o Geoscientific Thinking/Learning
York University Fall Term 2013 Examination: NATS 1780 Weather & Climate Content: o Geoscientific Thinking/Learning o Clouds and Fog o Condensation o H2O o Stability and Structure o Cloud Formation and Precipitation o Composition o Radiation o Aerosols o Temperature Geoscientific Thinking/ Learning Time Space Practice Systemsorientation Time Weather vs. Climate Weather: The condition of the atmosphere at a particular time and place o Temperature o Pressure o Humidity o Clouds o Precipitation o Visibility o Wind o …. Climate: The ‘average weather’ o Accumulation of daily and seasonal weather events over a long period of time o Extreme events o Heat waves o Cold spells o The frequency of evens distinguishes climates as much as the averages
Space “Geoscientists use spatial thinking extensively whenever they acquire, represent, manipulate, or reason about objects, processes, or phenomena in space.” Practice • Professional vision o The ability to see features that are important to professional practice • “… field experiences provide practice in transforming the raw material of nature into the words, signs, and symbols that geoscientists use to capture and communicate their observations.” Systemsorientation What is a scientific model? • A representation of empirical objects, phenomena, and physical processes that is logical and objective • Models are simplified reflections of reality • Building and disputing models is fundamental to science Summary: Time, space, practice and systemsorientation are key to geoscience thinking/learning Geoscience thinking/learning skills are of use beyond geoscience courses… BUT… geoscience needs thinking/learning skills developed outside of the geosciences. Clouds & Fog Classification of Clouds • Luke Howard (1803) • Introduced a classification system o Based on observations from the ground • Four basic cloud forms o Stratus (Latin for layer) o Cumulus (Latin for heap) o Cirrus (Latin for curl of hair) o Nimbus (Latin for violent rain) • Abercromby and Hildebrandsson o Modified Howard’s system o Resulting classification system still used today • 10 basic cloud forms o 4 primary cloud groups High Clouds Middle Clouds Low Clouds
Vertical Clouds
1. High Clouds • Altitude of 613km (in midlatitudes) o At this altitude the air is cold and dry Favors formation of ice clouds (almost exclusively) These clouds appear thin • Three forms o Cirrus (Ci) Thin wispy clouds blown by high winds Mares tails Most common of the cirrus group Mostly comprised of ice crystals o Cirrocumulus (Cc) Has a rippled appearance Looks like fish scales o Cirrostratus (Cs) Thin, sheetlike high clouds Often cover the entire sky Can see the Sun and/or Moon through them Light is refracted by ice crystals Results in halos Often form in advance of a storm Used to predict an approaching storm 2. Middle Clouds • Altitude of 27km (in midlatitudes) • Composition o Ice water droplets (primary) o Ice crystals (secondary) o Altocumulus (Ac) Grey fluffy masses Can see individual puffy mass At arms length are about the size of your thumb o Altostratus (As) Extensive gray mass Often covers the entire sky 3. Low Clouds • Altitude under 2 km • Almost always composed of liquid water droplets o In cold weather, may contain ice or snow • Three forms o Stratus (St)
A uniform gray cloud Often covers the entire sky Resembles a fog, that doesn’t quite reach the ground Has a low base No rain falls from stratus (normal) Light mist or drizzle (occasional) o Stratocumulus (Sc) A low, lumpy cloud layer Lumps appear larger than from Ac) Has a lower cloud base than Ac At arms length, is about the size of ones fist Rain or snow rarely fall from Sc o Nimbostratus (Ns) A dark, ‘wetlooking’ cloud layer Associated with rainingallday days Associated with light to moderate precipitation Rain or snow Often low visibility beneath the cloud deck Blocks light Low, irregular cloud base 4. Vertical Clouds • Clouds having vertical structure • Flat cloud bases and puffy, domed tops • 2 forms o Cumulus (Cu) ‘Fairweather’ cumulus Look like floating pieces of cotton Clouds are separate and distinct Show only slight vertical growth Often form on warm, sunny days If liquid water falls, its only showery Cumulus congestus Towering cumulus o Cumulonimbus (Cb) As cumulus congestus continues to grow it becomes Cb Associated with thunderstorms, thunder and/or lightning Violent updrafts Vertically extensive Base can be as low as 300m Can top out in the tropopause at over 11km Low in the cloud, liquid water present High in the cloud, ice can be formed
