It is the stratus clouds that are of interest in the case of nimbostratus, because nimbostratus is so thick that it stretches from near the ground up into the middle etage. It is stratus and altostratus combined into one thick layer. It is so thick that its bottom is very dark, even black. It is so laden with water that precipitation is inevitable. The “nimbus” in the name has many meanings, but in the case of the cloud, it indicates that it’s a rain cloud.
Credit Indrajit Das – CC-BY-SA
Nimbostratus is the cloud that gives sustained rain over a wide area. It is the bringer of those dark, gloomy, wet days. Often there are fractus clouds scudding about underneath it. It’s the kind of weather that’s good for the lawn, and good for watching from inside, warm and dry.
You will recall that jet streams form at the boundaries between major air masses, and at the boundary between the troposphere and the stratosphere. The Polar Jet, for instance, marks the boundary between the colder air mass to the north and the warmer one to the south. As a rule, the greater the difference in temperature of the air masses, the higher the wind speed in the jet stream. The Subtropical Jet is in addition affected by el niño and la niña. which are in turn affected by the Southern Oscillation. Whereas el niño and la niña are ocean events having to do with the warming and cooling of the surface waters of the eastern tropical Pacific Ocean, the Southern Oscillation is an equivalent atmospheric effect. When the two events synchronize they reinforce each other. During an el niño event the subtropical jet stream trends further south, and further north during la niña. When the periodic oscillations of the ocean waters and the atmosphere reinforce each other, the effect on the jet stream is greater and we experience more extreme weather modification. When they tend to cancel each other out, the weather events are less pronounced. This description is vastly simplified and once again I’m impressed by the persistence and attention to detail shown by meteorologists and other scientists.
The polar vortex has made it into the news recently as the eastern part of North America has been subjected to outflows of frigid arctic air. Strangely, people tend to blame the polar vortex, as if its existence were responsible for their frostbitten noses. The fact is, as you will see, it is the polar vortex that normally holds the cold air in, preventing their nasal misery.
Both poles have a polar vortex. They are persistent, large-scale low pressure areas generally lying poleward of sixty degrees latitude. These vortices are upper air phenomena, with their bases in the upper troposhpere and lower stratosphere, around the tropopause, where jet streams live. The strength of the vortex depends on the temperature differential between the equator and the poles. This is greater in winter, which is good for keeping that cold air in and saving our noses.
Unfortunately the vortices are more commonly ill- rather than well-defined. There is not always a strong jet stream wrapped around them. They can break up into two or more vortices, resulting in the flow of arctic air becoming disorganized, sometimes breaking out and spilling southward.
Climate change, for various reasons, is resulting in a decrease in the pressure and temperature differential between the equator and the poles, resulting in weaker vortices and less containment of arctic air. This causes the apparent paradox of localized cold snaps brought on by global warming. The atmosphere is dynamic and complex. There’s more to it than the nightly weather report.
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.
Here are the ten most viewed posts of 2017, not including permanent site components such as the home page, Downloads, Welcome, etc. Once again it seems I’ve become the Internet gateway for people wondering about spanking their wives.
In the past, meteorologists refused to include human-made phenomena in their classifications of cloud types. Yes, they said, the steam and smoke coming out of our smokestacks can appear like clouds or fog, but they’re not really. While weather observers might observe reduced visibility and even attribute it in part to our activities, there was no place for them on the reporting forms. If they were going to mention smog or condensation trails, it would be in the comments only. In the case of condensation trails, they became abbreviated in common language as “contrails.” On the reporting forms they appeared in the comments section as “COTRA.”
Credit Acabashi – CC-BY-SA
Now, with the updating this year of the International Cloud Atlas, hosted by the World Meteorological Organization, as reported on the Green Comet blog, a number of new cloud types have been included. I’ve already reported on asperitas, volutus and flumen, which are natural cloud types that have been included in this edition of the Atlas. Today I present another inclusion, this time a cloud type that results from human activity: cirrus homogenitus. Literally, cirrus made by humans. Condensation trails can now come out of the comments and take their rightful place in the form proper.
Credit Adam Jones Ph.D – CC-BY-SA
Cirrus homogenitus is the new name for contrails that have persisted for at least ten minutes. It comes in the one type only, with no sub-types or varieties. That’s because contrails are usually quite ephemeral and either disappear or change rapidly.
Credit Blue Stahli Luan – CC-BY
Cirrus homogenitus are like other cirrus clouds in that they don’t result in any precipitation or other weather. Unlike cirrus, they can’t even be credited with foretelling the approach of a weather system. They’re just the result of an airplane flying in the stratosphere, portending nothing more than its arrival, hopefully at its destination.