chapter 6 Copyright © 2011, 2015 by Roland Stull. Meteorology for Scientists and Engineers, 3rd Ed. clouds contents Clouds have immense beauty and variety. They show weather patterns on a global Processes Causing Saturation 159 scale, as viewed by satellites. Yet they are Cooling and Moisturizing 159 6 made of tiny droplets that fall gently through the Mixing 160 air. Clouds can have richly complex fractal shapes, Cloud Identification & Development 161 and a wide distribution of sizes. Clouds are named Cumuliform 161 according to an international cloud classification Stratiform 162 scheme. Stratocumulus 164 Clouds form when air becomes saturated. Satu- Others 164 ration can occur by adding water, by cooling, or by Clouds in unstable air aloft 164 mixing; hence, Lagrangian water and heat budgets Clouds associated with mountains 165 Clouds due to surface-induced turbulence 165 are useful. The buoyancy of the cloudy air and the Anthropogenic Clouds 166 static stability of the environment determine the Cloud Organization 167 vertical extent of the cloud. Fogs are clouds that touch the ground. Their lo- Cloud Classification 168 cation in the atmospheric boundary layer means that Genera 168 Species 168 turbulent transport of heat and moisture from the Varieties 169 underlying surface affects their formation, growth, Supplementary Features 169 and dissipation. Accessory Clouds 169 Sky Cover (Cloud Amount) 170 Cloud Sizes 170 Fractal Cloud Shapes 171 processes causing saturation Fractal Dimension 171 Measuring Fractal Dimension 172 Clouds are saturated portions of the atmosphere Fog 173 where small water droplets or ice crystals have fall Types 173 velocities so slow that they appear visibly suspend- Idealized Fog Models 173 ed in the air. Thus, to understand clouds we need to Advection Fog 173 understand how air can become saturated. Radiation Fog 175 Dissipation of Well-Mixed (Radiation and Advection) Fogs 176 cooling and Moisturizing Summary 178 Unsaturated air parcels can reach saturation by Threads 178 three processes: cooling, adding moisture, or mix- Exercises 179 ing. The first two processes are shown in Fig. 6.1, Numerical Problems 179 where saturation is reached by either cooling until Understanding & Critical Evaluation 181 the temperature equals the dew point temperature, Web Enhanced Questions 183 or adding moisture until the dew point tempera- Synthesis Questions 183 ture is raised to the actual ambient temperature. The temperature change necessary to saturate an air parcel by cooling it is: TT T ∆ = d − (6.1) “Meteorology for Scientists and Engineers, 3rd Edi- Whether this condition is met can be determined by tion” by Roland Stull is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike finding the actual temperature change based on the 4.0 International License. To view a copy of the license, visit first law of thermodynamics (see the Heat chapter). http://creativecommons.org/licenses/by-nc-sa/4.0/ . This work is available at http://www.eos.ubc.ca/books/Practical_Meteorology/ . 159 160 chAPTEr 6 ClOUDS The moisture addition necessary to reach satura- tion is ∆r = rs − r (6.2) Whether this condition is met can be determined by using the moisture budget to find the actual humid- ity change (see the Moisture chapter). In the real atmosphere, sometimes both cooling and moisturizing happen simultaneously. Sche- matically, this would correspond to an arrow from parcel A diagonally to the saturation line of Fig. 6.1. DMPVEZPS Clouds usually form by adiabatic cooling of ris- O GPHHZ P UJ ing air. Air can be rising due to its own buoyancy SB UV (making cumuliform clouds), or can be forced up B L1B T T over hills or frontal boundaries (making stratiform NPJTUVSJ[JOH F " clouds). Once formed, infrared radiation from cloud DPPMJOH top can cause additional cooling to help maintain VOTBUVSBUFE the cloud. Solved Example 5 $ Air at sea level has a temperature of 20°C and a mixing ratio of 5 g/kg. How much cooling OR mois- Figure 6.1 turizing is necessary to reach saturation? Unsaturated air parcel “A” can become saturated by the addi- tion of moisture, or by cooling. The curved line is the saturation Solution vapor pressure from the Moisture chapter. Given: T = 20°C, r = 5 g/kg Find: ∆T = ? °C, ∆r = ? g/kg. Use Table 4-1 because it applies for sea level. Other- wise, solve equations or use a thermo diagram. At T = 20°C, the table gives rs = 14.91 g/kg . At r = 5 g/kg, the table gives Td = 4 °C . Use eq. (6.1): ∆T = 4 – 20 = –16°C needed. Use eq. (6.2): ∆r = 14.91 – 5 = 9.9 g/kg needed. Check: Units OK. Physics OK. Discussion: This air parcel is fairly dry. Much cool- ing or moisturizing is needed. Mixing # Mixing of two unsaturated parcels can result in a DMPVEZPS saturated mixture, as shown in Fig. 6.2. Jet contrails GPHHZ and your breath on a cold winter day are examples of clouds that form by the mixing process. 