Manual Weather Manual Weather ObservationsObservations

Hargeisa, May 2009Hargeisa, May 2009

Commonly Accepted DefinitionsCommonly Accepted Definitions

WeatherWeather refers to the current refers to the current atmospheric conditions (including atmospheric conditions (including temperature, precipitation, wind, temperature, precipitation, wind, humidity, barometric pressure) at a humidity, barometric pressure) at a particular time and place.particular time and place.Climate Climate refers to the general weather refers to the general weather patterns expected in a given area patterns expected in a given area (sometimes based on the 30 year average (sometimes based on the 30 year average weather). Climate may also be applied weather). Climate may also be applied more generally to large-scale weather more generally to large-scale weather patterns in time or space (e.g., an Ice Age patterns in time or space (e.g., an Ice Age climate or a tropical climate).climate or a tropical climate).

Weather Parameters/elementsWeather Parameters/elements• CloudCloud• Water VaporWater Vapor• Relative HumidityRelative Humidity• PrecipitationPrecipitation• Max/Min/Current Air and Soil TemperatureMax/Min/Current Air and Soil Temperature• Maximum, Minimum, and Current Air Maximum, Minimum, and Current Air

TemperatureTemperature• Surface TemperatureSurface Temperature• Surface OzoneSurface Ozone• Wind speed/directionWind speed/direction

Atmosphere MeasurementsAtmosphere Measurements

Dry Bulb Temperature - Dry Bulb Temperature - TTdbdb

• The The dry-bulb temperaturedry-bulb temperature is the is the temperature of of air measured by a measured by a thermometer freely exposed to the air but freely exposed to the air but shielded from shielded from radiation and and moisture. In . In construction, it is an important , it is an important consideration when consideration when designing a building for a building for a certain climate. a certain climate.

• it is one of "the most important climate it is one of "the most important climate variables for human comfort and building variables for human comfort and building energy efficiency.energy efficiency.

• The temperature is usually given in degrees The temperature is usually given in degrees Celsius (Celsius (ooC) C)

• Should be read 2x per dayShould be read 2x per day

Wet Bulb Temperature - Wet Bulb Temperature - TTwbwb

• This is the temperature indicated by a This is the temperature indicated by a moistened thermometer bulb exposed to the moistened thermometer bulb exposed to the air flow. air flow.

• Wet Bulb temperature can be measured by Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped using a thermometer with the bulb wrapped in wet muslinin wet muslin

• The wet bulb temperature is always lower The wet bulb temperature is always lower than the dry bulb temperature but will be than the dry bulb temperature but will be identical with 100% relative humidity (the air identical with 100% relative humidity (the air is at the saturation line). is at the saturation line).

dry & wet bulb thermometersdry & wet bulb thermometers•The container should not affect the humidity of the air flowing past the wet-bulb.•Air which has been in contact with the wet-bulb does not flow past the dry-bulb, and,•The temperature of the water flowing up the cotton strands (wick) is reduced to the wet-bulb temperature by the time it reaches the bulb.

Maintenance of Dry & Wet-Bulb Maintenance of Dry & Wet-Bulb Therms.Therms.• Both instruments must be kept clean and Both instruments must be kept clean and

dust free. Water supplied to the wet-bulb dust free. Water supplied to the wet-bulb must remain as pure as possible to ensure must remain as pure as possible to ensure accuracy. This means that distilled water, accuracy. This means that distilled water, must be used to fill the container.must be used to fill the container.

• muslin must be changed as often as muslin must be changed as often as necessary to ensure cleanliness. necessary to ensure cleanliness.

• The use of distilled water obtained from The use of distilled water obtained from motor garages is not recommended as this motor garages is not recommended as this is very often contaminated with sulphuric is very often contaminated with sulphuric acid otherwise it must be checkedacid otherwise it must be checked

Relative Humidity (RH)Relative Humidity (RH)• Moisture in the form of water vapor is always Moisture in the form of water vapor is always

present in the atmosphere present in the atmosphere (Humidity)(Humidity) • RH is the ratio of actual water vapor in the RH is the ratio of actual water vapor in the

atmos. compared to the amount of water atmos. compared to the amount of water vapor that would saturate the atmosphere at vapor that would saturate the atmosphere at that temperature. eg 40 %, it means that that temperature. eg 40 %, it means that the atmos. has 40 % of the moisture that it the atmos. has 40 % of the moisture that it could contain at that same temperature. could contain at that same temperature.

• highest in the early morning & lowest during highest in the early morning & lowest during the early afternoon; the diurnal cycle. it is the early afternoon; the diurnal cycle. it is changed by temperature.. As temperature changed by temperature.. As temperature goes up, relative humidity goes down and goes up, relative humidity goes down and vice versa.vice versa.

