Types of Fronts: What kind of weather is on the way?

types of fronts weather map header imagePhoto: Petrovich12 - stock.adobe.com
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Ever wonder what the types of fronts you see on the weather map mean? As part of our continuing weather education series, we thought an explainer would be helpful. So let’s get started.

The polar and subtropical jets and the role separate air masses. This boundary is known as a front. If it sounds a bit militaristic, that’s because it is: meteorologists gave these boundaries this name for the similarity to a military unit moving across the battlefield.

Although not a hard and fast rule, a majority of these fronts will ride along ripples on the polar and subtropical jets, bringing a change in the weather as they pass. There are four types of fronts – cold, warm, occluded, and stationary.

types of frontsPhoto: The Weather Station Experts
The four types of fronts that are most commonly seen on a weather map.

Some cold fronts may tear through violently with high winds and storms with noticeably cooler air, while others will pass with barely any noticeable difference except a change in humidity and more pleasant weather. Warm fronts are typically much less benign. Severe storms with tornadic activity can occur near warm fronts, thanks to increased amounts of spin in the air. Occluded fronts are mixtures, any combination of weather, temperature, and/or humidity change. Their effects are widely variable.

Stationary fronts, when a boundary stays in place, sit over the same area for days at a time, and frequently are the cause of excessive rainfall in the summer months.

Cold fronts on a weather map are typically depicted as a blue line with triangles on it, the triangles indicating the direction of movement. Warm fronts are red lines with half-circles, those half-circles pointing in the direction of movement, while occluded fronts are shown in purple with alternating triangles and half-circles.

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Types of Fronts: Cold Front

The cold front is typically the most active. There is a noticeable wind shift and increase in wind speed as it passes, and both the temperature and humidity drop – sometimes quite significantly. It typically moves from west to east over the US, replacing warmer, more humid air. Because colder air is denser than warmer air, a cold front ‘scours’ warm air out, as shown below in the diagram.

types of fronts - cold front diagramPhoto: The Weather Station Experts
Cross-section of a cold front.

What weather does a cold front bring?

Before PassingWhile PassingAfter Passing
Windssouth-southwestgusty; shiftingwest-northwest
Temperaturewarmsudden dropsteadily dropping
Pressurefalling steadilyminimum, then a sharp riserising steadily
Cloudsincreasing: Ci, Cs and CbCbCu
Precipitationshort period of showersheavy rains, sometimes with hail, thunder, and lightningshowers then clearing
Visibilityfair to poor in hazepoor, followed by improvinggood, except in showers
Dew Pointhigh; remains steadysharp droplowering

Note: Ci = cirrus, Cs = cirrostratus, Cb = cumulonimbus, Cu = cumulus, As = altostratus, Ns = nimbostratus, St = stratus,  Sc = stratocumulus, Tcu = towering cumulus

Types of Fronts: Warm Front

Warm fronts do the opposite of cold fronts, so warmer air replaces colder air. Warmer air is less dense, so it will move over the top of colder air as it travels (the opposite of a cold front — see diagram). Warm fronts typically move from south to north in the Northern Hemisphere, with an increase in air temperature and humidity behind them. In some cases, severe weather may break out just behind a warm front, due to increased instability.

types of fronts warm front diagramPhoto: The Weather Station Experts
Cross-section of a warm front.

What weather does a warm front bring?

Before PassingWhile PassingAfter Passing
Windssouth-southeastvariablesouth-southwest
Temperaturecool-cold, slow warmingsteady risewarmer, then steady
Pressureusually fallingleveling offslight rise, followed by fall
Cloudsin this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summerstratus-typeclearing with scattered Sc; occasionally Cb in summer
Precipitationlight-to-moderate rain, snow, sleet, or drizzledrizzle or noneusually none, sometimes light rain or showers
Visibilitypoorpoor, but improvingfair in haze
Dew Pointsteady risesteadyrise, then steady

Note: Ci = cirrus, Cs = cirrostratus, Cb = cumulonimbus, Cu = cumulus, As = altostratus, Ns = nimbostratus, St = stratus,  Sc = stratocumulus, Tcu = towering cumulus

Types of Fronts: Occluded Fronts

By definition, an occlusion is where a cold front catches up to the warm front. As a result, the air out ahead of an occlusion will be different in some way from the air behind it. Speaking generally, in a warm occlusion, the air behind an occluded front is even warmer than the air out ahead of the warm front. In a cold occlusion, the most common, the air behind the occlusion is colder than the air out ahead of the warm front (not colder than the air ahead of a cold front). See the chart below.

types of fronts occluded frontPhoto: The Weather Station Experts
Cross-section of an occluded front.

What weather does an occluded front bring?

Before PassingWhile PassingAfter Passing
Windssoutheast-southvariablewest to northwest
Temperature
Cold Type 
Warm Type

cold-cool 
cold

dropping 
rising

colder 
milder
Pressureusually fallinglow pointusually rising
Cloudsin order: Ci, Cs, As, NsNs, sometimes Tcu and CbNs, As or scattered Cu
Precipitationlight, moderate or heavy precipitationlight, moderate or heavy continuous precipitation or showerslight-to-moderate precipitation followed by general clearing
Visibilitypoor in precipitationpoor in precipitationimproving
Dew Pointsteadyusually a slight drop, especially if cold-occludedslight drop, although may rise a bit if warm-occluded

Note: Ci = cirrus, Cs = cirrostratus, Cb = cumulonimbus, Cu = cumulus, As = altostratus, Ns = nimbostratus, St = stratus,  Sc = stratocumulus, Tcu = towering cumulus

Types of Fronts: Stationary Fronts

As the name implies, stationary fronts are cold or warm fronts that are no longer moving. The blue triangles of the cold front point towards the warmer air, and the red half-circles of the warm front towards the cold. One of its most noticeable features is a marked temperature and wind direction difference on each side. These stationary fronts can also act as a focal point for atmospheric lift, which creates clouds and precipitation. These fronts are a common cause of excessive precipitation year-round.

Types of Fronts: Drylines

Most areas of the country don’t experience these often, but since this is a book on American weather it’s important to mention the dryline. These fronts are most commonly found in the Plains during the summer. You can think of these drylines as a difference in humidity: warm, humid air on one side and dry, hot air on the other. This is typically shown as a dashed yellow or orange line on a weather map. In the summer months, drylines are a trigger for thunderstorms, often severe.

Want to learn more?

This post is an excerpt from my book, Weather Watch: An Introduction to America’s Weather and Climate, available on Amazon, and free for Kindle Unlimited subscribers!

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05/21/2022 12:02 am GMT
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Ed Oswald

Ed Oswald has nearly two decades of experience in technology and science journalism, and specializes in weather stations and smart home technology. He's written for Digital Trends, PC World, and TechHive. His work has also appeared in the New York Times. When he isn't writing about gadgets, he enjoys chasing severe weather and winter storms.

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