UNIT 2(Water Vapour & Visibility)

Describe water vapour in the atmosphere ?

Water vapour is very important in the atmosphere. Rainfall, snow, mist, fog, cloud formation depend on watervapour. Watervapour absorbs outgoing radiation from earth and moderates the heat of the earth. Watervapour controls the heat of living bodies. The amount of watervapour which air can hold is limited solely by the temperature of air. At each temperature there is a definite maximum value to the amount of watervapour which air can contain as vapour. Any parcel or sample of air that is fully saturated is called Saturated air. Any sample of air that is not fully saturated is called dry air. Air that contains some watervapour is called moist air. If the volume was increased, the temperature of the parcel would decrease and vice-versa. If a parcel of air is made to rise, its volume would increase in accordance with the rarer air at that height. This expansion causes the parcel of air to cool, though no exchange of heat has taken place with the surround air.

Explain briefly equation of state for dry air and moist air ?

The  equation  of  state  is  a  general  gas  law  for finding pressure, temperature, or density of a dry gas rather  than  using  volume,  this  formula  uses  what  is called  gas  constant. The   following   formula   is   an   expression   of   the equation of state: P = rRT, P = pressure in millibars, r = density , R = specific gas constant T = temperature (absolute). This formula is the equal sign that separates two side of formula. If same value exists both side then equation are equal. If left side pressure changes, corresponding change must occur on right side (either density or temp.) to make the equation equal again (Note: Since R is constant it will always remain) unchanged in any computation. The right side of the equation can balance out any change in either density  or temperature without having  change on the left side (pressure).  From this relationship, we can draw the  following conclusions:

(a) A    change    in    pressure,    density    (mass    or volume), or temperature requires a change of volume,  or  temperature  requires  a  change  in  one  or both of the others.

(b) With  the  temperature  remaining  constant,  an increase in density results in  atmospheric pressure.  Conversely, a decrease in density results in a decrease in pressure. (Note: Such a change could occur as a result of a change in the water vapor)

(c)With  an  increase  in  temperature,  the  pressure and/or density must change.

What is meant by Adiatic Process ?

· Adiabatic Process: In a system where there is no exchange of heat with the surrounding is called adiabatic process. No heat comes from surroundings nor any heat goes out from the system to outside. In the atmosphere, most of the process is nearly Adiabatic.

Adiabatic Process

· Process is adiabatic if there is no exchange of heat between system and environment.

· In an adiabatic process, the system is insulated from the surroundings and heat absorbed or released is zero. Since there is no heat exchange with the surroundings.

· When expansion happens temperature falls and when gas is compressed, temperature rises.

· An air parcel is a hypothetical volume of air that does not mix with its surroundings.(Parcel is a closed system)

· Parcel moves adiabatically if there is no exchange of heat with surroundings(Parcel does not interact with surroundings)

· If a parcel rises adiabatically, its pressure decreases(Parcel cools). If a parcel sinks adiabatically, its pressure increases(Parcel warms).

Define lapse rate ?

The temperature decreases with height. The rate of fall of temperature with height is called lapse rate. The lapse rate is 6.5 degree C per KM or 2 degree C/1000 feet or 3.5 degree F/1000 ft.

Define various Lapse rate ?

(a) DALR(Dry Adiabatic Lapse Rate): In meteorology, in general, unsaturated air is considered as dry air. It has a lapse rate of 10 Deg/Km that is if the dry air is rising adiabatically, temperature falls by 10 Deg C/Km.& 3 Deg C/1000 ft. 

(b) SALR(Saturated adiabatic Lapse Rate): The air full of watervapour which is saturated if lifted adiabatically, its lapse rate is called SALR. Its value is half that of DALR. Because when saturated air is lifted, watervapour becomes liquid or solid ice due to upper level cooling. Due to this change of state, latent heat or hidden heat is released which prevents the cooling at DALR, and falls by 5 Deg/Km. 

ELR(Environmental lapse rate): Whatever lapse rate exists in the atmosphere at a given place at fixed time, as monitored by the ascent of a weather balloon at an Observatory is called ELR. This rate will be dependent on many factors, although it is usually between SALR & DALR.

