{"id":1560,"date":"2022-07-10T11:25:27","date_gmt":"2024-07-10T11:02:27","guid":{"rendered":"http:\/\/ialert.com\/blog\/?p=1560"},"modified":"2026-04-28T01:36:40","modified_gmt":"2026-04-28T01:36:40","slug":"glossary-of-weather-terms-beginning-with-d","status":"publish","type":"post","link":"https:\/\/ialert.com\/blog\/weather-articles\/weather-glossary\/glossary-of-weather-terms-beginning-with-d\/","title":{"rendered":"Glossary of Weather Terms – Beginning with “D”"},"content":{"rendered":"

Searching for what is a derecho<\/strong>, what a Dense Fog Advisory<\/strong> means, the dew point meaning<\/strong>, what causes dust storms<\/strong>, or how Doppler radar<\/strong> works?<\/p>\n

Jump to weather terms beginning with the letter:<\/h3>\n

A<\/a> | B<\/a> | C<\/a> | D<\/a> | E<\/a> | F<\/a> | G<\/a> | H<\/a> | I<\/a> | J<\/a> | K<\/a> | L<\/a> | M<\/a> | N<\/a> | O<\/a> | P<\/a> | Q<\/a> | R<\/a> | S<\/a> | T<\/a> | U<\/a> | V<\/a> | W<\/a> | X<\/a> | Y<\/a> | Z<\/a><\/p>\n

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Weather Terms Beginning with “D”<\/strong><\/h3>\n

Dense Fog Advisory<\/strong><\/h4>\n

A Dense Fog Advisory is a National Weather Service product issued when fog is reducing visibility to 1\/4 mile or less over a widespread area. Drivers should slow significantly below posted speed limits, use low-beam headlights (not high beams, which reflect back off the fog and reduce visibility further), and increase following distance to at least three to four times the normal gap. Dense Fog Advisories are most commonly issued overnight or in the early morning hours when radiation fog forms as the ground cools rapidly under clear skies. Unlike some hazards that have both an Advisory and a Warning level, there is no separate “Dense Fog Warning” in the standard NWS product suite, the Dense Fog Advisory is the highest level of alert issued for fog, and it should be taken seriously.<\/p>\n

Derecho<\/strong><\/h4>\n

A derecho is a widespread, long-lived, fast-moving windstorm associated with a line of severe thunderstorms that produces straight-line (non-rotational) winds of 58 mph or more over a continuous damage path at least 240 miles long. The word comes from Spanish meaning “straight ahead,” contrasting directly with the rotating winds of a tornado. Derechos are often more destructive over a larger geographic area than a single tornado, a single event can topple millions of trees, knock out power to millions of people across multiple states, and produce wind gusts exceeding 100 mph. On radar, a derecho is associated with a characteristic bow echo signature, a bowed line of reflectivity with a rear-inflow notch. The key distinction from a tornado is that derecho damage follows a broad, mostly straight corridor, while tornado damage is confined to a narrow, often curved path. Read our full Derecho article<\/a> and the 2020 Iowa Derecho case study<\/a>.<\/p>\n

Dew Point<\/strong><\/h4>\n

The dew point is the temperature to which air must be cooled at constant pressure for condensation to begin, the point at which the air becomes saturated and water vapor converts to liquid water. It is the most reliable single measure of atmospheric moisture content because, unlike relative humidity, the dew point does not change as temperature rises and falls throughout the day. Dew points above 70\u00b0F feel oppressively humid to most people because sweat cannot evaporate efficiently; dew points in the 55-65\u00b0F range feel comfortable; and dew points below 30\u00b0F indicate very dry air. Heat index calculations rely on dew point rather than relative humidity alone because dew point more accurately reflects how hot and humid the atmosphere truly feels on the human body.<\/p>\n

Doppler Radar (NEXRAD)<\/strong><\/h4>\n

The WSR-88D (Weather Surveillance Radar 1988 Doppler) is a national network of 160 radar sites operated by the National Weather Service across the United States. Unlike older radar systems that only detected precipitation intensity, Doppler radar uses the Doppler effect to simultaneously measure both precipitation intensity and the velocity and direction of wind within storms, making it a fundamentally different and far more powerful forecasting tool. Between 2011 and 2013, dual-polarization capability was added to the entire network, allowing the radar to distinguish rain from snow from hail from biological returns such as birds and insects. The most critical application of Doppler radar is tornado detection, the rotating mesocyclone signature within a supercell thunderstorm can be identified on radar several minutes before a tornado actually touches down, providing life-saving warning time.<\/p>\n

Downburst<\/strong><\/h4>\n

A downburst is a strong, localized downdraft that strikes the ground and spreads outward horizontally as straight-line, non-rotational winds. Downbursts are divided into two types based on size and duration: a microburst<\/strong> is under 2.5 miles wide and lasts under 5 minutes but can produce winds of 60-150 mph or more, making it extremely dangerous to aircraft during takeoff and landing; a macroburst<\/strong> is wider than 2.5 miles, lasts 5-30 minutes, and typically produces winds of 60-130 mph. Wet downbursts produce visible rain shafts all the way to the ground, while dry downbursts occur in arid regions where rain evaporates entirely before reaching the surface, sending only a blast of cold, dense air downward. Microbursts in particular have been directly responsible for several catastrophic airliner crashes and remain among the most serious aviation weather hazards.<\/p>\n

