Moisture and Atmospheric Stability … and Instability How does is relate to cloud development and precipitation?

Water: A Unique Substance! What is Water: What is Water: –Water is the only liquid on the surface of the Earth in large quantities –It exists in all forms on Earth –Ice (solid state) is less dense than liquid (which is why icebergs ‘float’) (which is why icebergs ‘float’) Ice Forms on top of Water because it is less Dense

Moisture in the Atmosphere Water is part of a distinct system called the hydrological cycle Water is part of a distinct system called the hydrological cycle Water is removed from the surface & into the atmosphere by two processes: Water is removed from the surface & into the atmosphere by two processes: –Evaporation: Water removed off a free water surface, like a lake, river, ocean, or even soil –Transpiration: Water released into the air by the stomata in leaves

Hydrologic Cycle Hydrologic Cycle

Terms to remember! Evaporation - liquid to gas Evaporation - liquid to gas Condensation - gas to liquid Condensation - gas to liquid Precipitation - liquid/solid falling to ground Precipitation - liquid/solid falling to ground Sublimation - solid to gas Sublimation - solid to gas Deposition – gas to solid Deposition – gas to solid Transpiration - gas released by plants Transpiration - gas released by plants Runoff/Infiltration - precipitation soaking Runoff/Infiltration - precipitation soaking into soil into soil

‘Phases’ of Water Solid (ice): Water molecules are arranged and bonded together in a lattice network and are fixed in place Solid (ice): Water molecules are arranged and bonded together in a lattice network and are fixed in place Liquid (water): Water molecules are loosely bonded together but are able to move around freely Liquid (water): Water molecules are loosely bonded together but are able to move around freely Gas (water vapor): Water molecules are not bonded to each other and are freely flying around in the atmosphere Gas (water vapor): Water molecules are not bonded to each other and are freely flying around in the atmosphere

Change of Phase of Water Very Important Concept! Change of Phase of Water Very Important Concept! *

Phase Changes of Water Melting: Going from a solid to a liquid Melting: Going from a solid to a liquid HEAT IS ABSORBED BY THE SOLID (ICE) BUT REMOVED FROM THE AIR AROUND IT! HEAT IS ABSORBED BY THE SOLID (ICE) BUT REMOVED FROM THE AIR AROUND IT! When an Ice Cube melts it cools the environment slightly as it takes Heat from the atmosphere in order to melt the ice. The surrounding air becomes cooler during this process

Change of Phase of Water Very Important Concept! Change of Phase of Water Very Important Concept! *

Phase Changes of Water Melting: Going from a solid to a liquid Melting: Going from a solid to a liquid As the Ice ABSORBS Heat and takes it from the Atmosphere (thus cooling the air!) the Atmosphere (thus cooling the air!) Evaporation: Going from a liquid to a gas also Evaporation: Going from a liquid to a gas also takes heat from the atmosphere which takes heat from the atmosphere which results in a cooling effect For example: As standing water/puddles on streets and sidewalks dries up during a sunny morning, it slows the normal rise in

Change of Phase of Water Very Important Concept Change of Phase of Water Very Important Concept *

Phase Changes of Water Melting: Going from a solid to a liquid Melting: Going from a solid to a liquid Evaporation: Going from a liquid to a gas Evaporation: Going from a liquid to a gas Sublimation: Going directly from a solid to a gas without entering the liquid phase ALSO TAKES heat energy from the atmosphere – thus cooling the air Sublimation: Going directly from a solid to a gas without entering the liquid phase ALSO TAKES heat energy from the atmosphere – thus cooling the air Example – Snow melting on a sunny day Example – Snow melting on a sunny day often goes directly from a solid to water often goes directly from a solid to water vapor state vapor state

