Meteorology Lec 3 - Humidity and Condensation

Hydrologic Cycle

The continuous movement and transformation of water molecules between liquid, vapor, and solid states

Evaporation

Process where water molecules escape from liquid to vapor phase

Condensation

Process where water vapor molecules return to liquid phase

Saturation

State where the number of evaporating molecules equals the number condensing

Absolute Humidity

Mass of water vapor per volume of air (g/m³); represents vapor density

Specific Humidity

Ratio of mass of water vapor to total mass of air (g/kg)

Mixing Ratio

Ratio of mass of water vapor to mass of dry air (g/kg)

Vapor Pressure

Partial pressure exerted by water vapor molecules in the air

Actual Vapor Pressure

The vapor pressure currently present in the air

Saturation Vapor Pressure

The vapor pressure when air is saturated at a given temperature

Relative Humidity (RH)

Ratio of actual vapor pressure to saturation vapor pressure, expressed as a percentage

Dew Point Temperature

Temperature to which air must be cooled (at constant pressure) to reach saturation

Supersaturation

Condition where RH exceeds 100%

Psychrometer

Instrument with wet-bulb and dry-bulb thermometers to measure humidity

Wet-Bulb Temperature

Lowest temperature reached by evaporating water into air

Wet-Bulb Depression

Difference between dry-bulb and wet-bulb temperatures

Hygrometer

General term for instruments measuring humidity

Hair Hygrometer

Uses human/horse hair that lengthens with increasing RH

Electrical Hygrometer

Measures humidity via changes in electrical resistance

Infrared Hygrometer

Measures IR energy absorbed by water vapor

Dew-Point Hygrometer

Cools a mirror to detect condensation and measure dew point

Condensation Nuclei

Tiny particles (e.g., dust, salt) that serve as surfaces for condensation

Hygroscopic Nuclei

"Water-seeking" particles that attract water vapor (e.g., salt, acid)

Hydrophobic Nuclei

"Water-repelling" particles that resist condensation (e.g., oils, waxes)

Dry Haze

Suspended particles that scatter light without condensation; appears yellowish or bluish

Wet Haze

Particles with condensed water that scatter light efficiently; reduces visibility

Fog

Cloud at ground level with visibility <1 km; formed by cooling or evaporation/mixing

Radiation Fog

Forms due to radiational cooling on clear, calm nights; also called ground or valley fog

Advection Fog

Forms when warm, moist air moves over a cooler surface

Upslope Fog

Forms when moist air ascends and cools adiabatically over elevated terrain

Evaporation (Mixing) Fog

Forms by mixing two unsaturated air masses

Steam Fog

Type of evaporation fog when cold air moves over warm water

Precipitation/Frontal Fog

Forms when warm rain evaporates into cold, moist air near the surface

Heat Index

Combines temperature and RH to estimate apparent temperature and human discomfort

Boiling Point

Temperature where saturation vapor pressure equals atmospheric pressure

Dalton’s Law

Total air pressure equals sum of partial pressures of individual gases

Phase Change

Transformation between solid, liquid, and gas states of water

Transpiration

Release of water vapor from plants into the atmosphere

Interception

Capture of precipitation by vegetation before reaching the ground

Percolation

Movement of water through soil and rock layers

Groundwater

Water stored underground in aquifers

Runoff

Surface water flow into streams, rivers, and oceans

Spatial Humidity Variation

Specific humidity highest in tropics, decreasing toward poles

RH Diurnal Variation

RH highest at night (cooler temps), lowest in afternoon (warmer temps)

Human Discomfort

High RH reduces evaporative cooling, increasing perceived temperature

Heat Cramps

Muscle spasms due to loss of water and salt from sweating

Heat Exhaustion

Fatigue, headache, nausea from prolonged heat exposure

Heatstroke

Life-threatening condition with body temperature >41°C; circulatory failure

Density of Humid Air

Humid air is less dense than dry air due to lighter water vapor molecules

Valley Fog

Radiation fog that forms in low-lying areas due to cold air drainage

Clear Night Condition

Promotes rapid cooling and dew/fog formation due to lack of cloud cover

Calm Night Condition

Allows coldest air to remain at ground level, aiding condensation

Light Breeze

Enhances radiation fog by bringing more moist air into contact with cold surfaces

