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Clouds are great visual indicators of atmospheric conditions and upcoming weather changes.

This article begins by explaining different cloud types and their names, helping you identify clouds when you see them in the sky. We then explore how clouds form and their influence on surface winds. Finally, we examine real-world examples of how clouds impact sailing.

Three factors can be used to classify clouds:

The cloud base is represented in the picture below.

The height of the cloud base is the distance between the Earth's surface and the bottom of the cloud. The cloud base height is represented in the picture below and it should not be confused with the cloud height, which is the thickness of the cloud.

Cloud heights are broken in 3 categories provided in the table below.

So basically, a cloud very high in the sky has a name that starts with "Cirro". A cloud at mid level has a name that starts with "Alto". A cloud at low level has no prefix.

There are 2 major shapes:

Here, it is simple. If a cloud is precipitating, then we add the prefix Nimbo.

Based on the 3 factors above, we can define 10 major types of clouds. Those are listed in the table below.

Note about Stratocumulus: It is a combination between stratus and cumulus. You can see individual clouds and almost count them, but they are close to each other with very little empty sky between them. It also happens to be the most common cloud in the world.

The picture below presents the 10 essential clouds:

To see pictures of all clouds, visit The Cloud Appreciation Society.

A cloud is created by rising air. As the air rises, the air temperature drops. The temperature cools down until the water changes state from gas to liquid. Those liquid water droplets are visible, which is the cloud we see. Those droplets are very small at first (microns), and they will need to grow in size (millimetres) to precipitate and fall to the ground. The droplets grow either by coagulating together or by freezing.
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Two key things to remember: condensation happens when the air temperature reaches the dew point temperature, and the condensation releases energy.
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This release of energy helps the air continue its upward motion because the air is warmer than its surroundings and, therefore, more buoyant. The cloud will then extend vertically, and the cloud will build in height and grow.
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At one point, the air will stop rising because there is no more air temperature difference; this is the cloud top. This is also where the air temperature inversion happens. This is called cloud ceiling.

A logical question is how the air rises in the first place. This question has multiple answers corresponding to different modes of cloud formation. We will now cover three ways the air rises and creates a cloud.

This method forms the sea breeze when the sun heats the air over land more than the air above the sea.

During the day, the sun heats the earth, and warmer air bubbles develop at the surface, e.g. over rocks and sand. This warmer air has a lower density than its surrounding air, which heats less (over water, forest, etc.). This warmer and more buoyant air rises and creates a cloud if temperature and humidity conditions are met.

If surface winds encounter a topography (hill or mountain), the air rises, and a cloud may form. Depending on the stability of the air above the topography, two types of clouds will be generated:

When two air masses with different temperatures meet, the warm air mass will be pushed up by the cold air. This rising air will create a line of clouds where those air masses meet, and this is called a front.
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There are two types of fronts depending on whether the warm air mass moves toward the cold air mass or the opposite.

Source: MeteoSwiss

Low clouds are the closest to the ground and sea. Therefore, those clouds have the most direct influence on surface winds, especially in the short term. So, a sailor should prioritise low-level clouds to determine how the wind may change in the next few hours.

Mid-level and high-level clouds are further away from the surface and, therefore, have less influence on surface winds. Typically, mid-level clouds may influence in the next 3 to 6 hours. High-level clouds may influence after 12 hours or a day.

That is why we will focus now on low-level clouds because they have the greatest influence on surface winds relevant to sailors. An important thing to consider is if the cloud is precipitating or not, as it dramatically influences the surface winds.
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See below the three stages of a cumulonimbus, and we will use this picture to start our discussion on the surface winds around clouds.

The rising air creates an inflow at the surface. This convergence surface flow is basically sucking air in, and we call this a Sucking Cloud. (Left side of the picture above)
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The strength of the surface flow will depend on a few factors:

See below two cumulus clouds:

A sailor will feel little or no effect on the sucking wind.
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This strong cumulus cloud creates a significant sucking surface airflow.