High winds at upper levels can shape the cloud to form an anvil Can produce all types of precipitation
What are clouds? Droplets of liquid water and/or ice crystals Observing clouds from above (eg. Aircraft) o Light is reflected off the cloud (cf. albedo) The cloud appears bright Observing clouds from below (eg. Ground) o Light has passed through the cloud …and has been attenuated by the cloud The denser and larger the droplets, the more light is attenuated, and the darker the cloud appears. Scattering of Light • Redirection process • Scatterers (particles/gases) determine: o Quantity o Geometry o Wavelength dependence Classification of Fogs • Processbased classification o Radiation Fog Processed by radiative cooling of Earth’s surface As Earth cools after sunset, air above it cools As air temperature decreases, RH increases until saturation results in condensation As temperature continues to decrease, condensation results in droplet growth o Advection Fog Warm, moist air (water) moves over colder surface (land) Air cools, saturation occurs, and condensation o Upslope Fog When air is forced to rise due to topography • Eg. The presence of a hill or mountain As the air mass rises, it expands and cools • If it cools to saturation, condensation may begin (if CCN present) o Evaporation Fog When mixing of two, unsaturated air masses results in the formation of a saturated air mass
In cold weather bodies of water are often warmer than the surrounding air A layer of air above the water can be moist When this layer of air mixes with the cold air above it, the mixture can become saturated Condensation occurs, fog results
Condensation Latent Heat o Hidden energy o Energy that is used to break molecular structure, not bonds o When changing phases, temperature remains constant o Heat will be released or required Stored energy o Energy from the Sun can be stored o Solar energy is absorbed by liquid water o Results in some of the liquid water becoming water vapor o Stored energy can be released, elsewhere in the atmosphere o When the water vapor condenses In a system containing two or more phases o There is a constant exchange of molecules between the phases o Equilibrium is established when the exchange between the phases is equivalent Condensation o The rate of water vapor being deposited to the surface is faster than the rate of water molecules leaving the surface. Energy is released, and the surroundings are warmed. Evaporation o The rate of water molecules leaving the surface is faster than the rate of water molecules being deposited to the surface. Energy is required, and the surroundings are cooled. Vapor Pressure o Molecular collisions exert a force (force can be exerted on a wall, person, etc.) o Therefore, pressure = force / area o Therefore, pressure is dependent upon the number of molecules colliding with an area of a surface Saturation Vapor Pressure o The pressure of the vapor of a substance in equilibrium with its condensed phase (solid or liquid) is called the saturated vapor pressure o This is the maximum amount (with super saturation) of a molecule that can be found in the vapor phase at a particular temperature and pressure Saturation o Equilibrium between the liquid and vapor phase. For every molecule that evaporates another must condense. This is dependent upon the temperature and pressure
Unsaturated Air o Air that is holding less water than it could possibly hold. Therefore: non equilibrium evaporation is occurring. o Note: objects will dry in unsaturated air, but not in saturated air.