Strong Breeze

Prevents radiation fog by mixing moist surface air with drier air above

Coastal Advection Fog

Forms when warm ocean air moves over colder coastal waters

Example RH Calculation

For T=30°C, Td=15°C: es=42 mb, e=18 mb, RH=42.9%

Example Dew Point Calculation

For T=32°C, RH=60%: es=47 mb, e=28.2 mb, Td=23°C

Frozen Dew

Dew that freezes when surface temperature drops below 0°C

Cloud Droplet Formation

Billions of vapor molecules condensing onto nuclei to form tiny droplets

Pollution and Nuclei

Highly polluted air can contain over 1 million nuclei per cm³

Haze Color Appearance

Dry haze appears yellowish against light backgrounds, bluish against dark

Visibility Reduction

Wet haze and fog significantly reduce visibility, especially in mornings

Boiling at High Elevations

Water boils at lower temperatures due to reduced atmospheric pressure

Cooking at High Elevations

Requires longer time due to lower boiling point of water

Radiosonde

Instrument package including electrical hygrometer, used for atmospheric sounding

Carbon Film Sensor

Used in electrical hygrometers; resistance changes with humidity

Mirror Cooling Method

Dew-point hygrometer detects condensation on a chilled mirror surface

Nighttime Cooling

Rapid cooling on clear nights leads to dew and fog formation

Adiabatic Cooling

Cooling of air as it rises and expands, leading to cloud/fog formation

Evaporative Cooling

Cooling effect when water evaporates, used in wet-bulb thermometer

Salt Particles as Nuclei

Hygroscopic nuclei like salt allow condensation at RH <100%

Oil Particles as Nuclei

Hydrophobic nuclei like oil resist condensation even at RH >100%

Formation of Dew

Occurs when surfaces cool below dew point, causing vapor to condense

Formation of Frost

Occurs when surfaces cool below freezing point, depositing ice crystals

Heat and Evaporation

Warm water evaporates more readily than cool water

Wind and Evaporation

Wind enhances evaporation by removing saturated air near surface

Saturation and Temperature

Warmer air requires more vapor molecules to reach saturation

Ice vs. Water SVP

Saturation vapor pressure is lower over ice than over liquid water

Binding Effect of Solutes

Dissolved salts reduce evaporation by binding water molecules

Body Cooling Mechanism

Evaporation of sweat is primary cooling method in warm weather

Low RH Comfort

Low RH allows rapid sweat evaporation, feels cooler than actual temperature

High RH Discomfort

High RH hinders evaporation, feels warmer than actual temperature

Water Vapor Weight

Water vapor molecules are lighter than nitrogen or oxygen molecules

Lifting of Humid Air

Less dense humid air rises more easily, leading to cloud formation

Precipitation Formation

Rising humid air cools, condenses, and forms precipitable droplets

Industrial Cities and Haze

High concentration of nuclei in polluted air leads to persistent haze

Nighttime Haze Transition

Dry haze transitions to wet haze as RH increases overnight

Fog Classification

Categorized by formation mechanism: radiation, advection, upslope, evaporation

Valley Collection of Cold Air

Cold, dense air flows downhill and collects in valleys, forming fog

Mixing of Air Masses

Combining unsaturated air masses can lead to saturation and fog

Warm Rain Evaporation

Rain falling through cold air evaporates and raises dew point, causing fog

Steam Fog Appearance

Rising condensate resembles steam, often over lakes or rivers in cold conditions

Frontal Fog Formation

Associated with weather fronts where warm and cold air masses meet

Health Risks of High Heat

Includes cramps, exhaustion, and stroke due to impaired cooling

Heat Index Chart

Used to determine apparent temperature based on T and RH

Importance of Dew Point

Better indicator of actual moisture content than relative humidity

Use of Psychrometric Charts

Graphs to determine RH, mixing ratio from wet/dry-bulb temperatures

Role of Nuclei in Condensation

Essential for condensation to occur at realistic humidity levels

Impact of Humidity on Density Explains why humid air rises and influences weather patterns

Nighttime Radiation Loss

Clear skies allow maximum IR radiation loss, cooling the surface