Chapter 6, Lesson 1 Notes
*Clouds form when water vapor in the air condenses to form liquid water or ice crystals.
*Condensation occurs when water vapor becomes liquid water.
*Two conditions are required for condensation: cooling of air, and the presence of particles in the air.
*The temperature at which condensation begins is called the dew point.
*Before water vapor can condense and form clouds, it must have a surface on which to condense. These surfaces are small particles of dust, smoke, and salt crystals.
*Liquid water that condenses from the air onto a cooler surface is called dew. Ice deposited on a surface that is below freezing is called frost.
*Scientists classify clouds into three main types based on their shape: cirrus, cumulus, and stratus.
*Each type of cloud is associated with a different type of weather.
*Cirrus clouds are wispy and feathery. They form at high altitudes, usually above 6km, and at low temperatures. Cirrus clouds are made of ice crystals and indicate fair weather.
*Cumulus clouds look like cotton. They form less than 2km above the ground, but they may extend upward as much as 18km. Short cumulus clouds usually indicate fair weather, but towering clouds with flat tops often produce thunderstorms. They are then called cumulonimbus clouds. (The suffix –nimbus means “rain”)
*Stratus clouds form in low, flat layers covering most or all of the sky. They are dull gray and may produce drizzle, rain, or snow. They are then called nimbostratus clouds.
*Clouds are further classified by their altitude. Clouds that form between 2 and 6 km above Earth’s surface have the prefix alto-, which means “high”.
*Altocumulus and altostratus clouds are higher than regular cumulus and stratus clouds, but lower than cirrus clouds. Altocumulus and altostratus clouds indicate precipitation.
*Clouds that form near the ground are called fog.
Chapter 6, Lesson 2 Notes
*Any form of water that falls from clouds and reaches Earth’s surface is precipitation.
*Common types of precipitation include rain, sleet, freezing rain, snow, and hail. Rain is the most common.
*Drops of water are called rain if they are at least 0.5 millimeters in diameter. Smaller drops of water are called drizzle, and even smaller ones are called mist.
*An open-ended can or tube that collects rainfall is called a rain gauge.
*There are four types of freezing precipitation: freezing rain, snow, sleet, and hail.
*Freezing rain is rain that freezes when it hits a cold surface.
*When clouds are colder, water vapor can convert directly into ice crystals, forming snow.
*When raindrops fall through a layer of air colder than 0 degrees Celsius, they can freeze into ice particles.
*Ice particles smaller than 5 millimeters in diameter are called sleet.
*Round pellets of ice larger than 5mm in diameter are called hailstones. Hail forms only inside cumulonimbus clouds during thunderstorms. It forms when strong updrafts repeatedly carry ice pellets through cold regions of a cloud, adding another layer of ice each time.
Chapter 6, Lesson 3 Notes
*An air mass is a huge body of air in the lower atmosphere that has a similar temperature, humidity, and air pressure at any given height.
*Four major types of air masses influence the weather in North America: maritime tropical, continental tropical, maritime polar, and continental polar.
*Maritime air masses form over the ocean and can be very humid.
*Continental air masses form over land, and are drier than maritime air masses.
*Tropical air masses are warm, form in the tropics, and have low air pressure.
*Polar air masses are cold, form near the poles, and have high air pressure.
*In North America, most air masses move from west to east.
*The jet stream is a band of high-speed wind about 10 kilometers above the surface of Earth that pushes air masses along.
*Fronts occur along the boundaries between air masses. Changeable weather develops along fronts.
*Colliding air masses can form four types of fronts: cold fronts, warm fronts, stationary fronts, and occluded fronts.
*When a faster cold air mass runs into a slower warm air mass, a cold front forms. The cold air slides under the warm air. As the warm air rises, it cools and condenses, often resulting in heavy rain or snow.
*When a faster warm air mass runs into a slower cold air mass, a warm front forms. The warm air slides up over the cold air, possibly causing light rain or snow.
*When a cold air mass and warm air mass collide, but neither displaces the other, a stationary front occurs. Water vapor in the warm air condenses into rain, snow, fog, or clouds, lingering for days.
*When a warm air mass is caught between two cooler air masses, the warm air is pushed up and an occluded front forms. (The warm air mass is cut off, or occluded, from the ground.)
*Temperatures at the ground get cooler, and it may get cloudy and rain or snow.
*A swirling center of low-pressure air is called a cyclone. In the Northern Hemisphere, cyclones spin counterclockwise when viewed from above.
*Cyclones and decreasing air pressure are associated with clouds, wind, and precipitation.
*An anticyclone is the opposite of a cyclone. The descending air in an anticyclone generally causes dry clear weather.
Chapter 6, Lesson 4 Notes
*A storm is a violent disturbance in the atmosphere. Storms involve sudden changes in air pressure, which cause rapid air movements.
*Winter storms, thunderstorms, hurricanes, and tornadoes are all types of severe storms. Winter storms involve snow.
*All year round, most precipitation begins in clouds as snow. If the air is colder than 0 degrees Celsius all the way to the ground, the precipitation falls as snow.
*A thunderstorm is a small storm often accompanied by heavy precipitation and frequent thunder and lightning.
*Lightning is a sudden spark, or electrical discharge, that jumps between parts of a cloud, between nearby clouds, or between a cloud and the ground.
*Thunderstorms form in large cumulonimbus clouds, also known as thunderheads.
*A hurricane is a tropical cyclone with winds of 199km/h or higher.
*A hurricane begins over warm ocean water as a low-pressure area, or tropical disturbance.
*The low pressure and winds of a hurricane can cause storm surge, a “dome” of water that sweeps across the coast where the hurricane lands.
*A tornado is a rapidly whirling, funnel-shaped cloud that reaches down from a thunderstorm to touch Earth’s surface.
*Tornadoes most commonly develop in thick cumulonimbus clouds-the same clouds that bring thunderstorms.
Chapter 6, Lesson 5 Notes
*The first step in forecasting is to collect data. This can be done either through direct observations, such as recognizing that cumulonimbus clouds may produce a thunderstorm, or through the use of instruments such as a barometer.
*Meteorologists are scientists who study and try to predict the weather.
*Meteorologists use maps, charts, computers, and other technology to analyze weather data and to prepare weather forecasts.
*Weather reporters get their information from the National Weather Service, which uses weather balloons, satellites, radar, and surface instruments to gather data.
*Weather balloons carry instruments into the troposphere and the lower stratosphere to measure temperature, air pressure, and humidity.
*Satellites in the exosphere, the uppermost layer of the atmosphere, collect data on temperature, humidity, solar radiation, and wind speed and direction. They also include cameras that can make images of clouds, storms, and snow cover. Automated weather stations in 1,700 surface locations gather data on temperature, air pressure, relative humidity, rainfall, and wind speed and direction. Computers help process all of this weather data quickly to help forecasters make predictions. Currently, forecasts are fairly accurate up to five days in the future.
Weather Maps
The National Weather Service maintains weather maps that are snapshots of conditions at a particular time over a large area. Some show curved lines that connect places where certain conditions are the same. Isobars (-bar as in barometer) are lines joining places on the map that have the same air pressure. Isotherms are lines joining places that have the same temperature. Standard symbols on weather maps show fronts, areas of high and low pressure, types of precipitation, and temperatures.