Atmospheric

Moisture and

Precipitation

What is Moisture ?

A small amount of a liquid (such as water) that makes something wet or moistWetness caused by water; "drops of wet gleamed on the window"

Water is known to exist in three different states; as a solid, liquid or gas.

Clouds, snow, and rain are all made of up of some form of water. A cloud is comprised of tiny water droplets and/or ice crystals, a snowflake is an aggregate of many ice crystals, and rain is just liquid water.

The solid state of moisture is represented by solid forms of precipitation – snow – hail – sleet and tiny ice crystals - Solid state (e.g. ice) occurs at temperature below freezing point 00 C.

The liquid state of moisture consists of rain and of tiny drops of tiny droplets of water that aggregate to form most clouds. Liquid state (e.g. sea water) occurs between freezing and boiling point temperatures (00 C – 1000 C).

Water existing as a gas is called water vapor. When referring to the amount of moisture in the air, we are actually referring to the amount of water vapor. If the air is described as "moist", that means the air contains large amounts of water vapor - Gaseous state (e.g. water vapor) occurs at temperatures 1000 C or 2120 F

Water’s Changes of State

Solid to LiquidThe process of changing of state – such as melting ice – requires that energy be transferred in the form of heat.Latent Heat – is the energy absorbed or released during a change of state.

Water’s Changes of State

Liquid to GasEvaporation – is the process of changing a liquid to a gas

Condensation – is the process where a gas – like water vapor – changes to a liquid - water

Water’s Changes of State

Solid to Gas

Sublimation – is the conversion of a solid directly to a gas without passing through the liquid state.Deposition – is the conversion of vapor directly to a solid

What is Water Vapor?Water vapor is water in its gaseous state-instead of liquid or solid (ice). Water vapor is totally invisible. If you see a cloud, fog, or mist, these are all liquid water, not water vapor. Water vapor is extremely important to the weather and climate. Without it, there would be no clouds or rain or snow, since all of these require water vapor in order to form. All of the water vapor that

evaporates from the surface of the Earth eventually returns as precipitation - rain or snow. Water vapor is also the Earth's most important green house gas, accounting for about 90% of the Earth's natural greenhouse effect, which helps keep the Earth warm enough to support life. When liquid water is evaporated to form water vapor, heat is absorbed. This helps to cool the surface of the Earth. This "latent heat of condensation" is released again

when the water vapor condenses to form cloud water. This source of heat helps drive the updrafts in clouds and precipitation systems, which then causes even more water vapor to condense into cloud, and more cloud water and ice to form precipitation. It is restricted to the lower part of the troposphere – half of all water vapor is found within 1.6 km of the earth’s surface and only a tiny fraction exists above 6.4 km.

Water vapor is unevenly distributed and may be very low over desert areas and may be up to 4% by volume over equatorial areasEvaporation of liquid water is the main source of water vapor (gas) in the atmosphere

Water Vapor Cools and Warms the Climate System? When water evaporates from the surface of the Earth, it cools the surface. This keeps the surface from getting too hot. But because that water vapor is also the atmosphere's primary green house gas, water vapor acts to keep the Earth's surface warmer than it would otherwise be. The surface cooling effect of evaporation (which creates water vapor) is stronger than its greenhouse warming effect.

Water vapor varies by volume in the atmosphere from a trace to about 4%. Therefore, on average, only about 2 to 3% of the molecules in the air are water vapor molecules. The amount of water vapor in the air is small in extremely arid areas and in location where the temperatures are very low (i.e. polar regions, very cold weather). The volume of water vapor is about 4% in very warm and humid tropical air. 

Amount of Water Vapor in the AirThere is a limit to how much water vapor can enter the atmosphere. As more water vapor enters the atmosphere, the amount of pressure exerted by that water vapor increases. We call this pressure the vapor pressure. The higher the temperature of the atmosphere, the more vapor pressure it can withstand. When the vapor pressure maximum is reached, no more water can enter the atmosphere. At this point, we say that the atmosphere is completely saturated

Because the maximum vapor pressure increases with temperature, warmer air can hold more water vapor before becoming saturated.

