VATSIM Conference: Connexion 2017
29 & 30 July, London

by Bill Stack

Explain what weather minimums are and how they affect flight decisions.

At the end of this lesson the student will generally understand how to include weather conditions in flight planning and what to do when changes in weather force changes in flight activities.


  • Airspace Classification
  • Basic Meteorology


To understand weather minimums, pilots must understand definitions, theories, meteorology, airspaces, and the nature of aviation rules and regulations. Some of these topics are explained here. Links are provided to relevant topics explained in other VATSIM lessons.

Weather Minimums Defined: Weather minimums are the lowest (worst) visibility conditions under which an aircraft may legally be flown under visual flight rules (VFR). When visibility is above (better than) specified minimums, the aircraft may be flown VFR. When visibility is or below (worse) than specified minimums, the aircraft may not be flown VFR. It must fly under instrument flight rules (IFR) or not at all. 

Conditions that allow visual flight are called "visual meteorological conditions" (VMC). They are officially defined as: Meteorological conditions expressed in terms of visibility, distance from clouds, and ceilings equal to or better than specified minima. FAA Aeronautical Information Manual (AIM).

Conditions that prohibit visual flight and therefore force instrument flight are called "instrument meteorological conditions" (IMC). They are officially defined as: Meteorological conditions expressed in terms of visibility, distance from clouds, and ceilings less than the minima specified for visual meteorological flight. AIM 

Although two terms are used (VMC and IMC), the minimums that prohibit visual flight are the same minimums that dictate instrument flight. Thus, there is only one set of minimums.

Weather minimums are higher (more stringent) for busier airspaces and at night for all airspaces.

Theory: The basic theory behind weather minimums is that visibility must be adequate for visual flight and that instrument flight is required otherwise. They must be able to see other aircraft, terrain, and obstacles such as buildings, bridges, and radio towers. If a pilot can see enough to fly visually, visual flight is allowed. If the pilot cannot see enough to fly visually, instrument flight is required. If the pilot cannot fly on instruments, no flight is allowed. But enough visibility isn't for pilots to decide; it is specified by the minimums.

Meteorological Conditions: Pilot visibility is affected by clouds (including fog), precipitation, and haze and smog.

Clouds reduce visibility to little as a few hundred feet or even less, thus inhibiting visibility more than precipitation, haze and smog. Fog is basically clouds at ground level.

Precipitation reduces visibility in varying degrees. It includes rain, sleet, and snow. Light precipitation generally reduces visibility to several miles. Moderate precipitation generally reduces it to thousands of feet. Heavy precipitation can reduce visibility as much as clouds.

Haze and smog can generally reduce visibility as much as light-to-moderate precipitation. Haze is atmospheric moisture, dust, smoke, and vapor that diminishes visibility.  Dictionary.Com. Smog is basically haze induced or exacerbated by air pollution. In extreme conditions, haze and smog can reduce visibility enough to make visual flight very difficult, even if visibility is above specified VFR minimums. This is especially true in smog-prone areas such as the Los Angeles basin and in haze-prone areas such as the southeastern United States during summer. It was a factor in the death of John F. Kennedy, Jr., in the northeastern United States in July 1999.

Airspace Classes: Airspaces are generally classified according to the relative volume and type of air traffic in them, and weather minimums differ among them. Class A airspace is altitudes between 18,000 feet (5,490 meters) and flight level 600 (60,000 feet, 18,300 meters) MSL. Class B airspace is generally over the world's busiest airports such as Heathrow Airport in London, England, and Dulles Airport in Washington, D.C., USA. In the huge urban area of New York City, three Class B airspaces overlap one another to make a large Class B airspace over most of the metropolis. Class C airspace is generally over medium commercial airports, and D airspaces are generally over small commercial or large general-aviation airports. Class G airspaces are generally over small airports. All the rest of the airspaces are Class E. Airspaces are usually identified on aviation charts. For details about airspaces, please refer to VATSIM's Airspace Classifications.

Aviation Laws: Like most laws, aviation rules and regulations tell pilots what they are required to do and what they are not allowed to do. By implication, they also tell pilots what they are allowed to do or should do. Laws and regulations are often stated in the negative, such as "no aircraft flying VFR may be flown into clouds." This prohibition means the same as "stay out of clouds when you're flying VFR." Thus, weather minimums tell pilots what types of weather they may and my not fly in visually.

Pilot Decisions: Because weather minimums restrict flights, they affect decisions pilots make about their flights. If, for example, a pilot plans to fly in Class C airspace, and visibility is greater than 3 miles, and the sky has broken clouds at 5,000 feet, he has the following choices:

  1. Fly VFR as long as he stays below the clouds,
  2. or fly VFR where there aren't any clouds,
  3. or fly IFR with an IFR rating and a properly filed IFR flight plan,
  4. or stay on the ground and not fly today.

When clouds occur or develop ahead of VFR pilots who are already aloft, they must change course to fly around, below, or above them, if possible. They may not legally fly through any clouds enroute to a destination or while climbing or descending to a desired altitude. If they cannot fly around, above, or below clouds in their path, they must land at the nearest airport and wait for conditions to improve above the VFR minimums.

Pilots may and should decide to alter flights if visibility is marginal or poor even if it is above specified minimums. The minimums prohibit pilots from flying when conditions are less then specified. They neither mandate flight when conditions are marginal or poor nor disallow pilot judgment in such cases.

Flight simmers have different choices because they have the luxury of influencing simulated weather conditions. Flight simmers who want to simulate a visual flight may 1) select weather with sufficient visibility, or 2) select a high enough ceiling to enable plenty of clear space below. Simmers who want to simulate instrument flight would naturally select weather that would require IFR somewhere along the route. Flight simmers do not need IFR ratings and are not required to file IFR flight plans. To simulate flight realistically, however, flight simmers should make every effort to abide by the regulations regarding weather minimums.

Details about VFR weather minimums and IFR weather limitations are explained separately.

Related Information
The following VATSIM articles will help understand and apply weather minimums.


  • Airspace Classification
  • Basic Meteorology
  • VFR Limitations In Weather


  • VFR Limitations In Weather
  • IFR Limitations In Weather

As an overview of weather effects on flight simulations, these articles are written in generalized terms, refer to other sources, and are therefore not intended to be complete or comprehensive explanations of the subject. They are for flight simulation only and are not intended for use in real-world aviation.

The Author
Bill Stack is an avid flight simmer, author of several books and magazine articles about flight simming, and president of TopSkills.