Thermometer Essay

iii. The readings of the two thermometers should, as far as possible, be taken simultaneously, and it should be ascertained that the wet bulb is receiving a sufficient water supply (WMO, 2008).

The relative humidity was calculated using the following formulae by de Laaat and Savenije (2015); es = 0.6108e(17.27Ta)/(237.3+Ta) ed = es – γ(Tdry-Twet)

Where

es Saturation vapour pressure for air temperature at 2m height in kPa

Ta 24 hour mean temperature in 0C γ is the psychometric constant = 0.067kPa.0C-1 at sea level

The sling or whirl psychrometer

A small portable type of whirling or sling psychrometer consists of two mercury-in-glass thermometers mounted on a sturdy frame, which is provided with a handle and spindle,

Thermometers should be read at once after aspiration ceases because the wet-bulb temperature will begin to rise immediately, and the thermometers are likely to be subject to insolation effects (WMO, 2008).

The relative humidity is determined from a set of tables that make use of the dry bulb temperature and the difference between the dry and wet bulb temperatures.

Data logger

The data logger automatically recorded the parameters at 15 minutes interval, and relative humidity was given in percentage.

As mentioned above, three instruments were used to determine relative humidity (RH); i.e. the sling psychrometer, automated data logger system and the dry and wet bulb thermometers. Readings from the sling psychrometer and the wet and dry bulbs where taken twice a day; between 08:15 and 08:45 in the morning, and from 13:00 to 13:30 in the afternoon. During each observation session a team of two took turns to take readings from the instruments in order to minimize misreading values.

Rainfall Rainfall was measured using a standard raingauge having a diameter of 203mm. The rainfall measurements were done at 8:30Am every morning. The rainfall collected in the collector was poured into a calibrated measuring cylinder and values read in mm were collected in a table.

a. Physical

ii. Makkink Equation (Radiation method)

ETMakkink= CCM

Where CM of 0.8 is considered as a standard to determine open water evaporation

S is the slope and Rs is the net short wave radiation in W/m2.

Wind Velocity

a. Wind Speed measurement

The wind was measured using the anemometer at the meteorological station of IHE, and recording taken by recording average readings of wind speed for a 10 minute interval as well as for every 24 hour period. This involved taking the meter readings of the anemometer. The method applied for the Anemometer calculating is difference and multiplying it by a factor of 100 to change into meters and divide it by the total time taken in seconds. As given equation below;

U = (UT – U0) * 100/T

Where

U is the average wind speed during the measurement period [m/s]

U0 is the first anemometer reading at start [m/min]

UT is the second anemometer reading after time [m/min]

T is the time interval [seconds] Figure 2: Anemometer

Radiation

The daily sunshine duration (hours) were recorded from the Campbell-Stokes sunshine recorder and used in determining the amount of sunshine hours in a day (24 hours) and in turn compute the daily radiation. The daily sunshine hours were read off from the length of the burnt portion of paper strip placed on the instrument every day at 08:30 hours