Sources of Moisture in the AtmosphereEvaporation from oceans, lakes, rivers & soilTranspiration from plants and other vegetationPerspiration from people and animals. Sublimation: change from ice to water vapor

Evaporation is the process by which water changes from a liquid to a gas or vapor. Water boils at 212 degrees F (100 degrees C), but it actually begins to evaporate at 32 degrees F (0 degrees C); it just occurs extremely slowly. As the temperature increases, the rate of evaporation also increases.The amount of evaporation depends on the temperature, and it also depends on the amount of water there is to evaporate. For example, there is not much evaporation occurring in the Sahara Desert, but why? Although it can get extremely hot in the Sahara, it's just sand - there's just not much water to be evaporated.

What is Evapotranspiration?Evapotranspiration is an important process in the water cycle because it is responsible for 15% of the atmosphere’s water vapor.  Without that input of water vapor, clouds couldn’t form and precipitation would never fall.  Evapotranspiration is the combined name for the processes of evaporation and transpiration.  When water vapor is released into the atmosphere both processes are involved, so they have been combined into one word to cover all bases. The evaporation in evapotranspiration refers to water evaporated from over land.  This includes evaporation from soil, wetlands, and standing water from places

like roofs and puddles.  It can also refer to direct evaporation of liquid water from the leaf surface of the plant.Transpiration happens when plants release water vapor from tiny holes, called stomata, in their leaves.  This is caused in part by the chemical and biological changes that occur as the plant undergoes photosynthesis and converts carbon dioxide into oxygen.  Transpiration performs the same function as human sweating because plants do it to cool down their leaves.

Factors that affect evapotranspiration: Temperature – As temperature increases, the rate of evapotranspiration increases.  Evaporation increases because there is a higher amount of energy available to convert the liquid water to water vapor.  Transpiration increases because at warmer temperatures plants open up their stomata and release more water vapor.  Humidity – If the air around the plant is too humid, the transpiration and evaporation rates drop.  It’s the same reason sweat does not evaporate from our skin when it’s too humid.Wind speed – If the air is moving, the rate of evaporation will increase.  The wind will also clear the air of any humidity produced by the plant’s

transpiration, so the plant will increase its rate of transpiration.Water availability – If the soil is dry and there is no standing water there will be no evaporation.  If plants can’t get enough water they will conserve it instead of transpiring by closing their stoma.Soil type – Soil type determines how much water soil can hold and how easy it is for the water to be drawn out of it, either by a plant or by evaporation.  For areas where the ground is covered by vegetation, the rate of transpiration is considerably higher than the rate of evaporation from the soil.

Plant type – Some plants, like cacti and other succulents, naturally hold onto their water and don’t transpire as much.  Trees and crops are on the other end of the spectrum and can release copious amounts of water vapor in a day.  For example, an acre of corn can release 4,000 gallons of water vapor a day and a single large oak tree can transpire 40,000 gallons of water vapor in a year.

HumidityHumidity is the amount of water vaporin the atmosphere

It’s may be measured and expressed as:

- Absolute Humidity (gm/m3) - Specific Humidity (gm/kg) - Relative Humidity (%)

A device to measure humidity is called a hygrometer. Hygrometers may be designed for indoor or outdoor use (or both).

The psychrometer which is also called the “wet- and dry-bulb thermometer” is a type of hygrometer that measures the water vapor present in the air. A psychrometer is a kind of hygrometer and is used for the elite reasons of finding out the relative humidity, or moisture content, in the air. Relative humidity refers to the moisture content in the air compared to how much moisture the air can grasp at a given temperature. Information about the relative humidity is obliging in understanding the weather. A psychrometer is an instrument that computes both the wet bulb and dry bulb temperatures. Two thermometers are required to compute these constraints. From the values obtained, the relative humidity can be known.

