A front is a weather system that is the boundary separating two different types of air. One type of air is usually denser than the other, with different temperatures and different levels of humidity.
A front is a weather system that is the boundary separating two different types of air. One type of air is usually denser than the other, with different temperatures and different levels of humidity. This clashing of air types causes weather: rain, snow, cold days, hot days, and windy days.
Two major types of
fronts
are cold fronts and warm fronts.
Cold
fronts
often come with thunderstorms or other types of extreme
weather
. They usually move from west to east.
Cold
fronts
move faster than
warm
fronts
because cold air is
denser
, meaning there are more molecules of material in cold air than in warm air.
Strong, powerful
cold
fronts
often take over warm air that might be nearly motionless in the atmosphere. Cold,
dense
air squeezes its way through the warmer, less-
dense
air, and lifts the warm air. Because air is lifted instead of being pressed down, the movement of a
cold
front
through a
warm
front
is usually called a low-pressure system.
Low-pressure systems
often cause severerainfall or
thunderstorms
.
Warm
fronts
usually show up on the tail end of precipitation and fog. As they overtake cold air masses,
Fronts on Weather Maps On weather maps, cold fronts are illustrated by blue lines with sharp "teeth" pointing in the direction of the wind. Warm fronts are illustrated by red lines with rounded bumps pointing in the direction of the wind.
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Writers
Hilary Costa
Erin Sprout
Santani Teng
Melissa McDaniel
Jeff Hunt
Diane Boudreau
Tara Ramroop
Kim Rutledge
Hilary Hall
Illustrators
Mary Crooks, National Geographic Society
Tim Gunther
Editors
Jeannie Evers, Emdash Editing
, Emdash Editing
Kara West
Educator Reviewer
Nancy Wynne
Producer
National Geographic Society
other
Last Updated
October 19, 2023
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Common Front Derailleur Problems. Front derailleurs may cause sluggish or inaccurate shifting because (A) the derailleur body is not positioned properly, (B) the derailleur limit screws are not adjusted correctly, (C) the mechanism is dirty or (D) the cable is damaged or improperly tensioned.
Use the barrel adjuster, either on the derailleur or shifter. If you're finding the downshifting is generally responsive, but upshifting isn't, there's a high chance the problem is dirty inner cables or housing, in which case you should try lubricating or replacing them.
Use the barrel adjuster to fine-tune the position of the derailleur. Turning it anti-clockwise will increase the tension, making it shift into the big chainring more readily. Don't forget to shift across the rear cassette as well to make sure each individual gear combination works.
Inspect height alignment of front derailleur. The outer cage plate must be positioned over the largest chainring when viewing height. Simply grab the exposed front derailleur inner wire until outer cage plate is directly over outer chainring teeth. Use care to not shift the derailleur cage past this point.
Look for the high and low adjusters on the derailleur—often marked with an H and an L. Adjust the high-gear screw by turning it until the jockey pulleys line up with the smallest cog. Your chain should move smoothly around the cog and pulleys when you turn the pedals.
If you notice that your bike can't shift down from one gear to another, loosen the derailleur cable adjuster slowly, a quarter-turn at a time, by turning it counterclockwise. If you can't shift up, slowly tighten the cable adjuster by turning it clockwise.
In addition to cleaning and lubricating the cable you can also try: Checking the B-screw to make sure the jockey pulley isn't too far from the cassette cogs. Turn the adjusting barrel clockwise (screw it in) a half turn. This will bias the rear derailleur towards the downshift.
It is generally a good idea to lubricate the pivot point of the front derailleur with a light lubricant. The cable inner wires should also be lubricated. If you have loosen the inner wire pinch bolt, lubricate those threads as well. When tightening this pinch bolt, secure to approximately 30-40 inch-pounds.
If the derailleur won't move far enough to the left to allow the chain to go to the smallest chainring, either the L screw is too tight, or the shift cable is too tight. Given that you tested L and have a new cable, it sounds like the cable is too tight.
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