Phase Changes of Water Melting: Going from a solid to a liquid Melting: Going from a solid to a liquid Evaporation: Going from a liquid to a gas Evaporation: Going from a liquid to a gas Sublimation: Going directly from a solid to a gas without entering the liquid phase Sublimation: Going directly from a solid to a gas without entering the liquid phase ALL OF THE ABOVE: AS HEAT IS ABSORBED BY SOLID ICE, OR WHEN LIQUID WATER IS REMOVED BY EVAPORATION, THE ATMOSPHERIC ENVIRONMENT COOLS SLIGHTLY SNOW MELTING AS IT FALLS THROUGH THE ATMOSPHERE WHEN THE AIR TEMPERATURE IS ABOVE FREEZING WILL CAUSE THE AIR TEMPERATURE TO FALL. THAT’S WHY EVEN WITH AN INITIAL TEMPERATURE OF 40°, SNOW CAN FALL AND WITHIN AN HOUR, THE TEMPERATURE CAN DROP TO NEAR FREEZING! SNOW MELTING AS IT FALLS THROUGH THE ATMOSPHERE WHEN THE AIR TEMPERATURE IS ABOVE FREEZING WILL CAUSE THE AIR TEMPERATURE TO FALL. THAT’S WHY EVEN WITH AN INITIAL TEMPERATURE OF 40°, SNOW CAN FALL AND WITHIN AN HOUR, THE TEMPERATURE CAN DROP TO NEAR FREEZING!

Change of Phase of Water Very Important Concept Change of Phase of Water Very Important Concept *

Phase Changes of Water Deposition: Going directly from a gas to a solid Deposition: Going directly from a gas to a solid HEAT IS RELEASED INTO THE ATMOSPHERE HEAT IS RELEASED INTO THE ATMOSPHERE FROST will form on a sub-freezing surface when there is a high level of moisture in the lower level of the atmosphere (RELATIVE humidity close to 100%) FROST will form on a sub-freezing surface when there is a high level of moisture in the lower level of the atmosphere (RELATIVE humidity close to 100%)

Phase Changes of Water Deposition: Going directly from a gas to a solid Deposition: Going directly from a gas to a solid - Water vapor freezing into ice crystals in a cloud - Water vapor freezing into ice crystals in a cloud - Frost forming in a freezer is an example of deposition - Frost forming in a freezer is an example of deposition - Frost forming on grass/roads on a cold morning - Frost forming on grass/roads on a cold morning

Change of Phase of Water Very Important Concept Change of Phase of Water Very Important Concept *

Phase Changes of Water Deposition: Going directly from a gas to a solid Deposition: Going directly from a gas to a solid Condensation: Going from a gas to a liquid releasing Heat Energy Condensation: Going from a gas to a liquid releasing Heat Energy EXAMPLE: WATER VAPOR IN THE ATMOSPHERE CONDENSES INTO SMALL WATER DROPLETS WHICH FORM CLOUDS. HEAT ENERGY FROM THE GAS REMAINS WITHIN THE CLOUD, THUS WARMING THE ATMOSPHERE

Change of Phase of Water Very Important Concept Change of Phase of Water Very Important Concept *

Phase Changes of Water Deposition: Going directly from a gas to a solid Deposition: Going directly from a gas to a solid Condensation: Going from a gas to a liquid releasing Heat Energy Condensation: Going from a gas to a liquid releasing Heat Energy Freezing: Going from a liquid to a solid will release Freezing: Going from a liquid to a solid will release heat energy into the atmosphere heat energy into the atmosphere

THE HEAT ENERGY RELEASED or GAINED DURING A CHANGE OF PHASE IS A ‘HIDDEN’ HEAT THAT IS CALLED: LATENT HEAT THIS IS EXTREMELY IMPORTANT FOR THE DEVELOPMENT OF CLOUDS AND EVENTUALLY PRECIPITATION AND PLAYS A MAJOR ROLE IN THE INTENSIFICATION OF STORM SYSTEMS!

Latent Heat Latent heat: Latent heat: Adding heat to melt ice does not result in a temperature change BUT: Adding heat to melt ice does not result in a temperature change BUT: –Melting 1 gram of ice requires 80 calories of heat energy (taken from the atmosphere) heat energy (taken from the atmosphere) Latent heat of melting Latent heat of melting -Freezing 1 gram of water releases 80 calories of heat energy into the atmosphere of heat energy into the atmosphere

Latent Heat Latent heat is also involved with evaporation, the process of converting from a liquid to a gas Latent heat is also involved with evaporation, the process of converting from a liquid to a gas –The latent heat of vaporization is the energy absorbed by the water during evaporation. ~ 600 calories/gram required for water! ~ 600 calories/gram required for water! –Evaporation cools the air as all that heat energy is taken from the air to vaporize the liquid water!

