Weather Unit 6: Humidity and Clouds

Unit 6 - Weather, Part 2

• This unit covers relative humidity, clouds, air masses, fronts, and severe weather.

Atmospheric Moisture

• Water vapor is gaseous water in the atmosphere.
• Water enters the atmosphere through evaporation (liquid to gas) and transpiration (plants releasing water vapor).

Net Evaporation Rate Factors

• Insolation: Higher insolation provides more energy for evaporation.
• Surface Area: Increased surface area leads to a greater evaporation rate.
• Humidity: Drier air results in faster evaporation.
• Wind Speed: Increased wind speed accelerates evaporation

Humidity

• Relative humidity expresses how "full" the air is with water, expressed as a percentage.
• Hotter air can hold more water vapor than colder air (due to expansion).
• If the air temperature changes but the water vapor amount stays the same, the relative humidity changes.

Temperature vs. Relative Humidity

• As air temperature increases, its ability to hold water vapor increases.
• If the air temperature of a parcel increases, and the amount of water vapor stays the same, the relative humidity decreases.

Dew Point

• Dew point is the air temperature when the air is full to capacity with water vapor (100% relative humidity).
• At 100% relative humidity, the air is saturated.
• When the air is saturated, some water vapor begins to condense (gas to liquid).
• Relative humidity increases as the air temperature gets closer to the dew point temperature, and the chance of precipitation also increases.

Measuring Moisture - Sling Psychrometer

• Sling psychrometers measure relative humidity and dew point using wet-bulb and dry-bulb thermometer readings.
• The wet-bulb temperature is always less than or equal to the dry-bulb temperature.

Determining Relative Humidity and Dew Point

• Use the reference tables (ESRT) provided to determine relative humidity and dew point.

Cloud Formation (R.E.C.C.)

• As air heats at the surface, it rises and expands.
• As air cools it contracts and reaches its dew point temperature.
• Water begins to condense or change from a gas to a liquid.
• Water droplets form on condensation nuclei (dust particles or aerosols in the air).

Precipitation

• Precipitation is the falling of liquid or solid water from clouds towards Earth's surface.
• The solid/liquid particles collide until they are heavy enough to fall under the influence of gravity.
• Types of precipitation include rain, drizzle, snow, sleet, freezing rain, and hail.

Measuring Precipitation

• A rain gauge measures liquid precipitation in depth (usually in inches or centimeters).

Air Masses

• Air masses are large bodies of air in the troposphere with similar characteristics throughout (pressure, moisture, and temperature).
• Air masses form when air remains stationary over an area, adopting the characteristics of its source region.

Characteristics of Air Masses

• Moisture Level: Land/Continental areas form dry air masses; Ocean/Maritime regions form wet air masses.
• Temperature: Polar regions create cold air masses; Tropical regions create warm air masses.

Air Mass Symbols

• Reference tables (Page 13) provide symbols for cA (continental arctic), cP (continental polar), cT (continental tropical), mP (maritime polar), and mT (maritime tropical) air masses.

Jet Streams & Planetary Winds

• Jet streams and planetary winds move air masses around the globe, influencing storm paths across the U.S.

Weather Fronts

• Weather fronts are boundaries between different air masses with contrasting densities.
• Four types of fronts include cold fronts, warm fronts, occluded fronts, and stationary fronts.
• Cold front: Boundary where a cold air mass moves into a warm air mass. Rapidly pushes warm air aloft; thunderstorms are common.
• Warm front: Boundary where a warm air mass moves into a cool air mass; warm air rises over cool air, leading to widespread rain.
• Occluded front: Warm air gets trapped between two cold fronts.
• Stationary front: Air masses push against each other but neither moves.

Precipitation & Fronts

• Cold fronts bring precipitation at the frontal boundary.
• Warm fronts often bring precipitation well ahead of the front.

Severe/Extreme Weather

• Storms are violent or severe atmosphere disturbances causing dangerous or destructive conditions on Earth's surface.
• Examples include hurricanes, thunderstorms, tornadoes, hail, and blizzards.

Mid-Latitude Cyclone Storms

• Severe weather events with precipitation are low-pressure storm systems, where winds move counterclockwise and inward. Air rises from Earth's surface into the atmosphere, following R.E.C.C. (rising, expanding, cooling, and condensing).

Hurricanes

• Hurricanes are strong storms of tropical origin with sustained winds exceeding 74 mph.
• For a hurricane to form, four conditions are needed: humid air, warm ocean water, a weak jet stream, and extremely low pressure.
• Hurricanes gain energy from warm tropical ocean waters, and they typically form in late summer or early fall because of temperature lag.
• Hurricanes commonly originate off the coast of West Africa, and are pushed westward by planetary winds, then northeastward by southwesterlies.
• Hurricanes lose strength when they travel over land or cool water.

Saffir-Simpson Scale

• The Saffir-Simpson Scale categorizes hurricanes based on wind speed and resulting damage (1-5).

Preparing for a Hurricane

• Prepare an escape route.
• Tape/board up windows.
• Stock up on food, water, and candles.
• Move to higher ground.

Thunderstorms

• Thunderstorms are heavy rain storms accompanied by thunder and lightning.
• They form from rising warm air associated with fronts (and hurricanes, as part of their cycle), featuring cumulonimbus clouds (anvil clouds).
• Hazards include flooding, hail, high winds, and lightning.

Tornadoes

• Tornadoes are low-pressure funnel clouds that extend down towards the Earth's surface.
• Most tornadoes only last a few minutes.
• They are the most violent of storms.
• Tornado Alley is the most prone area in the U.S. for tornadoes, where mT air from the South meets cP air from the North.

Fujita Scale

• The Fujita Scale categorizes tornadoes based on wind speed and damage.

Blizzards

• Blizzards are intense winter storms with heavy snowfall, high wind speeds, low visibility, and below-freezing temperatures.
• To prepare: Stock up on supplies (food, water, batteries, blankets) and stay inside or off the road.