• Fog commonly occurs in the cold air ahead of a warm front.fog image by Lytse from Fotolia.com
  
  A warm front is a boundary or transition zone separating warm air from colder air . According to the United Kingdom Environmental Change Network online, warm fronts can be over 100 miles wide. The cold air must move out of the way when the warm front moves forward. Warm fronts move more slowly than cold fronts an when they arrive in a region, they may trigger specific weather effects that last for several days.
  
  

Fog

• Fog forms when warm, moist air cools to the saturation point.fog image by Vasiliy Koval from Fotolia.com
  
  Frontal fog is a factor that is associated often with warm fronts. When rain falls and starts evaporating beneath a cold air mass, fog often occurs in the cold air ahead of a warm front. Some conditions may produce fog within warm air, however. Warm, moist air blowing over a cold surface will also produce fog that may cover a wide area, making it one of the most dangerous weather problems associated with warm fronts, according to Integrated Publishing online. Fog usually clears after the warm front passes.
  
  

Wind Shifts

• Wind shifts indicate that a front is present.wind image by hanna kreis from Fotolia.com
  
  A wind shift, a change in the wind's direction, indicates that a front is present. As a low-pressure trough passes through a particular location, the wind direction shifts clockwise, according to the Forest Encyclopedia Network website. The wind action depends upon the speed of the front. Any local individual conditions also play a role in wind shifts. The change in wind direction with the arrival of a warm front is usually between 45 and 90 degrees.
  
  

Precipitation

• Warm fronts sometimes bring rain.Rain image by Herbert Gremmelmaier from Fotolia.com
  
  Precipitation in the form of drizzle, rain or snow often occurs ahead of a warm front. As wind flows between areas of high and low pressure, it creates an unbalanced system. A low pressure system of warm air can cause molecules to scatter, resulting in air that is less dense. This creates a situation perfect for clouds, rain showers with strong winds and thunderstorms.
  
  

Temperature

• As the temperature rises the stratus clouds will begin to break.sky image by cherie from Fotolia.com
  
  As the warm front replaces cool air, the temperature of the location begins to rise. Clouds begin to break and the sky begins to clear, allowing the sun to raise the temperature. The wind at this time is out of the south, allowing maritime tropical air to invade, according to University of Wisconsin Stevens Point.
  
  

Humidity

• NASA describes humidity as the measure of water vapor in the air.Rain in summer forest - fresh water image by JaNo from Fotolia.com
  
  As the warm temperatures that have resently risen are still unstable, relative humidity increases. High humidity causes an increases in body skin temperature, also.