HiSET Science: Weather Systems

Weather
Weather is the result of transfers of kinetic (heat) energy due to differences in temperature between objects as well as transfers of moisture in Earth's atmosphere. Meteorology, the study of weather, covers the same natural events as climatology, but observes them on a shorter time scale (usually no more than a few days). Rain, fog, snow, and wind are all examples of weather phenomena. The processes that occur at different stages in the hydrologic cycle form the basis of meteorological events. Most of the activity that produces the weather we experience on Earth takes place in the troposphere, the lowest level of the atmosphere. Atmospheric pressure, temperature, humidity, elevation, wind speed, and cloud cover are all factors in the study of weather.

Ozone Layer
The Earth's ozone layer is the region of the stratosphere with a high concentration of ozone (a form of oxygen) particles. These molecules are formed through the process of photolysis, which occurs when ultraviolet light from the sun collides with oxygen molecules (O2) in the atmosphere. The ultraviolet radiation splits the oxygen atoms apart; when a free oxygen atom strikes an oxygen molecule, it combines with the molecule to create an ozone particle (O₃). Ozone molecules may be broken down by interaction with nitrogen-, chlorine-, and hydrogen-containing compounds, or by thermal energy from the sun. Under normal conditions, these creative and destructive processes balance the levels of ozone in the stratosphere. The concentration of ozone molecules in the atmosphere absorbs ultraviolet radiation, thus preventing this harmful energy from reaching the Earth's surface. Ozone particles form in the region of the atmosphere over the equator, which receives the most direct sunlight. Atmospheric winds then disperse the particles throughout the rest of the stratosphere.

Air Mass

An air mass is a body of air that exhibits consistent temperatures and levels of moisture throughout. These (usually large) pockets of air tend to come together under relatively still conditions, where air can remain in one place long enough to adopt the temperature and moisture characteristics of the land below it; this often occurs above wide areas of flat land. The region in which an air mass originates and the course of its motion are used to name it. For example, a maritime tropical air mass (denoted mT) is formed over the Gulf of Mexico (a tropical climate) and moves across the Atlantic Ocean (a maritime area). The conditions of an air mass will remain constant as long as the body is still, but when it moves across surfaces with different conditions, it may adopt those qualities. For example, polar air that moves over tropical land areas will be heated by the conditions below.
Generally, maritime air masses contain high levels of moisture, and continental air masses are drier.

Meteorological Depression
A meteorological depression refers to a low-pressure zone (created by rising air) situated between 30 and 60 degrees latitude.
These zones vary from approximately 321-3,218 kilometers in diameter. The rising air associated with a depression usually condenses at higher levels in the atmosphere and causes precipitation. Depressions are formed when warm air masses and cold air masses converge. At first, a single front (boundary between converging masses of air with different temperatures) separates the air masses.
A distortion similar to the crest of a water wave develops, creating a small center of low pressure. Then, differentiated warm and cold fronts develop from that center. A mass of warm air forms and rises over the body of cold air. The cold front and the cold air eventually catch up with the warm air, creating an occluded front and causing pressure to rise, effectually slowing the depression's movement. Depressions usually have life spans of four to seven days.

Prevailing Winds and Wind Belts
Wind (the horizontal movement of air with respect to Earth's surface) forms due to pressure gradients (differences) in the atmosphere. Air tends to move from areas of high pressure (such as the poles) to areas of low pressure (such as the tropics). Prevailing winds, or trade winds, are the winds (named in meteorology for the direction they come from) that blow most frequently in a particular region. For instance, the prevailing winds most common in the region from 90 to 60 degrees north latitude blow from the northeast, and are generally called the Polar Easterlies. Wind belts are created in areas where prevailing winds converge with other prevailing winds or air masses. The Inter-Tropical Convergence Zone (ITCZ), where air coming from tropical areas north and south of the equator come together, is an example of a wind belt.

Coriolis Force
The Coriolis force, which gives rise to the Coriolis effect, is not really a force at all. Rather, it appears to be there to us because the Earth is a rotating frame of reference and we are inside it. In the atmosphere, air tends to move from areas of high pressure to areas of lower pressure. This air would move in a straight line but for the Coriolis force, which appears to deflect the air and cause it to swirl. Really, however, the Earth moves underneath the wind, which creates the impression of swirling air to someone standing on the Earth's surface. The Coriolis force causes winds to swing to the right as they approach the Northern Hemisphere and to the left as they approach the Southern Hemisphere.

