VFR/IFR/MMC/ IMC

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When dealing with aviation matters in the world of Forensic Meteorology weather obviously plays a crucial role in much of the work. The terms used can often be somewhat confusing to both the attorneys involved and the people culling and preparing the data needed for the cases.

Here’s some things to keep in mind regarding aviation weather terms and terms that pertain strictly to meteorological conditions encountered by pilots.

Probably the most important thing is to separate the categories for the sake of clarity.

IFR & VFR are terms and meteorological conditions that pertain to FLIGHT PLANS, filing flight plans and flying while under their conditions.

VFR: Flights operating under VFR are flown solely by reference to outside visual cues (horizon, buildings, flora, etc.) which permit navigation, orientation, and separation from terrain and other traffic. Thus, cloud ceiling and flight visibility are the most important variables for safe operations during all phases of flight The minimum weather conditions for ceiling and visibility for VFR flights are defined in FAR Part 91.155, and vary depending on the type of airspace in which the aircraft is operating, and on whether the flight is conducted during daytime or nighttime. However, typical daytime VFR minimums for most airspace is 3 statute miles of flight visibility and a cloud distance of 500′ below, 1,000′ above, and 2,000′ feet horizontally. Flight conditions reported as equal to or greater than these VFR minimums are referred to as visual meteorological conditions(VMC).

IFR:
Innstrument flight rules permit an aircraft to operate in instrument meteorological conditions (IMC) in contrast to VFR. They are also an integral part of flying in class A airspace. “Class A” airspace exists over and near the 48 contiguous U.S. states and Alaska from 18,000 feet above mean sea level to flight level 600 (approximately 60,000 feet in altitude depending on variables such as atmospheric pressure). Flight in “class A” airspace requires pilots and aircraft to be instrument equipped and rated and to be operating under Instrument Flight Rules (IFR). Most jet aircraft operate in “class A” airspace for the cruise portion of their flight and are therefore required to utilize IFR procedures.

Instrument pilots must meticulously evaluate weather, create a very detailed flight plan based around specific instrument departure, en route, and arrival procedures, and dispatch the flight.
In the US, weather conditions are forecast broadly as VFR, MVFR, IFR, or LIFR.
It is important not to confuse IFR with IMC. A significant amount of IFR flying is conducted in Visual Meteorological Conditions (VMC). Anytime a flight is operating in VMC, the crew is responsible for seeing and avoiding VFR traffic; however, because the flight is conducted under Instrument Flight Rules, ATC still provides separation services from other IFR traffic.

VMC & IMC are meteorological conditions.

For the sake of forensic meteorology work the definitions of VFR & IFR conditions, we use the ones on the weather depiction charts which we use as part our data collection. These may sometimes be different than pilots use or are familiar with.

Hurricane Marie strengthened to a Category 5 storm

On Sunday, August 24 Hurricane Marie strengthened to a Category 5 storm, the highest level on the Saffir-Simpson scale. It became the 5th major hurricane of the very active 2014 Eastern Pacific season.

The entire season (which runs until November 30) averages less than 4 major hurricanes, so the average seasonal total has already been exceeded, despite it still being rather early in the season. Actually, there is one more storm that reached major storm status–Genevieve formed in the Eastern Pacific and was only classified as a tropical storm until it crossed 140 W longitude, but it gained strength in the Western Pacific and became Super Typhoon Genevieve.

While early numerical weather prediction models had Hurricane Marie generating significant rainfall over Southern California, it is now expected to move farther out to sea, with the main effects in California being large surf and an increased risk of rip currents. This image shows the wave swell height forecast by Wave Watch III, NOAA’s state-of-the-art numerical wave forecasting model. It shows swells of 3-4 meters reaching the outer waters of Southern California. Actual wave heights at the beach will depend upon the orientation of the beach and the underwater topography.

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Mammatus Clouds Image

Mail-Attachment-1-680x380A strong and unseasonal cut-off low formed in Northern California and moved south along the California coast, before moving inland across Southern California. This brought unusual summer thunderstorms all the way from the San Diego beaches to the Colorado River on August 20-21. Some of the thunderstorms became strong and produced large hail–including marble-sized at WeatherExtreme headquarters!

Somewhat ironically, the American Meteorological Society was having a meeting in San Diego at the time, and morning thunderstorms forced them to move a lunchtime “luau” that they were going to have indoors, but by the time the luau took place, the thunderstorms had given way to sunshine and 75 degree temperatures.

This photo shows mammatus clouds pendant from a thunderstorm near Descanso, California. At the time this picture was taken, the National Weather Service had issued a Severe Thunderstorm Warning and a Flash Flood Warning for the storm. The photo was taken by Jim Means, one of WeatherExtreme’s meteorologists, who took a break at lunchtime to do a little storm chasing.

What is “elevated” convection?

What is “elevated” convection?

Meteorologists across southern California are talking about Wednesday’s thunderstorms. For many in an area from Encinitas to Palm Springs, frequent rumbles of thunder gave many a sleepless night late Tuesday night into Wednesday morning. Thunderstorms are already uncommon in southern California, but the amount of lightning in these storms is rare. The last time many locals remember such frequent, widespread lightning in coming in waves hour after hour dates back to October 2005. (See the lightning map from the National Weather Service, below.)

SoCal Lightning 8-20-14

So why did this happen?

For any thunderstorm to occur, a sufficiently deep warm moist bubble of air needs to condense as it rises past the freezing level in the cloud, building positive and negative charges. In southern California, especially near the coast in the summer, the air is generally cool and stable… so the threat of a thunderstorm is minimal.

But in this case a storm system approached from the southwest and rode up over the top of the dome of cool air. The thunderstorms began above, or were elevated, above the level where storms normally develop (called the boundary layer). Meteorologist call storms that develop this way elevated convection.

Computer forecast models are getting better at helping Meteorologists predict these storms, but they are still challenging. In a dry southern California summer, they also carry considerable wildfire risk, since the storms often do not produce sufficient, widespread rainfall to put out any lightning induced wildfires.

James K. Purpura
Certified Consulting Meteorologist
Director, Weather Forecasting
purpura@weatherextreme.com

Treading on Thin Air Official Release Summer 2015

Dr. Elizabeth Austin’s new book to be released summer 2015.

TREADING ON THIN AIR
Atmospheric Physics, Forensic Meteorology and Climate Change-How the Weather Shapes Our Everyday Lives 

The Atmospheric physicist, forensic meteorologist and founder of WeatherExtreme Ltd. will be writing a part memoir, part weather-related book. She will be explaining climate change, what’s happening with our stratosphere, and how the business of ‘weather’ has become a major economic factor in recent political history. Look for Dr. Elizabeth Austin’s forthcoming book to be published by Pegasus Books.