what is difference between RH and Humidity ratio?

Answer 1

I presume that you mean 'humidity mixing ratio' by 'humidity ratio'.
Humidity mixing ratio is the amount of water vapour in grams mixed with the one gram of dry air.That is ,when one gram of dry air holds 'x' gram of water vapour in it ,then the total mass of the moist air is '1+x' gram.Here 'x' is the humidity mixing ratio.In other words,humidity mixing ratio is the relative proportions by weight of water vapour and dry air.
Relative Humidity is defined as the ratio of the amount of water vapour present in the air to the amount of water vapour required to saturate the air at that temperature.In other words, the relative humidity is the percentage of moisture in the air.It is also known as the degree of saturation.