also be known. If the air is totally saturated at a particular level (e.g., the surface), then the dewpoint temperature is the same as the actual air temperature, and the relative humidity is 100 percent.

RELATIONSHIP OF DEWPOINT AND RELATIVE HUMIDITY TO CLOUDS AND PRECIPITATION:

If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog (at the surface) and clouds aloft (which consist of tiny water droplets suspended in the air). However, for precipitation to occur, the air must be rising at a sufficient rate to enhance condensation of water vapor into liquid water droplets or ice crystals (depending on air temperature) and to promote growth of water droplets, supercooled droplets, and/or ice crystals in clouds. Droplets grow through a process called "collision-coalescence" whereby droplets of varying sizes collide and fuse together (coalesce). Ice crystal processes (including deposition and aggregation) also are important for particle growth. In thunderstorms, hail also can develop. Once the suspended precipitation particles grow to sufficient size, the air can no longer support their weight and precipitation falls from the clouds. In humid climates, thunderstorms often cause heavier rain than general wintertime rainfall since moisture content in the air typically is higher in the spring and summer, and since air usually rises at a much more rapid rate within developing thunderstorms than in general winter systems. "Cloud microphysics" is the study of droplet and ice crystal production and growth within clouds and their relationship to precipitation.

PRECIPITABLE WATER:

Meteorologists are not just interested in dewpoint or absolute humidity at the surface, but aloft as well. Precipitable water (PW) is a measure of the total amount of water vapor contained in a small vertical column extending from the surface to the top of the atmosphere. However, as mentioned above, the majority of moisture in the atmosphere is contained roughly within the lowest 10,000 feet. Precipitable water values around or above 1 inch are common in the spring and summer east of the Rocky Mountains (including Kentucky). Values of 2 inches in the summer indicate a very high moisture content in the atmosphere, typical of a tropical air mass. In general, the higher the PW, the higher the potential for very rain from thunderstorms if they were to develop. However, another very important consideration is not only the amount of ambient moisture in a particular location, but also the amount of moisture advection and convergence which provides additional moisture to an area. If significant, these added factors help explain why rainfall totals from thunderstorms can exceed actual PW values of the air in which the storms are occurring. The movement of thunderstorms, called propagation, also is very important in determining the actual amount of rainfall in any one location. The slower the movement of thunderstorms, the higher the rainfall potential in one area.

NOW IT'S YOUR TURN. PLEASE ANSWER THE FOLLOWING QUESTIONS:

QUESTION 1: In the winter, if the air temperature was 40 F and the dewpoint was also 40, what would the relative humidity be? Now, in the spring, if the air temperature was 70 and the dewpoint was 70, what would the relative humidity be? In which situation would if feel more humid? What does this tell you about relative humidity? Answer to Question 1
QUESTION 2:   If the air temperature was 95 F with a dewpoint of 70, would the air's relative humidity be higher or lower than if the air temperature was 70 degrees with a dewpoint of 55? Which air mass would feel more uncomfortable to you? Answer to Question 2
QUESTION 3:   If the air temperature was 90 degrees with a relative humidity of 60 percent in the afternoon, would it feel more uncomfortable to a person than if it was 75 outside with a relative humidity of 100 percent in the morning? Answer to Question 3

These examples show how relative humidity can be quite misleading. In general, assuming the dewpoint or absolute humidity does not change, the relative humidity will be highest in the early morning when the air temperature is coolest, and lowest in the afternoon when the air temperature is highest.

HEAT INDEX:

While dewpoint is a more definitive measure of moisture content, it is the relative humidity that commonly is used to determine how hot and humid it "feels" to us in the spring and summer based on the combined effect of air temperature and humidity. This combined effect is called the " Heat Index." The higher the air temperature and/or the higher the relative humidity, the higher is the heat index and the hotter it feels to our bodies outside.

WIND CHILL INDEX:

In the winter, there is another index we use to determine how cold our bodies feel when we are outside. This is called the "Wind Chill Index" (also known as "Wind Chill Factor"). This index combines the effect of the air temperature with the speed of the wind. When it is cold outside and the wind is blowing, the wind carries away heat from our bodies faster than if the wind was not blowing. This makes it feel colder to us. Therefore, the stronger the wind is in the winter, the colder it feels to us and the lower is the wind chill index.

QUESTION 4:   If the temperature outside was 20 degrees with a wind speed of 20 mph, would this "feel" colder to you than if the temperature was 5 degrees with a 5 mph wind speed? Answer to Question 4

High humidity/dewpoints in the summer and cold wind in the winter are important because they affect how our bodies "feel" when we are outside. If the heat index is very high or the wind chill index is very low, then we must take safety measures to protect our bodies from possible effects of the weather, including heat exhaustion, sunstroke, and heat stroke in the summer, and frostbite in the winter.

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