Absolute HumidityAbsolute Humidity measures the total water vapor content of air.Absolute Humidity is the maximumamount of water vapor that a given columnof air can hold.It’s expressed as weight of water vapor in a given volume of air (gm/m3) Absolute Humidity is very sensitive to:

- temperature changes - changes in air volume

Specific Humidity

Specific Humidity measures the mass or weight of water vapor content in a given mass of air

It’s expressed as mass (gm) of water vapor in one kilogram (kg) of air (gm/kg)

For any given temperature, there is a maximum mass of vapor that a kilogram of air can hold

Specific HumidityIn cold polar air, specific humidity may be as low as 0.2gm/kg

In warm equatorial air, specific humidity may be as high as 16gm/kg

Specific humidity progressively increases from the poles to a single peak at the equator

Specific HumidityBoth absolute and specific humidity measure the quantity of perceptible water in the atmosphere

Relative HumidityRelative Humidity (R.H.) measures the percentage of water vapor present in the atmosphere relative to the maximum quantity the air could hold at the given temperature

Relative Humidity (R.H.) changes as air’s capacity to hold moisture (i.e. its vapor content) changes

Relative HumidityThere is an inverse relationship between temperature and relative humidity such that relative humidity is lower during the hot daytime and higher during the cooler night

Relative Humidity and Temperature ChangesRelative Humidity and Temperature Changes

Relative HumidityRelative Humidity (R.H.) increases if:

- evaporation increases vapor content of air- cooling reduces the holding capacity of air

Relative Humidity decreases if:- moisture is removed by condensation

or dispersal - heating increases air holding capacity

Relative HumidityRelative Humidity (R.H.) could be measured using this formula: Vapor pressure

R.H. = ---------------------------------- X 100

Saturation Vapor Pressure

R.H. is 100% at saturation

R.H. is 50% if only half of the total vapor is present

Relative HumidityAir that has R.H. of 100% (saturation) at cool temperature might be far from saturation at a warmer temperature even if the actual amount of moisture is unchanged

Dew point is the temperature at which air becomes saturated during cooling

Relative HumidityInstrument used for measuring relative humidity is called a hygrometer

A homemade hygrometer uses a strand of human hair attached to the end of a pointer to determine R.H.

The hair changes in length in response to changes in R.H.

Relative HumidityThe pointer falls whenever R.H. is high, or the hair lengthens when R.H. is high

Another method for measuring R.H. is through the use of a psychrometer whichconsists of 2 thermometers:

- a dry bulb thermometer - a wet bulb thermometer

Relative HumidityThe difference between the dry and wet bulb thermometer readings is called wet bulb depression

It involves the calculation of the wet bulb depression

The wet bulb depression is entered into a psychrometric table to obtain the R.H.

Pole-To-Pole Variation of Pole-To-Pole Variation of Relative Humidity (%)Relative Humidity (%)

Relative Humidity Values (%)

Dew PointThe dew point temperature is the temperature at which the air can no longer "hold" all of the water vapor which is mixed with it, and some of the water vapor must condense into liquid water. The dew point is always lower than (or equal to) the air temperature. If the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point where the dew point temperature equals the air temperature, the relative humidity is 100%. 

If there is then further cooling of the air, say because the air parcel is rising to higher (and thus colder) levels in the atmosphere, even more water vapor must condense out as additional dew, fog, or cloud, so that the dew point temperature then falls along with the air temperature. This is how precipitation forms...when water vapor is removed from the air so rapidly that the liquid water drops grow to a size where they fall out of the cloud. 

Dew Point

CONDENSATION Condensation is a process by which a gas such as water vapor is   changed into liquid water. When moisture cools and reaches saturation point, the tiny particles of water condenses into larger drops of water.

Forms of CondensationDew: Tiny drops of water formed when condensation of water vapor occur at or near the surface of the earth.Frost: It is a frozen condensation that occurs when air a ground level is super cooled below the freezing point.Fog: A mass of tiny drops of water that form when water vapor condenses on a nuclei near the earth's surface.Clouds: A cloud is a mass of tiny drops of water thatresults from condensation which takes place high up in the atmosphere.