Latent Heat Condensation is the reverse process, converting a gas to a liquid Condensation is the reverse process, converting a gas to a liquid –Water vapor changes from to a liquid state and releases latent heat to the atmosphere – ~ 600 calories/gram of heat energy ~ 600 calories/gram of heat energy –The energy released warms the surrounding air and helps drive storm intensification!

Phase Change Summary

WE WILL TALK MORE ABOUT CONDENSATION AND HOW IT RELATES TO CLOUD DEVELOPMENT AFTER WE DISCUSS HUMIDITY and the REAL difference between moist and dry air!

Humidity: Water Vapor in the Air Humidity is the amount of water vapor in the air. Humidity is the amount of water vapor in the air. –Absolute humidity is the actual mass or amount of water vapor in a given volume of air –Relative humidity indicates how close air is to saturation rather than the actual quantity of water vapor in the air ** Warmer air holds more moisture than colder air ** –Dew point is the temperature to which air needs to be cooled to reach saturatio n. When the air temperature drops to the dewpoint, the air is saturated and the relative humidity reaches 100%

Changes in Relative Humidity with Temperature Assuming there is no change in the actual water content of the atmosphere, as temperatures rise through the day, the relative humidity falls with no change in the absolute amount of water vapor in the air As the temperature rises… there is a drop in the relative humidity

Amount of Water Vapor Required for Saturation Rises with Temperature With an Air Temp of 15°C saturation is reached with 10g/Kg of water With an Air Temp of 25°C saturation is reached with 20g/Kg of water

Dew-Point Temperatures On a typical spring day Very Humid! VERY DRY

Relative Humidity the bottom line… How relative humidity changes: How relative humidity changes: –If water vapor is added, relative humidity goes up. –If water vapor is removed, relative humidity goes down. –100% relative humidity equals saturation –A decrease in temperature equals an increase in relative humidity.

CONSIDER THE ATMOSPHERE WITH ONLY THE PERMANENT GASES AND NO WATER VAPOR Is Humid Air Heavier than Dry Air?

Water vapor is more buoyant than dry air – BUT WHY? Water vapor is more buoyant than dry air – BUT WHY? The chemical make-up of a parcel of dry air is comprised of mainly of N 2 and 0 2, (78% and 21%) The chemical make-up of a parcel of dry air is comprised of mainly of N 2 and 0 2, (78% and 21%) Nitrogen has a molecular weight of 28 x (78%) = 22 Nitrogen has a molecular weight of 28 x (78%) = 22 Oxygen has a molecular weight of 32 x (21%) = 7 Oxygen has a molecular weight of 32 x (21%) = 7 Therefore  molecular weight of dry air = 22+7 = 29 Therefore  molecular weight of dry air = 22+7 = 29 H 2 0 molecular weight: H 2 0 molecular weight: H 2 molecular weight = 1 x 2 = 2 H 2 molecular weight = 1 x 2 = 2 O molecular weight = 16 O molecular weight = 16 Therefore  molecular weight of water = 2+16 = 18 Therefore  molecular weight of water = 2+16 = 18 Is Humid Air Heavier than Dry Air?

Water Vapor has a molecular weight of 18 Water Vapor has a molecular weight of 18 Dry Air has a molecular weight of 29 Dry Air has a molecular weight of IS < than IS < than 29 So Water vapor is more buoyant than dry air! Therefore, when there is water vapor in the atmosphere, it becomes buoyant and thus begins to rise! Therefore, when there is water vapor in the atmosphere, it becomes buoyant and thus begins to rise! Is Humid Air Heavier than Dry Air?

