Written by Amanda Delaney
Many mariners have heard the saying, “red sky at night, sailors’ delight. Red sky in the morning, sailors take warning.” Although there is some truth to this saying, how many times have you heard good weather predicted the following day, to then wake up to an ominous sky? Which clouds or weather conditions are safe to stay offshore, and which conditions can warn you to immediately turn back to port? Observing clouds and weather phenomenon can be a precursor for upcoming weather in the next few hours or next day.
Clouds form in different levels above the surface and their location can indicate what type of weather is moving into the area. Clouds that form greater than 20,000ft or more above sea level are called cirrus clouds. These clouds form approximately 24-36 hours ahead of warm fronts or a tropical cyclone. Strong winds aloft will push these clouds well ahead of these systems where ice crystals condense.
Usually if the presence of cirrus clouds begin to thicken and the cloud deck begins to lower, this indicates that a warm or occluded front is approaching the region and expect light to moderate rain to develop within 24 hours. Warm fronts are boundaries where warm air rises above colder air. Moisture condenses as the warm air permeates the higher atmosphere first (cirrus clouds), then as the warm air reaches the surface, the moisture condenses lower in the atmosphere. Clouds that form between 6,500ft and 20,000ft above sea level typically begin with the prefix alto, which also means mid-level clouds. Examples of these are altocumulus and altostratus. When the warm front is imminent, gray, sheet like clouds called stratus develop below 6,500ft.
Once a warm front moves through, the clouds that develop behind it are typically cumulus or cumulonimbus. The warm air allows moisture to rise and cumulus clouds develop vertically in the atmosphere. Substantial moisture can surge well up to as much as 50,000-60,000ft in a cumulonimbus cloud.
In the wake of cold fronts, skies typically clear. The lack of clouds is due to cold, dry air in the atmosphere sinking to the surface. Ahead of high pressure and near the center of high pressure, the air will sink, whereas to the west of the high, the air gradually rises allowing for moisture aloft to condense, and develop clouds high in the atmosphere (either cirrus or cumulus clouds).
Below is a table of the most common cloud types and the weather that either precedes or is associated with them.
|Cloud Name||Description||Advance Warning or Imminent Weather|
|Cirrus||White, wispy and thin high clouds. Also known as “mare’s tails”||Rain can be expected in 24-36 hours.|
|Cirrocumulus||High, puffy clouds that can create a “mackerel” or fish scale like sky||Fair weather for the next 24 hours.|
|Cirrostratus||High gray, thin sheet clouds||Rain can be expected in 24 hours.|
|Altocumulus||Mid-level puffy gray clouds||Rain will develop in 6-12 hours.|
|Altostratus||Mid-level gray, sheet clouds||Rain will develop in 6-12 hours.|
|Stratus||Low sheet like clouds||Continuous light to moderate rain and fog.|
|Stratocumulus||Low puffy gray clouds||Light rain or drizzle.|
|Nimbostratus||Dark, low clouds that may look ragged||Moderate to heavy rain.|
|Cumulus||Low and large puffy gray or white clouds||Fair weather for the next 24 hours.|
|Cumulonimbus||Clouds that rise rapidly, develop an anvil on top and a dark underside||Thunderstorms, heavy rain and, at times, severe weather|
Sea breezes can generate localized weather near the coast. As the land warms through the day, the thermal difference between the warmer land and cooler ocean water will generate a breeze that moves from the ocean to the land. The warm air rises and is transferred by winds aloft back out to sea where it cools and lowers to the sea surface to repeat the cycle again. At times this sea breeze will converge with warm winds trying to blow offshore. This convergence allows air to rise and create cumulus and, if the atmosphere is unstable enough, cumulonimbus clouds. A line of clouds will develop and will at times generate thunderstorms. This is known as a sea breeze front and this front will typically move from offshore waters inland over the coast. The sea breeze front will weaken in the evening when the land begins to lose the heat it retained during the afternoon.
At night, the winds will blow from the land to water. This occurs due to colder air over land moving out towards the warmer ocean waters. If the atmosphere is unstable over the water, then clouds will build offshore and generate thunderstorms that will move from the land out over the water. These thunderstorms will dissipate in the morning once the land begins to warm and the land breeze weakens.
