Wondering what the polar vortex is and why it causes extreme cold, what a PDS tornado watch means for your safety, the difference between partly cloudy and mostly cloudy, or what a pyrocumulonimbus cloud does to wildfire behavior?
Jump to weather terms beginning with the letter:
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
Get Severe Weather Alerts for Your Location
When any NWS watch, warning, or advisory is issued for your area, iAlert sends you an immediate notification by email or text, 24 hours a day, 7 days a week.
Sign Up for Severe Weather Alerts →Weather Terms Beginning with “P”
Particularly Dangerous Situation (PDS)
Particularly Dangerous Situation is a designation added by the Storm Prediction Center to Tornado Watches and Severe Thunderstorm Watches when conditions are especially favorable for violent, long-track tornadoes or widespread severe weather. It is indicated by the phrase “THIS IS A PARTICULARLY DANGEROUS SITUATION” in the watch text. PDS Tornado Watches are issued only approximately 2-5 times per year, reserved for the most extreme setups. Past PDS dates include April 27, 2011 (155 tornadoes, 250 deaths in Alabama alone, the deadliest US outbreak since 1974), May 3, 1999 (Bridge Creek-Moore EF5 tornado), and May 20, 2013 (Moore, Oklahoma EF5). When a PDS Watch covers your area, you should be positioned near or in your shelter, not just monitoring the situation.
Polar Vortex
The polar vortex is a large area of cold, low-pressure air that normally sits over the Arctic year-round, contained and organized by the polar jet stream. When the polar jet weakens or develops extreme southward meanders, lobes of the polar vortex can split off and dip far into the continental US, delivering extreme Arctic cold outbreaks. The January 2014 and January 2019 polar vortex events sent temperatures plunging to -20 to -40 degrees F across the Midwest, with wind chills reaching -50 to -70 degrees F in some areas. The polar vortex is technically a feature of both the stratosphere and the troposphere. The term became widely known after the 2014 event, though meteorologists had used it for decades before that. See also: OSHA Cold Temperature Stress guidance and Wind Chill Chart.
Precipitation
Precipitation is any form of water, liquid or solid, that falls from the atmosphere and reaches the ground. Types include rain (liquid drops over 0.5mm), drizzle (liquid drops under 0.5mm), sleet or ice pellets (precipitation that freezes completely before reaching the ground), freezing rain (liquid drops that freeze on contact with surfaces at or below 32 degrees F), snow (ice crystals that form in clouds and aggregate into flakes), graupel (rimed snow pellets, soft and opaque), and hail (layered ice balls formed in thunderstorm updrafts). Precipitation is measured in inches of water equivalent. Snow is measured by depth and converted to water equivalent, typically a 10:1 ratio for fluffy snow and roughly 3:1 for wet, heavy snow. The driest inhabited place on Earth is the Atacama Desert in Chile, which averages less than 0.04 inches of rain per year.
Types of Precipitation
| Type | Form | Freezes When | Surface Effect | Most Common Season |
|---|---|---|---|---|
| Rain | Liquid drops >0.5mm | Never (above freezing) | Wet roads, flooding | Year-round |
| Drizzle | Tiny liquid drops <0.5mm | Never (above freezing) | Light wet surfaces | Fall/Winter/Spring |
| Snow | Ice crystals / flakes | In the cloud | Accumulation, reduced traction | Winter |
| Sleet (Ice Pellets) | Small ice pellets | Freezes in the air | Slippery accumulation | Winter |
| Freezing Rain | Liquid drops | On contact with surfaces | Glaze ice, black ice | Winter |
| Graupel | Soft, rimed snow pellets | In cloud (partial) | Ball-bearing-like slipperiness | Winter/Spring |
| Hail | Layered hard ice balls | In thunderstorm updraft | Crop and property damage | Spring/Summer |
Pressure Gradient Force
The pressure gradient force is the atmospheric force that drives air from high pressure to low pressure. The greater the pressure difference over a given distance, the stronger the pressure gradient and the stronger the resulting wind. On surface weather maps, the pressure gradient is represented by the spacing of isobars (lines of equal pressure), closely spaced isobars indicate a strong gradient and high winds. The wind speed is proportional to the pressure gradient. A storm with a very tight pressure gradient can produce hurricane-force winds even without being a tropical system. The pressure gradient force is modified by the Coriolis effect and surface friction, which cause wind to flow nearly parallel to isobars at upper levels and at an angle across isobars near the surface.
