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GPH110-5-AtmosphericWaterandWeather-Spring2015.ppt
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Atmospheric Water and Weather

Chapter 5

Conditions in the Atmosphere

  • Weather: short-term day-to-day conditions
  • Climate: long-term average of conditions
  • Meteorology: scientific study of the atmosphere
  • As water changes from one state of matter to another, the heat energy absorbed or liberated drives circulation of atmosphere and weather patterns

Ice

  • Water reaches greatest density at 39°F
  • Water expands up to 9% in temps from 39°F to -20°F
  • Cracking pavement and water pipes
  • Ice is less dense than water (it floats)

  • Calorie: Amount of energy required to raise the temp of 1g of water 1°C
  • Latent heat: the energy that is either absorbed or released in phase change of water
  • Ice  liquid  vapor

Evaporation and Condensation

  • As water evaporates, it absorbs latent heat and cools the air around it
  • We feel this cooling as water evaporates from our skin
  • When vapor condenses back into water the latent heat is released
  • Humidity: Amount of water vapor in the air
  • Relative humidity: The percentage of the air at a given temperature that is filled with water vapor
  • Warmer air can hold more water than colder air
  • Air is saturated at 100% relative humidity

Relative humidity = (Actual water vapor in the air / maximum water vapor possible at that temperature) * 100

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  • Dew-point temperature: The temperature at which air becomes saturated
  • As temp rises throughout the day, RH falls
  • RH typically highest at dawn

Daily Relative Humidity Patterns

Expressing Humidity

  • Vapor pressure: portion of air pressure (mb) that coming from water vapor molecules
  • Saturation vapor pressure: is max capacity at a temp

  • Air at 68°F has a SVP of 24mb (RH would be 100%)
  • If water vapor is only exerting 12mb of pressure, RH is 50%

Expressing Humidity

  • Specific humidity: mass if water vapor (grams) per kg of air at any specified temp
  • Maximum specific humidity: max amount of water vapor a kg of air can hold at a specified temp
  • Useful measure because it remains the same as temp and pressure changes
  • Information used in weather forecasting

Measuring Humidity – Hair Hygrometer

Measuring Humidity – Sling Psychrometer

  • The bigger the difference in the dry-bulb and wet-bulb temperatures, the lower the humidity

Atmospheric Stability

  • Parcels of air will rise or fall to areas of equal density/temp
  • Example: hot air balloons
  • Air cools/expands as it rises, warms/compresses as it descends
  • This is called the Adiabatic process
  • Air parcels are stable if they stay in place
  • Parcels are called unstable as they rise

Dry Adiabatic Rate (DAR)

  • The lapse rate at which dry (less than saturated) air cools/heats adiabatically
  • 5.5°F / 1000ft
  • 10°C / 1000m

Moist Adiabatic Rate (MAR)

  • The lapse rate at which moist (saturated) air cools/heats adiabatically
  • The average rate is:
  • 3.3°F / 1000ft
  • 6°C / 1000m
  • Can range from 2°F to 5.5°F depending on temp and moisture content
  • Lower than DAR because as water condenses, latent heat becomes sensible heat
  • Relationship between the DAR, MAR, and ELR (Environmental lapse rate) make parcels unstable, conditionally unstable, or stable

Clouds and Fog

  • Clouds are groupings of moisture droplets and ice crystals
  • Fog is a cloud on the ground
  • It takes 1M+ moisture droplets to make 1 raindrop
  • As air rises/cools it condenses on condensation nuclei (particles) in the atmosphere

Classifying Clouds and Understanding Their Names

  • Clouds care classified by:
  • Altitude
  • Shape
  • How they develop and form
  • Stratus – “layered”
  • Cumulus – “heap”
  • Nimbus – “rainy/stormy”
  • Alto – “high”
  • Cirrus – “curl / hair”

Cumulonimbus

  • “Rainstorm”
  • “Thundercloud”

Advection Fog

  • Surface air migrates from one place to another and is cooled to the dewpoint

  • Evaporation fog:
  • Forms over water
  • Steam fog or “sea smoke” develops as water evaporates in to the cooler air

Fog: a cloud layer on the ground with less than 1km (3300ft) visibility

Advection Fog in Mountainous Areas

Radiation Fog

  • When radiative cooling of the surface chills the air layer directly above the surface to dew-point
  • Occurs over moist ground (not water) on clear nights
  • This is not an advection fog because it doesn’t involve the migration of air

