The oceans, in combination with air masses and warm temperatures play a vital role in patterns of precipitation. It is the interaction of these three processes that determine the global scale variation in evaporation, which shadows those variations of precipitation. Latent heat is developed when water changes from its liquid state to its gaseous state, this energy is hidden. Appendix B (Linacre, 1997) shows us that latent heat and thus evaporation is quite high along the tropics and this incombination with maritime air masses (as previously stated) feeds the tropical cyclones. Also oceanic currents e.g. the Gulf stream aid in providing Europe with not only warm waters but also warm and humid maritime air masses (Lee, 2003) which, when they reach land usually yield rain because the warm air is forced to rise over the colder coastal land and this causes condensation and rain. But we must also note that evapotranspiration from the continents and fauna adds water vapour to the atmosphere and in some places e.g. the Amazon Rainforest this evaporation actually develops its own localised hydrosphere and is almost self-sufficient. Without evaporation, the hydrological cycle would not continue and thus there would be no precipitation.
This is also the case for areas in the shadow of a mountainous region as most of an air masses moisture is precipitated as the air masses rises over the obstacle. This is called Orographic precipitation and is characterised with the western coasts of North and South Americas due to their coastal mountain ranges and also around the Himalayas but of course it can occur near most upland areas. As stated the further inland your go typically you get less moisture in the air and thus less precipitation, this is called continentality but along the plains of America and in tropical regions (Arnell, 2002) but also along fronts such as the ITCZ (Buchdahl, 2000) and in anti-cyclones (Henderson-Sellers, 1986), you can receive convective or adiabatic rain which results from air rising in warm conditions and as it rises it cools due to expansion spreading heat over a wider volume of air, this usually results in localised precipitation which is intense. Also cumulonimbus clouds are associated with this convection and they may result in thunderstorms (Niehoffa, 2002) or hailstorms (Buchdahl, 2000). So elevation also takes a key role in denoting where precipitation can occur on land.
Anti-cyclones yield little in the way of precipitation (apart from the occasional thunderstorm) but it is worth mentioning them in the way that they divert depressions. Anti-cyclones are, unlike depressions, an area of descending high pressure and may last for many weeks. There are bands of air which generally have permanent areas of high pressure and these are on the latitudes of 30ºN and 30ºS, the area of the descending limb of Hadley. Anti-cyclones act as “blocking Highs” (Buchdahl, 2000); areas of high pressure which divert depression around them and thus change their distribution.
Potentially the most pre-eminent factor affecting precipitation variation is Global warming, which is the warming of the atmosphere due to the increasing amount of greenhouse gases in the atmosphere e.g. carbon dioxide, methane (Loaiciga, 1996) and water vapour (Arnell 2002), these gases aid in reflecting heat back down to earth and thus warming the planet. This is called the greenhouse effect (Arnell 2002) for naturally induced warming and enhanced greenhouse effect (Linacre 1997) for human induced warming. With an increase in temperature more evapotranspiration from land and evaporation from seas, would provide more water vapour, which also adds to the effect (Kovach, 2003), to the hydrosphere and thus increase potential precipitation rates, by as much as 10% (Loaiciga, 1996). Already, as can be seen in appendix D (page 68 of climate and environmental change), global precipitation has increased much in the latitudes of 40º-60ºN and 0º-20ºS’ Snowfall has also increased in Antarctica by 5-20% (Harvey, 2000). This increase in precipitation is not received everywhere though as with changes in the worlds circulation systems means some areas will receive much more than others (Arnell, 2002) e.g. the subtropics are receiving less of a major change in rates (Loaiciga, 1996).The increase in temperature will also cause a variation in types of precipitation across the world: some places my receive distinctly less. The (IPCC) has stated that global warming and increases in the distribution of acid rain is due to humans – one of the main factors acting on today’s precipitation (and indeed all of meteorological) patterns is down to humans (IPCC secretariat, 2001).
This warming will also affect oceanic circulation (Loaiciga, 1996) and thus the movement of potentially precipitable maritime air masses and also the migration of the ITCZ. Evidence of the effects of global warming on precipitation patterns is shown in the more frequent occurrences of El Nino, this probably due to the warming of sea surface temperatures which prompts the migration of the warm band of Pacific water eastwards (Ritter, 2003). The Clausius-Clapeyron effect (, 1997) states that warm air holds more water and is more likely to precipitate thus this band of air brings devastating cyclones to the Americas and it also causes the variations in monsoons and tropical cyclones.
The factors I have looked at all affect the variations of precipitation across the world. The ITCZ depicts the specific areas that will receive precipitation but its patterns of associated precipitation vary with its seasonal migrations, which also bring monsoonal seasons to the tropics, so with this in mind precipitation vary with altitude but also time of year. I also included anti-cyclones in my work which act as obstacles to depressions and causes their course to change and thus for precipitation patterns to fluctuate. Oceans also provide moisture for maritime air masses and feed the depressions that plague the tropics during the monsoon season and oceanic circulation provides areas with precipitation e.g. the Gulf Stream provide southern England with precipitation. Land-use, in the way of cities and rainforests was not included in this essay due to the fact that even though they do have significant effects on weather it is all reasonable localised and should not be considered one of the major factors. All these factors and the others which I have stated are all being increasingly affected by the increase in global temperatures as a result of the enhanced greenhouse effect (and greenhouse effect), which is promoting higher temperatures and a decrease of water stores, through the melting of polar ice caps, which means the global hydrological cycle will contain more water and thus not only will precipitation increase, in some areas, but also variation in rainfall and snowfall will increase still further. Thus the impacts of global warming are, or will, affect all the other factors: wind circulation will be altered in the way that an increase in global temperatures will allow the migration of the ITCZ to higher latitudes at times in the year and with this monsoonal variation and El Nino variation will become more erratic. Simply put because of a current increase in greenhouse gases by humans we ourselves, in cooperation with the greenhouse effect have become the pre-eminent factor in determining precipitation variation.
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Reference:
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