what is the relationship between PRESSURE TENDENCY, RELATIVE HUMIDITY and FORECAST TIME FOR PRECIPITATION ?
109204
it's all a bit relative since these days we also have "k"-value which is essential in forecasting precipitation.  it refers to the lift in areas of an airmass. for example, a thunderstorm can form, grow to 40,000 feet, drop its precipitation and go away in only a few minutes.  an area such as this would have a negative value for k, say minus 40 while a very stable area could be plus 40.  while the factors you refer to are important and useful indicators, consider that you can have any combination of humidity and pressure trend and it does not necessarily nean it will rain.  those parameters are the basis of centuries old meteorology and the principles have not changed all that much over time but then the forecasts delivered with these tools were marginal, at best.  the relationship principles you speak of were the basis for a guage to do precisly what you speak of. it's called a barometer and you can find all kinds of information on one on these any even probably find one to purchase for only a few dollars on e-bay or something. a borometer takes pressure trends, considers humidy and does give an estimated time in which precipitation would be expected.  accurancy?  well if i remember right only maybe about 60% or so but don't quote me there.
33065
The meteorological conditions: temperature and wind and clouds and precipitation
30909
They may calculate the odds of precipitation, how long it will take for the temperature to rise and fall based upon influences and when those influences will occur, how fast storms are travelling, how fast wind speed are, changing Fahrenheit to Celcius and vice versa.
548046
To an extent it is true.  If dew has settled by morning, then that means there is less water vapor in the air.  If there is less water in the air, then it is less likely to condensate and fall.  However, after dew has settled, if it gets cold enough, it can still rain.
683254
http://www.bom.gov.au/lam/Students_Teachers/pressure.shtml\nhttp://en.wikipedia.org/wiki/Weather_front
853322
Portland (Oregon, I presume) is close to optimal; farther south on the  California coast (San Diego) might be better.  Anyplace east of the Rockies will have more front-related weather, with corresponding pressure changes.
516442
it means amount of rainfall in mm or cm or inches.
27238
Dew point isn't calculated, it's measured by an instrument.  The dew point is the temperature that the air has to be cooled to in order for condensation to occur.  Either a dew point sensor (like on the ASOS sites located at airports across the country) or a psychrometer (sometimes called a sling psychrometer) measures wet-bulb and dry-bulb temperature.  The difference between the dry bulb and wet bulb temperature is the relative humidity.
937278
Geographic location and altitude of the air mass, also the proximity of large bodies of water. Water tends to have a moderating effect on temperature. Moisture content changes at altitude and lower temperatures as moisture is lost in the form of precipitation.\n\nFor example you have on a mountain peak several zones one of which is the zone of precipitation, the altitude at which precipitation is likely to occur. The second is the zone of accumulation which is the altitude at which accumulation of precipitated materials is likely to occur (snow line).
517417
65\n100% humidity\nsunny morning, moderate to heavy rainy afternoon, calm muggy evening.\n\nI'm taking a shower by just standing outside.\n\nsouthern NH
891401
"temperatures range from 32 °C (90 °F) during the daytime to 21 °C (70 °F) in the evening practically year-round. Humidity is always high at about 80 percent. The rainy season takes place between October and November"
367121
They try to predict the weather.
560715
If you mean on the surface, then yes. There is a big misconception about this.\n\nIt needs to be 100% relative humidity where clouds are developing NOT on the surface.\n\nThe dew point is the temperature at which saturation occurs (100% humidity). When the dew point is reached at cloud condensation level, then clouds will form & precipitation may occur.\n\nHumidity is measured in several different ways. Relative humidity is how much water vapor compared to how much it could hold at a certain temperature. This is the one we use most often.
979367
An Air Mass is a large volume of Air having fairly uniform characteristics of temp., atmospheric pressure, and water vapor content.\n\nA Front is a boundary or transition zone between two air masses of different density, and thus (usually) of different temperature. A moving front is named according to the advancing air mass, e.g., cold front if colder air is advancing.\n\nDewpoint is a measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation. A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature.\nSaturated or Saturation is the cooling of the temp.\n\nRelative humidity is  the ratio of the partial pressure of water vapor in a gaseous mixture of air and water to the saturated vapor pressure of water at a given temperature.\n\nDewpoint is a measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation. A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature.\n\nA Hurricane is a storm system fueled by the heat released when moist air rises and the water vapor in it condense. It has sustained winds of at least 33 m/s (64 kt, 74 mph, or 118 km/h). A hurricane of this intensity tends to develop an eye, an area of relative calm at the center of circulation. Surrounding the eye is the eye wall, an area about 10-50 mi wide in which the strongest thunderstorms and winds circulate around the storm’s center.\n\nTornado is a violently rotating column of air in contact with the ground and extending from the base of a thunderstorm. \n\nThese are self explanatory.
31056
No, it means that you should take your umbrella with you, because the humidity in the air, and the pressure system suggest it could rain.  Weather prediction isn't exact science, so basically... they never KNOW for a fact if it is raining or not... no matter what percent they give.
342058
If they predicted a 30% chance of rain, and it does rain, that means that they were 30% = 0.3 correct in this instance.  If you average these correctness values over, say, a year...then you'll get a value for how accurate your weatherman is.  Of course, you have to remember...That a person guessing 0 or 100% (yes it will rain or no it won't) should average to a 50% = 0.5 accuracy.\n\nSo, if the weatherman's accuracy is greater that 0.5 over several days...then the weather is more right than he is wrong.  That seems intuitive, eh?\n\nRemember that you can't just compare to what all the other weathermen say...and take the average 'risk' as the actual risk...because all weathermen get their information from Environment Canada, and the numbers will be skewed because they're all using the same information.
754142
It seems like you could figure the relative humidity based on the pressure, temperature, specific volume, etc.  After that you could figure the volume of condensation on a per cubic meter basis.  You would need a total volume to come up with the total mm/day.  Hope that helps get you going on the right path to an answer.  If not, you should re-post your question with additional information- I get the feeling you left something out.