The weather will remain dry and mild, if not cloudless, through the President’s Day weekend before a storm system finally returns in the middle of next week. For Saturday, morning clouds will likely clear out by the afternoon and temperatures will top out in the low 60s, staying there through Tuesday. Wednesday, I am becoming more confident that a moderate winter storm will push through the region bringing rain and snow to the mountains, which should turn to all snow in the mountains by Thursday, although it’s unlikely snow levels will drop enough to cause any accumulations in the valleys.

jet_stream globe

Joe wondered: “Is there a corresponding jet stream in the southern hemisphere? Do large land areas that are not present in the southern hemisphere affect the jet stream?”

Jet stream 3

Yes, there are corresponding jet streams in the southern hemisphere, and they act much like the ones up here do. They even flow the same way (generally west to east.)

As for large land areas’ effects on the jet stream, it’s kind of hard to answer that in real simple terms, because if you really want to get down to brass tax, anything anywhere that touches air can have some effect on the chaotic system that is the atmosphere. “If a butterfly in Tokyo flaps its wings, it can cause rain a week later in New York” is an old adage that at least holds some truth in theory, if not being absolutely provable. Having said that, landmasses do have a limited effect on jet streams.

Earth wind patterns, artwork

Earth’s wind patterns, schematic view. Atmospheric circulation in each hemisphere consists of three cells. The Hadley Cell (labelled) dominates the tropical atmosphere and is intimately related to the trade winds, tropical rain belts, subtropical deserts and the jet streams. The Polar Cell (labelled) produces the polar easterlies. The outflow from the Polar Cell creates waves in the atmosphere known as Rossby waves which play an important role in determining the path of the jet stream. The polar cell also balances the Hadley Cell in the Earths energy equation. The Ferrel Cell (labelled) is dependent upon the Hadley Cell and the Polar Cell and just as the trade winds can be found below the Hadley Cell, the westerlies can be found beneath the Ferrel Cell. *** THIS PICTURE MAY NOT BE USED TO STATE OR IMPLY ESA ENDORSEMENT OF ANY COMPANY OR PRODUCT ***

Land receives and releases heat from the sun more rapidly than the oceans do, and therefore the vertical convective air currents (called Hadley, Ferrel and Polar Cells) which on a large scale help form somewhat stable weather patterns will have differing strengths over the oceans versus over continents. If your eyes are beginning to glaze over by all of this, suffice it to say that the continents can have an effect on the positioning of large scale persistent weather patterns, which also help determine the strength and position of the jet streams. But in terms of direct interaction between land and jet streams, there’s very little. Since the jet stream is usually found above 25,000’ of elevation, you’d have to get to the Himalayas before the land has any real touch on the jet.

And even though we often talk of the jet stream as a single entity, there are really four of them typically circling the globe. There’s a polar and a sub-tropical jet stream in each hemisphere, each generally following a sinuous path from the west to the east. The polar jets are lower in the atmosphere (generally 25-35,000’ altitude vs. 35-60,000’ for sub-tropical jets), and they tend to be stronger in the winter when the contrast of the polar and tropical airmasses is the greatest.

And at times you can get other types of jet streams, including one in the summer that goes the opposite direction (east to west) down in the tropics, and low-level jet streams can affect the strength of winter storms hitting the west coast. That’s one kind of jet stream on which the land does have a large effect.

 

 

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