Atmospheric Stability Adiabatic Processes The concept of a parcel Parcel and environmental lapse rates Atmospheric dry stability Determining stability

Air parcels A parcel is a “blob” of air Small enough to have only one value of T, p, ρ, etc. Large enough to contain a significant number of molecules. (Are there enough particles to talk about temperature as average kinetic energy, for example?)

Lapse Rates Parcel lapse rate – the rate at which temperature changes as the parcel is lifted to a higher altitude Environmental lapse rate – the rate at which the air surrounding the parcel changes as altitude increases

The Adiabatic Lapse Rate An adiabatic process is one during which no heat is exchanged between the substance in question and its surroundings Many atmospheric motions occur rapidly enough that parcels do not exchange a significant amount of heat with the environment Examples: rising air in a thunderstorm Air rising over a topographic barrier (like a mountain)

Adiabatic Processes (Chalkboard)

The Adiabatic Lapse Rate The adiabatic lapse rate for DRY air on Earth is Γd = g/cp Γd = 9.81 m s-2 / 1004 J kg-1 K-1 Γd = 0.00977 K m-1 Γd = 9.77 K km-1

The Adiabatic Lapse Rate This means that a rising(sinking) air parcel will cool(warm) at a rate of about 10 oC per km of ascent(descent) unless: It exhanges significant mass or heat with the environment It becomes saturated with respect to water vapor It rises(sinks) so slowly that radiation heat transfer is possible

The Adiabatic Lapse Rate What is the dry adiabatic lapse rate (Γd = g/cp) in these atmospheres? Atmosphere g (m s-2) cp (J kg-1 K-1) Venus 8.87 844 Mars 3.71 Titan 1.352 1039

The Adiabatic Lapse Rate We have thus far only discussed the DRY ADIABATIC LAPSE RATE Water vapor condensation releases 2.5 MJ of energy for each kg of water condensed – this latent heat changes the adiabatic lapse rate for condensing air parcels to the MOIST ADIABATIC LAPSE RATE

Atmospheric Stability

Atmospheric Stability stable and unstable equilibria air parcels adiabatic process adiabatic lapse rates Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

Determining Stability (Chalkboard)

A Stable Atmosphere environmental lapse rate absolute stability stabilizing processes Stable air provides excellent conditions for high pollution levels.

An Unstable Atmosphere absolute instability warming of surface air destabilizing processes superadiabatic lapse rates Unstable air tends to be well-mixed.

Conditionally Unstable Air conditional instability dry and moist adiabatic lapse rates are different Environmental lapse rate is between the two

Atmospheric Moisture Twice now, we’ve mentioned moist adiabatic lapse rates. Maybe we should talk about atmospheric moisture before we go down that road any further…

Humidity, Condensation and Clouds Circulation of water in the atmosphere Evaporation, condensation and saturation Humidity Dew and frost Fog Clouds

Circulation of Water in the Atmosphere

Circulation of Water in the Atmosphere evaporation condensation precipitation hydrologic cycle The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of precipitation for the planet.

Figure 4.1: The hydrologic cycle. Fig. 4-1, p. 80

Figure 4.1: The hydrologic cycle. Stepped Art Fig. 4-1, p. 80

Evaporation, Condensation and Saturation

Evaporation, Condensation and Saturation condensation nuclei In very clean air, about 10,000 condensation nuclei are typically found in one cubic centimeter of air, a volume approximately the size of your fingertip.

Humidity

Mixing Ratio The ratio of the mass of water vapor in air to the mass of dry air: w = mv / md Usually expressed in g kg-1 Some typical values: Tropical marine boundary layer air: w ≈ 18 g kg-1 Polar air: w ≈ 1 g kg-1 Stratospheric air: w ≈ 0.1 g kg-1

Specific Humidity The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor): SH = mv / (md + mv) w = SH / (1 – SH) SH = w / (1 + w)

Vapor Pressure actual vapor pressure saturation vapor pressure “Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that water vapor is condensing.

Vapor Pressure Actual vs. Saturation (or equilibrium) vapor pressure… (Chalkboard)

Vapor Pressure Formula: Saturation vapor pressure depends only on temperature… Formula: Saturation vapor pressure Saturation vapor pressure at 273 K = 6.11 mb Latent heat of vaporization = 2.5x106 J kg-1 Gas constant for water vapor = 461 J kg-1 K-1 273 K Temperature

Vapor Pressure Formula: Saturation vapor pressure depends only on temperature… Formula:

Vapor Pressure Saturation vapor pressure depends only on temperature… Graph:

Relative Humidity definition of relative humidity saturation and supersaturation condensation relative humidity and temperature When the general public uses the term “humidity”, they mean “relative humidity.”

Relative Humidity The ratio of the actual vapor pressure to the saturation vapor pressure. rh = e / es Since es depends on temperature, the relative humidity measures closeness to saturation, not actual water vapor content.