Air Stability in the Atmosphere
Air stability is the tendency for air to rise or fall through the atmosphere under its own power. Heated air rises because it is less dense than the surrounding air. As a pocket of air rises, however, it will expand and become cooler with changes in atmospheric pressure. If the ambient air into which rising air ascends does not cool as quickly with altitude as the rising air does, that air will rapidly become cooler (and heavier) than the surrounding air and descend back to its original position. The air in this situation is said to be stable. However, if the air into which the warm pocket rises becomes colder with increased altitude, the warm air will continue its ascent. In this case, the air is unstable. Unstable air conditions (such as those that exist in depressions) lead to the formation of large clouds of precipitation.

Clouds
The four main types of clouds are cirrus, cumulous, nimbus, and stratus. A cirrus cloud forms high in a stable atmosphere, generally at altitudes of 6,000 meters or higher. Temperatures at these altitudes (in the troposphere) decrease with increased altitude; therefore, the precipitation in a cirrus cloud adopts the form of ice crystals. These usually thin traces of clouds may indicate an approaching weather depression. A cumulous cloud is a stereotypical white, fluffy ball. Cumulous clouds are indicators of a stable atmosphere, and also of the vertical extent of convection in the atmosphere—condensation and cloud formation begin at the flat base of a cumulous cloud. The more humid the air, the lower a cumulous cloud will form. A nimbus cloud is, generally speaking, a rain cloud. Nimbus clouds are usually low, dark, and formless, sometimes spanning the entire visible sky. A stratus cloud is basically a cloud of fog which forms at a distance above the Earth's surface. This type of cloud forms when weak convective currents bring moisture just high enough to initiate condensation (if the temperature is below the dew point).

The four cloud subtypes are cumulonimbus, cirrostratus, altocumulus, and stratocumulus.
A cumulonimbus cloud is produced by rapid convection in unstable air. This type of cloud (which is often dark) is formed as a large, tall 'tower.' Collections of these towers (squall lines) often signal a coming cold front.
Thunderstorms often involve cumulonimbus clouds.
A cirrostratus cloud is an ultra-thin formation with a white tint and a transparent quality.
An altocumulus cloud forms at an altitude from 1,980 to 6,100 meters. Clouds of this type, which appear to be flattened spheres, often form in clumps, waves, or lines.
A stratocumulus cloud forms as a globular mass or flake. Stratocumulus clouds usually come together in layers or clumps.


Lightning
Lightning is a natural electrostatic discharge that produces light and releases electromagnetic radiation. It is believed that the separation of positive and negative charge carriers within a cloud is achieved by the polarization mechanism. The first step of this mechanism occurs when falling precipitation particles become electrically polarized after they move through the Earth's magnetic field. The second step of the polarization mechanism involves electrostatic induction, the process whereby electrically charged particles create charges in other particles without direct contact. Ice particles are charged though this method, and then energy-storing electric fields are formed between the charged particles. The positively-charged ice crystals tend to rise to the top of the cloud, effectively polarizing the cloud with positive charges on top and negative charges at the middle and bottom. When charged clouds conglomerate, an electric discharge (a lightning bolt) is produced, either between clouds or between a cloud and the Earth's surface.

Thunderstorms
A thunderstorm is a weather phenomenon that includes lightning, thunder, and usually large amounts of precipitation and strong winds. Thunder is the noise made by the rapid expansion and contraction of air due to the heat energy produced by lightning bolts. A thunderstorm develops when heating on the Earth's surface causes large amounts of air to rise into an unstable atmosphere. This results in large clouds of rain and ice crystals. The associated condensation releases high levels of heat, which in turn power the growth cycle of the cloud. The clouds created during thunderstorms are immense, sometimes reaching widths of several miles and extending to heights of 10,000 meters or more. The precipitation in such clouds eventually becomes heavy enough to fall against the updraft of unstable air; the consequent downpour is often short but intense. The differential speeds at which light and sound travel through the atmosphere enable one to estimate the distance between oneself and the storm by observing the interval between a lightning bolt and a thunderclap.

Hurricanes
Hurricanes form when several conditions are met: Oceanic water must be at least 26 degrees Celsius, the general circulation pattern of wind must be disrupted (this disruption usually takes the form of an atmospheric wave in the easterly trade winds), and the Coriolis force must be in effect. During hurricane season (June to November), easterly waves appear in the trade winds every few days. When such a wave occurs over a body of particularly warm, deep water, it is strengthened by the evaporation of warm air from below. Surrounding winds converge at the low-pressure zone created by the wave; air brought by these winds rises because it has nowhere else to go. The large body of warm, moist air rises high into the atmosphere and consequently condenses into huge clouds. As more and more humid air is drawn upward, this air begins to rotate around the area of low pressure. The storm continues to gain strength and may move toward land.