Partly Cloudy vs Mostly Cloudy
In official NWS public forecasts, “Partly Cloudy” means 3/8 to 4/8 sky coverage (Scattered or SCT in METAR), with intervals of sunshine expected during the forecast period. “Mostly Cloudy” means 5/8 to 7/8 sky coverage (Broken or BKN in METAR), with limited sunshine and predominantly gray skies. The distinction affects perceived temperature, solar energy production, and ground-level UV exposure. “Mostly Sunny” is functionally the reverse of “Mostly Cloudy”, the same cloud coverage range, but used when the emphasis is on the sunny periods rather than the cloudy ones. “Partly Sunny” is used equivalently to “Partly Cloudy.”
Pyrocumulonimbus (pyroCb)
A pyrocumulonimbus is a cumulonimbus cloud generated directly by an intense wildfire or volcanic eruption. It forms when extreme heat from a massive wildfire creates a powerful updraft that reaches the mid-troposphere, drawing in surrounding ambient moisture and generating a full thunderstorm above the fire. PyroCb storms are particularly dangerous because they produce lightning that starts new fires in locations separate from the original burn area, create powerful and erratic outflow winds that change fire behavior unpredictably, and can generate fire whirls, colloquially called fire tornadoes. The 2020 California Creek Fire generated a pyroCb intense enough to produce a documented EF1 tornado. PyroCb events represent the most dangerous and unpredictable conditions for wildland firefighters. Read more: Learn About Wildfires, Your Risk, and How to Prepare.
Freezing Level
The freezing level is the altitude at which the air temperature is 32 degrees F (0 degrees C). Above the freezing level, precipitation falls as snow; below it, precipitation melts to rain. The freezing level varies from near sea level in winter to 14,000 feet or higher in summer over mountainous terrain. During winter precipitation events, the precise height and depth of the freezing level determines whether surface precipitation falls as rain, freezing rain, sleet, or snow. Forecasting the exact rain-snow line requires precise knowledge of the vertical temperature structure, obtained from weather balloon soundings launched twice daily by the NWS. Even small errors in forecasting the freezing level can shift the snow-rain line by 50-100 miles.
Polar Front
The polar front is the semi-permanent boundary between cold polar air masses and warm tropical air masses in the mid-latitudes. It is the zone of greatest temperature contrast and atmospheric instability in the middle latitudes. Cyclones, extratropical storms, form and intensify along the polar front, fueled by the sharp temperature gradient. The polar front jet stream flows along and above the polar front, marking its position in the upper atmosphere at approximately 30,000-40,000 feet. The polar front migrates northward in summer and southward in winter, driving the seasonal shift in storm tracks across the United States. The most intense severe weather and winter storm development in the US occurs along and ahead of the polar front.
Rain Gauge
A rain gauge is the standard instrument for measuring liquid precipitation. The official NWS rain gauge, a 4-inch diameter standard gauge, is the reference measurement for all precipitation records. ASOS (Automated Surface Observing System) sites report precipitation automatically every hour using tipping bucket gauges. CoCoRaHS (Community Collaborative Rain, Hail, and Snow Network) recruits citizen scientists to take daily precipitation measurements using standard rain gauges, providing high-density coverage that official networks cannot match. Snow depth is measured by ruler at COOP (Cooperative Observer Program) sites. Hail size is reported in inches and compared to common objects: dime equals 0.75 inch; golf ball equals 1.75 inches; baseball equals 2.75 inches; softball equals 4.5 inches.
Probability of Precipitation (PoP)
Probability of Precipitation is the percent chance that at least 0.01 inches of precipitation will fall at any given point in the forecast area during the specified forecast period. PoP is commonly misunderstood, a 40% PoP does NOT mean it will rain 40% of the time or over 40% of the area. It means there is a 4-in-10 chance that any specific location in the forecast area will receive measurable precipitation. A PoP of 70% or higher generally means precipitation is likely at some point during the period. A PoP below 20% is generally not mentioned in public forecast language. PoP is calculated by combining the forecaster’s confidence that precipitation will develop with the expected areal coverage of that precipitation.
Public Information Statement (PIS)
A Public Information Statement is a non-warning NWS product that provides general public information about a weather event, climate record, or safety message that does not require a Watch, Warning, or Advisory. Examples include record temperature announcements, end-of-storm event summaries, significant weather event reviews, and seasonal climate outlooks. PIS products are issued on an as-needed basis rather than on a standard schedule. They help the public understand the meteorological significance of recent or upcoming events, for example, informing that yesterday’s temperature set an all-time station record, or that last week’s snowstorm ranked in the top five for the city, without triggering a formal alert response.
