Curious about what virga is and why it creates a hidden aviation hazard, how visibility is measured and what triggers a Dense Fog Advisory, what vorticity means in meteorology, how vertical wind shear drives tornado development, or what causes valley fog?
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Valley Fog
Valley fog is a form of radiation fog that preferentially collects in valleys and low-lying terrain overnight. It forms when cool air, which is denser and heavier than warm air, drains downslope under the force of gravity and pools in valley bottoms, where it continues to radiate heat upward to a clear sky and cools to its dew point. Valley fog can be 200 to 1,000+ feet deep, blanketing entire valleys while hilltops and ridges above remain in clear air. It burns off from the top down as daytime solar heating warms the upper layers, which typically clears the fog by late morning. Valley fog is common in the Central Valley of California, the Tennessee Valley, and river valleys across Appalachia. It persists longest in autumn when nights are long, skies are clear, and winds are calm, the three conditions that maximize overnight radiational cooling. The San Joaquin and Sacramento valleys in California experience some of the densest valley fog in the United States, called “Tule Fog,” which causes dozens of catastrophic multi-vehicle highway accidents each winter.
Veering Wind
A veering wind is wind that shifts clockwise with increasing altitude, for example, southerly at the surface shifting to southwesterly at 5,000 feet and westerly at 15,000 feet. In the Northern Hemisphere, veering winds with height are associated with warm air advection, the horizontal transport of warm air into a region, and the associated atmospheric destabilization. Veering winds with height are one of the key ingredients in severe thunderstorm and tornado environments because they create the clockwise directional wind shear that tilts horizontal vortex tubes in the environment upright into a convective updraft, generating the rotating mesocyclone that defines a supercell. When meteorologists examine a weather balloon sounding and see a strongly veering wind profile combined with high CAPE, they recognize the potential for supercell thunderstorms and tornadoes is elevated. A wind that backs, shifts counterclockwise with height, indicates cold air advection and a more stable, less tornado-favorable atmosphere.
Vertical Wind Shear
Vertical wind shear is the change in wind speed and/or direction with increasing altitude. It is the single most important atmospheric factor in determining whether thunderstorms become severe and tornadic. Low-level wind shear, the change in wind below about 6,000 feet altitude, is critical for tornado development: it creates horizontal rotating “vortex tubes” in the environment that are tilted upward into a storm’s updraft, generating the rotating mesocyclone that is the tornado’s parent circulation. Deep-layer shear, from the surface to 30,000 feet, determines whether storms organize into long-lived supercells or remain disorganized multicell clusters. Low wind shear environments produce short-lived pulse thunderstorms with little severe weather potential. Moderate to high shear combined with high instability (CAPE) produces the recipe for supercell thunderstorms. Excessive shear can tear storms apart before they can organize. The Storm Prediction Center’s daily outlooks explicitly evaluate wind shear parameters when assessing tornado potential across the country.
See also: How tornadoes form | What is a Supercell?
Virga
Virga is precipitation that falls from a cloud base but evaporates or sublimates completely before reaching the ground. It is visible as wispy, streaky “tails” or curtains hanging below a cloud base, often with a distinctive downward-curving shape caused by the wind catching the falling precipitation. Virga is most common in arid and semi-arid regions where the air below the cloud base is very dry, the Great Basin, Desert Southwest, and high plains. The evaporation of the falling precipitation cools the air below the cloud dramatically and rapidly, creating a dense, cold air downdraft that can reach the ground as a dry microburst, a sudden, violent outburst of wind spreading in all directions with speeds that can reach 60-150+ mph and no visible rainfall to warn of its approach. Dry microbursts from virga-producing storms are a leading cause of aircraft accidents in the Desert Southwest, where intense summer convection over mountain ranges generates virga showers that present invisible wind shear hazards to aircraft operating below cloud base.
