Surface Weather Observations
Jan 17, 2018
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Surface Weather Observations
Today, we're going to talk about moisture and precipitation, stable and unstable air, and then we're going to finish up with clouds.
Air contains moisture, or water vapor. The water vapor content of air can be expressed in two different ways: relative humidity and dew point. Relative humidity refers to actual water vapor present in the air relative to that which could be present.
Temperature largely determines the maximum amount of water vapor the air can hold. Warm air can hold more water vapor than cold air. As you can see on this graph, it basically takes a snapshot of an entire day.
As the sun comes up, air starts heating up. You can see that the relative humidity goes down. So the warmer the air, the more water that it can hold. Air with 100% relative humidity is said to be saturated, and air with less than 100% is unsaturated.
If you look at this picture right here, you can see we have a 12 ounce glass of water with 6 ounces of water in it. Therefore, its relative humidity is 50% full. On the other hand, if you have a four ounce glass, and you have four ounces of water in it, it is 100% humid, or 100% full. Dew point is the temperature to which air must be cooled to become saturated by the water already present in the air.
If you look at this graph here, you can see that 10 degrees Celsius can hold 9 grams of water, whereas 20 degrees can hold 17 grams, and then 30 degrees can hold 30 grams. So, as the air warms up, it expands and, therefore, it has more room for more water. When water vapor condenses on large objects, such as leaves, windshields, or aircraft, it will form dew.
Surface Weather Observations
When it condenses on microscopic particles such as salt, dust, or combustion byproducts, it will form clouds or fog. If the temperature and dew point spread is small and decreasing, condensation is about to occur. If the temperature is above freezing, fog or low clouds will most likely develop.
The growth rate of precipitation is enhanced by upward currents. Clouds particles collide and merge into larger drops in the more rapid growth process. As you look at this picture here, you can see, as we have updrafts, that air cools down. The water is able to condense, and then it also is able to impact with other water and grow bigger droplets, thus creating clouds.
This process produces larger precipitation particles and does so more rapidly than simple condensation growth. Upward currents also support larger droplets. Because the up-currents are pushing the water up, it has a more of a tendency to resist gravity, so therefore, the up-currents will hold the water nuclei up in the air a little bit longer until it gets relatively too big, and then it will fall and form rain.
If wet snow is encountered at your altitude, then the temperature is above freezing. Since melting snow has been encountered, the freezing level would be at a higher level. You can look at this graph over here. We've got cold air up above, and then you've got that little warm area, and that's probably where you guys are flying, and then you've got the cold air underneath.
As the moisture falls out of that cold air into the warmer air, it will start to melt. If it hits your aircraft, it will produce freezing rain, or sleet, or any number of things such as that. Rain falling through the colder air may become super cool, freezing on impact as freezing rain, or it may freeze during its descent, falling as ice pellets.
The presence of ice pellets at the surface is evidence that there is freezing rain at higher altitudes. Atmosphere stability is defined as the resistance of the atmosphere to vertical motion. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical or convective current.
If you look at this picture over here, as you can see, if we have unstable air, what happens is that air molecules are wanting to rise. As they continue to rise, it creates that unstable atmosphere. The water is able to condense, and it forms those clouds. We'll talk more about that when we get to thunderstorms, but that's basically how they start to form those thunderstorms.
Moving on over to the stable side there, it's resisting that upward motion. As it resists the upward motion, you don't get the vertical development and, therefore, the water molecules don't necessarily condense as quickly. Then, you've got the neutral there, which is basically every day.
Characteristics of unstable air are: cumuliform clouds, showery precipitation, rough air, which gives you turbulence, and good visibility, except for when you have blowing obstructions there.
Obviously, if you have turbulence, it's moving all those air molecules around, so it's going to get rid of any kind of haze, or any kind of pollution, or anything that might be out there, with the exception of the blowing obstructions, like it's talking about there. If you have large gust fronts or lots of wind, it can pick up particulates and move it around as well.
Surface Weather Observations
Characteristics of stable air are stratiform clouds and fog, continuous precipitation, smooth air, and fair to poor visibility in haze and smoke. Here's a couple good pictures there. So poor visibility, you've got those stratiform clouds, nice and flat, not much vertical development.
Determining the stability of the atmosphere requires measuring the difference between the actual existing temperature lapse rate of a given parcel of air and then comparing that to the dry adiabatic lapse rate, or 3 degrees per 1,000 feet.
The bigger the divergence between the dry adiabatic and the actual existing ambient temperature lapse rate, the more unstable the atmosphere is. The closer they are together, the more stable.
Let's talk about some clouds. Cloud types are divided into four families. We have the high clouds, middle clouds, low clouds, and clouds with extensive vertical development.
The first three families are further classified according to the way they are formed. Here's a good graph showing you the high, middle, and low clouds. The high cloud family is made up of cirroform, including cirrus, cirrocumulus, and cirrostratus. They are composed almost entirely of ice crystals. Clouds formed by vertical currents in unstable air are cumulus, meaning accumulation or heap. So, basically, they're heap clouds. That's a good way of thinking about it.
They are characterized by their lumpy, billowy appearance. Like the picture shows right here, you've got those big, billowy clouds. Clouds formed by the cooling of stable layer are stratus, meaning stratified or layered.
They are characterized by their uniform, sheet-like appearance. Here's a good picture of some stratus clouds. They're nice and flat. It's going to give you a nice day, like this picture shows right here. The prefix nimbo- or the suffix -numbus means raincloud. A heavy, swelling, cumulus-type cloud which produces precipitation is a cumulonimbus cloud, so here's a bunch of pictures of those.
Lenticular clouds are associated with a mountain wave. Crests of standing waves may be marked by stationary, lens-shaped clouds known as standing lenticular clouds. Mountain waves and lenticular clouds indicate strong turbulence.
Out here in Vegas, we get a lot of these coming over the mountains, and you can always be assured that there is definitely some turbulence with those. If you see those, try to stay clear of them. This is what we talked about in this little section here: moisture and precipitation, stable and unstable air, and then we finished up with some clouds.
Surface Weather Observations
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