Whitecaps set in essentially immediately with the onset of new wind, but the seas take time to develop. At 15 knots there are whitecaps readily visible here and there, but at 25 knots there are so many whitecaps that this white speckled surface of the sea is the first thing that catches your eye. See Sea State Atlas (G453) and Beaufort scale (G231).

Frontal waves of this type tend to move in a direction parallel to the isobars in the warm sector, which is nearly always generally to the east (NE to SE) in either hemisphere. This is a valuable rule when interpreting weather maps, especially when the direction of Low motion isn't shown. The warm sector isobars will be parallel to the winds aloft. This rule is also crucial to the application of the crossed winds rule for shipboard analysis and forecasting.

It's a way to determine the region to be avoided in the presence of a tropical cyclone — namely, avoid the area with winds greater than 34 kts, which is a forecasted boundary. See Resource 4.2-1, Mariners Tropical Cyclone Guide.

A sharp veer is a universal sign for the passage of a front since the isobars have a sharp kink at the trough line. As a Low approaches and passes, winds first back and then veer, but the veer is not sharp. As a rule winds always increase at a front, but usually more so in a cold front than in a warm front. See properties of frontal passage in G201.

All are potential indicators of strongest current flow. Water temp tells you only once you are in it, unless you have a recent satellite image. Wave steepness helps if there is wind — strong wind against the current makes much steeper waves. Low cumulus in otherwise clear skies is a real signal too: warm fast water rising into colder air can produce small clouds you can spot on the horizon. Depending on circumstances, any of these can be the best indicator.

All descriptions are valid. With nothing changing, the barometer should still be recorded at least every 4 hours. When anything changes (wind speed, direction, clouds, sea state, the barometer itself), record at that time. Ocean racing yachts typically record the barometer every hour to be certain not to miss the onset of any change. See ART-5 and 4-5-6.

"No change" means "no change" — at least for the short term. Look at typical isobars in the warm sector between a warm front and the cold front that follows it (G139): they are straight lines, generally parallel to the system's direction of motion. We expect the barometer to remain stationary after a warm front passes, and indeed this observation can confirm that a more serious front with heavy rain and strong wind will soon follow. See barometer section of G245.

This is a classic apparent wind computation that affects both sailors and powerboaters. Solve it with vector addition of true wind and your reversed boat-speed vector. See the video on several ways to solve this. For most powerboats, the practical takeaway is that a west wind eventually brings west seas — anticipate altering course to avoid beam seas before it's forced on you.

Those who practice can get remarkably accurate at it — though sensitivity varies. Lighter air, down to 1 knot, can be detected by smoke from a cigarette; some seasoned light-air sailors carry incense for this. Racers care about even 1 knot of wind to get the sails set on the right side of the boat. See Beaufort Scale (G231).

Actual rate depends on many factors including your vessel's speed and direction, plus the motion and steepness of the low. Averaging many published barograph traces of severe lows gives this answer. This is not the drop rate at the low itself (much steeper, 6 to 10 mb per 3 hours), but the initial down turn 12 to 24 hours before the low arrives. "Forecasting" is over once it drops faster than this. The key is a steady drop over several 3-hour periods. Treat 4–5 mb in 6 hours as fair warning. See ART-5 and 4-5-6.

1020 mb is a typical high pressure, 1050 mb is an extreme high — both typically lead to calm air and clear skies. 500 mb is far below any normal or even record surface pressure (it's what you'd expect at ~18,000 ft elevation). 980 mb is a clearly low surface pressure, the only stormy candidate. See ART-5.

In the Northern Hemisphere, wind circulates counterclockwise around a low. With wind at your back, the low is to your left. The "slightly forward" part accounts for surface friction, which pushes the wind slightly inward toward the low. See Buys Ballot's law and the crossed winds rule.

Mare's tails (cirrus uncinus) and mackerel sky (cirrocumulus or altocumulus in a rippled pattern) both indicate strong winds aloft and are classic warnings of approaching deteriorating weather. See the old saying: "mackerel sky and mare's tails make tall ships set low sails." See ART-6.

In the Northern Hemisphere, winds travel with low pressure on the left side at any level. See G180 looking at both north and south latitudes. For winds aloft, an alternative way to phrase it is that they travel with cold air on the left side. See Weather Maps Atlas (G455) for 500-mb maps.

In temperate latitudes, weather systems track generally west to east, so storms rarely arrive from the east. In the Tropics, the reverse is true — tropical cyclones tend to move westward (at least initially), and easterly waves move toward the west. See G218.

It's a genuine old weather adage worth noting. Red sunsets in mid-latitudes often imply clear, dry air to the west — and weather generally moves west to east, so good weather is coming. See ART-6 on old sayings explained.

A ring around the sun or moon is the most positive sign, especially when the weather has been good with clear skies and fair breeze. The ring is proof positive that there are now ice crystals high overhead — a transparent layer of cirrostratus that wasn't there before. This layer has been forced over you by an approaching warm front, usually the first step in a Low's approach. This is how early shamans earned their keep as forecasters. See RES-4.

A long low swell would likely be the first sign, especially in tropical waters as an indicator of a tropical cyclone — swells can reach out 1,000 miles or so ahead of the storm. See Shipboard Forecasting Atlas (G245) and the Sea State Atlas for the relative importance and sequence of these signs.

Sea breeze comes from land heating more than neighboring waters, creating a low over the land relative to over the water — air then flows from high (water) to low (land). If an offshore Low approaches, the reduced pressure along its fringes can outweigh the drop on land, canceling the pressure difference and the sea breeze. The same applies if a fresh sea breeze fades early in the afternoon rather than at sunset. See ART-4.

Quick conversion: 331 m/s ≈ 740 mph; 20 seconds = 0.0056 hr; 740 × 0.0056 ≈ 4.1 miles. Or even simpler: 331 m/s × 10 s = 3,310 m ≈ 2.05 statute miles, so every 5 seconds of delay ≈ 1 mile. 20 seconds = 4 miles.

The presence of whitecaps is very sensitive to current — whether the wind is blowing into the current or along with it — and to wave steepness (waves break around 1:10 steepness). So whitecap onset can't be pinned to a single wind speed. See Beaufort scale (G231) and Quick notes on waves (G217).

Beaufort force 5 corresponds to all three descriptions — it's defined by wind range (17–21 kt), the descriptive name ("fresh breeze"), and the associated sea state. See Beaufort scale (G231).

All apply. In most cases clouds are valuable for judging what's happening or coming, but sometimes they're not, and in extreme cases they can even be misleading. A quick look at clouds is available in G214 and G215.

The wind will sharply veer at every frontal passage. Veering wind shift along with gusty strong wind, clouds, and some form of rain are the common characteristics. The other options describe specific situations, not common features of all frontal types. See G201 and G245.

This is a classic warm front approach with backing wind followed by a veer. A helpful rule: the time from first signs of the front until rain starts is roughly how long it will rain until the front passes. See G245 and "long forecast, long to pass" in ART-6.

Get into the white caps — they mark an area with current going against the wind, which means current going with you (i.e., into the wind, which is what you want when sailing north against the wind).

Lows generally reach their peak depth just as their fronts occlude — or rather, the Low generally begins to dissipate after its fronts occlude, because occlusion cuts off the source of warm moist air which is the storm's fuel. See occluded front and ART-12.

In the Northern Hemisphere with wind at your back (NW wind blowing from NW), the low is to your left and slightly forward — which puts it to the NE. See Buys Ballot's law and graphic G168. Properties of Highs and Lows are compared in G153.