Visibility
Visibility is the greatest distance at which an observer can identify a prominent dark object against a bright sky (daytime visibility) or a moderately intense unfocused light source at night, using the unaided eye. In the United States, visibility is reported in statute miles or fractions of a mile in official METAR weather observations; the rest of the world uses meters. The NWS reports official visibility from automated weather stations and human observers at airports. At 10 miles, visibility is considered “unlimited” for most aviation purposes. Under 3 miles, conditions begin to attract attention for fog and precipitation advisories. Under 1/4 mile (440 yards) is the threshold for a Dense Fog Advisory or Blizzard Warning visibility criterion, a level at which driving is extremely dangerous and multi-vehicle highway crashes are common. Near-zero visibility in or near a severe thunderstorm can be caused by heavy rain, hail shafts, blowing dust, or debris lofted by a nearby tornado, the latter being an immediate life-threatening emergency requiring shelter immediately.
| Visibility | Condition | Typical Cause | NWS Product |
|---|---|---|---|
| 10+ miles | Unlimited / Clear | No significant obstruction | None |
| 3-10 miles | Mist / Haze | Light fog, dust, smoke, humidity | None (monitoring) |
| 1-3 miles | Restricted Visibility | Fog, light snow, drizzle | Special Weather Statement possible |
| Under 1/4 mile (440 yards) | Dense Fog / Blizzard level | Dense fog, blizzard snow, dust storm | Dense Fog Advisory or Blizzard/Dust Storm Warning |
| Near zero | Hazardous / Near-zero | Heavy precipitation, blowing dust/debris | Immediate shelter if near tornado |
Vortex
A vortex is any rotating mass of fluid, air or water, organized around a central axis of lower pressure. In meteorology, vortices range enormously in scale: from small dust devils (tens of feet in diameter) and waterspouts, to tornadoes (hundreds of yards wide), to tropical cyclones (hundreds of miles wide), to the polar vortex (thousands of miles in diameter). All atmospheric vortices are characterized by a low pressure center with rotating winds flowing inward and either upward (cyclonic vortex) or downward (anticyclonic vortex). The Earth’s rotation, the Coriolis effect, causes large-scale atmospheric vortices to rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. However, the Coriolis effect is negligible at small scales: dust devils and even tornadoes can spin in either direction, though most Northern Hemisphere tornadoes do rotate counterclockwise.
Vorticity
Vorticity is a mathematical measure of the rotation or “spin” present in a fluid, in this case the atmosphere. Positive vorticity (cyclonic spin, counterclockwise in the Northern Hemisphere) is associated with rising motion, convergence at the surface, and the development of clouds and precipitation. Negative vorticity (anticyclonic, clockwise) is associated with sinking motion, divergence, and fair weather. Forecasters examine 500-mb vorticity maps to identify areas where upper-level support for storm development exists, regions of positive vorticity advection (PVA) rotating around the base of upper-level troughs are prime areas for surface cyclogenesis and severe weather outbreaks. “Absolute vorticity” combines the Earth’s planetary vorticity (from its rotation) with the relative vorticity of the air motion itself. Concentrated regions of high positive vorticity at upper levels, called vorticity maxima or “shortwaves”, often serve as the critical trigger for tornado-producing thunderstorm outbreaks when they pass over favorable surface environments.
See also: Criteria for a Tornado Warning | Thunderstorm basics: structure, types, and forecasting
Vapor Pressure
Vapor pressure is the partial pressure exerted by water vapor molecules within a mixture of atmospheric gases. The atmosphere has a maximum amount of water vapor it can hold at any given temperature, the saturation vapor pressure, which increases dramatically and non-linearly with temperature. When the actual vapor pressure equals the saturation vapor pressure, the air is saturated and condensation begins, forming clouds, fog, or dew. The difference between the actual vapor pressure and the saturation vapor pressure drives either evaporation (when actual is less than saturation) or condensation (when actual equals saturation). Because the saturation vapor pressure roughly doubles with every 20-degree F rise in temperature, warm air can contain and transport dramatically more moisture than cold air. Saturated air at 80 degrees F contains more than twice the water vapor of saturated air at 60 degrees F, which is why tropical air masses can produce vastly more intense rainfall than polar air masses, and why warming climates produce more extreme precipitation events.
