So far we've discussed what role the various gases play in our atmosphere, and the dependence upon temperature and temperature differences to make it all work. Now we're going to focus in on the most important atmospheric gas to meteorologists: Water Vapor. Water vapor is the gaseous state of liquid water and solid ice. One cannot see water vapor, but it is always there. It is tough to ever find a circumstance where there is absolutely no water vapor overhead, but you can also find it concentrating mightily, like in a downpouring thunderstorm. You can sense an increase in water vapor by how it makes your skin feel, especially in the Summer.
Humidity has come to refer to, through any means possible, the amount of water vapor in the air. Most of the time we use for comparison purposes the rest of the atmosphere. High humidity would denote a high water content in the location and low obviously would be low water vapor content. Though gases in the atmosphere like Oxygen and Nitrogen are constant throughout, water vapor is highly variable, and thus so is the humidity. It can be measured several ways.
Absolute Humidity refers to the mass of the water vapor when compared to the volume of air. If there's 1 gram of water vapor (water molecules existing in the gas state) in 1 cubic meter of air, the absolute humidity would be 1 g/m3. Since we can't assess the entire atmosphere at once, and because that would be silly anyway because the atmosphere constantly moves around water vapor, the usual measurement of humidity would be for a localized air parcel, and so far it's been the size of one cubic meter. Keep in mind however, that when an air parcel is heated or if the pressure is lowered, the volume of the very same air parcel will expand and grow larger. Even though the air parcel's water vapor content has not changed, the absolute humidity would change, to reflect a change in the volume of air containing the water vapor. For this reason, absolute humidity is not often used in meteorology.
Specific Humidity is a measurement that isn't influenced by changes in air volume. It's a ratio of the mass of water vapor to the mass of the total air in a given parcel. Therefore, when an air parcel expands, the same masses of molecules are maintained, and thus the humidity remains constant. This is most convenient when assessing the Mixing Ratio, which is merely the mass of water vapor in an given air parcel compared to the mass of dry air. These concepts are quite useful in meteorology. These two ratios will not change in an air parcel no matter what happens to that parcel itself, so long as no water vapor is added or removed from the parcel itself.
Let's throw something else into the mix now: We know that warm air has a greater capacity for water vapor than cold air. It has less to do with the density or pressure of warmer air, as it does with the speed of the molecules.  Molecules have a more free range of movement as a gas than a liquid, so it's no surprise that the gas state of an element is the stage of highest average temperature. This is why, on average, it feels more humid in the tropics than in the mountains. The maximum capacity for water vapor (by mass) is fixed, and it increases exponentially with temperature.
The moisture content in the air can also be measured by pressure, because all molecules exert a pressure on the surrounding air. In fact, the total air pressure in the atmosphere is made up of the sum of all the pressures of the individual gases involved. Each is called a "partial" pressure. If we assigned a pressure of about 1000 millibars (mb), and then doled out the rest according the percentage each gas exerts in the atmosphere, then Nitrogen, being 78% of our air, would account for 78% of the pressure, or 780mb. Water vapor content in the air is very small, compared to the other gases, and it typically only exerts between 10 to 30 millibars of pressure. This assessment for a given air parcel is called the Actual Vapor Pressure. High vapor pressure shows a high number of water vapor molecules, while low vapor pressure indicates the opposite.
There is a point when an air parcel has reached it's capacity for water vapor, and no more water vapor can exist without massive condensation occurring. When an equilibrium is met between water molecules evaporating into the gas state and others condensing into the liquid state, it is said to be Saturated. The partial pressure of air that is at saturation, is called Saturation Vapor Pressure. This pressure is obviously dependent upon temperature, because air has a higher capacity for water vapor at higher temperatures. It also follows that when the saturation vapor pressure of water is equal to the actual total air pressure, the boiling point is reached. Normally this isn't an issue in meteorology, but high up in altitude, where the total air pressure is low, there is a point at which boiling occurs at room temperature. An egg would boil faster in Denver, CO than it would in Boston, MA, but I digress. It is the relationship of vapor pressure and saturation vapor pressure that is most crucial to understanding humidity, condensation, and precipitation.
