As the old say goes, “Everybody talks about the weather but nobody does anything about it.” For those in agriculture it should be changed to “but farmers and ranchers live it.” Fall harvest is wrapping up. Almost all of the 2016 wheat crop is in the ground and much of the area has experienced freezing temperatures at least briefly. We have access to and are bombarded with weather forecasts and data through all types of media. But how many of us really know where these facts and figures come from and what the terminology means? And those in agriculture, especially in crop production have an additional set factors they pay attention to every day. Over the next two weeks, this column will deal with this topic. Today’s column focuses on weather terminology, its meaning and its importance. Naturally, it’s impossible to cover everything so let’s assume a few terms such as temperature are pretty well understood.
• Relative humidity (RH) – Humidity denotes water vapor in the air. Rain, clouds, fog and dew are not directly correlated with humidity as they are liquids, not vapors. Relative humidity, expressed as a percentage, indicates how much water vapor is in the air “relative” to how much it could hold. If the RH is 100% the air is holding as much water vapor as it can and liquid water (dew, fog, clouds) forms. At 10% RH the air is holding only 10% of what it could. How much vapor the air can hold is a function of air temperature. The warmer the air, the more energy it possesses so the more vapor it can hold (water vapor is at a higher energy state than liquid – think boiling water to make steam). This means 40% relative humidity at 20° is less vapor than at 90° so you probably feel miserable and sweaty when it’s ninety and the air seems dry at twenty.
• Dew point – This term is more useful for all of us. It tells you for the amount of water vapor present in the air what temperature the air must cool to for the air to be saturated (100% RH). The closer this number is to the air temperature the “wetter” the air. During severe weather season, the higher the dew point, the more moisture is available for convective thunderstorm activity. If you are interested how fall the temperature may fall overnight, the dew point is an important clue as it tells you how low the temperature could potentially go. The lower the dew point the colder it can get.
• Frost point – This term isn’t used often but is similar to the dew point except it’s the temperature the air must cool to for vapor to come out of the air. The difference is that the water vapor undergoes “sublimation”. It goes directly from vapor to ice without becoming a liquid first. Again the temperature tells you how cold it could possibly fall.