CLOUDSA visible collection of particles of water or Ice suspended in the air, usually at an elevation above the earth's surface. They are a spectacular feature in the sky.They are the source of precipitation.Not all clouds precipitate – but all precipitation comes from the clouds.Clouds absorb – reflect scatter and re-radiate insolation. The function of the clouds in the global energy balance is important.

CLOUDSClouds are made of tiny drops of water or ice crystals that settle on dust particles in the atmosphere. The droplets are so small - a diameter of about a hundredth of a millimeter - that each cubic meter of air will contain 100 million droplets.Clouds will either be composed of ice or water droplets depending on the height of the cloud and the temperature of the atmosphere. Because the droplets are so small, they can remain in liquid form in temperatures as low as -30 °C. Extremely high clouds at temperatures below -30 °C are composed of ice crystals.

How do clouds form?Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals. There is water around us all the time in the form of tiny gas particles, also known as water vapor. There are also tiny particles floating around in the air - such as salt and dust - these are called aerosols.The water vapor and the aerosols are constantly bumping into each other. When the air is cooled, some of the water vapor sticks to the aerosols when they collide - this is condensation. Eventually, bigger water droplets form around the aerosol particles, and these water droplets start sticking together with other

droplets, forming clouds.Clouds form when the air is saturated and cannot hold any more water vapor, this can happen in two ways:The amount of water in the air has increased - for example through evaporation - to the point that the air cannot hold any more water.The air is cooled to its dew point - the point where condensation occurs - and the air is unable to hold any more water.The warmer the air is, the more water vapor it can hold. Clouds are usually produced through condensation - as the air rises, it will cool and reducing the temperature of the air decreases its ability to hold water vapor so that condensation occurs. The

height at which dew point is reached and clouds form is called the condensation level. Clouds mostly form and remain at some elevation above the earth’s surface- They can come into direct contact with the surface and this is known as fog.

What causes clouds to form?There are five factors which can lead to air rising and cooling and clouds forming.1. Surface heating - This happens when the ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals. Surface heating tends to produce cumulus clouds. 

2. Topography or orographic forcing - The topography - or shape and features of the area - can cause clouds to be formed. When air is forced to rise over a barrier of mountains or hills it cools as it rises. Layered clouds are often produced this way.3. Frontal - Clouds are formed when a mass of warm air rises up over a mass of cold, dense air over large areas along fronts. A 'front' is the boundary between warm, moist air and cooler, drier air.

4. Convergence - Streams of air flowing from different directions are forced to rise where they flow together, or converge. This can cause cumulus cloud and showery conditions.5. Turbulence - A sudden change in wind speed with height creating turbulent eddies in the air.The range of ways in which clouds can be formed and the variable nature of the atmosphere results in an enormous variety of shapes, sizes and textures of clouds.

CloudsClouds are composed of water droplets and

ice crystals

The base of a cloud is clear-cut and corresponds with the LCL (lifting condensation level)

Clouds grow upward from the base or LCL (lifting condensation level)

CloudsCloud may be classified on the basis of height of its base – general structure and appearance:- High cloud or cirrus cloud

(> 6 km cloud base)- Middle cloud or alto cloud

(2 – 6 km cloud base)- Low cloud (0 – 2 km cloud base)

Cloud NamesNames of specific types of clouds are created by combining the name of the cloud's shape with the name of the cloud's heightThere are 3 main types of clouds:Cumulus or fluffy cloudsStratus or layered cloudsCirrus or thin feathery clouds

Low Clouds •Stratus•Cumulus•Nimbostratus

Middle Clouds• Stratocumulus•Altocumulus•Altostratus

High Clouds• Cirrus• Cirrostratus• Cirrocumulus

Cirriform CloudsThe word cirrus comes from a Latin word and means a tuft or curl of hair. Cirrus clouds are very wispy and feathery looking.Cirrus generally occur in fair weather and point in the direction of air movement at their elevation. They are found at high elevations.

