Theory 
Climate and Weather at Earth| CONTENT - A useful view of climate and climate processes at Earth. A tutorial in our series about the Earth |
 | The Earth as seen by the Apollo 17 mission. That picture is the real Blue Marble image of the Earth, as we transposed it vertically relatively to the original one. picture NASA |
Large climate patterns are essentially the product of the interaction between Sun and oceans and land masses. The higher the Sun above a given area, the larger the quantity of heat this area is receiving. Whatever the season, Sun is always high in the tropics, a zone about 23° each side of the equator, or it is high above high latitude during northern or southern summer. A major large-scale air motion is the one which takes hot air rising at the equator to the poles, where the air sinks. A major process at work in the climate is the evaporation. When Sun heats a mass of ocean or of land, moist evaporates and forms water vapor. The water vapor is lighter in terms of weight than the dry air. Hence where there is evaporation, there are air masses with a low pressure as where there is no evaporation, air masses have a high pressure. This difference between air masses having diverging pressure brings winds. Winds are naturally flowing from the zones of high pressure to the zones of low pressure. Such winds are creating cyclones and anticyclones. Cyclones appear there where winds are coming into zones of low pressure. There winds are spiralling inward and counterclockwise. As anticyclones appear where winds are coming into zones of high pressure where they are spiralling clockwise and outwards. Motion of cyclones and anticyclones are the reverse in southern hemisphere. Such major regional air masses are yielding further air circulation: the air leaving anticylones is replaced by air sinking from above as in the cyclones, air converging to the center is rising there and forming clouds and precipitation. Due to Earth's rotation, a force which is called the "Coriolis force"is deviating winds right in northen hemisphere and left in southern. At middle and high latitudes, polar and tropical air masses are melting in low-pressure areas forming narrow zones called "fronts". At last, the higher the winds, the swiftier. At the top of the troposphere, they form the jet stream, which result from temperature constrast between air masses. As Earth spins on its axis, huge rivers of air called Rossby waves meander around the globe in a westerly direction. Currents in the center of these waves form the jet streams, fast-moving columns of air that push weather systems from west to east. Rossby waves aren’t uniform. They tend to undulate and have troughs and ridges. Areas of low-pressure typically develop in the troughs of the waves, while high-pressure areas form in their ridges. Parcels of warm air from the tropics and cool air from the poles swirl around those creating a complex tapestry of warm and cool fronts that meet and interact constantly. Abnormal highs in a Rossby wave may cause the jet stream to divide. Rossby waves appeared inside jet streams over the last 40 years, sending hot air flow to continents in the northern hemisphere and could account for heatwaves worldwide. All these conditions leads to the creation of cyclones and anticyclones. In terms of salinity, Earth's oceans feature a higher salinity in the subtropics, a higher average salinity in the Atlantic Ocean compared to the Pacific and Indian oceans and a lower salinity in rainy belts near the Equator worldwide. These features are related to large-scale patterns of rainfall and evaporation over the ocean, river outflow and ocean circulation. Other important regional features further are evident, like a sharp contrast between the arid, high-salinity Arabian Sea West of the Indian subcontinent, and the low-salinity Bay of Bengal to the East, which is dominated by the Ganges River and south Asia monsoon rains. A strong low-salinity water associated with outflow from the Amazon River is also observed. 'Atmospheric rivers' are relatively long, narrow, short-lived jets or tunnels of air at a mile high, that transport water vapor across significant portions of Earth's mid-latitude oceans, onto the continents and into Earth’s polar regions. Water vapor eventually turn snow or rain
Smaller scales climatological processes are seen under the form of main climate zones. These are zones of identical climate, extending parallel starting at the equator, and identical all along the planet. These zones may be seen like sub-parts of the large-scale air motion between the equator and the poles
->High Winds Over the Oceans
Late NASA studies have brought to a better understanding of such local conditions like high winds over the oceans:
- Earth's windiest ocean location is Cape Farewell, Greenland, where gale winds blow 16 percent of the time
- Half of the top 10 windiest spots occur where tall coastlines or high mountains meet the sea
- Strong winds are much more frequent on the warm side of cold-warm fronts formed where the Atlantic's warm Gulf Stream flows northward into cold ocean regions. This gives climate scientists important clues about how sharp differences in ocean surface temperatures affect the atmosphere, with warm ocean temperatures creating an unstable atmosphere that sucks strong winds down from aloft. What is most surprising is that narrow ocean currents have a large effect on the occurrence of high winds. For example, in cold meanders of the Atlantic's Gulf Stream, the frequency of high winds drops by an order of magnitude
High winds play an important role in Earth's climate. They remove heat from the ocean, leading to the formation of "deep water" -- cold, salty, dense water that helps drive global ocean circulation patterns. They also help exchange gases, such as carbon dioxide, between the oceans and the atmosphere, mix different types of ocean water, and pump nutrients up from the deep sea for plankton to feed on
->What 'Atmospheric Rivers' Are
Atmospheric rivers - narrow regions in Earth's atmosphere that
transport enormous amounts of water vapor across the Pacific or other regions- or 'rivers in the sky,' can transport enough water vapor in
one day, on average, to flood an area the size of Maryland 1 foot (0.3 meters)
deep, which is about seven times the average daily flow of water from the Mississippi
River into the Gulf of Mexico, and causing floods or, at the contrary, to beneficial increases in snowpack under the form of strong US winter storms over the Sierras, for example. As far as the USA are concerned, concentrated streams of tropical
