In last two weeks, I have talked about the climate patterns in the tropics and the subtropics. Because the sun heats the tropics most intensely, a Hadley cell forms, causing thunderstorms to form along a line called the Intertropical Convergence Zone. The poleward flow aloft begins to sink around 30 degrees north and south latitude. This sinking of air creates the world's largest deserts in this region, and also the world's hottest places. We also talked about how not every desert within the subtropics is actually hot, such as the Atacama Desert, the driest desert in the world. These kind of deserts form along the west coasts of most continents, mainly because of cold currents that cause fog and cool conditions along the western coasts. I also described how the Intertropical Convergence Zone can create a hurricane if it moves in the region of the subtropics. Since the wind flow in the tropics influences the wind flow in the subtropics, it also controls the wind flow within the middle latitudes, the stormiest place in the world. When the air flow begins to subside and sink around 30 degrees north and south latitude, the Coriolis force deflects the diverging winds to the right, causing some of the wind to bend southward and become the relatively weak trade winds. But some of the wind is also deflected northward and then finally westward when it reaches the middle latitudes. (in between latitudes of 30 and 60 degrees north.) This is why many places within the middle latitudes have a predominant wind flow from the west, also known as the prevailing westerlies. But as you all know, these winds occur less then half of the time, and this is because the traveling surface cyclones and anticyclones disrupt the generally westerly winds.
Unlike most places in the tropics and in the subtropics, middle latitude climates can change drastically from winter into summer, with spring and fall being the stormiest depending on where you live. The main and most important factor of the middle latitude climates is the position of the polar and subtropical jet streams, with the polar jet stream being the most important. These jet streams, as I have explained in past articles, create storm systems, destroy them, distribute heat energy, wind energy, and momentum; also distributing pressure differences between different areas of the earth. This is the article in which I will explain how the jet streams form in depth, without going in depth about cyclogenesis and storm formation. As I have said many times, the tropics are heated intensely while the poles have a net loss of solar radiation, causing warm temperatures in the tropics, while the poles experience generally extremely cold temperatures during most of the year, with the largest temperature difference being in the winter. But the reason why the jet stream forms is because, warm temperatures at the surface always mean, higher pressure surfaces or heights in the atmosphere while in a column of cold air, lower pressure surfaces or heights in the atmosphere. Since all things have the tendency of falling downhill, air that is over the higher pressure or heights in the atmosphere will flow towards the lower pressure or lower heights in the atmosphere. This means that aloft, the wind flows from the tropics toward the poles. Due to the Coriolis force, this mean south wind is eventually turned westward to flow parallel to the isobars over the middle latitudes. This is how a jet stream is formed. Now for the flow aloft to intensify, as stated in a past article, the temperature gradient has to increase over a short distance, which will strongly amplify the flow aloft. These jet streams form because this is the atmosphere's method of distributing not only heat energy, but wind energy and momentum in the atmosphere. For some reason distributing equally seems to be nature's main purpose in almost every aspect.