Humidity o Refers to the amount of water vapor in the air Absolute humidity, the weight of water vapor within a specific volume (g/m^3) o Specific humidity The weight of water vapor relation to the total mass of the air parcel (g/kg) Relative humidity o The ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor required for saturation at that particular temperature and pressure. o RH = (H2O (g) content) / (H2O (g) capacity) x 100% o Eg. An air parcel with RH = 25%, contains only 25% of the total water that the air parcel could hold. o Can alter RH by Changing the amount of water vapor in an air parcel Changing the capacity of the air parcel to hold water (change the temperature) Why Use Relative Humidity? • Importance WRT condensation/evaporation • Evaporation o The greater the difference between the amount of water vapour in the air and the amount that it could hold (lower the RH) The higher the rate or evaporation o On a very humid day (high RH)… Feel hotter, sweat on our skin does not evaporate as quickly, less cooling effect Wet clothes don’t dry as quickly, etc. Evaporation & Relative Humidity The lower the relative humidity, the faster the rate of evaporation – better cooling The higher the relative humidity, the slower the rate of evaporation – poor cooling Relative Humidity and Condensation Condensation o The higher the RH The closer the air is to saturation Less cooling will have to take place in order for the air to become saturated o Very important to cloud formation
Dew Point The temperature at which an air mass becomes saturated o RH = 100% Basis for comparison o Compare the ambient temperature with Td Means for assessing how close to 100% RH an air mass is The dew point is a physical property that serves to characterize an air mass Suppose that… o Day: T = 29 C and RH = 75% o Night: T = 23 C and RH = 100% o The air mass is saturated o Further cooling o Condensation starts o Liquid water (dew) is formed Can use the dew point to calculate RH RH = (SVP @ Td) / (SVP @ T) x 100 Where SVP = saturated vapor pressure Can use a wetbulb thermometer to measure the dew point
Frost Point o If the freezing point of water is reached before saturation… Then the saturation point is the frost point, not the dew point o When the air becomes saturated, water vapor is deposited to surfaces as a solid (frost, hoarfrost, white frost)
Measuring Relative Humidity o Wetbulb temperature The lowest temperature that can be reached by evaporating water into air Simulates the cooling effect of water on the skin The bulb of a thermometer is coated with a wick (a piece of cloth) and wetted (with water) Evaporation of the water results in cooling RH determined by a chart o Psychrometer – Wet and Dry Bulb Contains a wetbulb (wetted thermometer) and a drybulb thermometer Using the difference between these two temperatures • … and the dry bulb temperature • … the RH can be calculated eg. Suppose • A dry bulb temperature reads 27.5C
• • •
A wet bulb 24C Depression is 3.5 RH is 75% (via chart)
Measuring Dew Point Temperature o Psychrometer – Wet and Dry Bulb Same as to measure RH Obtain the wet and drybulb temperatures Calculate the wetbulb depression Difference between wet and dry Look up the dewpoint temperature on a chart
Humidex – Heat index o Accounts for humidity effect on temperature The higher the humidity … …. The less efficiently we can cool ourselves therefore hotter it ‘feels’ Apparent temperature Condensation o At 100% RH, clouds or fog are not always seen forming o A surface is required for the water vapor to condense onto o These surfaces are referred to as aerosols o Serve as nuclei for the formation of clouds OR cloud condensation nuclei (CCN)
Supersaturation o If no surface is available… the air mass becomes supersaturated o RH > 100%
Condensation Nuclei o Air contains aerosols Small particles suspended in air • Solid or liquid or a combination • Range in size from 0.01 um to 100’s of um o Similar to a liquid which also requires a surface before freezing o Can supercool (T
Space “Geoscientists use spatial thinking extensively whenever they acquire, represent, manipulate, or reason about objects, processes, or phenomena in space.” Practice • Professional vision o The ability to see features that are important to professional practice • “… field experiences provide practice in transforming the raw material of nature into the words, signs, and symbols that geoscientists use to capture and communicate their observations.” Systemsorientation What is a scientific model? • A representation of empirical objects, phenomena, and physical processes that is logical and objective • Models are simplified reflections of reality • Building and disputing models is fundamental to science Summary: Time, space, practice and systemsorientation are key to geoscience thinking/learning Geoscience thinking/learning skills are of use beyond geoscience courses… BUT… geoscience needs thinking/learning skills developed outside of the geosciences. Clouds & Fog Classification of Clouds • Luke Howard (1803) • Introduced a classification system o Based on observations from the ground • Four basic cloud forms o Stratus (Latin for layer) o Cumulus (Latin for heap) o Cirrus (Latin for curl of hair) o Nimbus (Latin for violent rain) • Abercromby and Hildebrandsson o Modified Howard’s system o Resulting classification system still used today • 10 basic cloud forms o 4 primary cloud groups High Clouds Middle Clouds Low Clouds
Vertical Clouds
