How to predict the weather onboard

Learn to predict the weather onboard by mastering the basics of weather forecasting. When you understand what your observations tell you about the weather, you can use this knowledge to refine a regional forecast.

First, let’s look at the signs of weather stability and change:

	Continuing good weather	Changing weather
Skies	Clear, light to dark blue; bright moon; dissipating contrails	Hazy; halo around the sun or moon; thick, lingering jet contrails
Clouds	Few puffy cumulus or high thin clouds, the higher the better	Veil of clouds; clouds at multilayers and directions; cirrus
Winds	Generally steady with little change during the day	Strong winds in early morning; wind shifting to the south
Seas	Sea swells in the same direction	Confused seas in varying directions
Temperature	Stable; heavy dew or frost at night	Marked changes; increased humidity
Dew Point	Marked spread between dew point and temperature = no fog	Close spread; probable fog if the temperature drops
Barometer	Steady or slowly rising	Falling slowly
Sunrise	Gray sky at dawn or sun rising from a clear horizon	Red sky; sun rises above the horizon because of cloud cover
Sunset	Red sky; sun “ball of fire” or sets on a clear horizon	Purplish or pale yellow; sun sets high above the horizon

Forecasters have difficulty predicting a low-pressure area’s precise path, but you can use Buys Ballot’s Law to get the local perspective.

According to Buys Ballot’s Law, if you are in the Northern Hemisphere standing with your back to the wind, atmospheric pressure is high to your right and low to your left.

Approaching low

When a low-pressure area is approaching,

cirrus clouds (wispy and hair-like) gradually lower and thicken;
backing winds could increase;
the barometer falls 2 to 10 millibars in three hours; and
offshore swells increase.

With an approaching low, you can expect rain within 15 to 24 hours. If the low is west to northwest passing to your north, you will see fronts. If the low is west to southwest passing to your south, you will not see distinct fronts.

Approaching warm front

With an approaching warm front, cirrus or mackerel clouds mean the warm front is more than 24 hours away. Lowering or thickening clouds (altostratus or nimbostratus) mean the front is less than 24 hours away.

Additionally, in an approaching warm front,

rain begins lightly, becoming steady and persistent;
the barometer falls steadily;
winds increase steadily and stay southeasterly; and
visibility deteriorates.

Passing warm front

When a warm front passes,

the sky lightens toward the western horizon;
the rain breaks;
winds veer from southerly to southwesterly and may decrease;
the barometer stops falling; and
the temperature rises.

Within a warm sector

A warm sector is the area between a warm front and a cold front. In the warm sector,

winds remain steady and primarily southwesterly, strengthening ahead of a cold front;
the barometer remains steady (but may drop shortly ahead of a cold front); and
you might see mist or drizzle.

Approaching cold front

When a cold front approaches,

southwesterly winds increase, with line squalls possibly more than 100 miles ahead of the front;
the barometer begins a brief, possibly rapid fall;
cumulonimbus clouds build to the west;
the temperature stays steady; and
the rain begins to fall and intensify for one to two hours.

Passing cold front

When a cold front passes,

winds veer rapidly to the northwest;
the barometer begins to rise, usually quickly;
cumulonimbus clouds become nimbostratus and then clear;
the temperature drops suddenly and then slowly; and
the rain ends, giving way to rapidly clearing skies, possibly with leftover altocumulus clouds.