Downburst Types: Microburst vs Macroburst<\/strong><\/h4>\n\n\n\n\n\n\n\n\n\n\n
Feature<\/th>\nMicroburst<\/th>\nMacroburst<\/th>\n<\/tr>\n<\/thead>\n
Size<\/td>\nUnder 2.5 miles wide<\/td>\n2.5 miles or wider<\/td>\n<\/tr>\n
Duration<\/td>\nUnder 5 minutes<\/td>\n5-30 minutes<\/td>\n<\/tr>\n
Peak Winds<\/td>\n60-150+ mph<\/td>\n60-130 mph<\/td>\n<\/tr>\n
Radar Appearance<\/td>\nLocalized burst pattern<\/td>\nLarger outflow signature<\/td>\n<\/tr>\n
Aviation Danger<\/td>\nExtreme (takeoff\/landing)<\/td>\nHigh<\/td>\n<\/tr>\n
Most Common Region<\/td>\nSoutheast US, Gulf Coast<\/td>\nPlains, Midwest<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Drought<\/strong><\/h4>\n

A drought is a prolonged period of abnormally low precipitation that causes water shortages affecting agriculture, municipal water supply, and wildfire risk. The U.S. Drought Monitor, a weekly national product produced jointly by NOAA, USDA, and the National Drought Mitigation Center, classifies drought severity on a scale from D0 (Abnormally Dry) through D1 (Moderate), D2 (Severe), D3 (Extreme), and D4 (Exceptional Drought), the most severe level. Meteorological drought refers simply to below-normal precipitation, agricultural drought focuses on insufficient soil moisture for crops, and hydrological drought means depleted reservoirs and groundwater supplies. Flash Droughts are a particularly dangerous subtype that can develop in just a few weeks due to a combination of abnormally high heat, low humidity, and persistent wind that rapidly dries out soils even in the absence of long-term precipitation deficits. See also: Drought Defined<\/a>.<\/p>\n

Dry Line<\/strong><\/h4>\n

The Dry Line is a moisture boundary that separates warm, moist Gulf of Mexico air to the east from hot, dry desert air to the west, and it sets up most frequently across the Texas and Oklahoma Panhandles on spring and summer afternoons. Dew point temperatures can drop 20-30\u00b0F in just a few miles across the Dry Line, one of the sharpest horizontal moisture gradients in the atmosphere. This boundary acts as one of the most important triggers for severe thunderstorm and tornado development in Tornado Alley, because the contrast between the air masses creates intense atmospheric instability. Storms that fire along or just east of the advancing Dry Line in May and June are responsible for many of the most violent tornadoes in U.S. history, and forecasters watch Dry Line position and movement with great care during the spring severe weather season.<\/p>\n

Dust Devil<\/strong><\/h4>\n

A dust devil is a small, rotating column of air and dust that forms on hot sunny afternoons when intense solar heating of the ground creates strong convective thermals, columns of rising hot air, that begin to rotate due to slight horizontal wind shear near the surface. Dust devils are typically 10-300 feet wide and 100-1,000 feet tall, lasting anywhere from a few seconds to a few minutes, with wind speeds that rarely exceed 60 mph. They are not<\/strong> tornadoes, dust devils form from the ground upward under clear or nearly clear skies, driven purely by surface heating, while tornadoes descend from the base of a thunderstorm cloud. In most cases dust devils are a nuisance rather than a true safety hazard, though large ones in desert environments can occasionally cause minor property damage.<\/p>\n

Dust Storm (Haboob)<\/strong><\/h4>\n

A dust storm is a wall of blowing dust that can rise 1-3 miles high and extend 100 or more miles wide, reducing visibility to near zero within seconds of its arrival. The word “haboob”, from the Arabic word “habb” meaning wind, refers specifically to a dust storm generated by the outflow winds from a thunderstorm’s downdraft, which scour dry, loose desert soils and hurl the debris forward as a rolling dust wall. Haboobs are most common in the Sonoran Desert of Arizona, the Southern Plains, and the Sahara. They can arrive with almost no warning at speeds of 35-60 mph and trigger Dense Fog Advisory-level visibility restrictions even though the surrounding sky may be partly sunny. The leading edge of a large haboob can be a dramatic and terrifying sight, a brown wall filling the entire horizon.<\/p>\n

Dust Storm Warning<\/strong><\/h4>\n

A Dust Storm Warning is issued by the National Weather Service when sustained blowing dust will reduce visibility to 1\/4 mile or less. These events are extraordinarily dangerous for highway travel, multiple-vehicle pile-up crashes on interstates in Arizona and Texas have killed dozens of people when drivers entered a dust storm at highway speeds and rear-ended stopped or slow-moving vehicles they could not see. The official safety motto taught by Arizona highway officials is “Pull Aside, Stay Alive”, meaning pull completely off the pavement, turn off all lights (so other drivers do not follow your lights into the stopped vehicle), and keep your foot off the brake pedal to avoid drawing rear-end traffic into your position. A Dust Storm Warning should be treated with the same urgency as a Tornado Warning in areas where they are common.<\/p>\n