9 straight line in Mixing essentially occurs along a L1B T F this graph connecting the thermodynamic states of VOTBUVSBUFE the two original air parcels. However, the satura- JPO tion line (given by the Clausius-Clapeyron equation) UVSBU TB $ is curved, so a mixture can be saturated even if the original parcels are not. 5 $ Let mB and mC be the original masses of air in parcels B and C, respectively. The mass of the mix- Figure 6.2 ture (parcel X) is : Mixing of two unsaturated air parcels B and C, which occurs along a straight line (dashed), can cause a saturated mixture X. The curved line is the saturation vapor pressure from the Mois- mXB= m + mC (6.3) ture chapter. r. STUll • METEOrOlOGy FOr SCIENTISTS AND ENGINEErS 161 The temperature and vapor pressure of the mix- Solved Example ture are the weighted averages of the corresponding Suppose that the state of parcel B is T = 30°C with values in the original parcels: e = 3.4 kPa, while parcel C is T = –4°C with e = 0.2 kPa. Both parcels are at P = 100 kPa. If each parcel contains mBB⋅ T + mCC⋅ T 1 kg of air, then what is the state of the mixture? Will TX = (6.4) the mixture be saturated? mX Solution mBB⋅ e + mCC⋅ e (6.5) Given: B has T = 30°C, e = 3.4 kPa, P = 100 kPa eX = mX C has T = –4°C, e = 0.2 kPa, P = 100 kPa Find: T = ? °C and e = ? kPa for mixture (at X). Specific humidity or mixing ratio can be used in place of vapor pressure in eq. (6.5). Use eq. (6.3): mX = 1 + 1 = 2 kg Use eq. (6.4): T = Instead of using the actual masses of the air par- X [(1kg)·(30°C) + (1kg)·(–4°C)]/(2kg) = 13°C. cels in eqs. (6.3) to (6.5), you can use the relative por- Use eq. (6.5): eX = tions that mix. For example, if the mixture consists [(1kg)·(3.4 kPa) + (1kg)·(0.2 kPa)]/(2kg) = 1.8 kPa of 3 parts B and 2 parts C, then you can use mB = 3 P hasn’t changed. P = 100 kPa. and mC = 2 in the equations above. Check: Units OK. Physics OK. Discussion: At T = 13°C, Table 4-1 gives es = 1.5 kPa. Thus, the mixture is saturated because its vapor pres- sure exceeds the saturation vapor pressure. This mix- cloud identification & ture would be cloudy/foggy. developMent You can easily find beautiful photos of all the clouds mentioned below by pointing your web- browser search engine at “cloud classification”, “cloud identification”, “cloud types”, or “Interna- tional Cloud Atlas”. You can also use web search engines to find images of any named cloud. To help keep the cost of this book reasonable, I do not in- clude any cloud photos. cumuliform Cumuliform clouds form vertically in up- drafts, and look like fluffy puffs of cotton, cauli- flower, rising castle turrets, or in extreme instabil- ity as anvil-topped thunderstorms that look like big mushrooms. These clouds have diameters roughly equal to the height of their cloud tops above ground (Fig. 6.3). Thicker clouds look darker when viewed from underneath, but when viewed from the side, the cloud sides and top are often bright white dur- ing daytime. The individual clouds are often sur- [ LN /BNF "QQFBSBODF rounded by clearer air, where there is compensating DVNVMVT DVNVMPOJNCVT subsidence (downdrafts). DPOHFTUVT DVNVMVT Cumulus clouds frequently have cloud bases NFEJPDSJT within 1 or 2 km of the ground (in the boundary lay- DVNVMVT TUSBUP er). But their cloud tops can be anywhere within the IVNJMJT DVNVMVT troposphere (or lower stratosphere for the strongest thunderstorms). Cumuliform clouds are named by their thick- ness, not by the height of their base (Fig. 6.3). Start- Figure 6.3 ing from the thinnest (with lowest tops), the clouds Cloud identification: cumuliform. These are lumpy clouds are cumulus humilis (fair-weather cumulus), caused by convection (updrafts) from the surface. 162 chAPTEr 6 ClOUDS S HLH cumulus mediocris, cumulus congestus (tow- ering cumulus), and cumulonimbus (thunder- storms). DBTUFMMBOVT Cumuliform clouds develop in statically un- stable air. The unstable air tries to turbulently stabilize itself by creating convective updrafts and downdrafts. Cumuliform clouds can form in the 1 L1B DMPVEUPQXIFSFDMPVEZQBSDFMDSPTTFTTPVOEJOH tops of warm updrafts (thermals) if sufficient mois- ture is present. Hence, cumulus clouds are convec- DVNVMVT tive clouds. These clouds are dynamically active DMPVECBTFBU -$- -$- in the sense that their own internal buoyancy-forces (associated with latent heat release) enhance and support the convection and their vertical growth. m m If the air is continually destabilized by some ex- 5 $ ternal forcing, then the convection persists.