The rain falls and creates a downward flow. When this downward flow hits the surface, it creates an outflow at the surface, and we call this a Blowing Cloud. (Right side of the picture above)

This cumulus is precipitating and is now a blowing cloud. At the surface, the outflow wind can be strong. You will see white caps on the water.

A blowing cloud, as mentioned above, has only a down-draft, and therefore, this is a dissipating cloud. For the cloud to develop and mature, you still need an updraft to bring moisture into the cloud, which acts as fuel and provides energy. (Middle of the picture above)
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A cumulonimbus has at the same time a side with an up-draft and a side with a down-draft. Usually, the up-draft is at the front of the cumulonimbus (what you will encounter first if a cumulonimbus is coming straight at you), and the down-draft is at the back.

The cumulonimbus cloud, or thunderstorm, is a convective cloud or cloud system that produces rainfall and lightning. It often produces large hail, severe wind gusts, tornadoes, and heavy rain.

Major cumulonimbus shows a shelf cloud at the front. Such a cloud is a sign of an extreme weather event coming to you. Shelf clouds are often associated with squall lines. Remember that the main threat to any squall line is severe damaging winds associated with the shelf cloud.

Clouds sometimes organise in the sky in a line. On both sides of the cloud line, there are usually two different winds that meet, thus pushing the air upward and creating a cloud. Below is a picture of a cloud line.

Clouds line can be made of sucking clouds or blowing clouds.
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A cloud line of sucking clouds can be divided into two types depending if the winds are blowing perpendicular or parallel to the cloud line.

During the day, the sun warms up the land more than the sea. The warmer air over land rises due to convection. Clouds form on land, and the sea breeze blows from the sea to the land (onshore flow).
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The base of the clouds that develop on land defines the thickness of the sea breeze and is called the mixing layer.

Mixing layer: deeper in the daytime, shallower at night. The depth of mixing depends on heating.

The previous section gave general information on the impact of clouds on surface winds. This section will be more practical for sailors and present wind changes (shift, trend, volatility, gusts, etc) based on the clouds. We will present theoretical examples and case studies based on real cloud pictures.

The sucking cloud, as discussed before, relates to a non-precipitating cloud at low-level, such as a cumulus. Let's isolate one cumulus cloud and study 2 examples if the cloud comes straight at you or on your side.

Example 1: a sailor is experiencing a steady 10 knot wind. A sucking cloud is coming straight in the same direction as the leading wind. Now let's assume the inflow created by the sucking cloud is a 2-knot wind converging toward the centre of the cloud.

Finally, when the cloud is far away, the sailor will get back his initial 10 kn of wind.

Example 2: this time, the sucking cloud still moves toward the sailor but passes on his right-hand side instead of overhead. In this case, the wind direction will turn left as the cloud passes. Wind speed may decrease as the cloud approaches and increase a bit after, similar to example 1. When the cloud has left and is at a distance, the wind will return to the mean direction.

Let's put the above theory into practice by going into practical examples by using real cloud pictures and how we can interpret them.

Picture 1: Small cumulus

​Question: You are at sea and see these clouds. What do they tell you?
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Answer: Those are small cumulus, they are in up-draft mode, and they do not precipitate. The cloud's vertical extension is small, so the updraft is small, and the sucking wind is slight as well. As the cloud is approaching, the sailor is likely to expect:

Picture 2: Large cumulus

This is a large sucking cloud, which is developing vertically, as indicated by the puffy look on top. It has not been raining yet, so there is a strong updraft in this cloud. The cloud base height is close to the ground, so expect a strong influence on surface winds. As the cloud is approaching, the sailor is likely to expect:

Blowing clouds will have the opposite effect on the wind. So, the examples above will be applied, and we will get the following:
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Example 3: If a blowing cloud moves directly toward a sailor. He will experience first an increase in wind speed and then a decrease. Therefore, a racing sailor must be careful not to go to the back of the blowing cloud (upwind side) as the winds will be light.
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Example 4: If a blowing cloud moves toward the right-hand side of a sailor. He will experience a right-hand shift.
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Those four examples help us understand the concept. However, in reality, sailors will need to adapt those concepts to the reality of being on the water. We are not dealing with a single cloud but multiple clouds instead.