Stratiform CloudsLow clouds are of made of water droplets. However, when temperatures are cold enough, these clouds may also contain ice particles and snow. The word stratus comes from the Latin word that means "to spread out." Stratus clouds are horizontal, layered clouds that stretch out across the sky like a blanket.

Cumuliform CloudsCumulo- means “heaped” or “piled”Cumulus clouds are flat-based, billowing clouds with vertical doming. Often the top of cumulus clouds have a "cauliflower-like" appearance. Cumulus clouds are most prominent during the summer months.Cumulus or fluffy clouds form when air is forced up rapidly and therefore rises higher.

They can be associated with both fairweather and bad weather when they get really tall

High Clouds or Cirrus (Ci)clouds

- cloud base is above 6 km - small amount of water vapor present - low temperature - thin and white clouds of ice crystals - examples: cirrus, cirro-cumulus, cirro-stratus

Middle clouds or Alto clouds- occurs between 2 and 6 km - examples: altocumulus, altostratus

Low clouds or Stratus Clouds- occurs between 0-2 km - examples: stratus, stratocumulus,

nimbostratus and cumulonimbus-    cumulonimbus cloud has vertical extent extending up to 15 km or more

Vertically Developed Cumulus CloudsSome clouds do not fit into any one of the three height categories mentioned. Such clouds have their bases in the low height range but often extend upward into the middle or high altitudes. Probably the most familiar of the classified clouds is the cumulus cloud. Generated most commonly through either thermal convection or frontal

lifting, these clouds can grow to heights in excess of 39,000 feet (12,000 meters), releasing incredible amounts of energy through the condensation of water vapor within the cloud itself.Cloud types include: fair weather cumulus and cumulonimbus.They are also called the clouds with Anvil head.They indicate very active vertical air movements.

PrecipitationThe definition of precipitation is any form of water - liquid or solid - falling from the sky. It includes rain, sleet, snow, hail and drizzle plus a few less common occurrences such as ice pellets, diamond dust and freezing rain.Precipitation comes only from clouds that have the root “Nimb” in their name – Nimbostratus - Cumulonimbus

PrecipitationFor clouds to produce precipitation that will fall to the ground:

- the tiny water droplets in clouds must form large raindrop sizes

- the raindrop sizes must be large enough to overcome:

atmospheric turbulence and fall through evaporation

Precipitation forming Process:The mechanism for producing larger raindrops in clouds include:

- ice-crystal formation process or Bergeron process in cold clouds

- collision-coalescence process in warm clouds

Precipitation forming Process: Ice-Crystal Formation – Bergeron Process

- Super-cooled water evaporates to replenish the vapor supply

- Hence, ice-crystals grow in size at the expense of the super-cooled water droplets

- Ice-crystals grow large enough to fall and picks up more moisture en-route

- It occurs in cold clouds, especially in temperate regions

- In cold clouds, ice crystals and super- cooled water droplets coexist

- Ice-crystals attract most of the water vapor because of lower vapor pressure around them

The enlarged ice-crystals precipitate to form:

- snowflakes when atmospheric temperature is below freezing

- rain when the ice-crystals melt enroute to the ground

First proposed by the Swedish meteorologist Tor Bergeron in the early 1920s, the Bergeron Process takes place when ice crystals form high in the cloud tops. These small, often microscopic ice crystals attract more water vapor, causing them to increase in size. As the ice crystals increase in size, the vapor pressure drops. This allows surrounding water droplets to evaporate, becoming smaller and smaller as the ice crystals grow. Eventually these ice

crystals become large and heavy enough that they begin to fall towards the Earth’s surface. As they do, they pass through the lower warmer portion of clouds, attracting even more water vapor and growing larger still. These ice crystals can then fall to the surface of the Earth as snow, or they can melt, becoming rain drops. It is believed that most precipitation happens in this manner.

