Atmospheric Humidity and Lifting Mechanisms

Humidity

• Humidity is a measure of the amount of water vapor in the air
• Relative Humidity (RH) is the percentage of water vapor in the air compared to the maximum amount of water vapor the air can hold at a given temperature
• Specific Humidity (SH) is the mass of water vapor per unit mass of air

Spatial Patterns of Humidity

• Latitude: humidity decreases as latitude increases
• Land vs. Water: humidity is higher over water than over land

Air Temperature and Humidity

• As air temperature increases, RH decreases
• As air temperature decreases, RH increases

Dew Point

• Dew point is the temperature at which the air becomes saturated with water vapor and dew or frost begins to form
• Dew point is a measure of the absolute humidity of the air

Daily Pattern of RH and TPW

• TPW (Total Precipitable Water) is the total amount of water vapor in the atmosphere
• Daily pattern of RH: RH is highest in the morning and lowest in the afternoon

Fog

• Fog is a type of cloud that forms at ground level
• Types of fog: radiation fog, advection fog, steam fog, and upslope fog

Lifting Mechanisms

• There are four lifting mechanisms: orographic, frontal, convergence, and updraft
• Each lifting mechanism leads to the formation of clouds and precipitation

Clouds

• Clouds are collections of water droplets or ice crystals suspended in the air
• Types of clouds: cumulus, stratus, cirrus, and nimbus
• Clouds are associated with different types of precipitation: cumulus clouds are associated with light precipitation, stratus clouds are associated with light to moderate precipitation, and nimbus clouds are associated with heavy precipitation

Precipitation Forming Mechanisms

• Precipitation forms when water vapor in the air condenses and falls to the ground
• Precipitation forming mechanisms: collision-coalescence, accretion, and ice-crystal process

Hail Formation

• Hail forms when updrafts in thunderstorms carry water droplets up into the freezing level of the atmosphere, where they freeze into small balls of ice

Latent Heat and Phase Changes

• Latent heat is the energy released or absorbed when water changes phase from liquid to solid (freezing) or from liquid to gas (evaporation)
• Latent heat is released during condensation and freezing, and absorbed during evaporation and melting

Air Masses

• Air masses are large bodies of air that have similar temperature and humidity characteristics
• Air masses are classified into four types: maritime tropical, maritime polar, continental tropical, and continental polar
• Air masses influence the weather in different regions of the US

Fronts

• A front is the boundary between two air masses of different temperatures and humidity
• Types of fronts: cold front, warm front, stationary front, and occluded front
• Weather associated with each type of front: cold front (clouds, precipitation, and wind), warm front (clouds, precipitation, and wind), stationary front (clouds, precipitation, and little wind), and occluded front (clouds, precipitation, and wind)

Mid-Latitude Cyclones

• Mid-latitude cyclones are low-pressure systems that form in the mid-latitudes and are associated with clouds, precipitation, and wind
• Basic structure of a mid-latitude cyclone: center of low pressure, cold front, warm front, and occluded front
• Relationship between mid-latitude cyclones and the polar jet stream: mid-latitude cyclones form when the polar jet stream is strong and moves southward

Thunderstorms

• Thunderstorms are intense storms that form when there is a strong updraft of warm air
• Three stages of thunderstorms: cumulus stage, mature stage, and dissipating stage
• Characteristics of thunderstorms: updrafts, downdrafts, lightning, thunder, and heavy precipitation
• Relationship between air masses and thunderstorm occurrence: thunderstorms are more likely to occur when there is a strong temperature gradient between two air masses