Frontal passage indicators
Read the trend, not a chart symbol: track pressure, wind, temperature, dew point, cloud, precipitation before, during, after passage.
Warning time and wind shift (PHAK ch.12):
| Front | Speed | Warning | Passage | Wind shift |
|---|---|---|---|---|
| Warm | Slow | Advance warning | Can take days to pass a region | Comparatively gradual |
| Cold | Fast | Little or none | Whole system clears in a few hours | Abrupt |
Passage trends (PHAK ch.12):
| Factor | Front | Before | During | After |
|---|---|---|---|---|
| Cloud | Warm | Cirriform or stratiform, often fog (Cb and thunderstorms possible in summer) | Stratiform | Stratocumulus |
| Cloud | Cold | Cirriform or towering cumulus, Cb may develop | Towering cumulus or Cb | Dissipates to cumulus |
| Cloud | Occluded | Cirriform and stratiform | Nimbostratus and Cb (towering cumulus may form) | Nimbostratus and altostratus |
| Precip | Warm | Light to moderate rain, sleet, snow, or drizzle | Drizzle | Possible rain showers |
| Precip | Cold | Rain showers possible | Heavy rain showers, possible lightning, thunder, hail (severe fronts: tornadoes) | Decreasing |
| Precip | Occluded | Light to heavy | Light to heavy | Decreasing |
| Visibility | Warm | Poor | Poor but improving | Improving (brief haze may linger) |
| Visibility | Cold | - | Poor | Good |
| Visibility | Occluded | Poor | Poor | Improving |
| Wind | Warm | South-southeast | Variable | South-southwest |
| Wind | Cold | - | Variable and gusty | West-northwest |
| Wind | Occluded | - | Variable | - |
| Temp | Warm | Cool | Rising steadily | Rises |
| Temp | Cold | - | Drops rapidly | Sharp drop, stays cooler |
| Dew point | Warm | Rising | Steady | Rises, then levels off |
| Dew point | Cold | High | Drops rapidly | - |
| Dew point | Occluded | Steady | - | - |
| Pressure | Warm | Falling steadily | Leveling off | Slight rise, then decrease |
| Pressure | Cold | Falling | Bottoms out, then rises gradually | Keeps rising |
| Pressure | Occluded | Dropping | Leveling off | - |
Warm front (PHAK ch.12):
- Flying toward it: gradual, multi-hour deterioration. If temperature and dew point converge, expect fog.
- Giveaway: cloud sequence, hours before arrival (TP 13014 sample exam Q65, key 65-(3)):
- High cirrus
- Cirrostratus (halo)
- Altostratus (watery sun)
- Nimbostratus (steady rain)
Cold front (PHAK ch.12):
- Flying toward it: faster deterioration than warm front:
- Scattered cloud building vertically
- Overcast at 1,000 ft, thunderstorms, heavy rain
- Rapid improvement behind it
- Fast-moving variant:
- Ground friction steepens frontal slope into a very narrow band.
- Overtaken warm air stable: overcast and rain persist some distance behind front.
- Overtaken warm air unstable: scattered thunderstorms, sometimes a squall line ahead of or along front.
- Behind it: skies clear rapidly, gusty turbulent colder wind.
Occluded front (PHAK ch.12): faster cold front catches warm front, lifts warm air off ground, mixes both weather types around the low. Which air is colder decides the type:
| Type | Condition | Result |
|---|---|---|
| Cold-type | Overtaking cold air colder than air ahead of warm front | Cold air undercuts, lifts warm front aloft. Typically stable mix of warm- and cold-front weather |
| Warm-type | Air ahead of warm front colder than cold front's air | Cold front rides up and over warm front. If lifted air unstable, more severe: embedded thunderstorms, rain, fog likely |
Trap: don't wait for rain or wind shift; pressure and dew point trends warn earliest, before cloud or precipitation, e.g. cold front close = high dew point with steadily falling pressure.
References
- PHAK ch.12 Warm Front
- PHAK ch.12 Flight Toward an Approaching Warm Front
- PHAK ch.12 Cold Front
- PHAK ch.12 Fast-Moving Cold Front
- PHAK ch.12 Flight Toward an Approaching Cold Front
- PHAK ch.12 Wind Shifts
- PHAK ch.12 Occluded Front
- TP 13014 sample exam Q65 (key 65-(3))