Ice-Crystal Formations in CloudsIce-Crystal Formations in Clouds

The collision-coalescence processThe collision-coalescence  process is an important  mechanism in forming raindrops in warmer clouds (those with tops warmer than -15°C = 5°F).  In these warm clouds raindrops form exclusively by this process. Most tropical rain is formed in this way.  The collision-coalescence process is of relatively little importance in middle and high latitudes where, even in the summer, most precipitation begins high in the clouds

where temperatures are well below freezing and the dominant precipitation-producing mechanism is the so-calledice-crystal or Bergeron process. However falling raindrops inthese clouds do grow by the collision-coalescence process.

In many parts of the world the air is too warm for ice crystals to form. This being the case, rain and snow cannot develop following the Bergeron Process. Instead, tiny droplets form as they collide into one another creating larger and larger droplets. The coalescence of droplets into larger droplets can only take place if the droplets have an opposite electrical charge. That is to say that if one droplet has a positive charge and the other a negative charge, then they will coalesce (combine) upon collision. Otherwise they will just bounce off of one another.

Raindrop Formation By Collision and Raindrop Formation By Collision and CoalescenceCoalescence

Forms of CondensationDew - is a type of precipitation where water droplets form on the ground, or on objects near the ground in a process called condensation of moisture. Dew forms during calm, clear nights, when the ground surface and other exposed objects, such as tips of grass or leaves, lose heat by radiation to the sky. The greater the amount of water vapor in the air the lower the temperature of the ground – the more abundant the dew. The dew point is the temperature at which the water vapor in the air becomes saturated

Mist -  is tiny droplets of water hanging in the air. These droplets form when warmer water in the air is rapidly cooled, causing it to change from invisible gas to tiny visible water droplets. Mist can reduce visibility to between 1 and 2 kilometers (0.6 - 1.2 miles).

Fog - There is absolutely no difference between fog and clouds found high in the air. Fog is simply a cloud that has formed near the surface of the Earth. Fog shows up when water vapor, or water in its gaseous form condenses. During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. You can see fog because of these tiny water droplets. Water vapor, a gas, is invisible. 

There are four main types of fog: Radiation fog, advection fog, upslope fog and evaporation fog. Radiation FogAs the ground becomes cooled by the night sky, the air above the ground also gets cooled. This can bring the air down below its dew point, causing water vapor to condense around the dust and other particulates in the atmosphere creating fog.

Radiation Fog

Advection FogSimilarly, advection fog takes place when warm air moves in horizontally over a cool surface, such as when air from the ocean blows in over land. The land cools the air down below the dew point, causing fog to occur.

Advection Fog

Upslope fogUpslope fog takes place as warm air passes over the slope of a cool mountain as it rises. The mountain cools the air below the dew point causing it to condense, forming fog.

Upslope fog

Evaporation fogThis form of fog is caused when additional water vapor enters air that is already near the maximum vapor pressure. Because the air is already nearly saturated, the additional water vapor causes the air to reach the dew point, forming fog.

Evaporation fog

FrostIs water vapor, or water in gas form, that becomes solid. Frost usually forms on objects like cars, windows, and plants that are outside in air that is saturated, or filled, with moisture. Areas that have a lot of fog often have heavy frosts. Frost forms when an outside surface cools past the dew point. The dew point is the point where the air gets so cold, the water vapor in the atmosphere turns into liquid.

This liquid freezes. If it gets cold enough, little bits of ice, or frost, form. The ice is arranged in the form of ice crystals.

Forms of PrecipitationSnow Snow forms when water vapor turns directly into ice without ever passing through a liquid state. This happens as water condenses around an ice crystal. Snow can take the form of ice pellets or snow flakes. As snow falls to the ground, it often melts on the warm surface of the Earth. If the surface of the Earth is

chilled sufficiently, snow begins to pile up, creating snow drifts. In some locations, such as mountains, these snow drifts can reach several feet or meters in depth.

SleetSleet refers to a mixture of snow and rain as well as raindrops that freeze on their way down. Unlike snow, the raindrops pass through a liquid form before freezing. The result is that they are not light and fluffy.

Glaze (Freezing Rain)Freezing rain, which is sometimes referred to as glaze, takes place when water droplets become super-chilled. They do not freeze in air, but rather freeze the instant they strike an object, such as a road or car. The result can make roads very slippery, and can cause car doors to become frozen shut.

