Clouds and What They Tell You
Guest Post by Greg Zielinski
Imagine being on a long Grand Canyon rafting trip and you are out of touch with weather forecasts, but you would like to know if there is a chance that you will encounter some precipitation or especially thunderstorms in the next 12 hours or several days. There is a way that you can make an educated guess of that possibility. That way is by looking at cloud types and especially the sequence of clouds moving into the area and around you. This article summarizes the different classification of clouds and how they can be used to predict the likelihood of impending precipitation including how jet contrails or lack thereof can be used in that process.
In addition to the use of clouds in helping to forecast, they have always been fun for individuals in picking out forms or pictures that they can see especially with fluffy cumulus clouds. Consequently, they can provide a form of entertainment. The Hopi have thought of clouds in a more spiritual manner and as a significant part of their life. As noted by Hays-Gilpin and Gilpin (2018), people who attend dances are clouds as they, too, have life, moisture, and breath. In fact they note that clouds gathering and people gathering are the same and people gather with good thoughts. They also note that the whitewashed village looks like a cloud on the mesa top and invites clouds and people to gather. The Hopi also relate clouds to the dances that occur in the plazas (kiisonvi literally meaning house-inside). At these dances the priests leave in four directions and the clans earned the privilege to petition the clouds in all four directions. Thus, clouds are a significant part of the Hopi culture.
Figure 1: Classification and examples of different cloud types going from High (H) to Middle (M) to Low (L) clouds from top to bottom of the chart. Chart available at: http://www.ncei.noaa.gov/sites/default/files/sky-watcher-cloud-chart-noaa-nasa.jpg.
How Clouds Form
To form clouds and ultimately precipitation, air must start to rise. Air will rise either by daytime heating, or by air being forced to rise over mountains or up onto the Colorado Plateau or when contrasting air comes together as along frontal boundaries. Although not technically correct, cold air cannot hold as much water vapor as warm air. Consequently, water vapor in the rising, and thus cooling, air will condense into small water droplets. Those droplets will attach to pieces of dust and other small particles, thus forming clouds. Once enough water has accumulated in the cloud, precipitation can occur. Liquid or solid particles must reach the ground to be called precipitation, but there are times when those particles are observed falling from a cloud without making it to the surface. In that case, the air in which the water particles are passing through is not saturated and the water particles will evaporate before hitting the ground. Those streak-like features that occur under this scenario are referred to as virga. If high enough, meaning cold enough, clouds are made of ice crystals, but those do not produce precipitation.
Classifying Cloud Types
There are two characteristics of clouds that are used to classify them. One is the altitude in which they form, the second is whether or not they show signs of vertical development. Clouds are placed into one of three groups depending on whether they form at high (H), middle (M) or low (L) altitudes (Figure 1). Although exact heights are not known by the casual observer, one can approximate their elevation based on the overall form they take.
For example, high clouds are referred to as cirrus clouds and the prefix cirro is used when combined with other characteristics. Mid-level clouds have the prefix alto, which again is combined with other terms to describe the cloud more completely. Low level clouds are referred to as stratus, although that term is also used to describe layered clouds. Clouds that show vertical growth are referred to as cumulus. Whenever precipitation is falling from the cloud, the term nimbus is added to the name.
Several examples of how clouds types are combined for classification purposes include the following, cloud types that are discussed next in forecasting: altocumulus (M3-M9; Figure 1) = mid-level cumulus clouds, cirrostratus (H5-H8;Figure 1) = high-level layered clouds and one of the most well known cloud types, cumulonimbus (L3 and L9; Figure 1) = thunderheads, that is, vertically grown clouds that are producing precipitation.
Prior to the formation of thunderheads, cumulus clouds that are growing vertically are classified as cumulus congestus (Figure 2). The classic “fair-weather” clouds are cumulus meaning that they show only a small amount of vertical growth (L1 and L2; Figure 1). Nimbostratus are the low thick clouds (a specific type of stratus clouds, L6; Figure 1) that usually cover the sky while producing rain or snow.
