EG4508: Issues in environmental science Meteorology and Climate Dr Mark Cresswell Evaporation, Humidity, Fog, Cloud & Precipitation

Phases of water Water (H 2 O) is the most important material on the planet Water can exist in solid, liquid and gas phases Water molecules are free to move in a gas, are closer together in a liquid and are locked in an orderly pattern as a solid

Phases of water As a solid, water forms hexagonal (6- sided) crystals we call ice In freezing air, if enough energy is available, ice can change directly into gas (water vapour). This is called sublimation If a water vapour molecule combined with ice crystals it is deposition

Phases of water Applying warmth (energy) to an ice crystal means that the molecules vibrate faster - so much so that they can vibrate out of their hexagonal crystal structure - the ice melts At the surface of water, some molecules have just enough energy to break free from the rest - called evaporation

Phases of water Some water vapour molecules with very little energy can combine with other water molecules on the surface of water - called condensing If a cover is placed over a beaker of water, eventually an equilibrium between escaping and returning water molecules is reached.

Phases of water When this state of equilibrium is reached the air in the beaker is said to be saturated with water vapour Removing the cover from the beaker would allow some molecules to be blown away - so the air would no longer be saturated and more would have to evaporate to take their place

Phases of water This explains why evaporation occurs more readily when there is wind than on a still day Temperature also affects evaporation Warm water means that molecules have more energy and speed up. These molecules are more likely to escape from the liquid surface

Phases of water Higher temperatures lead to enhanced evaporation Conversely, condensation is more likely to occur when the temperature is lowered

Hydrological (water) cycle Water is in constant motion in the atmosphere, in the oceans and on land Water evaporates over the oceans, then condenses at altitude to form clouds Wind carries clouds over land where they release their water as precipitation Over land, water is released into the atmosphere by transpiration and evaporation

Linacre et al, 1997

Hydrological (water) cycle Evaporation and transpiration over terrestrial areas accounts for only about 15% if the 1.5 billion billion gallons that annually evaporate. The other 85% evaporate over the oceans If all atmospheric water vapour were to condense and fall as rain it would cover the globe 2.5 centimetres thick

The mass of water vapour in a given volume (parcel) of air Represents water vapour density - usually expressed as g/m 3 Absolute humidity AH = mass of water vapour / volume of air

Mass of water vapour compared to the total mass of the parcel of air (including water vapour) usually expressed as g/kg Specific humidity SH = mass of water vapour / total mass of air

Vapour pressure The air’s moisture content may also be expressed in terms of the pressure exerted by the water molecules within it Air pressure at sea level is the result of pressure exerted by all gas molecules (nitrogen and oxygen included). The total pressure is equal to the sum of all pressures from all gases - known as Dalton’s law of partial pressure

Vapour pressure An increase in the number of water vapour molecules will tend to increase the total vapour pressure Actual vapour pressure indicates the air’s total water vapour content. Saturation vapour pressure describes how much water vapour is necessary in order to make the air saturated at any given temperature (remember the beaker hypothesis).

Vapour pressure Saturation vapour pressure is the pressure that the water vapour molecules would exert if the air were saturated with vapour at a given temperature

Relative humidity A common measure that is often misunderstood It tells us how close the air is to being saturated It is the ratio of the amount of water vapour actually in the air compared to the maximum amount of water vapour required for saturation at that particular temperature and pressure

Relative humidity Put more simply, it is the ratio of the air’s water vapour content to its capacity: RH = water vapour content / water vapour capacity RH(%) = (actual VP / SVP) * 100

Dew point The dew point is the temperature to which air must be cooled (with no change in air pressure or moisture content) for saturation to occur When the dew point temperature is reached on a surface, dew, frost or fog forms Lifting condensation level for air aloft

Measuring humidity Humidity is measured using a psychrometer (whirling or clockwork) Wet and dry bulb thermometers based in a Stevenson screen use the same principle Difference between wet and dry bulb temperatures indicates water vapour content of the air

Formation of FOG #1 The process of condensation that forms fog and clouds is not so simple. It is not simply the case that saturation (dew point) must be reached There must be airborne particles on which water vapour can condense

Formation of FOG #2 Although air looks clean - it never really is. Air contains many tiny particles (impurities) many of these particles serve as a surface on which condensation can occur These particles are called condensation nuclei Some condensation nuclei are very small with a radius of < 0.2µm (Aitken nuclei) Particles µm are called large nuclei Particles > 1µm giant nuclei

Formation of FOG #3 As the relative humidity reaches % (saturation) water condenses onto condensation nuclei As the air cools and becomes more saturated the droplets of suspended condensed water get larger until visible to the naked eye We can see these clouds of droplets as fog

Radiation fog

Formation of FOG #4 City air (with its extra impurities) produces a thicker fog as there are more condensation nuclei London often suffered from very thick fog as a result of pollution and industrial activity until legislation was introduced early in the 20th century

City fog – exacerbated by pollutant particulates

Formation of FOG #5 Fog often forms near the ground on a natural surface (e.g. football pitch) This is exacerbated on clear nights when radiation leaves rapidly and cools the ground down and the moist air directly above it This is known as radiation fog

Formation of FOG #6 Fog is usually seen in low lying areas as it is denser than the surrounding air and is pulled to the surface by gravity When fog is seen to "burn off" by sunlight it is actually the heating of the ground which rises the air temperature above causing the air to become unsaturated and the fog dissipates