Because the jet stream's position is determined by the position of the temperature gradient at the surface, we can assume that the jet stream's position changes based on the position of the sun, mainly because the position of the sun usually determines which areas are going to be the hottest. Because of this, southern areas of the middle latitudes always have the most widespread storminess during the winter and spring. During the summer, the most extreme southern portions of the middle latitudes have dry season during the summer, because of the subtropical high. Even though they have a dry season, quite a few of these areas experience hurricanes during the summer, making for a very wet season. Farther north, such as in areas of the Southern United States north of Florida, these places are wet year round, because during the winter, they experience widespread storminess, but during the summer, a type of monsoonal flow causes daily afternoon thunderstorms because of the proximity of the ocean and the high heat in these areas.(this type of pattern is most predominant over of the Four Corners region of the United States, where daily afternoon thunderstorms can cause heavy rainfall.) North of this region, including the Ohio Valley, southern Great Lakes, and the Mid-Atlantic states, storminess is usually year round (even though the new pattern over the last several years has been extremely wet springs, followed by extremely hot, dry summers, typical of the Deep South.). The jet stream generally brings heavy snowstorms and heavy ice storms to these regions in winter, followed by heavy rain events that lead to flooding in spring, followed by severe weather outbreaks during the early and mid summers. This makes these regions the most storm ridden region of the United States. Along the Pacific Coast, rainfall patterns usually bring extremely wet and stormy winters, followed by cool, dry summers, with the occasional heat wave farther inland. In Alaska, the northern branch of the polar jet stream causes extremely stormy conditions year-round. This is because of the Aleutian low, which usually cause waves of low pressure to hit the southern coast of Alaska daily. This causes extremely heavy snowfall for these areas, with many locations picking up as much 200 inches of snow in one winter season! Northern Alaska is influenced by the polar easterlies, allowing for extremely cold and dry conditions. For the Northern Plains, Great Lakes, and New England, conditions are usually wet year round around the Great Lakes and New England, but in the Northern Plains, winters are usually dry, even though this region experiences the most blizzards of any region of the United States. The snowfall to water to ratio is very low in these areas, which means that 20 inches of snow can fall in one day, but the amount of precipitation in that snow could be 0.5 inches. During the summer, all three of these regions experience a summertime severe weather phenomena called MCCs, which are Mesoscale Convective Complexes, which can cause widespread severe weather and heavy rainfall.
Temperatures contrasts are the most extreme in the middle latitudes than in any other part of the world. These temperature contrasts are brought about by the shifting highs and lows of the ever meandering jet stream. In winter, areas in the extreme Deep South (southern portions of the Gulf Coast states, Florida, Texas, and the Southwest) usually bask in mild temperatures ranging from the 70's in Florida, while in Texas cold weather is much more common allowing for average temperatures to be in the 40's in Dallas and Houston, while extreme southern Texas cities can experience 80's all winter long. Sometimes extreme cold outbreaks reach these areas, causing orange crop damage in Florida and Texas. In the winter of 2009-2010, cold outbreaks cause billions of dollars in damage to citrus fruit in Florida and Texas. During the summer, temperatures usually rise into the middle and upper 90's over Florida, along with very humid conditions, which causes high heat indices making for a very uncomfortable summer. In Texas, temperatures become even hotter, usually reaching the 100's and the occasional 110 in western Texas, but with much lower humidity, while east Texas's climate is virtually the same as Florida's. Over the mid-south, temperatures range in the 40's and 30's for wintertime highs, meaning that freezes are very common to these areas. Several times every winter, a cold wave descends out of Canada, causing temperatures in the twenties and teens, which many people in these areas are not used to. Also, the occasional snowstorm during the winter can really paralyze the region, since most cities do not have the equipment to remove the snow and ice. Summertime commonly brings extremely hot and humid conditions, also making for an uncomfortable summer. Over the Ohio Valley, Mid-Atlantic states, and the Southern Great Lakes and Midwest, frequent snowstorms hit these areas, causing as much as three feet of snow to fall, making it one of the snowiest regions of the countries. These areas are almost always below freezing during the winter and the occasional arctic outbreak can bring low temperatures below zero. In the summer, these regions have daytime averages in the upper 80's, with frequent middle 90's temperatures. Sometimes during intense heat waves, temperatures can pass the one hundred degree mark, especially in July. In the Northern United States, (Northern Plains, Great Lakes, and New England) temperatures are usually bitterly cold in these regions, with the coldest temperatures residing over the Northern Plains, where daily minimums can be lower than negative 40 degrees. Daily highs during the winter are usually just above zero, while during a arctic outbreak, daily highs don't even get above negative 10. Summertime highs are generally mild with temperatures mostly in the upper 70s to 80's, with the occasional heat wave bringing nineties to the Northeast but 100's to North and South Dakota.
The climate over the middle latitudes is so complicated mainly because it can be different and can change from one year to the next. It is also so complicated that it would many of my articles to explain and a lot of the factors that influence the climate in the middle latitudes is to difficult for even meteorologists to understand. For this reason, forecasts are most difficult to make in the mid-latitudes and is still trying to be understand.
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