Drizzle<\/strong><\/h4>\n

Drizzle is a form of light liquid precipitation consisting of uniformly tiny water droplets less than 0.5 millimeters in diameter that fall slowly through the air and create a fine, steady wetness. It falls primarily from low stratus or stratocumulus clouds and can produce light but persistent surface accumulations. Freezing Drizzle (FZDZ) is a particularly hazardous form that occurs when drizzle falls onto subfreezing surfaces, creating a thin layer of glaze ice, commonly called black ice, that is nearly invisible on roads and extremely treacherous for drivers and pedestrians. Heavy Drizzle at the upper end of the intensity scale can produce accumulation rates of 0.1 to 0.2 inches per hour, enough to create measurable standing water and significantly reduce road traction even without an icing threat.<\/p>\n

Downdraft<\/strong><\/h4>\n

A downdraft is a column of sinking air within a thunderstorm, and the two primary types play critical roles in both thunderstorm structure and tornado formation. The rear-flank downdraft (RFD)<\/strong> in a supercell thunderstorm wraps around the back side of the rotating mesocyclone, and meteorologists believe it is frequently the mechanism that brings the rotating air column into contact with the surface, triggering tornado formation, making the RFD one of the most closely watched features in storm surveys and real-time Doppler radar analysis. The forward-flank downdraft (FFD)<\/strong> is associated with the heavy precipitation falling in front of the storm and helps shape the overall structure of the supercell. Understanding both downdraft regions is essential to forecasting tornado development and to storm spotters correctly positioning themselves near a supercell.<\/p>\n

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Frequently Asked Questions, “D” Weather Terms<\/strong><\/h3>\n

What is a derecho?<\/h4>\n

A derecho is a widespread, fast-moving windstorm produced by a long line of severe thunderstorms, delivering straight-line (non-rotational) winds of 58 mph or more over a continuous damage path at least 240 miles long. Unlike a tornado, a derecho’s damage is spread across hundreds of miles rather than a narrow path. Derechos can down millions of trees, knock out power to millions of people, and produce wind gusts exceeding 100 mph. The term comes from Spanish meaning “straight ahead,” contrasting with tornado-type rotating winds.<\/p>\n

What is the difference between a microburst and a macroburst?<\/h4>\n

Both are types of downbursts, strong downdrafts that hit the ground and spread outward as straight-line winds, but differ in size and duration. A microburst is under 2.5 miles wide and lasts under 5 minutes, but can produce winds of 150 mph or more, making it extremely dangerous to aircraft. A macroburst is wider than 2.5 miles, lasts 5-30 minutes, and typically produces winds of 60-130 mph. Microbursts are responsible for several catastrophic airline accidents and remain a serious aviation hazard.<\/p>\n

What is dew point and why does it matter?<\/h4>\n

Dew point is the temperature at which air becomes saturated and moisture begins to condense. It is the most accurate measure of actual moisture content in the air, unlike relative humidity, which changes as temperature rises and falls, dew point stays constant. A dew point above 70\u00b0F feels oppressively humid because sweat cannot evaporate efficiently; below 30\u00b0F feels very dry. Heat index calculations use dew point because it accurately reflects how hot and humid the air actually feels.<\/p>\n

What causes a dust storm (haboob)?<\/h4>\n

Most dust storms are caused by thunderstorm outflow winds, when a thunderstorm’s downdraft hits the ground and spreads outward at 35-60 mph, it kicks up a wall of dust from dry desert soils. This type is called a haboob. Dust storms can also form from strong sustained winds during dry, drought-affected periods without any associated thunderstorm. The Dust Bowl of the 1930s was caused by years of drought combined with deep soil disturbance from farming. Modern dust storms remain most common in Arizona, New Mexico, and the Texas Panhandle.<\/p>\n

What is the Dry Line and why is it important for severe weather?<\/h4>\n

The Dry Line is a boundary across the Texas and Oklahoma Panhandles that separates warm, moist Gulf of Mexico air to the east from hot, dry desert air to the west. In spring and early summer, when the Dry Line pushes east in the afternoon, it creates an unstable boundary along which severe thunderstorms and tornadoes frequently develop. Many of the most violent tornadoes in U.S. history formed along or near the Dry Line. Forecasters watch Dry Line movement closely during severe weather season.<\/p>\n

What should you do when a Dense Fog Advisory is issued?<\/h4>\n

When the NWS issues a Dense Fog Advisory, meaning visibility is at or below 1\/4 mile, drivers should slow significantly below posted speed limits, use low-beam headlights (not high beams, which reflect back and reduce visibility further), increase following distance to at least 3-4 times normal, use fog lights if equipped, and avoid stopping on roadways. Multi-vehicle pile-up crashes are one of the most common fog-related fatalities. If conditions become too dangerous, pull completely off the road at a rest stop or exit ramp.<\/p>\n

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