Lets dig Deeper into the TWO things that make up HUMIDITY Temperature and Dewpoint Lets dig Deeper into the TWO things that make up HUMIDITY Temperature and Dewpoint

Temperature vs Dewpoint Temperature vs Dewpoint Temperature is the average kinetic energy of the molecules of the atmosphere. It measures the degree of heat in the air Temperature is the average kinetic energy of the molecules of the atmosphere. It measures the degree of heat in the air Dewpoint is the actual amount of moisture in the atmosphere. It measures the degree of moisture in the air Dewpoint is the actual amount of moisture in the atmosphere. It measures the degree of moisture in the air

Water Vapor in the Air For the purposes of this discussion, we will often refer to a ‘parcel’ of air – best thought of as a ‘balloon’ filled with moist air As the parcel of air moves up or down, the moist air does NOT mix with the surrounding environment

AS YOU MOVE UP FROM THE SURFACE OF THE EARTH THE TEMPERATURE DECREASES IN THE TROPOSPHERE…..SO HOW DOES THAT AFFECT THE TEMPERATURE OF THE RISING AIR PARCEL?

What actually happens within an enclosed “air parcel” What actually happens within an enclosed “air parcel” Because the atmosphere is NORMALLY cooler as you rise in altitude, the temperature of the parcel should also decrease as it rises – but why? 70% 85% 100%

HOW DOES LOWERING PRESSURE HELP CLOUDS DEVELOP? HOW DOES LOWERING PRESSURE HELP CLOUDS DEVELOP?

RECALL THAT AIR PRESSURE LOWERS AS THE ALTITUDE INCREASES. THIS ALLOWS AIR ‘PARCELS’ THAT RISE FROM THE SURFACE TO INITIALLY “EXPAND”

Time to expand our minds! Time to expand our minds! In physics, work is defined as a force being applied over a distance In physics, work is defined as a force being applied over a distance

Time to expand our minds! Time to expand our minds! In physics, work is defined as a force being applied over a distance In physics, work is defined as a force being applied over a distance In order to do work, energy must be expended In order to do work, energy must be expended When our parcel expands while being lifted, it is doing work by pushing out against the environment a certain distance When our parcel expands while being lifted, it is doing work by pushing out against the environment a certain distance

Time to expand our minds! Time to expand our minds! Therefore, the parcel must use up energy in order to expand. Therefore, the parcel must use up energy in order to expand.

Time to expand our minds! Time to expand our minds! Therefore, the parcel must use up energy in order to expand. Therefore, the parcel must use up energy in order to expand. The energy that is available to be used up is the kinetic energy of the molecules The energy that is available to be used up is the kinetic energy of the molecules Otherwise known as: Otherwise known as: TEMPERATURE ! TEMPERATURE !

Time to expand our minds! Time to expand our minds! Therefore, the parcel must use up energy in order to expand. Therefore, the parcel must use up energy in order to expand. The energy that is available to be used up is the kinetic energy of the molecules. The energy that is available to be used up is the kinetic energy of the molecules. Otherwise known as: Otherwise known as: TEMPERATURE ! TEMPERATURE ! Therefore, the Temperature in a parcel also decreases as it rises in the atmosphere.

Finally, Lets Talk About Stability and Instability Specifically, this refers to the temperature profile of the atmosphere

Air that is Warmer than its Surroundings Rises It’s said to be more buoyant …

Generally Speaking… Instability: Warm air at the surface and cold air above allows air parcels that are warm at the surface to rise…

Stability: Cool air at the surface but ‘warmer’ air above. This inhibits surface parcels from rising and normally forces the parcel to return to lower atmosphere

Adiabatic Temperature Changes

Air Cools at Dry Adiabatic Rate until Reaching Dew Point Then Cools at Wet Adiabatic Rate

Rising Air Cooler than Environment Tends Toward Stability Stable air does not allow warm air parcels to rise. Unstable air allows warm air parcels to rise.

Rising Air that is warmer than its environment will continue to rise and the air is said to be unstable Air that is not saturated will rise and cool at the ‘dry adiabatic’ rate of about 5°F/1,000 ft. Once it cools to the dewpoint (saturation) the air parcel will cool at the ‘wet adiabatic’ rate of 3°/1,000’. Remember, when saturation occurs, so does condensation - and that releases heat which warms’ the parcel.