# Relationship between relative humidity and rainfall

All three -- relative humidity, temperature and dew point -- are bound together in the mathematical relationship below. A source of confusion. Nice question! When relative humidity is low, rainfall is reduced. In some cases, raindrops even do not reach the ground. The second effect is that the rainfall. Short answer: humidity is not a proxy for rain starting and no, it does not start When the dew point depression (the difference between temperature and dew point) is high, the No, it doesn't start raining as soon as relative humidity (RH) is %. For example, Tripoli and Cotton created a relationship.

The critical supersaturation is the supersaturation needed to attain the critical drop size, and is generally small e. Droplets below the critical size are 'haze drops' and these make up the haze you see on very humid days. Drops that reach the critical size can continue to grow to become cloud drops. The condensed water is carried in the air but is no longer water vapor and is not part of relative humidity but does contribute to the parcel density So Drops will grow until the updraft can no longer support their mass and then they fall from the cloud as rain.

Your question asks at what humidity does it rain, but what surface humidity determines is how high the cloud bases are. When the dew point depression the difference between temperature and dew point is high, the cloud bases will be higher than when the dew point depression is small.

If there is forcing for vertical ascent, parcels can rise to this height and then to a height where they freely convect due to decreased parcel density caused by the release of energy during condensation see: It is these three things -- moisture, aerosols and cooling, that we need for a rain storm.

## Difference Between Humidity and Relative Humidity

We also have storms we call 'elevated convection' that are completely disconnected from surface conditions and when these storms cause rain is not related to surface humidity at all. An amount of water vapor in warm air will result to a lower relative humidity than in cool air. When the relative humidity is high, the evaporation of the moisture of the skin is reduced and decreases the effectiveness of sweating in cooling the body. The heat index which is used during the summer is used to measure this effect.

It is used in circumstances wherein the rate of water evaporation is necessary. Humidity is the amount of water vapor in the atmosphere while relative humidity is one type of humidity. Humidity is the water content of the mixture of water vapor and other elements found in the air while relative humidity is the percentage of water vapor in the air at a given temperature.

Humidity is used to determine the likelihood of precipitation, fog, or dew while relative humidity is used for climate control and how it affects the health, comfort, and safety of humans. Relative humidity is also used to ensure the safety of machines, vehicles, and buildings while determining the humidity of a certain place provides a way to gauge the weather.

### meteorology - At what humidity does it start to rain? - Earth Science Stack Exchange

Particles of dust, smoke and salt from the oceans are particularly good nuclei because they absorb water. Condensation in air itself can only take place if the air temperature is reduced to below the dew point. In contrast, when the relative humidity is low, a large amount of cooling is required to first reach the dew point and then the condensation. Condensation, therefore, depends upon— i the amount of cooling and ii relative humidity of the air.

Condensation occurs under varying conditions which, in some way or the other, are associated with change in any of these variables—air volume, temperature, pressure and humidity.

Thus condensation takes place: The most common circumstances favourable for condensation are those producing a drop in air temperature. When the air rises, it expands. Thus, heat; available per unit volume is reduced and, therefore, the temperature is also reduced. The vertical displacement of the air is the major cause of adiabatic and katabatic cold, dense air flowing down a slope temperature changes.

The rate at which temperature decreases in rising air depends upon the moisture content of the air. In unsaturated air, the decrease of temperature with height is twice that in saturated air.

This is mainly due to the release of latent heat of condensation after saturation occurs. The rate at which temperature decreases in rising unsaturated air is known as dry adiabatic rate and that in the saturated air is called wet adiabatic rate.

Non-Adiabatic Temperature Changes Non- adiabatic processes include cooling by radiation, conduction or mixing with colder air. The air may be cooled due to loss of heat by radiation. In case there is direct radiation from moist air, the cooling produces fog or clouds, subject to presence of hygroscopic nuclei in the air. Cooling may also be produced by conduction or advection of warm air across a cold surface.

Cooling by contact with a cold surface produces dew, frost or fog depending on other atmospheric conditions. Sometimes, the air is cooled due to its mixing with colder air. But the effect of cooling produced by radiation, conduction and mixing is confined to a thin layer of the atmosphere. The non-adiabatic processes of cooling produce only dew, fog or frost.

They are incapable of producing a substantial amount of precipitation. The only process capable of reducing the temperature of deep and extensive air masses, so that cloud-formation and appreciable precipitation may be possible, is the expansion associated with rising air currents or the adiabatic cooling. Forms of Condensation the forms of condensation can be classified on the basis of temperature at which the dew point is reached.

Condensation can take place when the dew point is— i lower than the freezing point, ii higher than the freezing point.

Whereas white frost, snow and some clouds are produced when the temperature is lower than the freezing point, dew, fog and clouds result even when the temperature is higher than the freezing point.

Forms of condensation may also be classified on the basis of their location, i. Dew, white frost, fog and mist come in the first category, whereas clouds are in the second category. Various forms of condensation are discussed below. Dew When moisture is deposited in form of water droplets on cooler surface of solid objects such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions -for its formation are a clear sky, little or no wind, high relative humidity and long, cold nights leading to greater radiation of heat from the earth for its cooling.

For the formation of dew, it is necessary that the dew point is above freezing point. It is called white frost.

The ideal conditions for formation of white frost are the same as those for formation of dew, except that the air temperature must be at or below freezing point. Fog is defined as a cloud with its base at or very near the ground. Fogs are of different kinds depending upon the nature of the cooling process. Radiation fog results from radiation, cooling of the ground and adjacent air.

These fogs are not very thick. Fogs formed by condensation of warm air when it moves horizontally over a cold surface, are known as advectional fog. These fogs are thick and persistent.

The Relationship between Relative Humidity and Moisture Content

Sometimes, due to convergence of warm and cold air masses, the warm air mass is pushed up by the heavier cold air mass. Then, if the warm air reaches saturation and some moisture falls down as precipitation, this moisture falling as precipitation may condense to produce fog at the boundary of the two air masses.

These are called frontal or precipitation fog. This is also a kind of fog in which the visibility is more than one kilometre but less than two kilometres. A cloud is a mass of minute droplets of water or tiny crystals of ice formed by the condensation of the water vapour in free air at considerable elevations. Clouds are caused mainly by the adiabatic cooling of air below its dew point.

Clouds can be classified on the basis of— a their appearance, i. On the basis or the appearance, the following cloud types may be identified. Cirrus clouds are high, white and thin.

They are composed of ice crystals. They have a fibrous and feathery appearance. Cumulus clouds exhibit a flat base and have the appearance of rising domes. These clouds have a cauliflower structure. Stratus clouds can be described as sheets of layers that cover much or all of the sky. All the clouds either reflect one of these three basic forms or are combinations or modifications of them.

## Humidity and Precipitation (Useful Notes)

On the basis of height, following categories of clouds can be identified. The International Cloud Code lists 28 types but ten fundamental genera are recognised. Condensation of water vapour in the air in the form of water droplets and ice another falling on the ground is called precipitation. This may take place in liquid or solid forms of water. It is only when the raindrops i. Precipitation is perhaps the most important stage of the hydrological cycle. Precipitation in the form of drops of water is called rainfall, when the drop size is more than 0.