Wind Backing
Wind backing is the counterpart to veering: a backed wind is one that shifts counterclockwise with increasing altitude, for example, southwesterly at the surface becoming southerly aloft. In the Northern Hemisphere, backing winds with height indicate cold air advection, the horizontal transport of cold air into a region, and the associated stabilization of the atmosphere. Because backing winds with height reduce or eliminate the clockwise directional shear that drives mesocyclone development, a strongly backed wind profile is less favorable for supercell thunderstorm and tornado formation compared to a strongly veering profile. When a forecaster examines a weather balloon sounding before a potentially severe weather event and finds the wind profile backing with height in the low levels, it reduces the tornado threat assessment even when other ingredients, instability, moisture, and a triggering mechanism, appear favorable.
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Get Weather Alerts →Frequently Asked Questions, “V” Weather Terms
What is virga and why is it dangerous for aircraft?
Virga is precipitation that falls from a cloud but evaporates before reaching the ground, visible as wispy, streaking tails beneath a cloud base. The evaporating rain or snow cools the air, creating a dense cold downdraft that, when it reaches the surface, spreads out as a dry microburst, a sudden, violent downburst of wind with speeds of 60-150+ mph and no visible rain to warn pilots. Dry microbursts from virga are a leading cause of aircraft accidents in the desert Southwest, where the atmosphere below clouds is often extremely dry. Pilots flying near virga-producing storms must be especially vigilant for sudden wind shear.
What is visibility in weather and what triggers a Dense Fog Advisory?
Visibility is the greatest distance at which a person can identify a prominent object in daylight, reported in miles or fractions. The NWS issues a Dense Fog Advisory when visibility is expected to drop to 1/4 mile or less, a level at which driving is extremely dangerous and multi-vehicle highway crashes are common. Blizzard Warnings also require visibility under 1/4 mile for at least 3 consecutive hours combined with 35+ mph winds. Near-zero visibility in a thunderstorm can signal the approach of a tornado, which can produce complete darkness and flying debris that makes escape extremely difficult.
What is vorticity in meteorology?
Vorticity is a measure of rotation or spin in the atmosphere. Positive vorticity (counterclockwise spin in the Northern Hemisphere) is associated with rising air and storm development. Negative vorticity (clockwise spin) is associated with sinking air and fair weather. Meteorologists use 500-mb vorticity maps to identify areas of upper-level forcing, where spin in the upper atmosphere is drawing surface air upward, promoting storm development and precipitation. Vorticity maxima “shortwaves” rotating around upper-level troughs are often the critical ingredient that triggers tornado outbreaks.
What is vertical wind shear and why does it matter for tornadoes?
Vertical wind shear is the change in wind speed and direction as you go up through the atmosphere. It is the single most critical ingredient for tornado development because low-level wind shear creates horizontal “tubes” of rotating air that are tilted upright by a thunderstorm’s updraft, generating the rotating mesocyclone that can produce a tornado. High-shear, high-instability environments are the recipe for supercell thunderstorms and violent tornadoes. Without sufficient vertical wind shear, even an extremely unstable atmosphere will produce only ordinary pulse thunderstorms, not rotating supercells.
What is valley fog and when is it most common?
Valley fog forms when cold, dense air drains downslope overnight and collects in low-lying valleys and basins, where it radiates heat and cools to its dew point. It is most common in autumn when nights are long, skies are clear, and winds are calm, conditions that maximize overnight radiational cooling. Valley fog can be extremely dense (near-zero visibility in the valley) while the hilltops above remain in clear air. It typically burns off by mid-morning as solar heating warms the air from the top down. The San Joaquin and Sacramento valleys in California experience some of the densest valley fog in the US, called “Tule Fog,” which causes dozens of multi-vehicle highway accidents each winter.
What is veering wind and why does it matter for severe weather?
Veering wind is wind that shifts clockwise with altitude, for example, from south at the surface to southwest at 5,000 feet to west at 15,000 feet. In the Northern Hemisphere, veering winds with height create the clockwise directional wind shear that is critical for severe thunderstorm and tornado development. This shear tilts the horizontal vortex tubes in the environment upright into the thunderstorm’s updraft, creating the rotating mesocyclone that defines a supercell. When meteorologists see a strongly veering wind profile in a sounding from a weather balloon, combined with high CAPE, they know the potential for tornadoes is elevated.
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