There is both science and art in the practice of weather analysis that studies the cause and effect of atmospheric changes over continents, coastal and ocean areas. It takes more than a basic grasp of weather concepts to properly understand and predict weather features and events because their are so many forces at work.
Weather patterns can be substantially modified by local topography and oceanographic features, which are difficult to account for in weather predictions. Local variables must be inserted, adjusting or modifying forecasting.
Levels of development
We often think of weather as a linear and uniform process but it is really a multi-level phenomenon. (See the table at the end of this article.) Each level operates independently as well as in conjunction with other weather levels and occasionally several levels of weather can be experienced simultaneously.
For example, thunderstorms (small scale) could occur along the leading edge of a squall line (intermediate scale) which is associated with the approach of a storm or hurricane (tropical or mid-latitude), which has developed in response to an upper level trough (global). Analyzing and forecasting weather requires looking at the big picture as well as the little picture. Focusing on just immediate events will often lead to missing dominant weather pattern.
The earth's atmosphere has a wavelike pattern. Disturbances occurring in the atmosphere develop fluid-like streams which are then classified according to the size and strength of these atmospheric waves. Large waves, encompassing the entire earth, are called global waves or Rossby waves, named after the meteorologist who discovered this phenomenon.
Rossbywaves act over large areas and time periods, controlling such events as droughts and extended rainy periods.
Synoptic waves extend over areas the size of the United States and generally last for about a week. Synoptic waves control development and movement of surface high and present the features seen on surface weather charts.
Mid-latitude weather events occur during transition between synoptic features, such as warm and cold front passage, lasting less than a week. Tropical scale events occur in latitudes between the equator and 23 degrees north and south. Hurricanes, typhoons and monsoons are examples of tropical events, featuring strong vertical motion that is more than found in mid-latitude activity. These events are also more intense than larger scale Rossby events.
Intermediate scale activity is found embedded in medium scale events and include squall lines preceding cold fronts and bands of showers within a broad area of rain. Intermediate events are analogous to ripples within larger waves.
Small scale weather lasts just a few hours or less, such as a thunderstorm, hail storm or local snow squall. This weather is brought on by strong vertical motion, convection that rapidly cools and warms small pockets of air, causing rapid condensation and precipitation.
Micro scale events last less than an hour and have extremely high vertical, or convective, speeds. Examples are tornadoes and waterspouts. Micro scale events, along with tropical hurricanes, are usually the most violent weather events.
Weather occurring in temperate regions (30N/S to 60N/S) is controlled by a strong current of air called the jet stream, which acts as a steering force on weather systems at the surface. There are actually two jet streams.
One, the tropical jet, is found at 18,000 feet near lattitude 30 degrees north and south. the other, the polar jet is found at an altitude of 14,000 feet near 60N and S. Both jets flow from west to east, following an undulating north-south path. Jet stream flow is so important to weather analysis and prediction that each day weather services map its flow at the 500 mb pressure level and make this available, along with a variety of other charts, via weather facsimile broadcasts and Internet services. Location of jet stream winds can be deduced from satellite imagery, noting movement of high-level cirrus clouds. The strongest jet stream flow is usually found along the poleward edge of well-defined cirrus cloud bands.
Heating and cooling
Our earth's surface consists of land and water, with water being a thermally stable substance, absorbing and holding heat without substantial changes in temperature. Land is unstable thermally, absorbing and releasing heat quickly, with appreciable change in temperature. Therefore weather patterns and systems over water tend to be more predictable and stable than over land.
Additionally, equatorial regions receive more heat than polar regions, and if there were not a method of transferring heat from the equator to the poles, equatorial regions would become hotter and hotter, and the poles colder.
Necessary heat transfer is accomplished through continual movement of air (wind) and water (ocean currents). Air warmed over equatorial regions rises and flows north and south toward the polar regions. As this air rises, it cools with some of this cooled air descending back to earth near 30 degrees north and south latitude, forming areas of high pressure.
Upon reaching the earths surface, air divides, moving north and south, and forming wind patterns known as prevailing south westerlies and north easterlies. Not all air rising from equatorial region descends at 30N and 30S, some continues toward the poles, descending in those regions and forming high-pressure areas. This cold and dense polar air flows toward the equator, and it is the earth's rotation which causes this flow to form into easterly and westerly prevailing winds.
Because cold air is denser than warm air, at the poles air flows southward from polar high-pressure areas, deflected to the westward by the earth's rotation into arctic northeasterlies in the Northern Hemisphere and southeasterlies in the Southern Hemisphere. These winds meet westerlies in an area known as the polar front, where warm westerlies rise over the south-flowing cold, dense polar air, condensing moisture.
Synoptic scale heating causes atmospheric pockets of uniform temperature, pressure and humidity, which are called air masses. An air mass that is cooler than an underlying surface will warm as it passes. This causes currents of rising air and unstable conditions. An air mass that is warmer than an underlying surface will cool and sink in a stable condition.
Air masses move across the earth, pushed by prevailing winds and steered by the jet stream. Areas separating air masses are called fronts or frontal zones. A front can be warm or cold, and signifies a change in the weather.
Fronts may be gradual or dramatic, depending on the difference between the advancing air mass and the one it is replacing. Fronts are transition zones, with winds associated with pressure differences causing mixing in the atmosphere. Because air masses differ in temperature, humidity, pressure and dew point, noticeable weather changes occur with frontal passage.
Isobars are lines of equal atmospheric pressure, a standard feature of weather maps, defining limits and strength of lows and high-pressure areas. Wind flows parallel to isobars. Due to surface friction and the earth's rotation, isobars angle inward around lows and outward around highs. Wind speed is proportional to the pressure gradient, reflected by isobar spacing. Isobars close together indicate a strong gradient, while isobars with wide separation indicate a low-pressure gradient. A geostrophic wind scale is used to calculate wind speed from isobar spacing.