Why SALR is half of DALR ?

Unsaturated Air is considered as Dry Air and lapse rate is 10 deg/KM. SALR is less than DALR because, when saturated air is lifted, watervapour becomes liquid or solid ice due to upper level cooling. Due to this change of state, latent heat or hidden heat is released whichprevents the cooling at DALR, by about 5 deg C instead of 10 deg C, therefore SALR is half of DALR(5 deg C/Km, 1.5 deg C/1000 feet)

What you understand by Fohn wind effect(Example for difference of DALR & SALR) ?

Fohn wind effect: is an effect, if a strong wind containing lot of moisture strikes a mountain and it rises on the windward side. The leeward side of mountain is drier and warmer than windward side. This is the direct result of difference between DALR & SALR. Imagine onshore breeze, of 25 deg C temperature & 15 deg C dew point temperature blowing against mountain range of 3 Km. The onshore breeze ascend 10 deg C when reaching 1 Km(DALR)and air fallen to 15 deg C, which is also its dew point. The air then saturated and on ascending further the temperature drops by 5 deg C per Km(SALR). On reaching the top, the excess moisture give off as orographic cloud and heavy rain falls on windward side. The air will still be saturated and its temperature would be 5 deg C . While descending on the leeward side, the temperature of air would increase at 10 deg C(DALR), and reaching sea level on leeward side the air would be 35 deg C . Since the same air was saturated at 5 deg C and no watervapour has been let in or taken out, the dew point of descending air would be 5 deg C. It is clear that the leeward side is warmer and drier than windward side.{"fd":"","fn":"Foehn1.png","id":"vrrrj7wm8Q4ZHM:","is":"283\u0026nbsp;\u0026#215;\u0026nbsp;198","isu":"en.wikipedia.org","ity":"png","lu":"/imgres?tbnid=vrrrj7wm8Q4ZHM:\u0026tbnh=94\u0026tbnw=134\u0026sa=X\u0026imgurl=http://upload.wikimedia.org/wikipedia/en/a/aa/Foehn1.png\u0026imgrefurl=http://en.wikipedia.org/wiki/Foehn_wind\u0026h=198\u0026w=283\u0026sz=9\u0026tbm=isch#imgrc=vrrrj7wm8Q4ZHM%3A%3BM1mCEMz3ktjuZM%3Bhttp%253A%252F%252Fupload.wikimedia.org%252Fwikipedia%252Fen%252Fa%252Faa%252FFoehn1.png%3Bhttp%253A%252F%252Fen.wikipedia.org%252Fwiki%252FFoehn_wind%3B283%3B198","md":"/search?tbs=sbi:AMhZZisIQIvDdh5e-Wx-mRAycBd8qP2c-UzE_1fBbI3avCX-gBzoyqV6aN5lqlUAmpFJNnH_1cb6h757HllLfxDyHseY2hCI83-1x8L1GBPaaFJaXkSeHuVNQOOI8mcx0C7IBF51eZDMh-geFqM7gdB0Klwz600T0z--nhvQYByBUUMGWQg8LF0eE3KW-pl5qGv2Vfw-07oeSYuCbAWSM9Ilfb5ZLyW8jLMA\u0026ei=Fs6IUruCCsykrQfQkIDwCw","msm":"More sizes","msu":"/search?sa=X\u0026imgurl=http://upload.wikimedia.org/wikipedia/en/a/aa/Foehn1.png\u0026imgrefurl=http://en.wikipedia.org/wiki/Foehn_wind\u0026h=198\u0026w=283\u0026sz=9\u0026tbm=isch\u0026tbs=simg:CAQSEgm-uuuPvCbxDiEzWYIQzPeS2A","oh":198,"os":"9KB","ow":283,"pt":"Foehn wind - Wikipedia, the free encyclopedia","s":"From Wikipedia, the free encyclopedia. Jump to: navigation, search","sc":1,"si":"/search?sa=X\u0026imgurl=http://upload.wikimedia.org/wikipedia/en/a/aa/Foehn1.png\u0026imgrefurl=http://en.wikipedia.org/wiki/Foehn_wind\u0026h=198\u0026w=283\u0026sz=9\u0026tbm=isch\u0026tbs=simg:CAESEgm-uuuPvCbxDiEzWYIQzPeS2A","sm":"Similar","th":158,"tu":"http://t3.gstatic.com/images?q=tbn:ANd9GcTXYWEaFp7xgj4MYIzvg6m11jOFhsu4wApf77uPTIJ9mKXUyH-D","tw":226}

Describe the change of state of the water vapour ?