1. High Clouds • Altitude of 613km (in midlatitudes) o At this altitude the air is cold and dry Favors formation of ice clouds (almost exclusively) These clouds appear thin • Three forms o Cirrus (Ci) Thin wispy clouds blown by high winds Mares tails Most common of the cirrus group Mostly comprised of ice crystals o Cirrocumulus (Cc) Has a rippled appearance Looks like fish scales o Cirrostratus (Cs) Thin, sheetlike high clouds Often cover the entire sky Can see the Sun and/or Moon through them Light is refracted by ice crystals Results in halos Often form in advance of a storm Used to predict an approaching storm 2. Middle Clouds • Altitude of 27km (in midlatitudes) • Composition o Ice water droplets (primary) o Ice crystals (secondary) o Altocumulus (Ac) Grey fluffy masses Can see individual puffy mass At arms length are about the size of your thumb o Altostratus (As) Extensive gray mass Often covers the entire sky 3. Low Clouds • Altitude under 2 km • Almost always composed of liquid water droplets o In cold weather, may contain ice or snow • Three forms o Stratus (St)
A uniform gray cloud Often covers the entire sky Resembles a fog, that doesn’t quite reach the ground Has a low base No rain falls from stratus (normal) Light mist or drizzle (occasional) o Stratocumulus (Sc) A low, lumpy cloud layer Lumps appear larger than from Ac) Has a lower cloud base than Ac At arms length, is about the size of ones fist Rain or snow rarely fall from Sc o Nimbostratus (Ns) A dark, ‘wetlooking’ cloud layer Associated with rainingallday days Associated with light to moderate precipitation Rain or snow Often low visibility beneath the cloud deck Blocks light Low, irregular cloud base 4. Vertical Clouds • Clouds having vertical structure • Flat cloud bases and puffy, domed tops • 2 forms o Cumulus (Cu) ‘Fairweather’ cumulus Look like floating pieces of cotton Clouds are separate and distinct Show only slight vertical growth Often form on warm, sunny days If liquid water falls, its only showery Cumulus congestus Towering cumulus o Cumulonimbus (Cb) As cumulus congestus continues to grow it becomes Cb Associated with thunderstorms, thunder and/or lightning Violent updrafts Vertically extensive Base can be as low as 300m Can top out in the tropopause at over 11km Low in the cloud, liquid water present High in the cloud, ice can be formed
High winds at upper levels can shape the cloud to form an anvil Can produce all types of precipitation
What are clouds? Droplets of liquid water and/or ice crystals Observing clouds from above (eg. Aircraft) o Light is reflected off the cloud (cf. albedo) The cloud appears bright Observing clouds from below (eg. Ground) o Light has passed through the cloud …and has been attenuated by the cloud The denser and larger the droplets, the more light is attenuated, and the darker the cloud appears. Scattering of Light • Redirection process • Scatterers (particles/gases) determine: o Quantity o Geometry o Wavelength dependence Classification of Fogs • Processbased classification o Radiation Fog Processed by radiative cooling of Earth’s surface As Earth cools after sunset, air above it cools As air temperature decreases, RH increases until saturation results in condensation As temperature continues to decrease, condensation results in droplet growth o Advection Fog Warm, moist air (water) moves over colder surface (land) Air cools, saturation occurs, and condensation o Upslope Fog When air is forced to rise due to topography • Eg. The presence of a hill or mountain As the air mass rises, it expands and cools • If it cools to saturation, condensation may begin (if CCN present) o Evaporation Fog When mixing of two, unsaturated air masses results in the formation of a saturated air mass
In cold weather bodies of water are often warmer than the surrounding air A layer of air above the water can be moist When this layer of air mixes with the cold air above it, the mixture can become saturated Condensation occurs, fog results
Condensation Latent Heat o Hidden energy o Energy that is used to break molecular structure, not bonds o When changing phases, temperature remains constant o Heat will be released or required Stored energy o Energy from the Sun can be stored o Solar energy is absorbed by liquid water o Results in some of the liquid water becoming water vapor o Stored energy can be released, elsewhere in the atmosphere o When the water vapor condenses In a system containing two or more phases o There is a constant exchange of molecules between the phases o Equilibrium is established when the exchange between the phases is equivalent Condensation o The rate of water vapor being deposited to the surface is faster than the rate of water molecules leaving the surface. Energy is released, and the surroundings are warmed. Evaporation o The rate of water molecules leaving the surface is faster than the rate of water molecules being deposited to the surface. Energy is required, and the surroundings are cooled. Vapor Pressure o Molecular collisions exert a force (force can be exerted on a wall, person, etc.) o Therefore, pressure = force / area o Therefore, pressure is dependent upon the number of molecules colliding with an area of a surface Saturation Vapor Pressure o The pressure of the vapor of a substance in equilibrium with its condensed phase (solid or liquid) is called the saturated vapor pressure o This is the maximum amount (with super saturation) of a molecule that can be found in the vapor phase at a particular temperature and pressure Saturation o Equilibrium between the liquid and vapor phase. For every molecule that evaporates another must condense. This is dependent upon the temperature and pressure
Unsaturated Air o Air that is holding less water than it could possibly hold. Therefore: non equilibrium evaporation is occurring. o Note: objects will dry in unsaturated air, but not in saturated air.
