Percentages are commonly used in meteorology. This writing will go over a couple of the common weather data where percentages can be found and the interpretation of those percentages. One area that percentages are used is relative humidity. This percentage ranges from 0% to 100%. 0% indicates air with no moisture and 100% represents air that is saturated with moisture with respect to a liquid water surface. When the relative humidity is 100%, then fog, cloud, dew, frost or precipitation are sometimes seen since at this point moisture will start condensing out of the air. Relative humidity of 30% or less indicates fairly dry air. Moisturizers and Chap Stick are good to use when the air is dry. 40% to 80% is generally comfortable although 60% to 80% can feel uncomfortable in warm air since the amount of moisture that can be in the air increases exponentially with increasing temperature. Higher than 80% relative humidity will generally feel humid outside, although at cold temperatures the air will feel damp instead of uncomfortably humid since cold air has a lower maximum moisture capacity. Relative humidity is a ratio of how much moisture is in the air over the maximum amount of moisture that could be in the air for a certain temperature. For example, a relative humidity of 50% indicates half the moisture is in the air for the amount of moisture that could be in the air for a certain temperature. Note there are many different combinations of values that can produce a relative humidity of 50%. Thus, relative humidity does not indicate the amount of moisture in the air, but only how much moisture is in the air relative to how much can be in the air for a certain temperature. For example, if the air can have 2 units of moisture and there is 1 unit of moisture in the air then the relative humidity is (1/2) which equals 50% when converted to a percent. In another example, the air could have 16 units of moisture and there are 8 units of moisture in the air. This will also produce a relative humidity of (8/16) which equals 50% when converted to a percent. Note the air that could contain a maximum of 16 units of moisture must be warmer than the air that could only have a maximum of 2 units of moisture since the amount of moisture that can be in the air increases with increasing temperature. Another area that percentages are used is in the probability of precipitation. The probability of precipitation ranges from 0% to 100%. 0% indicates no chance for precipitation while 100% indicates precipitation will occur. For precipitation to verify, a certain minimum amount must accumulate during a specific time period. In inch units, this minimum amount is 0.01 inches. If less precipitation falls than this then it counts as no precipitation being recorded. The probability of precipitation is specified for a time frame (such as noon to 6 pm) and for a forecast region. When the probability of precipitation is considered for a point location, it can be thought of as being the percent of the times that precipitation will occur for the total number of times precipitation had that same chance. For example, if on 10 different occasions the probability of precipitation was 30%, then it would be expected that on 3 out of those 10 times that precipitation will occur at that station. A couple of potential misconceptions about probability of precipitation are explained in the next couple of sentences. A 30% chance does NOT mean that in any one event that 30% of the forecast area will get precipitation in that single event. A 30% chance does NOT mean there will be precipitation for 30% of the forecasted time frame such as raining for 3 hours of a 10 hour time frame. Probability of precipitation (POP) combines the chance of the probability of precipitation occurring anywhere in the forecast area and the areal coverage expected if precipitation does develop. In a long term average for any point location in the forecast region this works out to the POP value meaning that out of all the times the atmosphere has had a similar set up, there is a particular chance of precipitation. For example, if the POP is 80%, then in 80 of 100 of the situations in which the atmosphere has this same set up, 80% of the time precipitation will occur at that specific station. In practical terms, a higher POP indicates a greater likelihood that precipitation will occur. 0% to 30% indicates precipitation is not likely, 40% to 70% indicates to be on the lookout for precipitation, and 80% or higher indicates there is likely to be precipitation. Note that the POP value says nothing about how much precipitation to expect. On a day with 100% POP, there can be 0.01 inches of rain or 2 inches of rain and in both cases the POP verified. The POP is only interested in if precipitation is less than 0.01 inch (precipitation not verified) or 0.01 inch or greater (precipitation verified). |