Key Concepts 1 Weather is the atmospheric conditions, along with short-term changes, of a certain place at a certain time. Variables used to describe weather are air temperature, air pressure, wind, humidity, and relative humidity. The processes in the water cycle—evaporation, condensation, and precipitation—are all involved in the formation of different types of weather. Lesson 1: Describing Weather

What Is Weather Weather: the atmospheric conditions, along with short- term changes, of a certain place at a certain time. Temporary behavior of the atmosphere (what’s going on at any certain time) Small geographic area Can change rapidly Meteorologist: is a scientist who study and predict weather.

Weather Variables: Air Temperature Air Temperature: the measure of average kinetic energy of molecules in the air Temperature is high – molecules have a high kinetic energy Molecules in warm air move faster than molecules in cold air Varys with time of day, season, location and altitude

Weather Variables: Air Pressure Air Pressure: the force that column of air applies on the air or a surface below Air pressure decreases with altitude Barometric pressure Measured using a barometer Unit is millibars (mb)

Weather Variables: Wind Wind: air moving from areas of high pressure to low pressure Wind is noted with direction and speed Weather vane used to denote direction direction = where it is blowing from Anemometer is used to measure wind speed

Weather Variables: Humidity Humidity: the amount of water vapor in the air Measured in (g/m³) Higher humidity = more water vapor in air Humidity is what makes your skin feel sticky.

Weather Variables: Relative Humidity Relative Humidity: the amount of water vapor in the air compared to the maximum amount of water vapor the air could hold at that temperature When air is saturated, it holds the maximum amount of water vapor possible at that temperature Measured using an psychrometer Recorded as a percent

Weather Variables: Dew Point Dew Point: the temperature at which air is saturated and condensation can occur When air near the ground is saturated, water vapor condenses into liquid Temperature is above 0° C dew forms Temperature is below 0° C ice crystals or frost forms

Precipitation Rain: precipitation as droplets of water Snow: precipitation as solid, frozen crystals of water. Sleet: precipitation as melted snow that refreezes or freezing rain Hail: precipitation as large pellets of ice Precipitation: is water, in liquid or solid form, that falls from the atmosphere.

Key Concepts 2 Lesson 2: Weather Patterns Low-pressure systems and high-pressure systems are two systems that influence weather. Weather patterns are driven by the movement of air masses. Understanding weather patterns helps make weather forecasts more accurate. Severe weather includes thunderstorms, tornadoes, hurricanes, and blizzards.

Pressure Systems: High Pressure System High-Pressure System: a large body of circulating air with high pressure at its center and lower pressure outside of the system Air inside the system moves away from center Dense, high pressure air sinks at center and moves towards low pressure areas Brings clear skies and fair weather H

Pressure Systems: Low Pressure System Low-Pressure System: a large body of circulating air with low pressure at its center and higher pressure outside of the system Air inside the system rises Rising air cools and water vapor condenses Forms clouds and sometimes rain or snow Brings stormy weather and strong winds L

Air Masses: Classification Classified by: Temperature and Moisture Moisture Classification: Air Masses over land = CONTINENTAL Air Masses over water = MARITIME Temperature Classification: Air Masses in equatorial regions = TROPCIAL Air Masses in colder regions = POLAR Air Masses near poles (coldest regions) = ARTIC and ANTARCTIC DRY MOIST WARM VERY COLD COLD

Fronts Front: a boundary between two air masses Drastic weather changes occur Changes in temperature, humidity, clouds, winds and precipitation common

Fronts: Cold Front Forms when: Cold air mass moves toward warmer air mass Brings: severe storms and cooler temperatures

Fronts: Warm Fronts Forms when: Warm (less dense) air moves toward colder denser air Brings: Warmer temperatures and shifting winds Precipitation often occurs

Fronts: Stationary Front Forms when: Boundary between two air masses stalls Brings: Cloudy skies and light rain

Fronts: Occluded Front Forms when: Fast-moving cold front overtakes a slow-moving warm front Brings: Precipitation

Key Concepts 3 Thermometers, barometers, anemometers, radiosondes, satellites, and Doppler radar are used to measure variables. Computer models use complex mathematical formulas to predict temperature, wind, cloud formation, and precipitation. Lesson 3: Weather Forecasts