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. 

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. 

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.