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 leads to more energy for evaporation.
• Surface area: Larger surface area exposed increases evaporation.
• Humidity: Drier air results in faster evaporation rates.
• Wind speed: Increased wind speed increases evaporation rate.

What is Humidity?

• Relative humidity represents how "full" the air is with water.
• Expressed as a percentage.

Temperature vs. Relative Humidity

• Warmer air can hold more water vapor than cooler air.
• If the air temperature changes, but the amount of water vapor remains constant, the relative humidity changes. A rise in temperature decreases relative humidity.

The Dew Point

• Dew point is the air temperature at which the air is entirely saturated (100% relative humidity).
• When this happens, water vapor in the air begins to condense (gas to liquid). This often starts formation of cloud water droplets or dew.

Dewpoint and Precipitation

• Relative humidity increases as air temperature approaches the dew point.
• The chance of precipitation increases as air temperature approaches the dew point.

Measuring Moisture

• Sling psychrometer is used to measure relative humidity and dew point.
• Wet-bulb and dry-bulb thermometer readings are used.
• Wet-bulb temperature is always less than or equal to the dry-bulb temperature.

To Determine Relative Humidity and Dew Point

• Use provided charts on a reference table (page 12 of ESRT). Follow steps to find values.

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

• Rises and Expands: Rising air heats, expands, and cools.
• Cools: Rising air reaches dew point temperature, contracts and cools further
• Condensates: Water vapor changes from gas to liquid.
• Nuclei: Water droplets form on condensation nuclei (dust/aerosols).

Precipitation

• Precipitation is the liquid or solid water that falls from clouds to the Earth's surface.
• Different types of precipitation include rain, drizzle, snow, sleet, freezing rain, and hail

Measuring Precipitation

• Rain gauges measure liquid precipitation.
• Precipitation is measured in depth, in inches or centimeters.

Air Masses

• Large bodies of air with similar characteristics throughout (pressure, moisture, temperature).
• Characteristics of air masses determined by the source region (land or water; high or low latitude).

Characteristics of Air Masses

• Moisture Level:
  • Continental regions - dry
  • Maritime regions – humid/wet
• Temperature:
  • Polar / arctic regions - cold
  • Tropical/low latitudes - warm

Air Mass Symbols

• Use reference table (page 13 of ESRT).

4 Major Air Masses Affecting the U.S.

• Continental Polar
• Maritime Polar
• Maritime Tropical
• Continental Tropical

Jet Streams & Planetary Winds

• These forces help move air masses around the globe impacting storm tracks.

Weather Fronts

• Boundary between two air masses with differing densities (warm air rises, cold sinks).
• Types of fronts include: Cold, Warm, Occluded, Stationary.

Cold Front

• Cold air mass moves into a region of warmer air.
• Rapidly pushes up warm air, causing it to cool, condense, and often leads to thunderstorms.

Warm Front

• Warm air mass moves into a region of cooler air.
• Warmer air rises over cooler air which cools, condenses and leads to rain/snow.

Occluded Front

• A cold front overtakes a warm front.
• Warmer air is trapped between two cooler air masses.

Stationary Front

• Neither air mass is moving.
• Air masses push against each other, but neither advances.

Severe/Extreme Weather

• Storm is a violent or severe disturbance of the atmosphere that creates dangerous, destructive, or unpleasant conditions.
  • Examples include: hurricanes, thunderstorms, tornadoes, hail, and blizzards

Mid-Latitude Cyclone Storms

• Severe weather events that occur with low pressure.
• Winds move counterclockwise inward toward low pressure.

Hurricanes

• Strong tropical storms with sustained winds exceeding 74 mph.
• Form over warm tropical ocean waters.

Ingredients for Hurricanes

• Humid air
• Warm ocean water (above 80°F)
• A weak jet stream
• Extremely low pressure

Hurricane Formation

• Low-pressure centers form over warm tropical waters, providing energy.
• Hurricanes develop in late summer or early fall due to temperature lag.

Hurricane Paths

• Often form off the coast of western Africa.
• Pushed west by planetary wind; then northeast by the southwesterlies.

Hurricane Weakening

• Loss of strength over land or cool water.

Saffir-Simpson Scale

• Classifies hurricanes based on wind speeds.
• Categories range from 1 to 5, with Category 5 having the highest wind speeds and potential for catastrophic damage.

Hurricane Preparedness

• Develop an escape route and appropriate preparedness measures.

Thunderstorms

• Heavy rain storms consisting of thunder and lightning.
• Form due to rising warm air and fronts, often part of hurricanes.
• Hazards include flooding, hail, etc.

Tornadoes

• Low pressure funnels that descend from the base of thunderstorms.
• Last only a few minutes.
• The most violent storms.

Tornado Alley

• Area in the U.S. experiencing most tornadoes.
• Where mT air from South meets cP air from North.

Blizzards

• Intense winter storms with snow, high winds, and low visibility.
• Hazards include dangerous travel conditions.