Vaporizing some errant notions about weather
- If it's raining, the humidity is 100 percent.
- Warm air can "hold" more water vapor than cold air because there's more room between the air's molecules.
- If the humidity is 100 percent, the air has more water vapor in it than when the humidity is 67 percent.
- Places around the Gulf of Mexico regularly see summer days with 95-degree temperatures and 95-percent humidity.
- Pilots don't have to worry about what dew point means.
All of these statements are false.
When you can explain why they are false you'll not only be able to win bets, but will also understand vital aspects of aviation weather.
Let's start with the first statement, the easiest one. When we talk about humidity, we're talking about the invisible water vapor in the air, not drops of liquid water falling as rain. The humidity is 100 percent in the clouds that are producing the rain. But rain often falls into drier air below the clouds. Sometimes enough water evaporates from the falling rain to push the air's humidity to 100 percent. When this happens, some of the water vapor begins to condense back into the tiny drops of water that create precipitation fog.
To see why the statement about warm air "holding" more water vapor than cold air is false, we'll have to take a closer look at water and its phases--solid, liquid, and gas--and what happens when water evaporates from liquid into vapor and then condenses back into liquid. When you understand how this works, you'll be well on the way to seeing why the other statements are also false.
The molecules of any substance are always moving; the warmer the temperature the faster the movement. The difference between the solid, liquid, and gas phases of anything is how strongly cohesive forces on the atomic level hold a substance's molecules together. When the cohesive forces are strong enough to hold the molecules in a definite shape the substance is solid--ice in the case of water--and the molecules are vibrating in place. A liquid's molecules are still bound together but freer to move around, allowing it to take the shape of the container holding it, such as a glass of water. Molecules of a gas, such as water vapor, are zipping around. Near the ground the molecules of the gases that make up air are moving at an average speed of about 1,000 mph.
As the temperature rises, the average speed of a substance's molecules increases. But, no matter what the temperature, some molecules are moving slower than the average speed, some faster.
The warmer air has more water vapor, not because the air can "hold" more water, but because the air's increased heat energy keeps water molecules moving faster, on the average, than when the air is cooler. Fewer water vapor molecules are going slow enough to condense back into liquid. If the water and the air above it cool, the average speed of the vapor molecules slows down and more and more of them condense back into the water until a new, lower saturation level is reached.
Saying that warm air "holds" more water vapor than cool air isn't inaccurate in terms of the effect, but the statement makes many meteorologists and science teachers cringe because it can lead to very wrong explanations of what's going on.
For example, people have produced diagrams showing air molecules moving farther apart as the air warms, allowing more water molecules to fit between them. As it happens, no matter how chilly the air is, there's always plenty of room between the molecules of nitrogen, oxygen, and other gases (including water vapor).
Scientists have calculated how much water vapor is required to saturate air at various temperatures and confirmed the figures with experiments. The most common measurement of the amount of water vapor in the air is grams of water vapor per each kilogram of air. This is known as the mixing ratio.
Table 1 shows the saturated mixing ratio of air at various temperatures; that is, the greatest amount of water vapor that can be in the air at each temperature.
We can use this chart to understand the two humidity measurements given in weather observations: relative humidity and dew point. We'll start with the dew point, or to be more correct, the dew point temperature. It's the temperature to which the air measured at a particular time and place must be cooled to be saturated.
Imagine you are a weather observer and that at 3 p.m. you record a temperature of 80 degrees Fahrenheit. You also take a humidity measurement (we won't worry about the details of how that's done) that shows the mixing ratio is 11.03 grams of water vapor per kilogram of air.
After the sun goes down, the air cools to 60 degrees, but the amount of water vapor in it stays the same. When the air reaches 60 degrees it's saturated and dew begins forming on the grass as water starts to condense out of the air. At 3 p.m. you knew this would happen, that the air would be saturated when it cooled to 60 degrees, which is why you reported the dew point temperature as 60 degrees.
