D.P., R.H., V.P. Calculator

This calculator will help you to find the DP, RH, VP

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Notes: D.P, R.H, V.P

Introduction

The most convenient method of determining the humidity of the air for meteorological purposes is by the use of wet and dry thermometers.
Angust's modification of Regnault's formula for deriving the vapour preasure, which has been in yse in the India Meteorological Department from 1876, (with temperatures in °C instead of °F) is as follows:-
For temperatures of the Vet Ball below 0°C

\sf{x = f' - {0.480(T-T')\above{1pt}671-T'} * P}

For temperatures of Wet Bulb above 0°c

\sf{x = f' - {0.480(T-T')\above{1pt}610-T'} * P}

Where,
x = pressure of vapour present in the air
f' = saturation vapour pressure at T'°C of the Wet Bulb
T = temperature of the Dry Bulk in °C
T' = temperature of the Wet Bulb in °C
P = pressure of air
It is well known that the readings of the Wet Bulb thermometer depend not only on the humidity but also on the strength of the wind which flows past the bulb, as that, at a particular humidity of air, the depression of Wet Bulb below the Dry Bulb is less in still air than when the air is in move sent. For temperatures of Wet Bulb higher than 0°C, Fernter modified Regnault'a equation so as to correspond to (1) calm, (11) light wind and (iii) strong wind. Under the conditions of expomre obtaining in a Stevenson Serena, the vele eity of air past the Wat hulb generally ennes under light wind'. Pernter's modification of Regnault's formula for light wind conditions almost exactly agrees with August's formula referred to earlier in this introduction, August's equation has, therefore, been used in the India Meteorological Department for a long time for calenlating the hygrometric tables.
It must be remembered that these tables are strictly applicable only when the thermometers are in a Steveneen Screen or are exposed to a gentle breeze at the time of observation. They are not to be used with any form of artificially ventilated psychronsters.
The realative humidity is given by the equation

\sf{U = {x \above{1pt}f} * 100}

Where,
U = relative humidity
f = saturation vapour pressure at temperature T °C of the Dry Bulb.
The Dew Point temperature is the temperature to which the air should be cooled in order just to cause condensation, or the temperature at which the anturation vapour pressure is equal to the pressure of the vapour present in the air.
In the Hygrometric Tables published by the India Meteorological Depart ment in 1949, the two arguments used vix., the Dry Bulb temperature and the depression of the Wet Bulb, as also the Dew Point temperatures were given in 7. With the adoption of the metric system of units in the India Meteg rological Department and the supply of thermometers reading directly in °C to observatories, it has become necessary to provide the sheervers with tables in which the temperatures appear in C instead of in P. The tables published in 1949 have been recast accordingly and brought out in this revised Edition.
In this edition of the Tables, Dew Point temperatures are given in addition to Relative Humidity values as is the previous editinn. The following important modifications should havever he noted :-
(1) The Dry Bulb and Wet Bulb temperatures instead of Dry Balb and depression of the Wet Bulb form the twe arguments of the tables.
(2) Values have been provided in steps of 0.2°C both in the Dry and Wet Bulb temperatures, as against 1°F for Dry Bulb and 0.5°F for depression of Wet Bulb in the previous edition.
The tables provide for ranges of Dry Bulb temperature from -10°C to 55°C and Vet Bulb temperatures -10°C to 30°C. The values of the saturation vapour pressure used are these given in tables 94 and 96 of the Saith sunian Net. Tables, sixth revised edition 1951.

Wet Bulb below the Freezing Point

When the Wet Bulb temperature falls below the freezing point it may be either covered with ice or with supercooled water. Alternative tables have been provided for use in either contingency
For use when the wet bulb is covered with ice a table is given for the range of Wet Bulb temperature -10°C to 0°C. In calculating the Relative Humidity in this table the saturation vapour pressure aver ice has used as the standard of reference for Dry Bulb temperature below 0°C. The been Tev Point in this table is the temperature as which the saturation vapeur pressure over ice is equal to the actual vapour present in the air. This temperature is the hoar frost point."
For use when the Wet Bulb is covered with superessled water, the main table has been extended down to a Wet Bulb temperature of -10°C, the vapour pressure over the supercooled water being used as the standard of reference for calentating Relative Humidity Dew Point temperatures below 0°C in the sain table represent the temperature at which the saturation vapour pressure over anpercooled water je equal to the actual vapour pressure.

INSTRUCTION FOR USING THE TABLES

The tables are calculated for a pressure of 1,000 abs, and are meant for use at stations whose heights above mean sea level do mut exceed 1,500 ft.
Dew point temperatures and Relative Humidities corresponding to age sified values of Dry Bulb and Wet Bulb temperatures, at intervals of 0.2°C, are printed at the top of each page. The Dew Point temperatures are given on the left side and Relative Humidities on the right side on each page.
The different ranges of Wes Bulb and Dry Balb temperatures occurring on one and the sane page bave been clearly divided by suitable headings. The tables can be estarad directly with readings of Dry Bulb and Wet Bulb temperatures, depression of the Wet Bulb need not be separately calculated for using the tables.
In order to find the Relative Humidity or the Dew Point turn to the page which contains the values of the Wet Bulb temperature and Dry Dulb temperature for which either of these is required. (In both cases the values can be read off directly from the budy of the tables, if the temperatures are the actual values contaianed there otherwise interpolation should be made to the nearest 0.1°C in both cases. It should be remembered that both the Dry bulb and Wet bulb readings should be corrected for instrumental errors before using the tables.
The Dew Point and Relative Humidity from the values of Dry bulb and Wet Bulb temperatures can be read directly from the tables when the decimal in the Dry Belb and Wet Bulb are even numbers. When one or both of them are odd the values can be got by simple interpolation of values on either side.

The following examples are given for completeness :-

S. No.	Dry Bulb in °C	Wet Bulb in °C	Page No.	Dew Point in °C	Relative Humidity (%)	Remarks
1	10.8	9.4	29	7.9	83
2	25.5	19.8	48	16.3	57	By interpolation of D.B.
3	17.4	13.7	36	10.5	64	By interpolation of W.B.
4	28.5	21.3	54	17.1	50	By interpolation of both D.B. and W.B.