Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

1. Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt Anthony J. Vega

2. Part 2. Water in the Atmosphere Chapter 6 Cloud Development and Forms

3. Introduction • Clouds are instrumental to the Earth’s energy and moisture balances • Most clouds form as air parcels are lifted and cooled to saturation • Mechanisms that Lift Air • Orographic Lift occurs as air is displaced over topographic barriers such as mountains and hills • On the windward side of the barrier, air is displaced toward higher altitudes and undergoes adiabatic cooling, possibly to saturation • On the leeward side, descending air warms through compression leading to a dry rainshadow • Exemplified by the Sierra Nevada mountain range and the dry eastward interior

4. Orographic uplift (left) and orographically induced clouds (below)

5. Frontal Lifting • When boundaries between air of unlike temperatures (fronts) migrate, warmer air is pushed aloft • This results in adiabatic cooling and cloud formation • Cold fronts occur when warm air is displaced by cooler air • Warm fronts occur when warm air rises over and displaces cold air A cold front (a) and a warm front (b)

6. Convergence • Atmospheric mass is non-uniformly distributed over Earth • Air advects from areas of more abundant mass to areas of less mass • Air moving into these low pressure regions converges • Stimulates rising motions and adiabatic cooling • Localized Convection • Localized surface heating may lead to spatially limited free convection • Vertical motions are stimulated from the surface upward resulting in towering clouds and a chance for intense precipitation over small spatial scales

7. Static Stability and the Environmental Lapse Rate • Static stability refers to atmospheric conditions as they relate to vertical air motions • An atmosphere which supports upward motions = statically unstable • A parcel pushed up in this condition will rise freely • An atmosphere which resists vertical motions = statically stable • A parcel pushed up in this condition will return to the origination point • An atmosphere that promotes neither situation = statically neutral • A displaced parcel will simply remain at rest • Static stability is related to temperature controlled buoyancy • Positive buoyancy in a parcel = less density than surrounding conditions, statically unstable situation • Negative buoyancy in a parcel = greater density than surrounding conditions, a statically stable situation

8. Absolutely unstable, and unsaturated, air Absolutely unstable, but saturated, air

9. Temperature and saturation conditions of rising parcels as they relate to ambient air lead to absolutely unstable, absolutely stable, and conditionally unstable atmospheres • Absolutely Unstable Air • When parcel temperature, considering the dry adiabatic lapse rate (DALR), is greater than the ambient air, taking into account the environmental lapse rate (ELR), positive buoyancy occurs • Vertical motions are supported throughout the atmosphere and buoyancy increases with height • Parcel cooling rates are less than that of the ambient air • Whether the parcel is saturated or not

10. Absolutely stable, unsaturated, air Absolutely stable, but saturated, air

11. Absolutely Stable Air • When the ELR is less than the saturated adiabatic lapse rate (SALR) • Negative buoyancy results as vertical motions are discouraged • No matter where a parcel is lifted, it will always be cooler than the ambient air • Conditionally Unstable Air • When the ELR is between the DALR and the SALR • Initially the atmosphere resists vertical motions • If a parcel is forced to rise and saturation occurs, parcel cooling at the lesser SALR will eventually create a situation where the parcel temperature will exceed that of the ambient air • The parcel will accelerate upward under a positive buoyancy situation • Parcel buoyancy is dependent on lifting to the level of free convection

12. A conditionally unstable situation with unsaturated air A conditionally unstable situation with saturated air

13. Factors Influencing the ELR • The ELR changes temporally and spatially as surface temperatures change • Heating or Cooling of the Lower Atmosphere • During the day, surface insolation gains result in greater heating near the surface than aloft • This leads to a greater ELR through the lower atmosphere • The ELR is greatest on clear days, above dry, low albedo surfaces • At night, the situation reverses as terrestrial radiation loss causes near surface chilling = a temperature inversion A diurnal profile of the ELR

14. Advection of Cold and Warm Air at Different Levels • Vertical temperature profiles may lead to varying wind direction with height • Advection of cold or warm air at different levels promotes such conditions and affects the ELR • Advection of an Air Mass with a Different ELR • Air masses are quantities of air with similar temperature and moisture characteristics • Those conditions are maintained as the air mass advects to other locations, although modifications occur • As temperatures change with the advection, so does the ELR Advection influences on the ELR

