Chapter 13 Weather

High-pressure system

A large body of circulating air with high pressure at its center and lower pressure outside the system. In this system, air moves away from the center. It brings clear skies and fair weather because the dense air sinks.

Low-pressure system

A large body of circulating air with low pressure at its center and higher pressure outside of the system. Causes the air inside this system to rise. The rising air cools and the water condenses, creating clouds and causing precipitation.

Meteorologist

A scientist who studies weather by using specialized instruments to measure conditions in the atmosphere. These include thermometer to measure temperature, barometers to measure the air pressure, psychrometers to measure relative humidity, and anemometers to measure wind speed.

Doppler radar

A specialized type of radar that can detect precipitation as well as the movement of small particles, which can be used to approximate wind speed. That can be important during severe weather like tornadoes or thunderstorms.

Blizzard

A violent winter storm characterized by freezing temperatures, strong winds, and blowing snow. Can reduce visibility, or cause frostbite or hypothermia from the freezing temperatures.

Tornado

A violent, whirling column of air in contact with the ground. Sometimes have a diameter that exceeds 1,500 m. The winds inside can reach up to more than 400 km/h. They are strong enough to lift cars, trees, and sometimes houses flying. Usually only last a few minutes, but some can last for hours.

Hurricane

An intense tropical storm with winds exceeding 119 km/h. They are the most destructive storms on Earth. They have a circular shape with intense, swirling winds. Form over warm, tropical ocean water. Larger than a tornado. At the center, there is an eye with clear skies and light wind. Results from these storms can be flooding, strong winds, and high waves. As it moves over land or colder water, the storm eventually dissipates.

Front

Boundary between two air masses. There are four types of fronts: Cold, warm, stationary, and occluded. Drastic weather changes can occur here. Changes like temperature, humidity, cloud types, wind, and precipitation. At fronts, two air masses collide.

Surface Report

Describe a set of weather measurements made on Earth's surface. The variables used in the measurement are temperature, air pressure, humidity, precipitation, and wind speed and directions. Cloud amounts and visibility are measured by human observers.

Upper-air Report

Describes wind, temperature, and humidity conditions above Earth's surface. They are measured by a radiosonde- a package of weather instruments carried many kilometers above the ground by a weather balloon.

Computer Model

Detailed computer programs that solve a set of complex mathematical formulas. They predict temperatures and winds that might occur, when and where it will rain and snow, and what types of clouds will form. They can created weather maps and forecasts.

Stratus Cloud

Flat, white, low lying clouds that bring wet conditions and are uniform in color. Their altitude is up to 2,000 meters. When these clouds are closer to Earth's surface, they reduce visibility and become known as fog. They have horizontal layering.

Cumulus Cloud

Fluffy, heaped, or piled up clouds that bring fair weather and sometimes rain. They have vertical growth and are up to 2,00 to 6,000 m in altitude. From Earth's surface, they look like fluffy cotton balls. The tops are rounded towers.

Stationary Front

Forms when the boundary between two air masses stalls. One side is cold air and the other side is warm air. Both air masses are directed toward the middle. They bring cloudy skies and light rain.

Rain Gauge

Instrument that measures how much rain has fallen.

Psychrometer

Instrument used to measure relative humidity as a percent. Two thermometers are used to measure the moisture content, one a wet bulb and one a dry bulb.

Air mass

Large bodies of air that have uniform temperature, humidity, and pressure. Forms when a large high pressure system lingers over an area for several days. The air in the system takes on the temperature and moisture characteristics of the surface below it.

Isobar

Lines that connect all places on the map where pressure has the same value. Show the location of high and low pressure systems, and provide information about wind speeds. Winds are strong when isobars are together.

Isotherm

Lines that connection places with the same temperature. They show which areas are warm and which are cold.

Lake effect snow

Occurs when a cooler air masses goes over a warm lake. The air above the lake then becomes warm and rises and condenses. When cloud formation occurs, wind blows the clouds over to land and precipitates down onto Earth's surface.

Cumulonimbus Cloud

Tall clouds that are high enough for airplanes to fly through them. They are known as thunderstorm clouds and bring heavy rain, snow, hail, lightning and sometimes tornadoes. They can reach up to 10 km in altitude. Formed from water vapor carried from upward air currents.

Relative Humitidy

The amount of water vapor in the air compared to the maximum amount of water vapor the air could contain at that temperature. Warm air can contain more water vapor than cold air. Air can contain a certain amount of water vapor depending on the temperature, but when it becomes too saturated it can't absorb any more.

Humitidy

The amount of water vapor in the air. Can be measured in grams of water per cubic meter of air. High humitidy means more water vapor in the air. This prevents the sweat from evaporating from skin as quickly.

Weather

The atmospheric conditions, along with short-term changes, of a certain place at a certain time. Weather can be a rainstorm, a sunny day, or a blizzard. Can change in just a few hours, or can stay the same for several days in a row. Temperature and rainfall are the two variables used to describe weather.

Air Pressure

The force that a column of air applies on the air or a surface below it. Air pressure decreases as altitude increases. Air pressure is greatest at low altitudes. Measured by barometric pressure, or a barometer. it can help meteorologists predict the weather.

Water Cycle

The series of natural processes by which water continually moves among oceans, land, and the atmosphere. Evaporated water droplets from oceans condense into clouds. Clouds produce precipitation, which is a form of weather, and the process repeats.

Dew Point

The temperature in which air is saturated and condensation can occur. When temperature decreases, the air can hold less water. At higher temperatures, clouds can form, and at lower temperatures, frost can form.

Continental Polar Air Masses

These air masses are fast moving and bring cold temperatures in winter and cool weather in summer. They can be found over Canada. The air is cold and dry.

Continental Tropical Air Masses

These air masses form in the tropics over dry, desert land and are hot and dry. Weather associated with them is clear skies and high temperatures. They usually form during the summer.

Arctic Air Masses

These air masses form over Siberia and the Arctic. They contain bitterly cold, dry air. Can bring temperatures down to -40 degrees C

Maritime Polar Air Masses

These air masses form over the northern Atlantic and Pacific Oceans, and are cold and humid. They bring cloudy, rainy, dull weather.

Maritime Tropical Air Masses

These air masses form over the western Atlantic Ocean, the Gulf of Mexico, and the eastern Pacific Ocean. They are moist and bring hot, humid air to the southeastern United states in summer. They also bring heavy snowfall in winter.

Cold Front

These fronts form when a colder air mass moves toward a warmer air mass. The cold air pushes underneath the warm air mass, and the warm air rises and cools. Brings showers and thunderstorms, and temperature can decrease as much as 10 degrees C. Wind also becomes gusty.

Occluded Front

These fronts form when a fast moving cold air form catches up with a slow moving warm air form. The cold air goes below and the warm air rises above. These fronts usually bring precipitation.

Warm Front

These fronts form when less dense, warmer air moves toward cold, denser air. The warm air rises and glides above the cold air mass. When the warm air condenses, it creates a wind blanket of clouds. Bring rain or snow for several hours or days. Also brings warmer temperatures and causes the wind to change direction.

Precipitation

Water, in liquid or solid form, that falls from the atmosphere. Examples are rain, snow, sleet or hail. Occurs when cloud droplets combine and become large enough to fall back to Earth's surface. They form around solid particles in the atmosphere like salt, dust, or smoke.

Cirrus Cloud

Wispy, upper atmosphere clouds that bring fair weather and are an indication of a warm front on the way. They can reach up to 6,000 meters in the atmosphere. They are the most common type of cloud, and are made up of thin ice crystals. These clouds build up larger over time.