· The terms used to describe the change of state of the water vapour:-

The termChange Process

Condensation Water vapour(Gas) into liquid(water)

or water into ice

Evaporation Liquid in to vapour(Gas)

Melting Solid ice into liquid

Freezing Water(Liquid) into ice

Deposiiton Water vapour(Gas) into ice

Sublimation Ice into Watervapour

However, one interesting point to note is that whereas the minimum ground temperature may be only couple of degrees below the air temperature, the maximum ground temperature may be as high as 40 deg. higher than the air temperature.

Three states of Change of Water: Latent Heat Absorbed/Released

31.What is Diurnal Variation of watervapour(Humidity) ?

· The relative humidity is a good representation of watervapour content of the atmosphere

· Surface evaporation which peaks around noon.

· Atmospheric large scale vertical motion, which tends to be downward from late morning and upward from midnight to early morning.

· The temperature of a mass of air increase, in which the moisture content remains constant, the relative humidity of that air will decrease.

· During morning time relative humidity will be high. During afternoon and evening relative humidity is low.

· During summer, R.H will be less, whereas in winter, it will be high.

· The diurnal variation changes little from -4 to 16 Km above the ground.

Describe global distribution of humidity ?

Maximum R.H is found near the doldrums close to the equatorial regions, where a low-pressure area is located. Around 30-40 deg latitude on both hemispheres, R.H will be minimum where a high-pressure area is located. At this latitude, the important deserts like Sahara desert are located. Around 60 deg latitude in both hemispheres, a secondary high R.H area is found. Around Artic and Antartica area RH is Minium.

Write short notes on the following: -

(a) Humidity : It is the amount of watervapour present in the atmosphere. It is expressed as grams/m3.

(b)Relative Humidity: It is the ratio between the amount of water vapour present and amount of water vapour that atmosphere can hold at given temperature which is expressed as :- R.H (%) = Present quantity of water vapour X 100

Maximum possible at that temp.

(c)Specific Humidity: Is defined as amount of watervapour present per unit weight of moist air in the atmosphere. It is expressed as gm/kg.

(d) Humidity Mixing Ratio: It is defined as the ratio of weight of watervapour contained in a mixture of unit weight of dry air. It is expressed as gm/kg.

(e) Daily Ranges: During morning time relative humidity will be high. During afternoon and evening RH is low. During summer RH will be less, whereas in winter, it will be high.

(f) Absolute Humidity: The actual amount of water vapour contained in a given volume of air expressed in gm/m3.

(g) Humidity & Human Comfort: When humidity is very high, breathing becomes difficult as human lungs are filled up with watervapour. When the humidity is low due to dry conditions, the human skin becomes very dry and starts cracking. The comfortable conditions for human beings is between 50-60 % of RH..

Explain the relationship between temperature, Dew point temperature & wet bulb temperature ?

Dew Point Temperature: It is the temperature of a parcel of air to which it must be cooled to become saturated at constant pressure and constant watervapour content.

(a) Dry Bulb Temperature: It is the air temperature at any time as read from a Thermometer.

(b) Wet Bulb Temperature: It is the lowest temperature of the air that can be attained by evaporating water into atmosphere. If air is already saturated, evaporation will not occur. Hence there will be no cooling. Wet bulb temp. of saturated air will be same as air temperature.

Explain the terms (a) Cargo Sweat (b) Ship Sweat ?