Relative Humidity
Relative Humidity, contrary to the other humidity measurements we've discussed, does not measure the actual amount of water vapor in the air. But rather, it's a relative measurement to how close air is to becoming saturated. Relative humidity (RH) can be expressed any one of the following ways:
| RH = WaterVapor Content / Water Vapor Capacity
RH = Vapor Pressure / Saturation Vapor Pressure
RH = Mixing Ratio / Saturation Mixing Ratio
|
So if we were to take an air parcel that was fully saturated at a certain temperature (relative humidity = 100%), and raise it's temperature, the relative humidity would decrease, because the air would then have a greater capacity for water vapor, and a higher saturation vapor pressure. Likewise in situations where the water vapor content in the air doesn't vary a whole lot, it's the temperature during the day that changes the relative humidity. When the air temperature is lowest (early morning), the relative humidity would be the highest. In the mid afternoon, the humidity would be the lowest with the warmest temperature.
Dewpoint
Another way of assessing the moisture content in the local atmosphere is by using what is known as the Dewpoint. If the air around you is at 40ºF, and the relative humidity is 100%, that air is completely saturated. If you then, maintaining the same amount of water vapor locally, heat the air to 60ºF, the relative humidity would drop because the air is no longer saturated with water vapor due to the increased capacity. If you were to try and find the temperature to which the 60ºF air would need to be cooled in order to become saturated with water vapor again, well, that would be 40ºF. At that level of water content in the air, the dewpoint would be 40ºF. It represents the temperature air must be cooled to (without any change in pressure or moisture content) for saturation to occur. The Frost point is similar to dewpoint, but it's determined with respect to ice, and quite helpful to determining the temperature for frost formation. At air temperatures below 32ºF, the dewpoint is colder than the frost point, because it is easier to form frost than dew at that level. Since the dewpoint is measured in degrees like temperature, it becomes the chief way of assessing water content in the atmosphere for meteorologists. When the dewpoint is very high, the air is quite muggy and humid, regardless of the difference between it and the air temperature, because of the high water content. Likewise if the air temperature is very high, and the dewpoint is quite low, then it is known as 'dry heat', whereby often stifling heat is made more bearable by the cooling effect of such a low dewpoint (remember, when sweat evaporates, it's adding water vapor to the air cooling your body as it does so. If the air is already saturated, this cannot happen easily. In dry air, perspiration is quite successful at cooling the body, even in the hottest temperatures). 
When the dewpoint and the air temperature are both quite high (high heat and relative humidity), then the air will actually feel hotter than it really is, because the heat is compounded by your body's limited ability to cool itself off. Meteorologists describe this apparent temperature increase as the "Heat Index". With each increase in either air temperature or relative humidity, your body becomes more susceptible to sunburns, heat exhaustion, heat cramps, and the like. Exposure to direct sunlight compounds this problem, by adding a few more degrees to the temperature. In days with a high heat index, it is always best to seek cooler air indoors. The places with typically the highest dewpoints and humidities are in the Southeast united states, with their proximity to the warm Gulf of Mexico and thick swampy vegetation. The places with the lowest dewpoints and humidities in the United States is the desert Southwest.
There is one more measurement involving humidity, and we'll throw this at the end here. It's called the Wet Bulb Temperature. The reason it has this name, is because of the instrument typically used to measure it, called a psychrometer. The wet bulb temperature measures the coolest point to which the air can be cooled through evaporation. This has many uses, not the least of which is it can tell you how much cooler you're going to feel through perspiration. It also helps the ski resorts, because even though air temperatures may be a few degrees above freezing, if the wet-bulb temperature is not (usually in drier days) you can still get those snow-guns going. The psychrometer is an instrument with two thermometers on it, except one is attached to a wick that has been wetted. The thermometer with the wick at the end of it would begin to cool evaporatively, until it settles at the wet-bulb temperature.
|
; "Click for Seven Day Forecast")