Relative Humidity - Relative Humidity - RHRH ( (%)%)

Dry Bulb Temperature - Dry Bulb Temperature - TTdb db ((ooCC))  TTdb db - -

TTwbwb

((ooCC)) 1515 1818 2020 2222 2525 2727 3030 333311 9090 9191 9191 9292 9292 9292 9393 939322 8080 8282 8383 8484 8585 8585 8686 878733 7171 7373 7575 7676 7777 7878 7979 808044 6262 6565 6767 6868 7070 7171 7373 747455 5353 5757 5959 6161 6464 6565 6767 696966 4444 4949 5252 5454 5757 5959 6161 636377 3636 4242 4545 4747 5151 5353 5555 585888 2828 3434 3838 4141 4545 4747 5050 535399 2121 2727 3131 3434 3939 4141 4545 48481010 1313 2020 2525 2828 3333 3636 4040 4343

Maximum TemperatureMaximum Temperature

• The highest temperature reached in any The highest temperature reached in any given period. Measured by a mercury-in-given period. Measured by a mercury-in-glass thermometer which has a constriction glass thermometer which has a constriction in the capillary.in the capillary.

• The action is as follows:The action is as follows: When the When the temperature raised, the mercury expands temperature raised, the mercury expands and forces its way past the constriction. On and forces its way past the constriction. On cooling, the mercury cannot readily contract cooling, the mercury cannot readily contract through the constriction and the thread through the constriction and the thread breaks at this pointbreaks at this point

• The thermometer is reset by gentle shaking The thermometer is reset by gentle shaking

Max temperatureMax temperature

Maximum temp

Minimum TemperatureMinimum Temperature• measures the lowest temperature during a measures the lowest temperature during a

given period of time. It is a alcohol-in-glass given period of time. It is a alcohol-in-glass thermometer . thermometer .

• As the air temperature drops, the liquid and As the air temperature drops, the liquid and marker move down the thermometer tube. marker move down the thermometer tube. At minimum temperature, the liquid and the At minimum temperature, the liquid and the marker stop moving; as the temperature marker stop moving; as the temperature starts to rise, the liquid moves back up the starts to rise, the liquid moves back up the tube, but the marker remains stationary. tube, but the marker remains stationary. Thus the level of the marker represents the Thus the level of the marker represents the minimum temperature. minimum temperature.

• To reset it, the thermometer is simply tipped To reset it, the thermometer is simply tipped upside down. upside down.

Minimum ThermometerMinimum Thermometer

Stevenson Stevenson screenscreen• The thermometer/ The thermometer/

Stevenson screen is a Stevenson screen is a shelter specially designed shelter specially designed for protecting weather for protecting weather instruments from sun instruments from sun rays and other natural rays and other natural heat radiations while heat radiations while allowing free flow of airallowing free flow of air • The wooden frame is The wooden frame is usually painted white and usually painted white and is buried 1-2 m above is buried 1-2 m above ground. A British civil ground. A British civil engineer Thomas engineer Thomas Stevenson designed the Stevenson designed the first Stevenson screen in first Stevenson screen in 18181818

• The Stevenson screen has been designed The Stevenson screen has been designed to endure, that as far as possible:to endure, that as far as possible:

– Instruments within it are effectively Instruments within it are effectively shielded from all sources of radiation, shielded from all sources of radiation, and especially from solar radiation;and especially from solar radiation;

– There is a gentle flow of air in and out There is a gentle flow of air in and out of the screen and around the of the screen and around the instruments;instruments;

– Instruments are protected from Instruments are protected from precipitation.precipitation.

Stevenson screen………..Stevenson screen………..

• Dig a hole one foot deep Dig a hole one foot deep • Doors must face true north and south. Doors must face true north and south. • The inner floor of the screen of which the The inner floor of the screen of which the

thermometer stand is placed should be level thermometer stand is placed should be level (check with spirit level), and about three feet (check with spirit level), and about three feet nine inches above the surface of the ground.nine inches above the surface of the ground.

• The inside and outside of the screen must be The inside and outside of the screen must be washed thoroughly at least once per month, washed thoroughly at least once per month, and more often if necessary, particular and more often if necessary, particular attention being paid to the removal of dust attention being paid to the removal of dust and dirt between the louvers.and dirt between the louvers.

Stevenson screen Installation……Stevenson screen Installation……practicalspracticals

Mounting ThermometersMounting Thermometers• Dry-bulb,we-bulb, maximum and minimum Dry-bulb,we-bulb, maximum and minimum

thermometers are all mounted on a moveable thermometers are all mounted on a moveable wooden stand within the screen. wooden stand within the screen.

• The dry –and wet-bulb mounted vertically about The dry –and wet-bulb mounted vertically about 5 to 6 inches apart with the bulbs about 5” 5 to 6 inches apart with the bulbs about 5” above the inner floor. above the inner floor.

