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•Weather is short-term changes in the atmosphere, such as changes in temperature, air pressure, humidity, wind, and precipitation. Climate is the average weather pattern of an area over a long period.
•The differences between weather and climate can be summarized as follows:
•Weather patterns are caused by the flow of the atmosphere. In the temperate zones, weather conditions usually move from west to east.
•The seasons—summer, winter, spring, and fall—are cycles of weather caused by Earth's tilt and orbit.
•The four main climate zones are tropical, subtropical, temperate, and polar.
•Temperate climate zones have two types of climate: continental (inland) and maritime (near oceans or seas).
•Clouds are water droplets that condense in the air. The droplets form around condensation nuclei, which may be salts or dust particles.
•Clouds have many effects. They can keep an area cool by reflecting the Sun's rays, keep an area warm by trapping heat emitted by Earth's surface, and they produce precipitation—rain, sleet, hail, or snow.
•You can usually tell what type of weather will occur by observing clouds. For example, if you see cumulonimbus clouds, you are almost guaranteed a major storm. If you see cirrus clouds, you will have good weather.
•Precipitation is any form of water that falls to the surface of Earth. Precipitation can be rain (liquid water), snow (ice crystals), sleet (water and ice), or hail (packed ice crystals).
•An air mass is a large body of air, at least 1,000 miles across. The air has a uniform temperature and humidity and flows as one unit.
•There are four main types of air masses. Air masses can be cold (polar) or warm (tropical). They can form over water (maritime) or land (continental). Combining these two dimensions produces four categories: maritime tropical, maritime polar, continental tropical, and continental polar.
•Where two different air masses meet, it is called a front. There are different types of fronts:
◦Warm front - When a warm air mass overrides a cold area, it rises slowly, creating nimbostratus, altostratus, or cirrostratus clouds. This can produce light precipitation.
◦Cold front - When a cold air mass overrides a warm area, it pushes the warm air up forcefully, creating cumulus and cumulonimbus clouds. Cold fronts often produce thunderstorms and very bad weather.
◦Stationary front - When a cold air mass and warm air mass flow side-by-side, and neither mass invades the other, weather can be unpredictable. It most often resembles a warm front, with light rain if any precipitation occurs.
◦Occluded front - When two cold air masses trap a warm air mass, they push the warm air up. The boundary that exists is between two similar air masses: cold and cooler air. Clouds and precipitation are likely.
•Mid-latitude cyclones are severe storms that develop between tropical and polar air masses in mid-latitude regions around low-pressure centers.
•All weather conditions on Earth are affected by the Sun's heat. The amount of heat or energy that comes into Earth or reflects back out into space is the earth's energy budget.
•The Sun provides energy for the water cycle, which begins with the evaporation of water to produce water vapor (humidity). Water vapor rises and cools into clouds, and, eventually, falls as precipitation.
•A yearly change in the intensity of solar radiation an area receives produces seasons. Changes in solar radiation are caused by Earth's orbit, the earth's tilt toward (summer) or away (winter) from the Sun, and the earth's spherical shape.
•The more direct solar radiation an area receives, the hotter its temperature and the lower its air pressure will be. If an area receives less solar radiation, it will be cooler in temperature and higher in air pressure. Air moves from high to low pressure, creating wind.
•A notable wind pattern is the jet stream. It occurs high in the atmosphere where warm, tropical air meets cold, polar air.
•Climate zones and their features are also related to the amount of solar radiation an area receives. For example, near the equator where the Sun's rays are most concentrated, it is always hot, humid, and raining. This produces a tropical environment.
•By the beach, a cool sea breeze comes in during the day and a cool land breeze blows out at night.
•Coastal areas have more moderate summer and winter temperatures than inland areas because ocean water both heats and cools more slowly than does the land.
•When cold, arctic air flows over warm lakes, a downpour of snow can fall. This is lake effect snow.
•The rain shadow effect occurs when tall structures, like mountain ranges, interrupt wind flow. The rain shadow effect creates a wet, tropical climate on the side facing the wind and a dry, desert climate on the leeward side of the mountain.
•The air above cities is often warmer than the surrounding area, called an urban heat island effect. The absorption of heat by streets and buildings produces this effect.
•Climate zones demonstrate the diverse conditions of temperature, precipitation, and humidity on Earth. The climate of an area depends on its latitude and its proximity to pressure belts, bodies of water, and other geographic features, like mountains.
•The atmosphere and oceans redistribute heat from the equator to the poles. They display similar patterns of air and water movement because air currents produce surface currents in the oceans.
•The thermohaline circulation current is the main transporter and distributor of heat in the ocean. This is a deep ocean current driven by temperature, salinity, and density differences in ocean waters.
•Ice ages are theorized to have happened due to orbital changes of Earth, called the Milankovitch cycles.
•Other factors that can produce climate changes are plate tectonics, volcanic activity, changes in solar output, and human activities.
•Thunderstorms are common in warm, moist climates. They usually produce rain or hail, heavy winds, thunder, and lightning.
•Late in the summer, hurricanes are common over warm tropical waters. Hurricanes have wind speeds over 74 mph, with a calm center or eye. They get their energy from the heat of tropical waters and die out once over cool water or land.
•When warm, moist air masses clash with polar air masses, a horizontal spinning column of wind can form—wind shear. As the warm air rises into the storm clouds, it can turn the spinning column into a vertical vortex, or tornado.
•Floods can be produced by tropical storms, violent wave activity along coasts, and earthquakes. Some scientists think global warming is causing more storm activity and flooding.
•Drought is low rainfall for an extended period. Any climate can have a drought if rainfall is below average. Water shortages can also occur whenever water consumption is greater than precipitation. In either case, water conservation efforts are needed.
•Stay up-to-date on appropriate safety tips for severe weather conditions because severe weather can happen quickly.
•Meteorologists use computer-generated data and their knowledge of their region to forecast weather.
•Weather prediction is usually accurate short-term (1-2 days out), but long-term weather prediction is less accurate.
•Meteorologists often use weather maps and satellite images to look for air masses and fronts. Doppler radars are used to determine the location and strength of precipitation, as well as wind speed and direction.
•Radiosondes are transmitters placed in weather balloons to send measurements of temperature and humidity to receivers in weather stations. They can measure wind speed and direction as well.
•Hygrometers, like the hair hygrometer and sling psychrometer, measure relative humidity. Psychrometers have two thermometers used to compare the difference between a dry bulb's temperature and a wet bulb's. Drier air produces a greater temperature difference.
•Basic weather measuring tools are:
◦barometers for air pressure,
◦anemometers for wind speed,
◦wind vanes for wind direction, and
◦thermometers for air temperature.