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Clouds & Particles More Unit 1: What happens in clouds? We saw in the 'basics' section that clouds are made up of millions of droplets which form as water condenses onto tiny suspended particles. In this 'read more' section on clouds we'll have a look in more detail at how these water droplets form, what happens inside clouds and how different cloud types vary. Formation of droplets Cloud characteristics Cloud chemistry Part 1: Formation of droplets A cloud forms when humid air is cooled enough so that the water vapour it holds becomes liquid. In this section we look at the relationship between temperature and the amount of water vapour the air can hold and what affects the size of water droplets in a cloud. Saturation of air The quantity of water vapour that air can hold depends on its temperature. For a given temperature, the relationship between the amount of water a given mass of air actually holds and the total amount it can hold is known as the relative humidity. Air is said to be saturated when it holds as much water vapour as it can. Saturated air, therefore, has a relative humidity of 100%. Supersaturated air has a relative humidity of more than 100%. The table below shows the maximum amount of water vapour the air can hold at different temperatures, before condensation begins. T°C -20 -10 0 +10 +20 +30 Amount of water vapour 1.1 2.3 4.8 9.4 17.3 30,5 (g water per m3 of air) See the water vapour saturation curve for more values. For example, imagine an air parcel at 20°C that contains 9.4 g m-3 of water vapour. The relative humidity of the air is therefore (9.4/17.3) x 100 = 54.3%. Now assume the air parcel cools down to 10oC (for example by rising in the atmosphere), at this point the air is now saturated and the relative humidity is 100%. Now imagine that the air rises further and the temperature falls to 0°C. At 0oC, air can only hold 4.8 g m-3 of water vapour and the air parcel is now supersaturated by (9.4 - 4.8) = 4.6 g of water. This extra water condenses on available aerosol particles forming cloud droplets and the relative humidity of the air parcel returns to 100%. ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 1 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics The warmer the air, the more water it can hold. This is why warm air is used to dry objects, it absorbs moisture. On the other hand, if saturated air is cooled down, the water vapour in the air is forced to condense into water droplets. This is why water droplets are seen on the outside of a cold soft drinks can . The cold can cools the air around it causing the water vapour in the air to condense. Water droplets inside clouds 1. A cold drink can 'sweating'. Source: C. Gourbeyre. Cloud droplet diameters vary from a few micrometers to more than 100 µm (0.1 mm), with the average droplet size being around 10 µm in diameter. Continental clouds are generally made up of many small droplets. Marine clouds are made up of fewer droplets but the droplets are large. There are usually between 25 000 and 1 million water droplets per litre of air. The distance between 2 droplets is around 1.4 mm, a distance about 70 times the diameter of each droplet (it's perhaps easier to visualise this as a football on the ground every 20 or 30 meters). In order to fall as rain, droplets need to grow until they reach a diameter of around 1 mm, that's about one hundred times bigger than the droplet was originally! For "warm clouds" (those which don't contain ice crystals), droplets grow to rain drop size by sticking to each others. As times passes, the droplets become larger and larger until they are too heavy to remain suspended in the air by the up-drafts that counterbalance the droplets falling. In cumulonimbus clouds, for example, the up-drafts are very strong. This means that droplets can grow to very large sizes before they become too large to remain suspended in the air. This is why the raindrops are so big during thunderstorms. 2. Author: J. Gourdeau. Colder clouds are made up of ice crystals, liquid water and water vapor. Water vapour condenses onto the ice crystals and liquid droplets freeze when they come into contact with the ice crystals. As the ice crystals become bigger and bigger, they start to fall as snow, or rain if they melt before reaching the ground. 3. Source: freefoto.com 4. Source: fond-ecran-image.com ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 2 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics Part 2: Cloud characteristics Characteristics of the different cloud types In the 'basics' section we looked at the different types of clouds and showed that clouds are classified into four groups depending on the height of the cloud base and the cloud thickness. In this 'read more' section we will look at the different cloud types in more detail. The different types of clouds: an overview The lower layer of the atmosphere, the troposphere, can be divided into three layers: the lower level, the mid level and the upper level. These levels don't have firm definitions and their height depends on the latitude considered. In Europe, low level clouds occur at altitudes of up to 2 km whereas medium level clouds extend to about 6 km. The highest clouds are seen at altitudes of up to 12 km. 1. The different types of cloud in the troposphere. St: stratus, Sc: stratocumulus, Nb: nimbostratus; Ac: altocumulus, As: altostratus; Ci: cirrus, Cs: cirrostratus, Cc: cirrocumulus; Cu: cumulus, Cb: cumulonimbus. Author: J. Gourdeau. Some clouds generate rain, others don't. You can only get drizzle from stratus clouds and can only get hail from cumulonimbus clouds. High level clouds and altocumulus clouds hardly ever rain. The table on the right shows what type of precipitation you can get from each cloud type. High level clouds: Ice crystals As Ns Sc St Cu Cb Rain Drizzle Snow Hail The highest level clouds, like cirrus, are made of tiny ice crystals rather than liquid water droplets. A typical small ice crystal contains between 1016 and 1018 water molecules. Although no two ice crystals are exactly alike, there are several basic types of crystals. The crystal shape depends mainly on temperature and this is shown in figure 2. Snowflakes are simply aggregates of ice crystals. As long as the snowflakes do not pass through a layer of air warm enough to make them melt, the snow flakes remain