• The max.and min. are mounted about 2The max.and min. are mounted about 200 out of out of the horizontal with the bulb ends lower. The the horizontal with the bulb ends lower. The instruments are held in place by means of light instruments are held in place by means of light metal brackets, cradles or spring clips.metal brackets, cradles or spring clips.

• face the northern door when the sun is in the face the northern door when the sun is in the southern sky (October to March), and southern southern sky (October to March), and southern door when the sun is in the northern sky (April to door when the sun is in the northern sky (April to September). The position of the stand is changed September). The position of the stand is changed around on March 31st, and on September 30th. around on March 31st, and on September 30th.

EvaporationEvaporation

• Evaporation is greatest Evaporation is greatest on hot, windy, dry days; on hot, windy, dry days; and is greatly reduced and is greatly reduced when air is cool, calm, when air is cool, calm, and humid. Pan and humid. Pan evaporation evaporation measurements enable measurements enable farmers and ranchers to farmers and ranchers to understand how much understand how much water their crops will water their crops will need need

• Class A evaporation pan Class A evaporation pan is the most common is the most common one used in the tropics.one used in the tropics.

Procedure for evap. pan: Procedure for evap. pan: (09:00am)(09:00am)

1)1) Remove wire mesh. Select of procedure 2. to 4. the Remove wire mesh. Select of procedure 2. to 4. the one which is true and apply it.one which is true and apply it.

2)2) If the end of the hook is below the water surface:,If the end of the hook is below the water surface:, remove water from the pan using the measuring cup. remove water from the pan using the measuring cup. Count the No. of cups “taken out of pan” till the top of Count the No. of cups “taken out of pan” till the top of the hook is exactly at the same level as the water the hook is exactly at the same level as the water surface and indicate the No. of cups. Continue with surface and indicate the No. of cups. Continue with procedure 5 ..procedure 5 ..

3)3) If the end of the hook is above the water surface:If the end of the hook is above the water surface:, add , add water into the pan using the measuring cups. Count water into the pan using the measuring cups. Count the No. of cups “added to pan” till the tip of the hook the No. of cups “added to pan” till the tip of the hook is exactly at the same level as the water surface and is exactly at the same level as the water surface and indicate the No. of cups. Continue with procedure 5 ..indicate the No. of cups. Continue with procedure 5 ..

4)4) If the end of the hook is at the same level as the water If the end of the hook is at the same level as the water surfacesurface indicate a dash in column (3) and (4) for the indicate a dash in column (3) and (4) for the corresponding date Continue with procedure 5 ..corresponding date Continue with procedure 5 ..

Procedure for evaporation pan Procedure for evaporation pan ………………5)5)Return the protective wire mesh properly.Return the protective wire mesh properly.6)6)Calculate the evaporation:Calculate the evaporation:

In case of 2. evaporation = Rainfall -0.5NIn case of 2. evaporation = Rainfall -0.5NIn case of 3 * evaporation = Rainfall + 0.5NIn case of 3 * evaporation = Rainfall + 0.5NIn case of 4 * evaporation = RainfallIn case of 4 * evaporation = Rainfall

Where N is the No. of cups removed or addedWhere N is the No. of cups removed or added

Daily check whether the evaporation pan is leaking. If the water in the tank is dirty (due to algae , etc.), clean the tank once every month after the 0900 reading. Put in column (Remark): Cleaning and at what time the tank was filled up to the level of the hook (eg 1045).

Piche evaporimeterPiche evaporimeter• The Piche evaporimeter is a type of The Piche evaporimeter is a type of

Atometer used to measure the rate ofAtometer used to measure the rate of• evaporation from a wet disc of absorbent evaporation from a wet disc of absorbent

paper. It is used mainly in hot, drypaper. It is used mainly in hot, dry• climates where water loss through climates where water loss through

evaporation must be observed regularly. evaporation must be observed regularly. • Since the results are dependent on wind Since the results are dependent on wind

speed past the disc, as well as the wet speed past the disc, as well as the wet bulb saturation deficit, it is almost bulb saturation deficit, it is almost essential to expose the evaporimeter essential to expose the evaporimeter inside a meteorological screen.inside a meteorological screen.

Piche evaporimeter……….Piche evaporimeter……….• CONSTRUCTIONCONSTRUCTION• A glass tube graduated in steps of 0.1ml will contain A glass tube graduated in steps of 0.1ml will contain

justjust• over 3 0ml of water. A metal c lip holds the p aper d over 3 0ml of water. A metal c lip holds the p aper d

iskisk• against the open end, while a ring at the closed endagainst the open end, while a ring at the closed end• allows the tube to be hung vertically.allows the tube to be hung vertically.• INSTALLATIONINSTALLATION• The evaporimeter is best suspended from a standThe evaporimeter is best suspended from a stand• available from Casella or from a hook fixed to the roof available from Casella or from a hook fixed to the roof

ofof• a meteorological screen.a meteorological screen.• Piche evaporimeter hanging from a stand.Piche evaporimeter hanging from a stand.