Advection: “In atmospheric science, change in a property of a moving mass of air because the mass is transported by the wind to a region where the property has a different value.
Climatology: “Is the study of the atmosphere and weather patterns over time. This field of science focuses on recording and analyzing weather patterns throughout the world and understanding the atmospheric conditions that cause them.”
CoCoRaHS: (Community Collaborative Rain Hail and Snow Network): “Grassroots volunteer network of backyard weather observers working together to measure and map precipitation.” These are a few terms associated with measuring precipitation and snow:
- Precipitable water: This includes rain and melted amounts of snow, hail, freezing rain, sleet, or any combinations.
- Liquid water equivalent: The measured amount of liquid in freezing or frozen precipitation.
- Snowfall: “The amount of snow that falls in a single storm or in a given period.”
- Total snow depth: “The depth of the new and old snow remaining on the ground at observation time.”
Convection: “Is used specifically to describe vertical transport of heat and moisture in the atmosphere, especially by updrafts and downdrafts in an unstable atmosphere.”
Convective Wind Gusts: Are normally associated with thunderstorm. As the storm collapses, the cold air at the top of the cloud starts to fall. As it falls it is surrounded by ever increasing warmer air. The increasing thermal gradient, increases the velocity. As it exits the base of the cloud it spreads out rapidly causing an outflow from the cloud. In extreme cases, this is accompanied by heavy rains and is referred to as a microburst.
Forecast Models: “Weather forecast models are computer programs that can help predict what the weather will be in the future, any time in the future from an hour to ten days out and even months ahead.” There are dozens of models. We’ll discuss two of them here.
- Euro forecast model: “The European Centre for Medium-Range Weather Forecasts (ECMWF) is an independent intergovernmental organization supported by most of the nations of Europe. It operates one of the largest supercomputer complexes in Europe and the world’s largest archive of numerical weather prediction data. It aims to provide accurate medium-range global weather forecasts out to 15 days and seasonal forecasts out to 12 months.”
- GFS forecast model: Global Forecast System is a weather forecast model produced by the National Centers for Environmental Prediction (NCEP). Dozens of atmospheric and land-soil variables are available through this data set, from temperatures, winds, and precipitation to soil moisture and atmospheric ozone concentration.
- Euro Model typically outperforms Euro but it is expensive to access the data. GFS has been making improvements. It’s best to monitor both models and verify performance of each run.
- Resolution of models: Resolution refers to the distance between grid points. The finer the resolution (shorter distance) the more computations necessary, the longer run time. Mid range models are 14 to 27km whereas short range models such as the HRRR are only 3km and more accurate in their computations.
Instability: “Condition in which air will rise freely on its own due to positive buoyancy.”
Isolated/spotty/scattered: Are terms in referencing the extent of coverage of thunderstorms, rainshowers or snowshowers. Scattered is more numerous than isolated or spotty. “The term “isolated” refers to showers that are few and far between; 10% to 20% coverage. “Scattered” refers to the range of 30% to 60% coverage. So, even with “scattered” showers, half or less of the neighborhoods are expected to “get wet.””
Monsoon: “A seasonal change in the direction of the prevailing, or strongest, winds of a region. Monsoons cause wet and dry seasons throughout much of the tropics.” Monsoon in Pagosa is a wet season that happens on average from July to September.
Orographic lift: “When a moving Air Mass meets a geographical feature such as a mountain, some of the air will have no option but to rise up over it. If the rising air is moist, clouds will form.”
QPF (quantitative precipitation forecast): “Expected amount of melted precipitation accumulated over a specified time period over a specified area.”
Radar: “NEXRAD (Next Generation Radar) obtains weather information (precipitation and wind) based upon returned energy. The radar emits a burst of energy. If the energy strikes an object (rain drop, snowflake, hail, bug, bird, etc), the energy is scattered in all directions.”
Satellite: There are three main satellite images that we look at.