When the air cooled to 60 degrees, mist might have started forming in the air a little above the ground as water condensed on tiny particles in the air known as condensation nuclei. If the air was rising when you took the 3 p.m. observation, tiny cloud drops would form when it rose far enough to cool to 60 degrees.
Relative humidity compares how much water vapor is actually in the air to how much could be in the air at a particular temperature. When you took the 3 p.m. observation with the 80-degree temperature and 60-degree dew point, you calculated the relative humidity by dividing the saturated mixing ratio by the actual mixing ratio and multiplying by 100 to make it a percentage. That is, 11.03 divided by 22.17 and multiplied by 100 gives you a 49.75 percent relative humidity.
When the air cooled from 80 to 70 degrees, 11.03 (the dew point mixing ratio) divided by 15.73 (the 70-degree mixing ratio) and multiplied by 100 would give the relative humidity as 70.1 percent. In other words, as the air cools, the relative humidity goes up if the amount of water vapor in the air stays the same. This is why relative humidity confuses many people, although they usually don't realize they are confused.
Before the Internet came along, you would have had to do some fairly complicated math or use charts in handbooks or meteorology texts to calculate dew point and relative humidity. Today, you can do these and many other calculations by going to Web sites such as the Weather Calculator maintained by the El Paso, Texas, National Weather Service office (www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml).
Back to the statements
Now we're ready to look at the third false statement: "If the humidity is 100 percent, the air always has more water vapor in it than when the humidity is 67 percent." Weather observations on the same day last winter at two very different places show why the statement is wrong.
Around 9 a.m. in Fairbanks, Alaska, the temperature was minus 29 degrees F and the dew point was also minus 29 degrees, making the actual and saturated mixing ratios both 0.26 grams of water vapor per kilogram of air. The relative humidity was 100 percent. Even with 100 percent relative humidity the sky was clear and visibility good--0.22 grams of vapor per kilogram of air isn't much moisture to make clouds or fog.
At the same time it was a steamy afternoon in Lagos, Nigeria, in the African tropics with a temperature of 90 degrees and a dew point of 77 degrees. The actual mixing ratio was 21.15 and the saturated mixing ratio was 31.74, making the relative humidity 67 percent.
This illustrates why relative humidity doesn't tell you anything at all about how humid it feels. You can be sure that with a temperature of minus 29 degrees, even with a relative humidity of 100 percent, no one in Fairbanks was complaining about the humidity, unlike in Lagos where the 67-percent humidity was definitely uncomfortable for anyone without air conditioning. The combination of heat and humidity made it feel like 102 degrees in Lagos.
The dew point is a much better guide to how uncomfortable humidity is. When the dew point is below about 60 degrees most people are comfortable. When it's between about 60 and 70 you start to feel the humidity. Above 70 most people will feel that it's definitely humid, while dew points above 75 are oppressive to most people.
Now, for the fourth statement: "Places around the Gulf of Mexico regularly see summer days with 95-degree temperatures and 95 percent humidity." A relative humidity of 95 percent with a temperature of 95 degrees would require a dew point of 93 degrees--which doesn't happen in the United States. Dew points in the high 80s and low 90s are found only near shallow, warm water such as the Red Sea, the Persian Gulf, and the Gulf of California. Places along the U.S. Gulf of Mexico Coast can see dew points in the low 80s in the summer, however.
Even a dew point of 85 degrees combined with a temperature of 95 would give you a relative humidity of "only" 73 percent. But, this combination would give you a heat index--how hot the heat-humidity combination makes it feel--of 126 degrees.
The dew point is even more important for pilots to understand because high dew points in the morning can be a sign that strong afternoon thunderstorms are possible, if the other ingredients are around. Depending on other conditions, high dew points can be a sign that low clouds are likely. In the late afternoon or early evening dew points that are 5 degrees less than the temperature, or closer to it than that, should be a warning that fog is possible after sunset when the ground begins cooling off.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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