Look up in the sky! Is it a cloud? Is it rain? No, it’s Droughtman! Yes, I’m afraid our blogger is at it again. Our water watchers have raised the advisory level to 2, which stands for dry conditions and the first signs of potential water supply problems. It is the time when some water suppliers might consider asking their customers to begin voluntary water conservation. The snowpack is gone and rainfall is still well below normal, so these are wise precautions. Not for Droughtman, though. To him it’s fear-mongering. Apparently the people we pay to keep watch for us are drought-crazy and they get some kind of mysterious benefit from scaring us unnecessarily.
I think I have gleaned a clue into Droughtman’s thinking now. He points out that the big lakes haven’t dried up. It seems that, to him, as long as we can pump water out of the lakes, there’s no drought. So he defines drought, not by precipitation, but by the availability of water for our use. I guess this means that we could go ten years without rain or snow and, as long as the lakes haven’t dried up, there’s no drought.
He didn’t mention the upper elevation reservoirs, though, where we get much of our water during the dry season. These reservoirs won’t be replenished without rain and snow, but maybe Droughtman would have us fill them by pumping water out of the big lakes.
The jet stream was discovered by Japanese meteorologist Wasaburo Ooishi when making over 1200 balloon observations of high altitude winds between 1923 and 1925. This information was later used when the Japanese launched nearly 9000 hydrogen-filled paper balloons to carry explosives across the Pacific Ocean to North America during the second world war. The remnants of one of these were found near Lumby, British Columbia, Canada as late as 2014.
Jet streams — see video here — form at the tropopause, the boundary between the two lowest layers of the atmosphere, the troposphere and the stratosphere. There are four major jets, two in each of the northern and southern hemispheres. They are referred to as the polar and subtropical jets and they form at the boundaries of the atmosphere’s major circulating air masses. The northern hemisphere’s polar jet flows at the mid- to northern latitudes and is a regular feature of television weather reports for many of us. The southern hemisphere’s polar jet mostly just circles Antarctica. The subtropical jets are weaker than the polar jets and don’t have as much effect on our weather patterns. There are other jets streams that form at particular times of the year or in particular places, but they don’t have much wider effect either.
Jet streams form at the boundaries of air masses where there are steep pressure and temperature gradients. The tendency of the air to move rapidly from high to low pressure down this steep pressure gradient, and its diversion by the Coriolis force results in a strong current of air at the boundary between the air masses. This current flows generally from west to east in prevailing westerlies. Since weather systems also tend to form at the interface between air masses, it is common for those systems to follow the jet stream. The polar jet streams track north and south with the seasons in concert with the Sun. The streams are quite concentrated phenomena, being only a few hundred kilometers wide and less than five thick.
The wind speed in a jet is often a hundred kilometers per hour and can exceed four hundred. It is easy to see how this could affect the flight of aircraft by reducing or prolonging flight time, depending on whether the flight was with or against the flow. Before this was understood, aircraft were known to take longer than anticipated to reach their destination, sometimes running out of fuel before arriving.
The jet stream is not straight, but rather meanders in its flow from west to east. These meanders look like waves and are called Rossby waves. These waves also travel from west to east, carrying the different weather on their north and south sides across the land below. Recently, probably due to climate change, Rossby waves have been stalling their eastward movement for unusually long periods, subjecting areas to prolonged rainfall or heatwaves. These extreme weather conditions are becoming more common.
In a future post we will cover related phenomena such as the Southern Oscillation, el niño/la niña, the polar vortex and the Dust Bowl.
In my area precipitation has been below normal for the last few months. This is the time of year when we would normally expect a good part of our annual rainfall, and the appropriate authorities have been warning us of the possibility of drought. The conditions are abnormally dry. If they continue abnormally dry then the criteria for moderate or worse drought conditions will be met, hence the warning. That’s what we pay them to do. We pay people to collect the data and we pay other people to interpret it for us so we can plan accordingly.
It’s not a perfect system. It doesn’t always get everything right. Sometimes the actual amounts of precipitation will differ from the forecasts used in their projections. It’s not a perfect system, but it’s the one we use. They have to work with the available data and this year the data is saying that it’s drier than normal. It would be wrong to criticize them for employing current best practises with an abundance of caution.