Cargo sweat: Condensation occurs on the surface of the cargo as warm, moist air enters the cargo hold containing a cold cargo. For example, if a cargo of steel is loaded in winter in the UK for discharge in Singapore, the temperature of the cargo will be low. If warm moist air is later introduced in the cargo hold, condensation takes place as soon as it comes into contact with cold cargo. To avoid the possibility of cargo sweat, all ventilators should be closed and no ventilation carried out. However, if the moisture content of the cargo is high, extraction of the moist air from within the cargo holds may be required. (If holdtemperature is less than dew point of out side air, Restrict Ventilation).

Ship sweat : This results when condensation occurs on the ship's structure as the ship becomes colder moving from a hot to a cold climate. The underwater parts of the hold would be cooled by contact with sea.The warm moist air within the cargo compartment condenses as it comes into contact with the cold structure of the vessel. For ship sweat to occur, the dew point in the cargo hold must exceed the temperature of the ship's structure. To eliminate ship sweat the cargo should be ventilated if the vessel is moving from a warm to a cold climate. (If hold temp. isgreater than dew point of outside air, ventilate freely). Using dunnage for cargo also help in reducing Relative Humidity in hold.

Describe The Parcel Theory of Atmospheric Stability ?

In physics equilibrium is defined as stable, unstable or neutral. The type is determined by the movement of an object, which is disturbed from its equilibrium position. These definitions are applied to layers of the atmosphere. In this case the object is a “parcel” of air. The parcel is disturbed from its original level. If the forces acting on the parcel tend to make it return to its starting level the atmospheric layer is described as stable. If the forces acting on the parcel tend to make it keep moving away from its starting level the atmospheric layer is described as unstable.

The key factor is the temperature of the parcel compared to the temperature of the air surrounding it. As the temperature of the parcel changes its density changes. A parcel warmer than its surroundings will be less dense and so will tend to rise and a parcel colder than its surroundings will be relatively dense and so will tend to sink. A temperature height diagram can be used to determine the stability of the atmospheric layer.

Write short notes on the following: -

· Stable Air(Absolute Stability): If air which has been forced to rise/fall from its initial level tends to return to that level (original Position) when ELR < SALR.

· Unstable Air(Absolute Instability): If air which has been forced to rise/fall from its initial level tends to continue its upward/downward movement when ELR > DALR

· Neutral Air(Neutral Equilibrium): If air which has been forced to rise/fall will have no tendency to continue its upward or downward movement, nor will it have any tendency to return to its original position. ELR is same as DALR(if Dry air) & ELR is same as SALR(if Saturated air).

· Conditional Instability: Occurs when stable conditions exist for dry air and unstable conditions exist when the air is saturated. ELR lies between SALR &DALR.

Discuss the stability of the atmosphere based on ELR ?

Dry air is stable if ELR < DALR

Dry air is unstable ELR > DALR

Saturated air is stable if ELR < SALR

Saturated air is Unstable if ELR > SALR

Whether air is dry or saturated, if unstable ELR < SALR the atmosphere is absolutely stable. Whether air is dry or saturated, if ELR >DALR the atmosphere is absolutely unstable.

If ELR lies between SALR & DALR the atmosphere is conditionally stable. That is with respect to dry air, atmosphere is stable and with respect to saturated air, atmosphere is unstable.

Write short notes on the following:-

(a) Latent Instability: If the layer of atmosphere which is stable at lower levels which when lifted mechanically upwards, can become unstable, if instability prevails aloft is called Latent Instability.

(b) Potential Instability: Suppose there is high relative humidity available at lower levels and low R.H is there at upper levels, if the lower level atmosphere is lifted mechanically upwards, potential Instability is achieved.

(c) Potential Temperature: It is the temperature of parcel of air which when brought dry adiabatically(ie. without transfer of heat or mass) to standard pressure level of 1000 mb. This has wide application in Meteorology because of its conservative property.

Explain the Tephigram and its uses ?