HailHail forms in a complex dance between moisture and wind. Deep within cumulonimbus clouds ice crystals form and begin to fall towards the Earth’s surface. As this happens, wind gusts pick up the ice crystals, pushing them back up high into the clouds. As they begin to again fall down, they continue growing in size. Again, a wind gust might catch the growing hail stones, pushing them back up high into

the clouds. This process may be repeated several more times until the hail stones become so large that they are too heavy for the wind to carry, causing them to fall towards the Earth. 

RainRain is by far the most common type of precipitation in our atmosphere. Rain takes place when drops of liquid water fall all the way to the surface of the Earth. Rain often takes one of two main forms. These two forms are showers and drizzles. A shower lasts just a brief period of time, and usually is made up of large heavy drops. Drizzles generally last much longer, and are made up of smaller, finer droplets of water.

Rain can either form as ice crystals melt or as a coalescence of many smaller water droplets. In drizzle the rain drops are extremely small like a fine spray – diameter of droplets 0.2 to 0.5 mm. Rain drops have a diameter of more than 0.5mm. Rain usually from the cumulonimbus clouds.

Types of RainfallPhysical/Orgographic/Relief Rainfall1. Prevailing winds bring warm, moist air to a mountain barrier2. Air is forced to rise over high areas.3. Air cools and condenses.4. Clouds form and it rains.5. Air descends on the other side of the mountains.6. It warms up and therefore becomes drier.

Frontal rainfallWhen warm and cold air meet, a depression forms:1.When a cold polar air mass meets a warm tropical air mass they do not mix – they form fronts.2.The colder air mass is heavier than the warmer air mass, therefore the lighter, warmer air rises over the top of the heavier, colder air.

3.As the warm air is forced to rise it cools. Also, the warm air is in contact with the cold air along the fronts, and this also cools.4.Condensation occurs and clouds form.5. Rain occurs along the front.

Convectional rainfallWhen the land warms up, it heats the air above it. This causes the air to expand and rise. As the air rises it cools and condenses. If this process continues then rain will fall. This type of rainfall is very common in tropical areas.

Coastal rainfallThis is caused by unequal heating and cooling of seawater and adjacent lands. When moist air comes from the sea – if the land of the coastal area is cool – it condenses and precipitates. Coastal rainfall often occurs within 10km of the coastline

Global Patterns of PrecipitationGlobal Patterns of PrecipitationISOHYET:  a line on a map or chart connecting areas of equal rainfall

Global Patterns of PrecipitationGlobal Patterns of Precipitation The global average annual precipitation is about 99 cm (39 inches)

Total annual rainfall tends to decrease from the equator toward the poles

Hence, equatorial region receives the highest amount of precipitation, with an annual average of 125 - 300 cm

MEAN PRECIPITATION BY MONTH

Global Average Annual PrecipitationGlobal Average Annual Precipitation

Global Average Annual PrecipitationGlobal Average Annual Precipitation

Global Pattern of PrecipitationGlobal Pattern of PrecipitationThe subtropics, between lat 15o and 35o

N & S are substantially drier due to air subsidenceHence, the continental west coasts of the

subtropics washed by cold currents are the world's driest deserts

However, the continent east coasts washed by warm ocean currents are often wetter

Global Pattern of PrecipitationGlobal Pattern of PrecipitationThe middle latitudes, between lat. 35o and

65o N and S, receive total precipitation that is close to the global average – rain is

heavier on the east coast and decreases travelling towards the westIt is the zone of Westerly wind with

winter cyclonesCalifornia in this zone have summer

drought when STH shifts pole ward

The high latitudes, between lat. 65o N &S and the Poles, receive low annual precipitation because high latitude cold air has very low capacity for moisture

The zonal distribution of precipitation described is often complicated by:

-   land and sea distribution-   ocean currents and-   topography

Global Pattern of Precipitation

Global Pattern of Precipitation• The coastal areas of the world receive a great amount of rain fall than the interior of the continents•