Formation of FOG #7 Fog may form when warm moist air travels over a cold surface. This is known as advection fog Fog may form by the mixing of two unsaturated air masses - leading to evaporation and enrichment of water vapour. This type of fog is called evaporation (mixing) fog. It forms when cold unsaturated air settles over warm water from which water may be evaporating - explains why fogs form over lakes and ponds in summer

Advection Fog in a valley

Formation of Clouds #1 Clouds form in a similar way to fog - except that the process takes place aloft In the case of cloud formation, the cooling required to cause water to condense on particulate nuclei is due to adiabatic cooling Clouds consist of tiny particles of ice or water droplets (formed around condensation nuclei) so small and light in weight that impacts from the air's randomly moving molecules are sufficient to keep them aloft

Formation of Clouds #2 Cloud formation may be convectional, orographic or frontal Convectional clouds form when moist air is carried upwards by the action of vertical convection (due to solar heating of the surface). Moist air cools as it ascends until it becomes saturated. At the point of saturation, the moist air condenses to form cloud (Convectional Condensation Level)

Formation of Clouds #3 Strong surface heating where there is very moist air can lead to the development of intense storm clouds - so called "anvil shaped" structures that produce large amounts of precipitation

Formation of Clouds #4 Orographic cloud forms when moist air is forced upwards - usually when it flows over a plateau or mountain. The air forced upslope cools until it becomes saturated forming clouds near to or above the mountainous structure (Lifting Condensation Level). The air (now free of moisture) flows down the lee side of the mountain at a higher temperature as energy lost during condensation is carried away by the wind. Rainfall usually occurs on the lee side, forming a rain shadow on the upslope side.

Formation of Clouds #5 With frontal cloud formation, moist warm air is forced above cooler air (the cooler air acting like a wedge). As this moist air is forced upwards it cools until the air becomes saturated and condenses into clouds. The clouds are usually easily seen as a visible ridge along the line of an active front.

Formation of Clouds #6 When winds are forced to converge at low level (the ITCZ near the equator for example) air will be forced upwards where it cools and condenses. This is known as convergence clouds and explains why the equatorial ITCZ is visible from space as a zone of active cloud systems along the equator.

Cloud Types NAMEABBR.HEIGHT (km)CATEGORY CirrusCi6 – 10HIGH CirrocumulusCc6 – 10HIGH CirrostratusCs6 – 10HIGH AltostratusAs3 – 6MEDIUM AltocumulusAc3 – 6MEDIUM StratocumulusAc< 3LOW StratusSt1 – 2LOW CumulusCu0.6 – 6CUMULIFORM CumulonimbusCbTo the tropopauseCUMULIFORM

Precipitation processes We have already seen how important condensation nuclei are for the formation of droplets when the air becomes saturated To keep a droplet in equilibrium, more water vapour molecules are needed around it to replace those that are constantly evaporating from the surface

Precipitation processes Small cloud droplets have a greater curvature which causes a more rapid rate of evaporation. As a result of this process (curvature effect) smaller droplets require an even greater vapour pressure to keep them from evaporating away. This requires the air to be supersaturated - with a relative humidity greater than 100%. The smaller the droplet, the greater the supersaturation needed to keep it in equilibrium

Precipitation processes One may ask, how do droplets with a diameter of <1µm grow to the size of a cloud droplet? The answer lies with the cloud condensation nuclei. Many of these nuclei are hygroscopic (have an affinity for water vapour) Condensation may begin when the vapour pressure is much lower than the saturated vapour pressure This reduces the equilibrium vapour pressure required and is known as the solute effect

Precipitation processes In warm clouds (tops warmer than -15ºC) the action of collisions between droplets is important Random collisions with already large droplets mediated by salt particles (hygroscopic condensation nuclei) produce larger droplets when they collide Large droplets begin to reach terminal velocity and collide with smaller droplets in their wake - merging together in a process called coalescence Falling droplets may evaporate on their way down, or reach the ground as drizzle if the air below is moist

Precipitation processes The range of droplet sizes The cloud thickness Updraughts of the cloud Electric charge of the droplets and the cloud itself Coalescence may be enhanced where strongly charged droplets exist in a strong electrical field The most important factor in the production of raindrops is the cloud’s liquid water content. In a cloud with enough water, other factors are:

Precipitation processes In very deep convective clouds the ice-crystal process is an important factor in precipitation Ice crystals may form nuclei upon which other ice crystals may form These are deposition nuclei as water vapour changes directly into ice without passing through the liquid phase The constant supply of moisture to an ice crystal allows it to enlarge rapidly, it becomes heavy enough to overcome updrafts and begins to fall If these crystals stick together (accretion) the icy matter (rime) that forms is called graupel (or snow pellets).

Ice crystals No precipitation

Altocumulus Cloud Water and Ice clouds – usually bring precipitation After 15 – 20 hours

Stratus Cloud Typically overcast or drizzle conditions

Cumulus Cloud Associated with gusty winds and heavy precipitation

ITCZ (Inter-Tropical Convergence Zone)

Observing Cloud #1 Clouds are observed at regular intervals at ground stations The degree of cloud cover in the sky is expressed as oktas (or eighths) A value of 4 oktas would mean that about half (50%) of the sky was covered with cloud A value of 8 oktas means that the entire sky is cloud-covered (totally overcast)

Observing Cloud #2 Sometimes it is preferable to estimate the cloud cover for the three main layers of the troposphere (low, middle and high)