Tephigram:For accurate forecasting upper air conditions are essential. If upper air tem8perature known, then the other conditions can be found . A Tephigram is a diagram or a graph with number of lines pertaining to temperature, dry adiabatic lapse rate, saturated adiabatic lapse rate, humidity mixing ratio and pressure surface lines. The upper atmosphere data as given by the balloon ascent, pertaining to temperature, dew point temperature, pressure values and the wind speed along with it directions are plotted in the Tephigram in which theoretical lines are already printed. This is called environmental lapse rate, which shows the present and actual real condition of the upper atmosphere.

The atmosphere, whether dry or saturated, is decided with the help of surface temperature and dew point temperature. If temperature & the dew point temperature are very close to each other, the atmosphere is saturated. On other hand, if they are far apart, then it can be considered as dry.

Accordingly, the actual environmental lapse rate is compared with printed theoretical DALR or SALR from which one can easily decide regarding the stability of the atmosphere. The degree or the intensity of instability can also be judged from the Tephigram. With sufficient moisture available in an unstable atmosphere bad weather like cloud formation and consequent rainfall, thunder and lightening will be produced. On theother hand, stable atmosphere will produce clear weather and if some moisture is available mist, fog, haze, etc are likely. These are the uses of Tephigram.

( -30oC -20 -10 0oC +10Tephigram)

Define Visibility ?

Visibility is a measure of degree transparency of the atmosphere and is defined as the maximum distance at which an object can be clearly seen by Naked eyes on Horizon. Visibility can be reduced by liquid or solid particles in the air as Mist/Fog, Precipitation, spray, smoke, dust, etc., Visibility is an important parameter in the Navigation of a ship in order to avoid collision with other ships, rocks, icebergs etc. The observer reports minimum visibility after scanning the visibility in all directions. Visibility at sea can not usually be ascertained accurately and is hence obtained by estimation.

What are the causes of poor visibility ?

(a)Dust haze, dust raising winds, dust storms, spray and fog: are some of the causes. When the atmosphere is dry or semi arid, due to steep pressure gradient when strong wind are caused over desert, lot of dust will be raised and visibility may reduce to even less than 1 Km and dust may be carried to 3-5 Km. This is commo n in North West India during summer.

(b)Haze: Visibility reduced by solid particles such as dust, sand, volcanic ash, etc. In rare cases 200 M.

(c)Spray: It is the name given to small droplets of water driven by the wind, from the tops of waves. Spray affects visibililty when wind force is 9 or more.

Definitions of fog mist and haze:

CompositionVisibilityRH

FOGwater dropletsless than 1000m95%

MISTwater dropletsmore than 1000m95%

HAZEsolid particlesmore than 1000mless than 95%

Condensation can occur on certain “hygroscopic” nuclei if the relative humidity is more than 90%, but not necessarily at 100%. It is possible for a doubt to exist as to whether the reduction in visibility is due to dust or due to water droplets. A value of 95% is used arbitrarily to distinguish mist from haze.

What is the difference between mist and fog ?

Mist & Fog: Due to the presence of very high quantity of watervapour, mist and Fog are caused. When visibility is <1 Km is called Fog, with high relative humidity. When visibility >1 Km is called Mist. In both the cases, that is mist and fog, R.H must be high and more than 90 %.

Write short notes on the following:

(a)Super cooling: also known as under cooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. This occurs when heat is removed from a liquid so rapidly that the molecules do not have enough time to align themselves in the ordered structure of a solid. Super cooled liquids are unstable, and gentle stirring will easily cause the substance to solidify.

(b) Frost: Is the name given to ice crystals deposited on exposed surfaces on or near the ground, when the ground temperature is much lower than freezing point. In this case, the water vapour directly turns into ice without becoming water.

(c) Frost point : is the temperature at which the air is saturated with respect to watervapor over an ice surface. It is more difficult, more water molecules to escape a frozen surface as compared to a liquid surface since an ice has a stronger bonding between neighboring water molecules. Because of this, the frost point is greater in temperature than the dew point.

(d)Hoar Frost: is the name given to ice crystals deposited on exposed surfaces on or near ground, when ground temperature is much lower than freezing point(water vapour directly turns into ice). The presence of ice particles on surface quickens the formation of hoarfrost.