intact and reach the ground. Although they are only made up of ice ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 3 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics crystals, high level clouds never produce snow because their crystals are too small, and therefore not heavy enough to fall. 2. The different shapes of ice crystals according to the air temperature and degree of supersaturation. Source: www.snowcrystals.com. The record for the most snow falling in a single day is held by Silver Lake, Colorado in the United States of America. On the 15th April 2001, 192 cm of snow fell! 3. A dog in the snow! Source, M. Ruinhart. Low level clouds: stratus, stratocumulus and nimbostratus and fog Clouds at low levels form as a result of condensation of water vapour into liquid cloud droplets. Stratus clouds form a layer near the ground that is typically only a few hundred meters thick. Only stratus clouds produce drizzle, which we define simply as water droplets less than 0.5 mm in diameter. They fall so slowly that they seem to remain suspended in the air. Cumulonimbus clouds and thunderstorms In summer, strong daytime heating warms the lower atmosphere making it warm and moist. This warm air rises by convection and, since the air above is ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 4 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics colder, this makes the atmosphere unstable. Water vapour in the rising air condenses into water droplets as the warm air cools. This condensation process releases more heat into the atmosphere and convection is further accelerated. A towering cumulonimbus cloud forms in just 30 minutes which can reach upto 15 km in height and millions tonnes of water are lifted from the ground into the sky. We still don't know the exact way lightning forms. Water droplets, ice crystals and hailstones inside the cumulonimbus cloud collide because of the strong air currents which exist in the cloud. Friction then creates static electricity. Positive charges build up at the top of the cloud and negative ones build up at the bottom. The ground underneath is positively charged. The difference between charges gets bigger and bigger until lightning sparks cross the gap. The amount of energy stored by a cumulonimbus cloud is huge, about the same amount as a small atomic bomb! 4. Illustration of the electrical structure of a thundercloud. Author: J. Gourdeau. Lightning travels at upto 40,000 km per second and can generate 100 millions volts of electricity. This heats the air in the path of the lightning flash to around 30,000°C! This temperature is so hot that the air expands violently, like popcorn, and creates sound waves which we hear as thunder. 5. Lightning strikes seen during a night-time thunderstorm. Source: NOAA. Part 3: Cloud chemistry Clouds are not an inert mixture of water droplets (or ice crystals) and particles. Particles that allow clouds to form are called cloud condensation nuclei (CCN). These particles have different chemical compositions depending on their origin. They can have natural sources and originate from the deserts, from the oceans, from volcanoes or ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 5 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics from living organisms or they can come from human processes. The cloud also contains gases which can change the chemical composition of the droplets. So a cloud is far from inert! Four main processes occur within the cloud droplet. These are shown in Figure 1: 1. the composition and size of the CCN particle changes after the droplet evaporates. 2. the soluble part of the particle dissolves. 3. chemical reactions occur inside the water droplet. 4. transfer between atmospheric gases and the liquid phase take place. 1. Various cloud chemistry processes. Author: Justine Gourdeau. The particle inside the droplet The water soluble fraction of an aerosol particle governs whether it can take up water and grow into a droplet. The chemical composition of CCN particles controls the initial chemical composition of a cloud droplet as its soluble content dissolves in the condensed water. The less water soluble particles, for example soil dust, pollen and particles from biomass burning, remain in the surrounding air. Most clouds don't lead to rain and simply evaporate. As a result of in-cloud chemical reactions, the particles which remain after the water evaporates have a different chemical composition to those which entered the cloud in the first place. 2. A sulphur dioxide molecule (1) reacts with ammonia in the air to form to ammonium sulphate (2). This then grows to form an ammonium sulphate particle (3). These particles are hygroscopic, meaning they rapidly grow in the presence of water (4). Author: J. Gourdeau. The atmospheric gases around the droplet Whether a chemical species stays in the gas phase or is absorbed by the water droplet is determined by the Henry’s law equilibrium: A(aq) = HA PA where: A(aq) is the aqueous phase concentration (mol L-1) PA is the partial pressure of A in the gas phase (atm) ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 6 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics HA is the Henry’s law coefficient of the gas considered. Some species go back to the gas phase and move away from the drop. Others, once captured, remain associated with the aqueous phase unless total evaporation occurs. Reactions inside the droplet At least a hundred chemical reactions take place in a droplet. These reactions can change the acidity of the rainfall resulting in acid rains which are hazardous to plants and animals living in lakes and streams and can contribute to the deterioration of buildings. The main chemical species involved in acid rain are sulphuric (H2SO4) and nitric acids (HNO3). 3. A scientist collecting water samples for acid rain analysis. Look at the damaged forest! Source: NOAA. All this complex chemistry modifies not only the cloud itself but also the atmosphere around the cloud. Only about one cloud in every seven results in rain and, as a result, a single particle acting as a CCN undergoes between 10 and 25 evaporation-condensation cycles before it reaches the ground. ESPERE Climate Encyclopaedia – www.espere.net - Clouds and Particles More - page 7 English offline version supported by the International Max Planck Research School on Atmospheric Chemistry and Physics