Humidity o Refers to the amount of water vapor in the air Absolute humidity, the weight of water vapor within a specific volume (g/m^3) o Specific humidity The weight of water vapor relation to the total mass of the air parcel (g/kg) Relative humidity o The ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor required for saturation at that particular temperature and pressure. o RH = (H2O (g) content) / (H2O (g) capacity) x 100% o Eg. An air parcel with RH = 25%, contains only 25% of the total water that the air parcel could hold. o Can alter RH by Changing the amount of water vapor in an air parcel Changing the capacity of the air parcel to hold water (change the temperature) Why Use Relative Humidity? • Importance WRT condensation/evaporation • Evaporation o The greater the difference between the amount of water vapour in the air and the amount that it could hold (lower the RH) The higher the rate or evaporation o On a very humid day (high RH)… Feel hotter, sweat on our skin does not evaporate as quickly, less cooling effect Wet clothes don’t dry as quickly, etc. Evaporation & Relative Humidity The lower the relative humidity, the faster the rate of evaporation – better cooling The higher the relative humidity, the slower the rate of evaporation – poor cooling Relative Humidity and Condensation Condensation o The higher the RH The closer the air is to saturation Less cooling will have to take place in order for the air to become saturated o Very important to cloud formation
Dew Point The temperature at which an air mass becomes saturated o RH = 100% Basis for comparison o Compare the ambient temperature with Td Means for assessing how close to 100% RH an air mass is The dew point is a physical property that serves to characterize an air mass Suppose that… o Day: T = 29 C and RH = 75% o Night: T = 23 C and RH = 100% o The air mass is saturated o Further cooling o Condensation starts o Liquid water (dew) is formed Can use the dew point to calculate RH RH = (SVP @ Td) / (SVP @ T) x 100 Where SVP = saturated vapor pressure Can use a wetbulb thermometer to measure the dew point
Frost Point o If the freezing point of water is reached before saturation… Then the saturation point is the frost point, not the dew point o When the air becomes saturated, water vapor is deposited to surfaces as a solid (frost, hoarfrost, white frost)
Measuring Relative Humidity o Wetbulb temperature The lowest temperature that can be reached by evaporating water into air Simulates the cooling effect of water on the skin The bulb of a thermometer is coated with a wick (a piece of cloth) and wetted (with water) Evaporation of the water results in cooling RH determined by a chart o Psychrometer – Wet and Dry Bulb Contains a wetbulb (wetted thermometer) and a drybulb thermometer Using the difference between these two temperatures • … and the dry bulb temperature • … the RH can be calculated eg. Suppose • A dry bulb temperature reads 27.5C
• • •
A wet bulb 24C Depression is 3.5 RH is 75% (via chart)
Measuring Dew Point Temperature o Psychrometer – Wet and Dry Bulb Same as to measure RH Obtain the wet and drybulb temperatures Calculate the wetbulb depression Difference between wet and dry Look up the dewpoint temperature on a chart
Humidex – Heat index o Accounts for humidity effect on temperature The higher the humidity … …. The less efficiently we can cool ourselves therefore hotter it ‘feels’ Apparent temperature Condensation o At 100% RH, clouds or fog are not always seen forming o A surface is required for the water vapor to condense onto o These surfaces are referred to as aerosols o Serve as nuclei for the formation of clouds OR cloud condensation nuclei (CCN)
Supersaturation o If no surface is available… the air mass becomes supersaturated o RH > 100%
Condensation Nuclei o Air contains aerosols Small particles suspended in air • Solid or liquid or a combination • Range in size from 0.01 um to 100’s of um o Similar to a liquid which also requires a surface before freezing o Can supercool (T