moisture that sometimes get connected with cold fronts and winter storms
approaching the U.S. West Coast - sometimes called the pineapple express, since
they often originate near Hawaii - can result in very intense rain
Due to the preceding conditions, most parts of the Earth are belonging to some characteristic weather zone. It is such zones which make that weather conditions in the desert zones of southwestern USA are somehow identical to weather conditions in the Sahara or in the Australian desert. Temperatures, precipitation or vegetation are some of these conditions
Several phenomenon are exerting a large-scale influence upon the Earth's weather. A example of which is the Madden-Julian Oscillation (MJO) with unusual variations of clouds, rainfall and large-scale atmospheric circulation moving slowly eastward from the tropical Indian Ocean into the Pacific Ocean over the course of weeks, ebbing and flowing like waves in cycles lasting about 40 to 50 days. This climate pattern typically spans more than half the distance around Earth's equator. In the disturbed portion of the "wave," air rises, triggering showers and thunderstorms; in the sinking portion, air subsides, inhibiting clouds and rainfall. Madden-Julian Oscillation events can strongly influence long-term weather patterns and have widespread impacts around the globe. They can help trigger the beginning and end of the Asian and Indian monsoons and influence the development and evolution of El Niño, hurricanes and weather in Earth's mid-latitudes. The MJO begins now to be better forecasted and integrated into weather prediction models
A simpler way still to understand the large-scale, weather phases occurring seasonally over the more important parts of the world, is to catch some simple elements for each of the Earth's hemispheres. As far as the northern hemisphere is concerned, there are to be found at the mid-latitudes, between the tropics and the poles, vast zones -which exist, or which form- as they are either of the anticyclone, or of the cyclone types. Such vastness is allowed for through that they are located above large stretches of land, or of water. According to the seasons further, those zones are moving somewhat, relatively to their primary location, as it's such a motion which brings to determine the weather characteristics on large parts of the Earth. For Americas, those zones are the following: an anticyclone over the North of North America which is forming during winter, the anticyclone of Hawaii, which forms during summer, and the cyclone of the Aleutians (for Europe, on the other hand, such equivalent zones are the anticyclone of Siberia, the anticyclone of the Azores, and the Icelandic cyclone). It's the cyclonic zone which defines the weather flux. The weather patterns, for Americas -or Europe- are further refined by additional elements as, during summer, the cyclone zone weakens, and one of the anticyclone increases in size, tending to halt the flux generated by the cyclone zone. As far as the European zone is concerned, moreover, one may know that, during summer too, complementarily, a vast cyclone is settling from Central Asia to the Sahara, in Africa
 | click to a view of the weather fronts at the northern mid-latitudes, monsoon excepted |
From a part of the Middle East up to Japan, it's the famed monsson which is determining the great cycles of the weather. Since about June to November, the summer monsoon -with an orientation southeast-northwest, is powering strong fluxes of heavy rain towards the mainland, providing agriculture there with the water necessary to the crops, as, from November to May, the winter monsoon, blowing then from the northwest to the southeast (as it is more or less associated to the vast anticyclon of Siberia) is blowing dry winds upon those lands. In the western Pacific Ocean, as they are linked to the warm and wet summer monsoon, do occur the 'typhoons", those devastating tropical tempests, which are affecting the regions located from Indochina to Japan mainly. It has to be noted that the 'hurricanes', in the Atlantic Ocean, are of the same type, at the exception that they're not typically associated to any monsoon weather regime. They are powered instead from the warm tropical water offcoast of Africa and then they are heading towards the Western Indies, the Gulf of Mexico, or Florida. The hurricanes, further, may be considered, when they eventuallay come to die in the northern Atlantic, like they power the Icelandic cyclone. A flow of hurricanes too may be found west of the coast of Central America, in the eastern Pacific Ocean, as they come to hit those coasts
As far as the tropics are concerned, those countries of an everlasting spirng, are featuring a great meteorological stability, at the exception of the rain season over their Sahelian margins and of a -relative- difference in the temperatures between the summer, and the winter solstices. The equatorials regions, due to the intense moisture generated by either the oceans or the rainforest, are regions where the solar intensity is stable all year long. The evaporation cycle is determining the climate, with the formation, in the morning of cumulus, which swiftly turn into cumulonimbus. Those do unleash their equatorial rains about midday (those rains, by the way, are called 'zenithal rains" due to that they are beginning when the Sun is at is higher). By the end of the afternoon, and at night, the rains cease, with a clear weather back, still filled however with the moisture of the rains which fell along the day!
The southern hemisphere now! The large-scale patterns there have to take in account that there are fewer landmasses and more water ones in that hemisphere. Australia only allows for the existence of an anticyclone during winter, as the Atlantic, Indian and Pacific oceans, respectively, are giving birth to the anticyclones of the southern Atlantic (or of the St. Helena island), the Mascarene Islands, and the Easter Island. Antarctica, on the other hand, is completely covered with a cyclonic area. It's those zones, like in the northern hemisphere, which are triggering the main weather flux -with the difference that North, the flux are moving eastwards, are they are westwards South. There are no monsoon regimes in the southern hemisphere
Website Manager: G. Guichard, site 'Amateur Astronomy,' http://stars5.6te.net. Page Editor: G. Guichard. last edited: 11/2/2017. contact us at ggwebsites@outlook.com