Great Bend Tribune
Published November 8, 2015
Weather Terminology and the Facts Part I: Humidity
As the old saying 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 20o is less vapor than at 90o 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.
Published November 15, 2015
Weather Terminology and Facts – Part 2 – Temperature & Wind
Last week’s column focused on humidity. This week focuses on temperature measurements with emphasis on agriculture. However, instead of two parts devoted to this topic there will be three with next week’s focus on where the long-term data comes from and what it tells us. First we’ll briefly discuss the temperature measurements everyone is familiar with.
- Daily high and low temperature – This is just what it sounds like. What was the highest and lowest recorded temperature for the period starting at 12:01 a.m. and ending at midnight of that day. Even if that temperature only occurred for a second it counts. The official temperatures are recorded at specific sites designated by the National Weather Service. These are typically at airports such as Eisenhower in Wichita. There are other sites part of the NWS grid with some being automated and some, where information is gathered manually that you won’t see unless you dig for them. K-State has a network of sites as do many Groundwater management Districts. Also temperature is recorded at a specific height, 1.5 meters or about five feet and should be above a natural surface such as grass.
- Soil temperature – Producers planting summer crops in the spring are concerned about the soil temperature they are planting into, especially with more tropical crops such as cotton, sesame, grain sorghum, and soybean. Crops need a minimum soil temperature to germinate, emerge, and grow vigorously. Some such as cotton can be “shocked” with cool soil, and air temperatures, and while not dying never recover and have poor season long growth.
- Heat accumulation – While you won’t find this on your nightly forecast, you can find it at the K-State weather data library website. Crops such as corn and grain sorghum grow based on the accumulation of heat and their growth stage is determined by this accumulation. This accumulation is determined by adding together the daily high and low, dividing by two and subtracting a base temperature (50 degrees for corn). Various formulas may vary a bit from this.
- Cold accumulation – Winter wheat has to accumulate cold before it can go ahead and flower. Different varieties have differing amounts of cold they need to accumulate. This prevents the plant, along with the need for increasing day length, from flowering too early.
- Wind – Wind is measured at sites where temperatures are measured. Wind is described by how fast the air is moving and where it is coming from. Not where it is going. We care where it is coming from since that helps in understanding what the condition of the air is – hot or cold, moist or dry.
Next week – precipitation and long-term data.