El Nino
El Niño refers to the unusual warming of surface waters near the equatorial coast of South America. This phenomenon occurs during the winter approximately every two to seven years, lasting from a few weeks to a few months. El Nino can cause torrential rains, violent winds, drought, and dangerously high temperatures in surrounding areas. El Nino is caused by a reversal of the atmospheric pressures on the eastern and western sides of the Pacific (normally, pressure is high on the eastern side near South America and lower on the western side near the Indonesian coast). This reversal causes a wave of warm water to flow eastward and sea levels to fall on the western side. The changes in air pressure and ocean temperature cause moisture levels in the western Pacific to rise drastically while the region east of the Pacific experiences drought. The air pressure changes also weaken the region's trade winds, which normally serve to distribute heat and moisture.

Monsoons and Savannahs
The term monsoon refers to a unique pattern of moving air and currents that occurs when winds reverse direction with a change in season. India and Southeast Asia experience the most intense monsoons. This area lies between tropical and subtropical climate zones. During the winter season, northeasterly winds (which are generally dry) move from high-pressure subtropical areas to lower-pressure tropical areas. During the summer season, the continents of India and Asia heat up, creating a low-pressure zone. This causes winds to reverse and blow southwesterly across the Indian Ocean, accumulating high levels of moisture, thereby creating large amounts of precipitation during this season.
Savannahs also exist between wet equatorial and dry subtropical climate zones. These regions are characterized by vegetation consisting mainly of shrubs and grass. Savannahs experience dry weather throughout most of the year. A single, brief rainy season that occurs when the Sun is directly above the region interrupts prolonged dry spells.

Influence of Mountains on Climate

At the level of local climate, the presence of mountains forces air to rise to travel above them; this contributes to increased formation of clouds and consequently, increases in levels of precipitation. Mountain chains can affect regional and even global climates by deflecting airflow. The Coriolis force causes most of Earth's atmospheric airflow to move east and west. Therefore, the presence of north-south–oriented mountain chains can alter general circulation patterns.
For example, the Rocky Mountains force air to move northward; the air cools near the North Pole before blowing back down. This causes winter temperatures in Canada and parts of the United States to be very cold.

Humidity and Cloud Cover
Humidity is a measure of the amount of water vapor in the air. Specific humidity is the expression of humidity as a ratio of aqueous vapor to dry air; it is expressed as a ratio of mass of water vapor per unit mass of natural (dry) air. Absolute humidity measures the mass of water vapor in a given volume of moist air or gas; it is expressed in grams per cubic foot or per cubic meter. The equilibrium (or saturated) vapor pressure of a gas is the vapor pressure (created by the movement of molecules) of water vapor when air is saturated with water vapor. Relative humidity, usually expressed as a percentage, is the ratio of the vapor pressure of water in air (or another gas) to the equilibrium vapor pressure. In other words, it is a ratio of the mass of water per volume of gas and the mass per volume of a saturated gas. Cloud cover refers to the amount of sky blocked by clouds at a given location.

Measuring Weather
Weather can be measured by a variety of methods. The simplest include measurement of rainfall, sunshine, pressure, humidity, temperature, and cloudiness with basic instruments such as thermometers, barometers, and rain gauges. However, the use of radar (which involves analysis of microwaves reflecting off of raindrops) and satellite imagery grants meteorologists a look at the big picture of weather across, for example, an entire continent. This helps them understand and make predictions about current and developing weather systems. Infrared (heat-sensing) imaging allows meteorologists to measure the temperature of clouds above ground. Using weather reports gathered from different weather stations spread over an area, meteorologists create synoptic charts. The locations and weather reports of several stations are plotted on a chart; analysis of the pressures reported from each location, as well as rainfall, cloud cover, and so on, can reveal basic weather patterns.

Global Warming
The natural greenhouse effect of the atmosphere is beneficial to life on Earth; it keeps temperatures on the planet 33 degrees higher than they would be without this phenomenon. Originally, this helped sustain life. However, it has been discovered in the last 20 years that this effect is being intensified by the actions of humans. In the twentieth century, certain activities of mankind, including the burning of fossils fuels like coal and oil, have resulted in an increase in the levels of greenhouse gases (such as methane and carbon dioxide) being released into the atmosphere. Also, increasing deforestation has affected the number of photosynthesis-practicing plants. The combined effect of these trends is a higher-than-normal concentration of greenhouse gases in the atmosphere. This, in turn, produces the effect of global warming. The average temperature at the Earth's surface has gone up 0.6 degrees Celsius in the last 100 years. Continuation of this trend is likely to have a detrimental effect on many of the planet's ecosystems, including that of human beings.