fronts and thunderstorms

What is a front

A transition zone between two different air masses where temperature changes over ~50–100 miles.

What is required for frontal cloud formation

Moisture.

What does stability determine along fronts

Cloud type, precipitation, and turbulence.

How does frontal slope affect weather

Steeper slope = narrower transition = more severe weather.

How does frontal speed affect weather

Faster front = more violent, shorter-lived weather.

What increases weather severity at a front

High contrast in temperature and moisture between air masses.

How are fronts named

By the colder air mass involved.

What are the four types of fronts

Stationary, warm, cold, occluded (TROWAL).

What is a stationary front

When warm and cold air masses meet but neither advances.

What weather is typical with a stationary front

Low ceilings, poor visibility, fog, drizzle (if warm air is moist).

How long can a stationary front last

Several days.

What is a cold front

Cold air advances and undercuts warm air.

What clouds are typical ahead of a cold front

Convective clouds (TCU, CB).

What precipitation is typical of a cold front

Showery precipitation.

What happens to wind as a cold front approaches

Wind speed increases due to higher PGF; winds veer.

What happens to temperature after a cold front passes

Temperature drops.

How does cold front speed affect severity

Faster cold front = more severe weather.

What is a warm front

Warm air overruns retreating cold air with a shallow slope.

What clouds are typical of a warm front

Extensive stratiform clouds.

What precipitation is typical of a warm front

Steady, long-duration precipitation.

How do winds change with a warm front

They veer gradually as it approaches.

How does temperature change after a warm front passes

Temperature gradually increases.

What is an occluded front

When a faster-moving cold front catches a warm front and lifts warm air from the surface.

What weather is typical along an occluded front

Combination of warm-front and cold-front characteristics.

What is a TROWAL

Trough of Warm Air Aloft associated with an occlusion.

What starts a frontal wave (cyclone)

Two air masses meeting along a stationary front.

What deepens a frontal wave

A pressure disturbance causing a wave-like motion.

What happens as a cyclone intensifies

Pressure drops and winds increase.

What causes the occlusion stage in a cyclone

The cold front catches the warm front.

What happens after a cyclone occludes

Front dissipates after 1–2 days and becomes stationary again.

What type of precipitation forms at a winter warm front

Rain, freezing rain, ice pellets, snow.

If you encounter ice pellets approaching a winter warm front, where is the freezing rain

Above you (in the warm layer overrunning the cold air). What hazards can CBs contain

Why must CBs be avoided in flight

They contain multiple phenomena capable of destroying an aircraft.

What three conditions are required for thunderstorm formation

Unstable air, lifting action, high humidity.

What lifting mechanisms can create CBs

Convection, orographic lift, convergence, fronts.

What are the two basic types of thunderstorms

Frontal and airmass.

Which front produces more violent CBs

Cold fronts (steeper slope).

Why are warm-front CBs dangerous

They may be embedded in stratiform cloud and hard to see.

Where do airmass thunderstorms form

Within an air mass (scattered).

What causes airmass thunderstorms

Convection, orographic lift, convergence.

What are the three stages of a thunderstorm

Cumulus, mature, dissipating.

What happens in the cumulus stage

Warm moist air rises, cools, condenses, and grows vertically.

What allows cumulus clouds to keep growing

Air remains warmer and moister than its surroundings.

How high can cumulus clouds grow

20,000 ft or more.

How long does the cumulus stage last

About 20 minutes.

What marks the start of the mature stage

Precipitation falling to the ground.

What causes downdrafts

Falling precipitation dragging air downward.

What is a gust front

The leading edge of outflowing downdraft air.

How far ahead can a gust front be felt

10–15 miles.

How high can a mature CB reach

Up to the tropopause.

What marks the start of the dissipating stage

Downwards spreading downdrafts cutting off updrafts.

Why is the dissipating stage still dangerous

Downdrafts remain strong and hazardous to aircraft.

What is fog

A cloud formed at or near the ground.

How does fog affect flight

Reduces visibility during landing and takeoff.

What is fog composed of

Water droplets, ice crystals, or both.

What is required for fog formation

High RH, condensation nuclei, and a process that reduces T–D spread to zero.

What two methods reduce temperature-dewpoint spread

Cooling air to dewpoint or adding moisture.

What are the types of fog

Radiation, advection, upslope, steam, frontal, ice fog, freezing fog.

When does radiation fog form

At night over land under clear skies with light winds.

What happens if there's no wind at night

Dew or frost forms instead of fog.

When does advection fog form

When warm moist air moves over a cold surface.

Is advection fog affected by wind

No.

When does upslope fog form

When moist air flows uphill and cools adiabatically.

What wind does upslope fog require

Light to moderate winds.

When does steam fog form

Early morning when warm water vapor meets cold air.

Where is steam fog common in Thunder Bay

When cold cA air meets warmer Lake Superior water.

Where does frontal fog form

50–100 miles ahead of a warm front.

How does frontal fog form

Rain from warm air falls into cold air, saturating it.

What is another name for frontal fog

Precipitation fog.

When does ice fog form

In extremely cold air (below –30°C) with high humidity and light winds.

What creates ice fog

Water vapor from combustion or cracks sublimating directly into ice crystals.

When does freezing fog form

Under radiation fog conditions when droplets are supercooled.

What happens when freezing fog contacts a surface

Droplets freeze on impact.