- Visual Satellite: “Images are photographs of the earth that provide information about cloud cover.”
- Water Vapor Satellite: Imagery is created using a wavelength sensitive to the moisture content in the atmosphere. In this imagery, bright blue and white areas indicate the presence of high water vapor or moisture content, whereas dark orange and brown areas indicate little or no moisture present.
- Infrared Satellite: “The weather satellite detects heat energy in the infrared spectrum (infrared energy is invisible to the human eye).”
Saturation: “The state of the atmosphere in which air contains the maximum amount of water vapor that it can hold at a specific temperature and air pressure. At saturation, the relative humidity is 100 percent, temperature and dew point are equal, evaporation of water ceases.”
Snow ratio: “How much liquid would result if you melted the snow. The average snow ratio is 10 to 1 (10 inches of snow for 1 inch of liquid water), which is usually seen with temperatures near 32° or just below.” It is not uncommon to see 15 to 1 in Pagosa during the winter and 20 to 1 at Wolf Creek.
Snotel (Snow Telemetry): Automated snow measuring device network throughout the western US.
Snow vs Hail vs Sleet vs Graupel
- Snow: “Precipitation in the form of small white ice crystals formed directly from the water vapor of the air at a temperature of less than 32°F (0°C).”
- Hail: “Hailstones start with water droplets that are carried high into the clouds (past the freezing level) by the updrafts of a thunderstorm. The frozen water droplets fall once more, sometimes melting a bit as they reach warmer layers of air, and then they’re carried back up into the freezing layer by yet another updraft. The more this happens, the larger the hailstones get. When the updraft can no longer support the weight of the hail, or it weakens, they fall.
- Sleet or ice pellets: “It’s very similar to hail, but much smaller and forming under different conditions. Sleet is a winter weather occurrence and usually appears as clear, hard pellets. Sleet starts out as snowflakes high in the clouds, then falls through a warm layer of air, where it melts and turns into partially melted snowflakes and raindrops. Before reaching the Earth’s surface, sleet must fall through another layer of below-freezing air, where it re-freezes into ice pellets. It hits the ground with that all-too-familiar sound that’s unmistakable to those of us who live in cold climates!”
- Graupel or snow pellets: “Looks a lot like sleet or small hailstones, but the small balls are made of snow, not ice, and they are white. They almost look like tiny Styrofoam pellets. This form of precipitation starts as snowflakes, then those snowflakes grow larger and larger as supercooled water funnels up into the clouds and bonds to the flakes. You can tell the difference between graupel and hail or sleet by picking up the tiny snowballs — if they’re soft and easy to crush, it’s graupel. In fact, graupel is often referred to as “soft hail.” It often bounces and breaks apart once it hits the ground.”
Snow level: “Refers to the lowest elevation (either above ground or above sea level, depending on your output choice) at which falling precipitation should be mostly (almost all) snow.”
Sublimation: “The conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. Sublimation is most often used to describe the process of snow and ice changing into water vapor in the air without first melting into water.”
Trough vs Ridge: “A ridge is an elongated area of relatively high pressure extending from the center of a high-pressure region. A trough is an elongated area of relatively low pressure extending from the center of a region of low pressure.”
Upper Air Charts: “Weather observations above the ground are collected by weather balloons. Data from one a single site are plotted on a sounding, but an upper air chart can show information from multiple observation sites. Most upper air charts are plotted in pressure coordinates instead of height coordinates.” Satellite derived data is also added to these charts. Standard height for 700mb is 10,000 feet. Standard height for 500mb is 18,000 feet. Standard height for 300mb is 30,000 feet.
Weather Advisory: “Are issued for weather conditions that are expected to cause significant inconveniences and may be hazardous, These situations are normally not life threatening if caution is exercised.”
Weather Watch: “The potential exists for the development of severe” weather, “depending upon the specific type of watch issued”
Weather Warning: “Severe weather is imminent in your area or is already occurring”.