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).
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.