Photo credit: https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/01-met_concepts/01g-vfr-ifr/index.html , courtesy of Prof Roland Stull

1. Visibility

Visibility is crucial to flying VFR since the pilot relies on looking out of the window to observe its weather conditions and surroundings. Pilots flying visually (VFR) must stay outside of clouds, and fly under appropriate cloud ceiling. However, when there is rain, a hazard associated with rain is reduced visibility, since heavier rains bring poorer visibility. So when flying VFR (visual) you might need to fly around heavy rain areas in order to remain in regions with sufficiently good visibility.  The exception is a squall line, where there is a nearly continuous line (100s to 1000s km long, but often only 15 km wide) of heavy rain.

Photo credit: https://www.eoas.ubc.ca/courses/atsc113/flying/met_concepts/01-met_concepts/01g-vfr-ifr/index.html, courtesy of Prof Roland Stull

2. Frontal Hazards

Fronts often have clouds, precipitation,  strong winds, and turbulence, all could be flight hazards, especially for flying VFR. VFR pilots may encounter clouds, precipitation, low ceilings (= low cloud base), and poor visibility near the front that can block their intended flight. They might even encounter thunderstorms as a result of fronts, and the VFR pilot could be flying between stratiform layers and have part of the route blocked by a thunderstorm updraft tower which is very dangerous. 

But what happens to pilots with only VFR skills if they accidentally fly into clouds, fog, or other obscuration that reduces their ability to see things outside the window:

• Most often pilots are not able to determine if the aircraft is right-side-up.  These VFR pilots are not trained to interpret and ignore their balance signals from their inner ear, or their inertial feelings (seat of their pants).  So the pilots inadvertently turn the steering wheel (control yoke) in the wrong direction.  This causes the aircraft to stall (lose aerodynamic lift, causing the plane to fall out of the sky), or spin (a rapid spiral corkscrew descent) until the aircraft crashes on the ground.  Namely, the pilot loses control of the aircraft.

• Pilots also lose track of where they are.  Because of this, they might accidentally fly into mountains or other high terrains that they cannot see (called "controlled flight into terrain" by accident investigations).  Or they might fly in the wrong direction and never find their airport, requiring them to make an emergency landing on a road or farm field before they run out of fuel.  Or they might hit tall TV towers or power lines.  Or they might hit other aircraft.  Or they might accidentally fly into more-dangerous weather such as thunderstorms.

• Psychology is another factor.  As pilots get into difficult situations, they can panic.  Namely, they are so scared about their situation that they become irrational, they forget their emergency training, and they make poor decisions.  The result: a bad situation can become worse.