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.
Reprinted with permission from America’s Boating Compass, United States Power Squadrons. Do you want to learn more about the weather and predicting the weather? Consider taking the Weather course with America’s Boating Club Houston. Check our calendar for course offerings.