(e) Glazed Frost: Is a thin, transparent, smooth layer of ice formed when rain or drizzle falls on a surface whose temperature is below freezing point. Glazed frost can also form if a warm moist current of air blows over a very cold surface. In UK, it is called “Black ice”

(f) Frozen Dew : Frozen dew or frozen supercooled dew is also know as White Dew or Silver Frost. Some consider it to be merely a type of Hoar Frost.

(g)Rime: Rime ice is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward side of tree branches, buildings, and any other solid object. Rime ice is similar in appearance to hoar frost but whereas rime ice is formed by vapour first condensing to liquid droplets and then attaching to a surface, hoar frost is formed by direct deposition from water vapour to solid ice.

Write short notes on different kinds of fog ?

Radiation Fog:- During the night, due to cooling of the ground, radiation fog occurs mainly during winter. The air has to be cooled below dew point temperature. The conditions favourable for the formation of radiation fog are as follows:-

(a) High relative humidity( Only a little cooling is required to reach the dew point temperature)

(b) Clear Sky( This permits maximum cooling during night due to outgoing radiation from earth) .

(c) Light wind( Enables the transfer of very low temperature from earth to aloft to bring down the air temperature to near about the dew point temperature 2 – 8 Kts wind speed is considered to be ideal for this purpose).

Vertical extent of radiation fog: Usually vertical thickness extends up to 1 Km. A slack pressure distributions with weak highs are favourable for fog formation. In India over northern parts, during winter, after the passage of a western disturbance radiation fog is common. It is frequent around sunrise time. If pressure pattern is conducive the fog can form one or two hours earlier than sunrise. However, it will thicken at the time of sunrise. Radiation fog dissipates about two hours after sunrise due to warming up.

Advection fog: When the moist air mass from elsewhere is transported over the cold surface, advection fog forms because moist adverted air, due to its contact with cold surface, the moisture condense into fog. This occurs when the cold and warm ocean current meet. (e.g) Convergence of cold Labrador current & Warm Gulf stream current.

Sea Smoke(Arctic Sea Smoke, Steaming fog : When the cold airmass flows over a warm sea surface, steaming fog occurs. Observations sugges that the air must be atleast 9 deg C. colder than the sea. The evaporation which is taking place over the ocean, when condensed instantly due to flow of cold air mass in the area, steaming fog occurs. This type of fog occurs when the prevailing temperature is very low. Sea fog usually lies in a shallow layer (10m) and it is often patchy. Sea smoke is most common in polar regions, hence the term arctic sea smoke, and off the east coast of continents in autumn and winter.

Frontal fog : When the Low Cloud is coming down to the surface with passage of a frontal boundary frontal fog occurs.

Smog: is radiation fog mixed with industrial smoke.

Note: Radiation fog can occur only over land only, during both night and day. Advection fog can occur both over land as well as over and sea during day or night. For mariners over the sea, it is the advection fog, which is frequent and hazardous for safe navigation of a ship.

Other Types of Fog:

Hill fog : Hill fog is stratus cloud observed by an observer on a hill.

Valley fog : Valley fog is a localised form of radiation fog. Cold air tends to flow into valley’s (katabatic flow). So radiation fog commonly forms in valley bottoms.

Mixing Fog : Mixing fog can occur in a col and at a front where different air masses meet.

What is the ideal wind speed required for advection fog?

Wind causes advection fog to form and also to spread. If the wind is quite strong, turbulence causes advection fog to form to considerable depth with an extreme velocity of 100 feet/sec. However, very strong winds carry the moisture too high, resulting in low clouds(stratus type) and no fog.

Which are the oceanic areas prone for the formation of advection fog ?

(a) On the Grand Banks of New Foundland where the warm, moist westerlies, blowing over the warm Gulf stream, cross over the cold Labrador current.

(b) Off the east coast of Japan where the warm, moist westerlies, blowing over the warm Kuro ship, cross over the cold Oya shio.

(c) The south coast of the UK in winter, whenever SW winds blow. These winds come from lower latitudes and blow over the sea and are hence warm and moist, compared toaa the cold land surface.