With Northeast Colorado still experiencing severe drought conditions, and the summer storm season approaching, many of us may be looking up to the sky for evidence of the clouds that we hope will bring much-needed rain. Most of us are probably hoping for moderate, sustained amounts of rain that will soak into the ground slowly. We may also be hoping to avoid the severe storms that produce dangerous lightning, loud thunder, and fast run-off when the ground cannot absorb the amount of rain that comes on quickly. Neither scenario would be possible without cloud formations.
Clouds are formed when air contains as much water vapor as it can hold, and the vapor transitions into water droplets or ice crystals. This saturation point can be reached either by evaporation or condensation, and the moisture becomes visible in the form of fog and clouds. The only difference between clouds and fog is altitude, with fog constituting a cloud that begins at a height below 50 feet. An interesting fact about clouds is that moisture needs a small amount of dust, smoke, or salt in the air around which to form a droplet.
Most clouds are formed by rising air. Water vapor that feels like humidity at the ground level, becomes water droplets and then ice crystals as it is carried up with air cooling as it ascends. Clouds are composed of large numbers of very small water droplets or ice crystals that can stay suspended for a long time as the air continues to rise. Complex wind patterns keep these saturated packages of air afloat, moving them and shaping them until the conditions become right for the water to fall back to the ground as precipitation, however, not all precipitation even reaches the ground.
Clouds are classified by their appearance and height from the ground. There are several basic types of cloud classifications, and more types that are created by combining the basic types. The basic classifications are:
Cirrus – these are clouds formed primarily of ice crystals and found above 20,000 feet; appearing feathery and wispy; and can appear as a veil-like layer, or as strung-out rows with lumpiness almost like old quilt batting.
Stratus – horizontal formations of primarily water droplets with a flat, uniform appearance that produce a blanket of grey and may cause periods of drizzle and can occur at higher or lower levels; generally stable.
Cumulus – vertically forming, flat-bottomed, with fluffy, rounded tops; these are the clouds we love to watch as they float and create identifiable images and stories in the sky; these also occur at higher or lower levels and are unstable often resulting in storms.
Nimbus – this is the term for a cloud formation that produces rain or snow, these can be either stratus or cumulus; nimbus is the ancient Latin word for “rain storm.”
What does this MEAN? It is the combination of these classes of clouds at the appropriate altitude for precipitation or storms to occur. Almost all rain is produced from low-level clouds. Precipitation is produced when the water droplets in a cloud have bumped into each other enough to condense into droplets too heavy to stay suspended so they fall to earth. Much of the planet’s precipitation starts as snow and becomes rain as it falls through warmer air.
Stratus clouds tend to produce steady precipitation, while cumulus clouds can become the thunderheads that produce severe storms and hail. If you see high-level, widespread, sheet-like clouds, these are not going to provide precipitation, but can be an indication of the rain or snow to come. These are also what create halos around the sun or moon and folklore picked up on the weather forecasting effect of seeing these. Have you heard: A ring around the sun or moon means rain or snow is coming soon?
Mid-level versions of this stratus type of coverage, wide swaths of flat looking grey clouds that create the overcast condition, don’t produce precipitation. As these descend, though, they can produce light drizzle or mist, and once they start producing consistent precipitation, they are reclassified as nimbostratus: stratus clouds may look gloomy but are providers of the steady rain that is so important for plant growth and reducing fire risk. Mid-level cumulus clouds may provide sprinkles or light showers.
Cumulus formations are unstable because upward warm, moist updrafts are interacting with strong downdrafts of cold air. Cumulonimbus clouds have a base as low as 2-5,000 feet and vertical development that can extend high into the atmosphere, upwards of 40,000 feet with the top becoming anvil shaped. These can produce intermittent, intense precipitation but can develop into strong storm cells that produce heavy rain, hail, lightning, and tornadoes. Beautiful to look at and create lovely shadows across the plains, but volatile.
Reading the clouds to determine expected precipitation is a skill that takes being familiar with the cloud types and spending some time watching them. The first determination is whether you can see the sun or moon through the cloud cover. If you cannot, it is probable that the clouds are mid- or low-level and precipitation is likely in time, hoping they don’t blow away in the meantime.
Another interesting cloud phenomenon are mammatus clouds: these are pouch-like lumps appearing to hang from the underside of a thundercloud. These clouds are formed by air that is sinking, rather than rising and are composed primarily of ice, even though they are lower in altitude. Mammatus appear ominous, are associated with thunderstorms but are not necessarily indicators of tornados and are generally short lived.
Something we may not get to see often here on the plains, but you can experience when visiting the mountains or flying, are lenticular clouds. They look calm or motionless but are caused by great instability in that layer of the atmosphere – wind that is flowing fast over the mountains like a river flowing fast over rocks. Visibly they look a bit like snow drift upside down in the sky, saucer shaped and layered parallel to the wind direction. They are often the cause of turbulence experienced when flying.
While we hope for the sustained, replenishing rains we need so badly, we also need to be prepared for severe weather. Stay safe and check out further emergency preparedness information at https://www.nchd.org/emergencypreparedness.
Sources used for this blog include NWS, USGS, NOAA, and National Geographic.