We’ve had some rain in the last couple of days. We’re still below normal for the period, and there are dry, sunny days in the forecast, but a local blog operator has made a post mocking the reports warning of possible drought conditions. He thinks it’s clever to sieze on two wet days and mock the efforts of the people we pay to watch out for us. This same blogger has used a cold snap in the winter as an opportunity to say, “So much for global warming, eh?”
What are you supposed to do with people like that?
The International Cloud Atlas fifth edition, published in 2017, has included cloud types that were not recognized before, including types that are caused by humans and other unexpected sources. I’ve already posted on cirrus homogenitus, upper etage clouds forming from the condensation trails of aircraft. Today’s post is about cumulus flammagenitus, cumulus clouds formed by convection caused by a heat source. The heat source could be a wildfire or a volcano, for example, but the cloud can’t be just smoke or ash. It must include water droplets to qualify as a cloud. This type of cloud is also known by the name “pyrocumulus,” or “fire cloud,” which combines fire and the basic cloud type.
Wikipedia has a more in-depth article on flammagenitus than does the Cloud Atlas. As with other cumulus clouds, the convective clouds of the lower etage, flammagenitus can vary in vertical development, with bigger clouds being named accordingly. So we can have cumulus congestus flammagenitus, also known as towering cumulus flammagenitus, and cumulonimbus flammagenitus, complete with the lightning, wind and precipitation associated with thunder clouds.
Credit Jan Knight – World Meteorological Organization
Here’s another example. This flammagenitus is also referred to as pyrocumulonimbus. That is, a cumulonimbus cloud formed from fire. Tap for large original.
Eric Neitzel – CC-BY-SA
I have seen flammagenitus rising above forest fires, and it is impressive. The heat is so intense that the smoke, combined with the water driven out of the burning trees, can be driven rapidly high into the atmosphere. These clouds easily rival regular towering cumulus and cumulonimbus in their size and appearance.
Cavum is one of the new clouds that show up in the latest edition of the World Meteorological Organization’s International Cloud Atlas. I reported on the release of the new edition in this post. Cavum is really just a new name for a cloud type previously known as a fall streak hole, which I reported on here. There are more great pictures in that post. The full name for the example shown in this post is altocumulus stratiformis perlucidus translucidus cavum. That is, the middle etage cloud altocumulus (my previous post on altocumulus) which is in a layer thin enough to allow light through, and which has gaps between its elements and a great big hole with virga in it. Here’s how cavum is described in the International Cloud Atlas.
A well-defined generally circular (sometimes linear) hole in a thin layer of supercooled water droplet cloud. Virga or wisps of Cirrus typically fall from the central part of the hole, which generally grows larger with time. Cavum is typically a circular feature when viewed from directly beneath, but may appear oval shaped when viewed from a distance.
When resulting directly from the interaction of an aircraft with the cloud, it is generally linear (in the form of a dissipation trail). Virga typically falls from the progressively widening dissipation trail.
Occurs in Altocumulus and Cirrocumulus and rarely Stratocumulus.
And here’s the description of the image from the International Cloud Atlas.
This thin, translucent and extensive layer of cloud is Altocumulus stratiformis translucidus. In the top part of the picture it also displays the variety perlucidus, as there are the gaps between the cloud elements. However, the most striking feature is the large, roughly circular hole beneath which there is virga. The large hole is the supplementary feature cavum, popularly known as a “fallstreak hole” or “hole-punch cloud”. The full classification for the cloud is therefore Altocumulus stratiformis perlucidus translucidus cavum.
Also of note is a linear gap in the cloud between the fallstreak hole and the horizon. This is an aircraft dissipation trail, or distrail, formed as a result of an aircraft flying through the cloud layer. Informally this is sometimes known as a “canal cloud”. It later transformed into a circular-type hole.
The supplementary feature cavum is formed when glaciation occurs in a thin cloud layer consisting of supercooled water droplets that are in a liquid state and at a temperature below 0 °C. As the supercooled water drops glaciate, the resulting ice crystals fall from the cloud layer to a lower level as virga, or fallstreaks. The resulting cloud hole typically grows larger with time while the glaciation process continues.