In this article, we will continue to do that by discussing the cloud itself, and what conditions create them. With this knowledge, you can better understand what you're seeing when you look at the sky, and how that relates to weather that could impact your plans. The objective is to give you a powerful tool in assessing conditions as you get out and spend time outdoors. Weather can be deadly, and shelter is one of our top "rule of three" priorities. Understanding how and why it develops is an important step in beating mother nature at her game... especially considering her game is killing you.
In meteorology, 'Cloud Genera' is a term we use to describe the types of clouds. As we progress, however, we learn that these types have significance; they tell us about the state of the atmosphere, which is a boon when you're afield and could be affected by surprise weather.
Plain speak: Clouds are a language, and you can learn to speak the basics.
In this article, we'll start with the very basics by breaking clouds down by their heights, and the conditions that create them. Put simply, we will ask "At what height are they in the atmosphere?" and "is the atmosphere stable or unstable?"
Speaking generally, we break cloud heights into three basic categories.
- Low: These clouds occur from the Surface (fog or mist) up to around the bottom of the freezing level. The majority of routine weather impacts are caused by low clouds.
- Mid level: Mid level clouds extend from the freezing level. Typically, somewhere between 6000 feet up to around 16,000 feet in the mid-latitudes.
- High: Clouds that occur above 16,000 feet in the mid latitudes are generally considered "high clouds". High clouds are ice particles, and can indicate weather that's coming.
Cloud height tells us quite a bit about what's going on in the atmosphere, but before we get to that, it's important to talk about stability. Stability isn't an easy concept to explain, so we won't spend a lot of time on it; suffice to say that when the temperatures increase with height, the atmosphere is conditionally stable. If the temperature cools with height, we have a conditionally unstable environment. How fast it heats or cools plays a part as well. Generally we expect that the atmosphere will cool 3.5 degrees F (2 degrees Celsius) for every 1000 feet (305 meters). This is considered a 'neutral' environment, and is a useful reference.
Clouds are a language, and you can learn to speak the basics.
Now, let's look at an example of our 6 basic cloud types:
Stable Cloud Types (Stratiform): Mild Weather
Stratus is a low level, stable cloud that creates drizzle or snow. Drizzle is 'continuous' rain, and generally feels more like mist. Very common in coastal climates, stratus tends to linger and drop a lot of rain a little bit at a time. The appearance of stratus is similar to a sheet; it appears as a smooth, covering layer.
Altostratus is a mid-level, stable cloud that generally doesn't produce much precip (occasionally virga, especially in high desert environments). It's found frequently ahead of fronts, provided there are no mountain ranges to interfere with them, and they can warn of an approaching front. Altostratus frequently brings warmer weather and higher humidity, and can be confirmed using a simple trick: if you can't see your shadow, there's altostratus present. This is because the clouds are highly reflective and the light that passes through the clouds is diffuse due to the thickness and altitude of the clouds.
Cirrus is a solid state cloud, being ice crystals and very high in the atmosphere. It typically appears under high pressure, or during transitional moments such as post-frontal periods. Likewise, it can be in wisps (horsetails), or more uniform swaths (shields) which ride over high pressure. Because we know something has to be driving that lift, when we see cirrus, we should be cautious of a weather change.
Unstable Cloud Types (Cumuliform): Wild Weather
Cumulus, which is latin for "heaps" is the puffy, popcorn looking clouds you see during fair weather, and if given enough time, develop into rainclouds and thunderstorms. The type pictured here is typically a sign that the weather is fair, but be alert; Cumulus like this generally means there's lift and moisture present. That's often times the first indication that a front is approaching. Cumulus commonly produces rain showers and snow flurries.
Towering Cumulus (TCU)
TCU's, or Towering cumulus clouds are the "core" of a storm cloud as it forms. They typically reach up in columns that can extend several thousand feet. TCU's themselves aren't major weather producers, however. What they are is a physical manifestation of a couple ingredients necessary for storm development: Moisture, and lift. TCU's mean the atmosphere lift, instability, and moisture is rising rapidly. However, they don't mean "watch for storms unless there are a couple other things in play; Dry air intrusion in the mid levels (this can be evidenced by a "tilt" to the TCU. If you see that, watch out for...
The Cumulonimbus, or "CB" is the traditional storm cloud, and produces strong winds, hail, lightning, and heavy rain/snow showers. Hail and Lightning are only produced by CB's, so if you see those, you've got a storm on your hands. While it's beyond the scope of this article to examine just how powerful a storm will become, tracking the development of the CB is a practical skill that can have serious impacts on your time afield. In the TCU section above, we discuss the necessary ingredients for the development of the CB. Remember the principles discussed in Reading the Sky, and consider the following acronym "MILE":
- Moisture: the presence of available moisture in the atmosphere; typically represented by a high dew point, high relative humidity, southerly winds.
- Instability: Instability occurs when the surface of the earth heats rapidly, and the temperature aloft is cold (remember the lapse rate discussed above, and our principles from "reading the sky")
- Lift: Lift occurs when wind flow is interfered with by topography (orographic lift), rapid surface heating (convection), and collision between air masses (frontal lift). Watch out for terrain effects! If you're near hills or mountains, lift is more likely.
- Exhaust: This is what causes the tilt we can see in developing TCU's. If you think of the TCU like a calm, what goes up must come down. Eventually, if there is no exhaust (and tilt) the mass will 'choke' development and the rain will fall straight down. If the TCU tilts, it can expand and the rain will fall ahead of the updraft core. The exhaust will also cause the spread of the anvil, which occurs when the storm has grown so tall that it's hit the stable layer called the tropopause. Strong winds and the low pressure cause the suspended ice crystals to spread out and become diffuse.
Strong thunderstorms are a matter unto themselves; they can cause many serious complications, including flooding, damage to structures and trees, lightning strikes, and of course, tornados.
While this doesn't cover all possible types of clouds, but combined with the information in the "Reading the Sky", it should enhance your ability to look up, and take what you see to forecast out. Keep in mind various other types of clouds exist, and while this is a useful primer,
Get out and give it a try, snap a picture, share it with us, or post it up on Facebook @IntegratedSkillsGroup!
Cheers, and happy adventuring,