WEATHER

               

Weather describes the condition of the atmosphere over a short period of time e.g. from day to day or week to week, while climate describes average conditions over a longer period of time. Step outside and you experience many facets of weather. Humidity, air temperature and pressure, wind speed and direction, cloud cover and type, and the amount and form of precipitation are all atmospheric characteristics of the momentary conditions we call weather.

The sun is ultimately responsible for the weather. Its rays are absorbed differently by land and water surfaces (equal amounts of solar radiation heat the ground more quickly than they do water). Differential warming, in turn, causes variations in the temperature and pressure of overlying air masses.

As an air mass warms, it becomes lighter and rises higher into the atmosphere. As an air mass cools, it becomes heavier and sinks. Pressure differences between masses of air generate winds, which tend to blow from high-pressure areas to areas of low pressure. Fast-moving, upper atmosphere winds known as jet streams help move weather systems around the world.

Large weather systems called cyclones rotate counterclockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere); they are also called “lows,” because their centers are low-pressure areas. Clouds and precipitation are usually associated with these systems. Anticyclones, or “highs,” rotate in the opposite direction and are high-pressure areas - usually bringing clearer skies and more settled weather.

The boundary between two air masses is called a weather front. Here, wind, temperature, and humidity change abruptly, producing atmospheric instability. When things get “out of balance” in the atmosphere, storms develop, bringing rain or snow and sometimes thunder and lightning too.

The weather you experience is influenced by many factors, including your location’s latitude, elevation, and proximity to water bodies. Even the degree of urban development, which creates “heat islands,” and the amount of snow cover, which chills an overlying air mass, play important roles.

CAUSES

On Earth, the common weather phenomena include wind, cloud ,rain, snow , fog and dust storms. Less common events include natural disaster such as tornadoes, hurricanes, typhoons and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere).Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.

         Weather occurs primarily due to air pressure, temperature and moistures differences between one place to another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. In other words, the farther from the tropics one lies, the lower the sun angle is, which causes those locations to be cooler due to the indirect sunlight The strong temperature contrast between polar and tropical air gives rise to the jet stream. Weather systems in the mid-latitudes, such as extra tropical cyclone, are caused by instabilities of the jet storm flow. Weather systems in the tropics, such as monsoons or organized thunderstorm systems, are caused by different processes.