Figure 4.5: Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C it is about 42 mb. The insert illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 4-5, p. 83

Figure 4.7: When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed. Fig. 4-7, p. 85

Relative Humidity and Dew Point dew point temperature: the temperature to which air must be lowered to reach 100% relative humidity dew point depression and relative humidity The dew point temperature is useful for forecasting heat index, precipitation probabilities, and the chance of frost.

Measuring Humidity psychrometers hygrometers

Dew and Frost

Dew and Frost dew frost frost point and deposition Frost is one of the few examples of deposition in nature.

Fog

Fog radiation fog advection fog upslope fog evaporation (mixing) fog Fog is an extreme hazard to aircraft.

Clouds

Classification of Clouds major cloud types cloud appearance cloud base It’s easy to identify clouds, but it takes practice. The ability to identify clouds allows you to forecast many aspects of the weather using nothing but your eyes.

Table 4-2, p. 98

Cloud Identification high clouds middle clouds low clouds clouds with vertical development

High Clouds cirrus cirrocumulus cirrostratus Cirrostratus clouds can sometimes be quite thick.

Middle Clouds altocumulus altostratus Altocumulus clouds are very pretty, especially just after sunrise or just before sunset.

Low Clouds nimbostratus stratocumulus stratus Marine stratocumulus is the most common cloud type in the world.

Clouds with Vertical Development cumulus cumulus congestus cumulonimbus Not all cumulus clouds grow to be thunderstorms, but all thunderstorms start out as cumulus clouds.

Some Unusual Clouds lenticular clouds pileus mammatus clouds contrails Several alleged ‘flying saucer’ reports have turned out to be lenticular clouds.

Cloud Development and Stability

Cloud Development and Stability surface heating and free convection uplift along topography widespread ascent lifting along weather fronts

Convection and Clouds thermals fair weather cumulus Fair weather cumulus provide a visual marker of thermals. Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

Topography and Clouds orographic uplift rain shadow The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Conditional Stability Environmental lapse rate between wet and dry adiabatic lapse rates Lifting Condensation Level (LCL) Level of Free Convection (LFC) (Chalkboard)

Summary of Atmospheric Stability Absolute Stability = environmental lapse rate greater than both wet and dry adiabatic lapse rates Absolute Instability = environmental lapse rate less than both wet and dry adiabatic lapse rates Conditional Stability = environmental lapse rate less than dry but greater than we adiabatic lapse rate. The environment is stable to moist but unstable to dry disturbances.

Precipitation Processes

Collision and Coalescence Process terminal velocity coalescence warm clouds A typical cloud droplet falls at a rate of 1 centimeter per second. At this rate it would take 46 hours to fall one mile.

Figure 5. 9: Conditionally unstable atmosphere Figure 5.9: Conditionally unstable atmosphere. The atmosphere is conditionally unstable when unsaturated, stable air is lifted to a level where it becomes saturated and warmer than the air surrounding it. If the atmosphere remains unstable, vertical developing cumulus clouds can build to great heights. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Stepped Art Fig. 5-9, p. 116

Ice Crystal Process cold clouds supercooled water droplets saturation vapor pressures over liquid water and ice accretion The upper portions of summer thunderstorms are cold clouds!

Figure 5. 22: The ice-crystal process Figure 5.22: The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 5-22, p. 124

Figure 5. 22: The ice-crystal process Figure 5.22: The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Stepped Art Fig. 5-22, p. 124

Cloud Seeding and Precipitation silver iodide It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

Precipitation in Clouds accretion ice crystal process

Precipitation Types

Rain rain drizzle virga shower Virga is much more commonly observed in the western US, because the humid climate of the eastern US reduces the visibility.

Snow snow fallstreaks dendrite blizzard Snowflake shape depends on both temperature and relative humidity.

Sleet and Freezing Rain rime Sleet makes a ‘tap tap’ sound when falling on glass.

Snow Grains and Snow Pellets graupel

Hail updraft cycles accretion A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

Figure 5.35: Hailstones begin as embryos (usually ice particles) that remain suspended in the cloud by violent updrafts. When the updrafts are tilted, the ice particles are swept horizontally through the cloud, producing the optimal trajectory for hailstone growth. Along their path, the ice particles collide with supercooled liquid droplets, which freeze on contact. The ice particles eventually grow large enough and heavy enough to fall toward the ground as hailstones. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Stepped Art Fig. 5-35, p. 134

Measuring Precipitation

Instruments standard rain gauge tipping bucket rain gauge It is difficult to capture rain in a bucket when the wind is blowing strongly.

Doppler Radar and Precipitation

Figure 5. 39: A microwave pulse sent out from the radar transmitter Figure 5.39: A microwave pulse sent out from the radar transmitter. The pulse strikes raindrops and a fraction of its energy is reflected back to the radar unit, where it is detected and displayed, as shown in Fig. 5.40. Stepped Art Fig. 5-39, p. 135