Airmass: A section of the atmosphere that covers a large geographical area and is defined in terms of temperature and moisture. Airmasses are fluid and can move from one region to another, but originate from "source" regions. Airmasses tend to fall into one or more of these categories: Maritime (wet), continental (dry), tropical (hot), polar (cold), and arctic (very cold).

Alberta Clipper: A fast-moving low-pressure system that originates along the eastern side of the Canadian Rockies before tracking east/southeast across the Great Lakes and into the Northeast (and occasionally further south into the Mid-Atlantic). These systems are typically cold and low in moisture content, resulting in relatively dry/powdery snow across the Great Lakes, New England, and occasionally the Mid-Atlantic. 

Altocumulus Clouds: Small, layered patchy clouds that form in the mid-levels of the atmosphere. If you see these clouds in the morning during the warm season, it's typically a sign of instability in the atmosphere and is often a precursor to afternoon thunderstorms. Example

Altostratus Clouds: Thin, gray, and relatively uniform overcast cloud layers that form in the mid-levels of the atmosphere, typically above mountain-top level. These clouds often form in advance of low pressure systems and/or warm fronts, and typically signal the arrival of precipitation in the next 12-24 hours. Example

Arctic Airmass: A region of dry and frigid air originating in Alaska and Northern Canada (above the Arctic Circle) that periodically intrudes southward into the mid-latitudes (including the U.S.) during the winter. These airmasses are typically very dry with very light if any snow within the airmass itself. However, when the leading edge of an arctic airmass (also known as an arctic front) interacts with a moist airmass, heavy snowfall and deep powder days can occur. 

Arctic Cold Front: A cold front involving an arctic airmass overtaking a comparatively warmer airmass, resulting in a substantial temperature drop. When an arctic cold front encounters a moist airmass, it can produce very heavy snowfall rates along with low-density snow quality. Eventually, cold and dry air behind an arctic cold front will scour out any lingering moisture, often resulting in clear and frigid conditions in its wake. 

Atmospheric Lift: Upward vertical motion (or lift) in the atmosphere is necessary for precipitation to form. Air parcels containing moisture that rise vertically will expand and cool with altitude, condensing into clouds and eventually precipitation. There are numerous mechanisms that can cause air parcels to "lift", including mountains/terrain, converging winds, fronts, low pressure systems, and upper air disturbances. 

Atmospheric River: A long, narrow corridor of moisture moving across the ocean that can reach coastal and inland areas, depositing heavy precipitation. Most common in fall, winter, and early spring. These often originate in the subtropics and can result in heavy snow, but also wet/dense snow quality, high snow levels, and sometimes heavy rain even at ski resort elevations. In North America, these patterns typically impact the Sierra, Cascade, and Coast Ranges, but can occasinally have downstream impacts in the Rockies. More Info

Avalanche Weather: Describes weather factors that influence avalanche danger. These factors include precipitation, new snowfall, snowfall rates, snow density, temperature, wind speed, wind direction, cloud cover, and solar radiation. More Info

Backdoor Cold Front: A cold front that arrives from the east or northeast, with winds blowing from similar directions behind the front. Most common along the eastern slopes of the Continental Divide in the Rockies and also in coastal regions of the Eastern U.S. and along the eastern slopes of the Appalachians. Behind these fronts, cooler temperatures, increased cloud cover, and increased precipitation chances are typical. 

Blizzard: A snowstorm that is characterized by strong winds and low visibility, rather than snowfall rates or amounts. According to the U.S. National Weather Service, a blizzard is defined as a snowstorm in which 1) wind speeds or frequent gusts are at least 35 mph and 2) visibility is 1/4 of a mile or less, and 3) both of these criteria are met for a sustained period of at least 3 hours. 

Cold Front: A boundary that involves a colder airmass overtaking a warmer airmass. Heavy precipitation (snow, rain, or thunderstorms) and gusty winds often develop along and ahead of a cold front when moisture is available, while a wind direction change typically occurs behind a front. Terrain-enhanced snow or rain showers may continue after a cold front passes if enough moisture remains. Snow behind a cold front typically becomes lower density (i.e. drier/more powdery) due to colder air arriving. 

High Pressure: A region of air that is denser than its surroundings, which promotes sinking motion in the atmosphere, typically leading to drier and more stable weather. Air within a region of high pressure can be either hot or cold depending on other factors. Winds in the Northern Hemisphere blow in a clockwise direction around a high-pressure center, which can transport moisture around a high-pressure center in the summertime, resulting in showers/thunderstorms along the periphery of a high-pressure region.

Low Pressure: A region of air that is less dense than its surroundings, which promotes rising motion in the atmosphere, typically leading to less stable weather including clouds and precipitation. Air within a region of low pressure can be warm or cool. Winds in the Northern Hemisphere blow in a counterclockwise direction around a low-pressure center. Low-pressure systems typically are associated with active weather, including rain, snow, and wind. 

Warm Front: A boundary that involves a warmer airmass gradually overtaking a colder airmass. As warmer air is forced to rise up and over colder and denser air near the surface, this often results in widespread steady precipitation. Warm fronts can result in heavy snowfall rates, but can also lead to rising snow levels and increasing snow density (i.e. wetter snow) depending on how warm the airmass is behind the warm front.

Weather: Mix of events that happen each day in our atmosphere. Weather is different in different parts of the world and changes over minutes, hours, days, and weeks. Most weather happens in the troposphere, the part of Earth’s atmosphere closest to the ground.