Fog, Haze and Halos:
Navigating through fog can be treacherous. The most common type of fog over coastal waters is called advection fog or sea fog. This fog typically develops overnight when moist air over the ocean moves over cooler land regions. The moisture condenses and generates fog along the coast, which will dissipate later in the morning or early afternoon, when the land begins to warm. Advection fog can also develop in areas where warm, moist air that originates over a warm current moves over a cold current usually by winds. The moisture condenses at the sea surface to create the fog and dissipates when the wind increases over the cold current or the air above the cold current cools.
Some fog formation is associated with frontal passages. After a cold front, cool air will move over warm ocean waters. When the warm air above the ocean surface cools and becomes saturated, fog will develop. In the wake of a warm front, rain will fall but evaporate prior to reaching the cooler water. Evaporation causes the air to cool and as the temperature drops, then the air becomes saturated allowing fog to form over the ocean surface.
Haze typically develops when high pressure builds over a region. Air sinks from aloft and the winds are usually calm within a high. This forces pollutants, dust and/or smoke to remain suspended in the atmosphere and lower visibility. The haze will dissipate when the high exits the area and cool air is allowed to lift from the surface. This allows the winds to increase and disperse the pollutants, dust or smoke which therefore allows visibility to improve.
Halos develop when ice crystals suspended in high cirrus clouds reflect light around the moon or sun. When halos develop, these are a good indicator that rain will arrive in the area approximately 24 hours later as the high clouds develop about a day ahead of a warm front.
There are several signs of an approaching thunderstorm. The one obvious sign is building cumulonimbus clouds on the horizon. A thunderstorm is within an hour away when the pressure suddenly drops 1-2 millibars, and the winds begin to gust for a short period of time. This is what is known as the gust front. When rain falls from a thunderstorm, cold air from the top of the thunderstorm rushes to the surface and spreads away from the thunderstorm. Stronger gust fronts can be seen on radar as a narrow arc spreading out ahead of the thunderstorm. At times, the gust front is accompanied by a low, dark horizontal cloud known as a shelf cloud. If this is seen, action must be taken quickly to prepare for strong winds arriving within minutes. Also, an unusual cloud called mammatus is associated with severe weather. These clouds protrude towards the bottom of a cumulonimbus cloud like pouches. If these clouds are seen, immediate action should be taken as severe weather is imminent. Once lightning is seen and thunder is heard, the thunderstorm is only minutes away and precautions should be taken to head back to port or to avoid the thunderstorm.
Fortunately, satellite imagery and computer models can provide ample warning of an approaching tropical cyclone. However there are signs that can be observed several days before a tropical cyclone. A drop in the barometric pressure of three millibars if below the average for a given time of year, is a sign that a tropical cyclone may be approaching. A drop of five millibars or more indicates that a tropical cyclone is approaching and immediate action should be taken. If the drop in pressure is slow then a tropical cyclone may be several hundred nautical miles away. A more moderate drop in pressure generally means that the tropical cyclone is approximately 50-100 nautical miles away and a rapid drop means that the tropical cyclone is within 10-50 nautical miles.
Another sign is a long period swell. These swells can extend 500 nautical miles to as much as 1000 nautical miles from the center of a tropical cyclone and can be observed several days before the arrival of a tropical cyclone. If the swells continue to build and the period gradually shortens, then action should be taken to seek shelter.
A lowering cloud deck, similar to what was described for an approaching warm front, will occur with high cirrus clouds developing first. This usually happens when the tropical cyclone is still several hundred nautical miles away. Then mid-level stratus (also known as altostratus) will appear in the direction of the tropical cyclone. Gradually the clouds will lower further to low stratus clouds. Finally a wall of cumulonimbus, which is the first outer band of the tropical cyclone will approach and with it, increasing winds and heavy rain.
Although the weather features described here provide a prediction for the next few hours to a day in advance, where appropriate precautions may be taken, but one should always double check an official forecast again. This will help in being aware of the changing conditions to help prevent a vessel from heading into dangerous weather. And at times, the weather proverbs can be right, and when visiting our weather proverb from earlier, this proverb is accurate in the lower latitudes where the trade winds are from east to west. The sunset reflects when dust/pollutants are in the atmosphere, which we know that linger in the atmosphere, the air is stable and high pressure is moving into the area. Therefore, fair weather is expected the next day. When the skies are red in the morning, that generally means more clouds and rain are approaching from the east to the west. Unfortunately this proverb doesn’t work in the mid-latitudes where the prevailing weather moves from west to east but knowing all of the weather features described above will still give a mariner a good idea of what to expect in the near future.