Severe Weather Alerts, Any Location, Any Time
From a Tornado Watch to a PDS Tornado Watch, iAlert monitors every NWS alert and notifies you the moment one is issued for your area. Set up alerts for home, work, school, or anywhere your family is.
Get Weather Alerts →Frequently Asked Questions, “P” Weather Terms
What is the polar vortex and why does it cause extreme cold?
The polar vortex is a large circulation of cold air and low pressure that normally sits over the Arctic, contained by the polar jet stream. When the polar jet weakens or develops large southward meanders, lobes of the polar vortex can break off and push deep into the continental US, bringing Arctic air that originated over Canada and the Arctic Ocean. During these events, temperatures across the Midwest and Great Plains can drop to -20 to -40 degrees F with wind chills of -50 to -70 degrees F. The January 2014 and January 2019 events brought record cold to dozens of US cities and triggered thousands of flight cancellations.
What does PDS mean in a tornado watch?
PDS stands for “Particularly Dangerous Situation”, a designation the Storm Prediction Center adds to a Tornado Watch when the atmospheric setup is unusually favorable for violent, long-track, potentially EF4 or EF5 tornadoes. It is issued only about 2-5 times per year for the most extreme setups. Past PDS days include April 27, 2011 (the deadliest US tornado outbreak since 1974), May 3, 1999, and May 20, 2013. When a PDS Tornado Watch covers your area, you should be positioned near or in your shelter, not just monitoring the situation.
What is the difference between partly cloudy and mostly cloudy?
In NWS forecasts, “partly cloudy” means 3/8 to 4/8 of the sky is covered by clouds, expect alternating periods of sunshine and clouds. “Mostly cloudy” means 5/8 to 7/8 sky coverage, predominantly gray skies with limited periods of direct sunlight. “Overcast” means complete cloud cover with no direct sunlight. In METAR weather observation codes, these correspond to SCT (scattered), BKN (broken), and OVC (overcast) respectively. The distinction matters for solar energy production, outdoor activities, and perceived comfort on warm days.
What is a pyrocumulonimbus cloud?
A pyrocumulonimbus (pyroCb) is a thunderstorm that develops directly over an intense wildfire when the extreme heat creates a powerful updraft that draws in surrounding moisture and builds to cumulonimbus height. PyroCb storms are dangerous because they generate their own lightning (starting new fires), create powerful and erratic outflow winds that change fire behavior unpredictably, and can produce fire whirls (colloquially called fire tornadoes). The 2020 Creek Fire in California generated a pyroCb that produced a documented EF1 tornado. These events represent the most dangerous conditions for wildland firefighters.
What are the different types of precipitation?
The NWS recognizes several precipitation types: Rain (liquid drops larger than 0.5mm falling through air above freezing all the way to the surface); Drizzle (tiny liquid drops under 0.5mm); Snow (ice crystals that form in clouds and fall as flakes); Sleet or Ice Pellets (rain that freezes completely before reaching the ground, bouncing on impact); Freezing Rain (rain that freezes on contact with surfaces below 32 degrees F, forming glaze ice); Graupel or Snow Pellets (snowflakes coated with rime ice, soft and opaque); and Hail (layered ice balls formed in thunderstorm updrafts, 5mm or larger).
What does “probability of precipitation” mean in a forecast?
Probability of Precipitation (PoP) is the percent chance that at least 0.01 inches of precipitation will fall at any specific point in the forecast area during the specified time period. A 40% PoP means there is a 4-in-10 chance of measurable precipitation at your location, not that it will rain 40% of the time or over 40% of the area. A 70% PoP or higher means precipitation is likely. Below 20% is usually not mentioned in forecast text. PoP combines the forecaster’s confidence that precipitation will form with the expected areal coverage of the precipitation.
Never Miss a Severe Weather Alert
iAlert monitors National Weather Service watches, warnings, and advisories around the clock and sends you an immediate notification the moment your area is threatened. Works for any U.S. location.
View All Alert Services → Severe Weather Alerts →Related Articles
- Get real-time Severe Weather Email & Text Alerts
- Learn About Wildfires, Your Risk, and How to Prepare
- Hot Weather Survival Tips in Extreme Summer Heat
- Protection Against Extreme High Temperatures
- Heat Index Calculator
- OSHA: Protecting Workers from Cold Temperature Stress
- Wind Chill Chart
- Flood Safety
- What is the Difference Between a Flash Flood and a Flood?
- Criteria for a Tornado Warning
- Criteria for a Severe Thunderstorm Warning
- Understanding the NWS Severe Weather Outlook
- WxData.com, Weather Data API for apps and systems