15. ELR changes with air mass replacement • Limitations on the Lifting of Unstable Air • Rising parcels are limited by atmospheric stability and entrainment • A Layer of Stable Air • A rising parcel may reach a stable upper air environment • The parcel cooling rate will exceed that of the ambient air • The parcel will slowly cease ascension and come to rest at some equal temperature level

17. Entrainment • Ambient air intrusions into parcels which limits vertical cloud development • Causes evaporation along cloud boundaries • The evaporation process uses latent heat which cools the cloud margins and reduces buoyancy • Extremely Stable Air: Inversions • An atypical situation when temperature increases with height through the troposphere • A negative buoyancy situation • Develop from a variety of situations • Most commonly they relate to terrestrial radiation loss on clear, calm nights • Near surface air chills diabatically more rapidly than air aloft • Radiation fogs are a common indicator of a radiation inversion

18. Frontal inversions form along cold/warm air boundaries as air associated with a front wedges into unlike air at some angle • Sleet and freezing rain are commonly associated with this situation • Subsidence inversions occur when air sinks toward the surface and undergoes adiabatic warming • Frequently found on the eastern sides of migratory or semi-permanent high pressure areas and in lee of mountains Profile of a frontal inversion

19. Profile of a subsidence inversion

20. Cloud Types • Occur in an unlimited variety of size, shape, and composition • Subdivided into classes based on appearance and/or height • High Clouds • Bases above 6000 m (19,000 ft) • Composed of ice • Cirrus is the most common • Wispy appearance due to low water content and cold temperatures • Fall streaks may appear below as ice crystals descend • Mares’ tails - horizontal swirls, occur in turbulent conditions • Cirrostratus occurs when cirrus thickens and stretch across the sky • May form a halo about the sun or moon as entering light is refracted 22o by cloud ice crystals • Cirrocumulus occurs due to thickening causing a billowy appearance which resembles fish scales - a mackerel sky

21. Cloud types

22. Cirrus Cirrus clouds with fall streaks

23. Middle Clouds • Bases between 2000 and 6000 m (6-19,000 ft) • Largely composed of liquid drops • Carry the “alto” prefix • Altostratus is typically thick enough to almost fully obscure the sun or moon and blanket the sky from horizon to horizon • Altocumulus, typically typified by a banded arrangement of billowy clouds • Low Clouds • Bases below 2000 m (6,000 ft) • normally composed of liquid water • Most commonly, stratus clouds which blanket large spatial areas • Shallow vertical extent (0.5-1 km) • Precipitation associated with nimbostratus is usually very light • Stratocumulus are low, layered clouds with some vertical development

24. Altocumulus Stratus

25. Stratocumulus

26. Clouds with Vertical Development • High vertical velocities in air that is unstable or conditionally unstable • Cumulus clouds occur • Cumulus humilis, or fair weather cumulus • Develop primarily from convection • Usually evaporate shortly after formation and are vertically limited • Cumulus congestus • More organized development as cloud towers appear • Each tower is indicative of uplift cells • Cells are short lived but are constantly replaced • Each tower progresses higher • Cumulonimbus • Most violent of all clouds = thunderstorms • Indicate inherently unstable conditions • From base to top may extend fully through the troposphere • Anvil top may form as ice crystals are blown horizontally

27. Cumulus humilis Cumulus congestus

28. Formation of fair weather cumulus

29. Cumulonimbus

30. Unusual Clouds • Not easily categorized as they occur in a variety of situations • Lenticular clouds form as a result of turbulence downwind of mountain ranges • Exhibit a lens shape • Banner clouds are similar to lenticular but are anchored to individual mountain peaks • Mammatus, or sack-like protrusions from the base of a cloud, indicate low level turbulence common in cumulonimbus clouds • Nacreous clouds, composed of supercooled water or ice, are stratosphere clouds sometimes called mother of pearl clouds • Noctilucent clouds form in the mesosphere and are typically illuminated after sunset

31. Lenticular Banner cloud

32. Nacreous Noctilucent

33. Cloud Coverage • When clouds comprise more than 9/10th of the sky = overcast • When coverage is between 6/10th and 9/10th = broken • When coverage is between 1/10th and 6/10th = scattered • Cloud coverage less than 1/10th = clear

34. End of Chapter 6Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt