Atmospheric Moisture

State Changes of Water Humidity Adiabatic Cooling What Makes Air Rise? Atmospheric Stability

Terms Adiabatic –Without the transfer of heat Exothermic –Heat-releasing Endothermic –Heat-absorbing

Terms Humidity –How much water vapor the air is holding Parcel –A volume of air assumed to have the same properties throughout (temp, humidity, etc.) –Assumed to behave independently of surrounding (ambient) air Environmental lapse rate (ELR)

Terms Environmental lapse rate (ELR) –Rate at which temperature drops with increasing altitude in the troposphere –Variable, but average = 6.5°C/km

State Changes of Water PSCI 131: Atmospheric Moisture

Water’s State Changes PSCI 131: Atmospheric Moisture Transfer of heat between water molecules & their surroundings Either endothermic or exothermic Heat source: reradiation of solar energy from ground

PSCI 131: Atmospheric Moisture: Water’s State Changes Blue arrows: exothermicRed arrows: endothermic

Water’s State Changes PSCI 131: Atmospheric Moisture Why does sweating cool you off? Sweat evaporates (endothermic), absorbing heat from your skin

Humidity PSCI 131: Atmospheric Moisture

Humidity PSCI 131: Atmospheric Moisture Evaporation from Earth’s surface (especially oceans) adds water vapor to the air A given mass of air at a given temperature can only hold so much water in vapor form If this limit is exceeded, excess vapor condenses to liquid

Ways of Expressing Air’s Humidity PSCI 131: Atmospheric Moisture: Humidity Relative humidity Dewpoint temperature

Relative Humidity PSCI 131: Atmospheric Moisture: Humidity Saturation content –Maximum amount of vapor an air parcel can hold –Controlled by air’s temperature

Relative Humidity PSCI 131: Atmospheric Moisture: Humidity Vapor content –How much vapor the parcel is actually holding RH = vapor content / saturation content

Relative Humidity PSCI 131: Atmospheric Moisture: Humidity Relative humidity will change if either of the following happens: –Temperature changes (more common cause) –Vapor content changes

Relative Humidity PSCI 131: Atmospheric Moisture: Humidity Example –Parcel’s vapor content is 10g (given) –Parcel temp: 25 degrees C –Therefore, saturation content is 20g (from table) RH = 10g/ 20g = 50%

Relative Humidity PSCI 131: Atmospheric Moisture: Humidity Example (cont.) –Parcel’s vapor content changes to 14g –Parcel temp remains 25 degrees C –Therefore, saturation content is still 20g RH = 14g/ 20g = 70% RH has risen because vapor content has risen

Relative Humidity PSCI 131: Atmospheric Moisture: Humidity Example (cont.) –Vapor content is still 14g –Parcel cools to 20 degrees C –New saturation content is 14g RH = 14g/ 14g = 100% RH has risen because temp has fallen Air is saturated

Relative Humidity: Summary PSCI 131: Atmospheric Moisture: Humidity Higher vapor content = higher RH Lower temp = lower sat. content = higher RH

Relative Humidity: Summary PSCI 131: Atmospheric Moisture: Humidity RH highest when temp is lowest & vice versa

Dewpoint Temperature PSCI 131: Atmospheric Moisture: Humidity Temp at which air parcel is saturated (100% RH) More vapor in parcel (more humid) = higher dewpoint

Adiabatic Cooling PSCI 131: Atmospheric Moisture

Adiabatic Cooling PSCI 131: Atmospheric Moisture: Adiabatic Cooling Air parcel rises, pressure drops, air cools (no heat energy transferred).

Adiabatic Cooling Rates Dry adiabatic lapse rate (DAR): 10° C /km –If RH less than 100% Wet adiabatic lapse rate (WAR): 5-9° C /km –If RH equals100% –Depends on vapor content –Always less than DAR PSCI 131: Atmospheric Moisture : Adiabatic Cooling

Condensation Level Altitude at which rising, cooling air’s temp reaches dew point and condensation begins PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Condensation Level PSCI 131: Atmospheric Moisture: Adiabatic Cooling

Several things are true when air has risen to its condensation level: –Air has cooled to its dewpoint –RH = 100% –Condensation can occur (usually as clouds) PSCI 131: Atmospheric Moisture: Adiabatic Cooling Condensation Level

Adiabatic lapse rates (WAR and DAR): rates at which air parcel cools as it rises Env lapse rate (ELR): rate at which temperature around parcel decreases with altitude PSCI 131: Atmospheric Moisture: Adiabatic Cooling Adiabatic Lapse Rates vs Env Lapse Rate

What Makes Air Rise? PSCI 131: Atmospheric Moisture

Four Processes Orographic Lifting PSCI 131: Atmospheric Moisture: What Makes Air Rise?

Four Processes Frontal wedging PSCI 131: Atmospheric Moisture: What Makes Air Rise?

Four Processes Convergence PSCI 131: Atmospheric Moisture: What Makes Air Rise?

Four Processes Convective Lifting PSCI 131: Atmospheric Moisture: What Makes Air Rise?

Atmospheric Stability PSCI 131: Atmospheric Moisture

What Is It? Balance between DAR, WAR, and ELR How likely a given air parcel is to rise, and how far up it will go The “weather maker” PSCI 131: Atmospheric Moisture: Atmospheric Stability

Three Combinations Absolute stability Absolute instability Conditional instability PSCI 131: Atmospheric Moisture: Atmospheric Stability

Adiabatic lapse rates (WAR and DAR): rates at which air parcel cools as it rises Env lapse rate (ELR): rate at which temperature around parcel decreases with altitude Adiabatic Lapse Rates vs Env Lapse Rate PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Instability ELR > DAR > WAR Surrounding air cools faster than parcel, so parcel is always warmer Parcel “wants” to keep rising Think of a hot-air balloon PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Instability PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Instability Weather effects –Lots of condensation –Heavy clouds –Lots of precip –Storms PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability DAR > WAR > ELR Rising parcel cools faster than surrounding air, so parcel is always colder “Wants” to descend PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability Will only rise if forced to –Orographic lifting, frontal wedging, etc. A “cold-air balloon” PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability PSCI 131: Atmospheric Moisture: Atmospheric Stability

Absolute Stability Weather effects –Fair weather –Light to moderate clouds and precip If forced above condensation level –No storms PSCI 131: Atmospheric Moisture: Atmospheric Stability

Conditional Instability DAR > ELR > WAR Rising parcel cools faster than surrounding air while RH is less than 100% and air is cooling at DAR –Parcel always cooler while below condensation level –Must be forced to rise PSCI 131: Atmospheric Moisture: Atmospheric Stability

Conditional Instability Surrounding air cools faster than parcel once RH = 100% –Parcel always warmer above condensation level Think of a cold-air balloon that is forced to rise to condensation level, at which it changes into a hot-air balloon PSCI 131: Atmospheric Moisture: Atmospheric Stability

Conditional Instability PSCI 131: Atmospheric Moisture: Atmospheric Stability

Conditional Instability Weather effects –Fair weather –Heavy clouds and precip/storms if forced above condensation level PSCI 131: Atmospheric Moisture: Atmospheric Stability

End of Chapter