Air Masses

  • Air masses carry the characteristics of their source regions and influence our weather patterns
  • Moisture:
  • m = maritime (wet)
  • c = continental (dry)
  • Temperature:
  • A = arctic
  • P = polar
  • T = tropical
  • E = equatorial
  • AA = Antarctic

Air Mass Modification

  • As air masses migrate they slowly take on the characteristics of the land they pass over
  • Temp and humidity will change
  • Example: Lake Effect Snow

Convergent Lifting

  • Air converges from different directions toward an area of low pressure
  • Example: ITCZ

Convectional Lifting

  • Air is heated over warm land and rises through convection
  • Example: Florida afternoon thunderstorms

Average number of thunderstorm days per year

Orographic Lifting

  • Air is forced over a barrier such as a mountain range
  • Precipitation on the windward side ; Chinook winds and rainshadow on the leeward side

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Frontal Lifting – Cold Fronts

  • Front: The leading edge of an air mass
  • Cold fronts advance faster than warm fronts
  • Can cause violent weather conditions: thunderstorms, cumulonimbus clouds, etc.

Frontal Lifting – Warm Fronts

  • Warm fronts advance more slowly
  • Create stratus clouds and drizzly rain showers

Midlatitude Wave Cyclone
Stage 1: Cyclogenesis

  • Cold and warm air masses converge around a low pressure center
  • Air begins to flow inward and counterclockwise (Northern hemisphere) around the low

Midlatitude Wave Cyclone
Stage 2: Open Stage

  • Counterclockwise spin draws in colder air masses from the north and west and pulls in the warm air mass from the south

Midlatitude Wave Cyclone
Stage 3: Occluded Stage

  • Faster moving colder air overtakes the warm front and lifts the warm air mass up
  • Cold fronts average a speed of 25mph
  • Warm fronts average a speed of 10-15mph

Midlatitude Wave Cyclone
Stage 4: Dissolving Stage

  • Lifting – the source of energy and moisture – is complete
  • Warmer, lighter air is layered over the cooler, denser air

Weather Forecasting

  • Data needed to prepare a weather forecast:
  • Barometric pressure and tendency (rising/falling)
  • Surface temperature
  • Dew-point
  • Wind speed, direction
  • Type and movement of clouds
  • Current weather
  • Sky conditions
  • Visibility
  • Precipitation since last observation
  • Meteorologists also use a variety of models, charts, and knowledge of the local landscape
  • Doppler radar is used to detect the movement, speed, and direction of moisture droplets. Also important for issuing severe weather watches and warnings

Thunderstorms

  • May develop within an air mass, in a line along a cold front, or on the windward slopes of mountains
  • Tremendous upward movements of air cause violent updrafts and downdrafts

How Thunderstorms Form

How Lightning Forms

How far away is lightning from you?

Thunderstorm Frequency

Lightning Strikes

Derechos

  • Straight-line winds of 58+mph associated with rapidly moving thunderstorms
  • Downward gusts of wind
  • Most common in the Midwest and Great Lakes in May-July
  • Sometimes called “inland hurricanes” because of the damage they cause

What is a Derecho?

Tornadoes

  • Formation is not totally understood
  • One idea is, a spinning column of air rises and forms a mesocyclone
  • Rotates up into a thunderstorm cloud
  • A change in wind direction (spinning) on radar is a sign for a meteorologist

Tornadoes tend to form in relatively flat areas where different air mass types come in contact

Scale for Rating Tornadoes

Tropical Cyclones

  • Powerful storms that originate in the Tropics
  • Intense low pressure systems with warm oceans as the energy/fuel
  • Full-fledged tropical cyclones are called “Hurricanes” or “Typhoons”

Classifying Tropical Cyclones

Tropical Cyclone Paths

Scale for Rating Hurricanes

Structure of a Hurricane

  • Eye: Center of the storm but is completely calm
  • Eyewall: most intense precip in this area
  • Strongest winds typically come in the storms right-front quadrant (relatively to storm’s direction)

Hurricane Landing

  • Hurricanes usually die out relatively quickly over land since they loose their warm water energy source
  • Very costly for coastal cities
  • Storm surges and subsequent floods cause the most fatalities and damage
  • Bangladesh:
  • 1970 cyclone killed about 300,000 people
  • 1991 about 200,000 people killed

  • Central America:
  • Hurricane Mitch (1998) killed 12,000
  • United States:
  • Galveston, TX (1900) 6,000 deaths
  • Hurricane Katrina (2005) 1,830 deaths along the Gulf Coast of Louisiana, Mississippi, and Alabama
  • Hurricane Andrew (1992) left 200,000 people homeless in south Florida

The Toll of Hurricanes / Cyclones