Using Clouds to Forecast Weather
The type of clouds that can be useful in predicting precipitation within the next 6 to 36 hours depends on time of year and the causes of precipitation that are most common during the year. During the winter and through most of the spring, the features that bring precipitation to the Grand Canyon region are usually low pressure systems and their accompanying cold and warm fronts. However, in July, August, September and sometimes late June, precipitation is usually associated with the monsoons and the thunderstorms that the monsoons bring.
When a front is approaching there is a common sequence of clouds that form during the 12-36 hours before it arrives. In the case of a cold front the classic sequence starts with cirrus, followed by cirrostratus, altocumulus, and cumulus congestus to cumulonimbus (Figure 3). As a warm front approaches the sequence is similar, but because there is not the vigorous lifting of air that occurs with a cold front, the approaching warm front sequence often ends with stratus and nimbostratus clouds as opposed to cumulus and cumulonimbus. During the summer and monsoon season, thunderstorm growth is frequently characterized by the presence of cirrus and altocumulus clouds early in the morning or even the day before, owing to the presence of available moisture, followed by cumulus congestus and ultimately cumulonimbus clouds (Figure 2).
Contrails
Jets are well known for leaving vapor trails (i.e., contrails), but they are not always present, and whether or not they are present, can be used as guide in forecasting. Contrails form when the very warm and moist jet engine exhaust comes into contact with the cold air at the heights in which jets often fly. That warm air cools, condenses and forms the contrails observed from the ground (Figure 4). Consequently, the jet has to be high enough for the air to be cold enough for condensation to take place, and there needs to be some water vapor, that is, humidity, for the contrail to form and be observed. As a result, if the jet is high enough, but no contrail is forming or it dissipates very quickly, that would suggest warm air aloft with not much moisture available for cloud formation and precipitation to occur later in the day. Warm air aloft means that any rising air will be more dense than the air around it so it will stop rising and will not form larger clouds and precipitation (since warmer air is less dense than colder air). If a contrail forms, then conditions could be favorable for precipitation to occur in the next 12-24 or 36 hours, because the colder air aloft is more dense than any rising air, meaning the rising air will continue to rise and may eventually form thick enough clouds for precipitation to occur. At the same time, when contrails have formed, that means there is enough moisture aloft to form larger clouds and eventually precipitation.
Conclusions
There are several ways that clouds can be very useful in forecasting weather conditions over the next 12-24 hours or so. For example, the general sequence of fronts approaching the Grand Canyon during the cooler times of the year are characterized by the sequence of cirrus, altocumulus and then either cumulonimbus or stratus/nimbostratus depending on type of front moving into the area. During the monsoon season of late June/early July through September, the vertical grown of cumulus clouds (cumulus congestus) with the ultimate formation of thunderheads is the classic sequence.
Another unlikely source of forecasting is the presence or absence of contrails from jets flying over the region. A lack of contrails if the jet is high enough, suggests a lack of humidity and warmer temperatures at altitude, a scenario not conducive to the vertical grown of clouds and thunderstorm formation. The presence of contrails indicates enough moisture at higher altitudes and cooler temperatures, thus conditions that would be favorable for the continued rising of air and eventually precipitation and particularly thunderstorm development during the summer. Consequently, the different types of clouds can be very useful as a tool in forecasting when out on the river or on the trails of the Grand Canyon, especially if on a multiple-day trip away from media sources.
References
Hays-Gilpin, K. and Gilpin, D., 2018, Becoming Hopi, p. 123-140, in Footprints of Hopi History, Hopihiniwtiput Kukveni’at, Kuwanwisiwma, L.J., Ferguson, T.J. and Colwell, C., eds., The University of Arizona Press, Tucson.
Guest Blogger: Greg Zielinski is a retired professor in the fields of climatology, meteorology and geology. Internationally known for his work on the impact of volcanic eruptions on climate through ice core research, he has been a featured scientist in many documentaries including NOVA. He has published several books on weather and climate and was the Maine State Climatologist.