Picture 3:

This is a large cloud with rain underneath. The rain indicates a down-draft mode.
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Note that there are still some puffy shapes on the top, so some parts of the cloud are still in up-draft mode but are weaker than the down-draft.

The clouds will blow air out quite strongly because of the rain. If such a cloud comes at you, the wind increases, and wind shifts will be significant, usually before the rain. After the cloud has passed, there will be a zone with little wind, and it will take time for the mean wind to re-establish as this cloud is significant in size.

As explained before, a cumulonimbus, also called a thunderstorm, is complex because it is a combination of a sucking and blowing cloud. This cloud can be extremely powerful and dangerous to sailors. We will describe here what sailors could expect so you can be prepared when you see one of these on the horizon.

Typically, if a cumulonimbus comes straight at you, you will first experience the up-draft and then the down-draft. This could be summarised in 3 stages:

To illustrate the gust front, please watch the following video recorded in Corsica, France, in August 2022. Pay attention to the Shelf Cloud approaching and the extreme blowing wind filling in.
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The physics above is important, but in practice, sailors are looking for warnings when violent and dangerous thunderstorms may develop so they can take pre-emptive actions, e.g. go to the port, change anchorage, add a chain, clean deck and reduce the boat windage.

PredictWind offers five tools to help you apprehend those violent thunderstorms and other extreme weather warnings. Those tools are:

PredictWind provides the following high-resolution (1-kilometer) regional weather model:

PredictWind Worldwide Coverage Map by Forecast Model

Those regional models have complex physics, such as non-hydrostatic equations, which simulate well vertical movements of the atmosphere and local topography, which helps forecast intense and localised weather phenomena.

Those regional models forecast more accurately extreme events than global models like GFS, which has a 25-kilometer resolution.

Valencia, Spain

CAPE stands for Convective Available Potential Energy and is the amount of fuel available to a developing thunderstorm.

If air rises and clouds start to form, then CAPE will boost the instability and turn a cloud into a strong cumulonimbus. So CAPE on its own is not a guarantee that something big will happen, but CAPE in conjunction with other parameters (such as high temperature, rain, formation of clouds in real-time, SST) is important to consider as it will represent favourable conditions for the weather to become violent potentially.

Read here to learn more about What does CAPE mean?

Using the Split Screen feature of PredictWind, we have put below the forecasted Rain on the left side and CAPE on the right side to easily identify the storms that can potentially become violent.

Valencia, Spain

Checking the weather forecast multiple times a day is time-consuming, and the reality is we sometimes do not have the time or the energy to look at all models and all weather parameters, and therefore, we can miss critical weather information. To solve this, PredictWind has developed Extreme Weather Warnings to present clear icons to the user on the potential danger forecasted by weather models.

Looking at weather output from all our weather models, PredictWind will show all warnings using this bright orange icon:

Those warnings are displayed in several places - Daily Briefing, Tables, Weather routing, Weather Planning, so you cannot miss them.

Here is an example showing two warnings, "Gust and High wind against current", on the PWG route (blue):

Read here to learn more about What are Extreme Weather Warnings?

GMDSS forecast is a marine weather forecast provided under the Global Maritime Distress and Safety System (GMDSS), a standardized international system developed by the International Maritime Organization (IMO) to enhance maritime safety. GMDSS forecasts are written and quality-controlled by humans, specifically by trained meteorologists at official national weather services. As an example, the GMDSS will cover extreme weather events such as hurricanes.
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PredictWind offers the standard GMDSS and the far superior Graphical GMDSS, which PredictWind developed in-house.
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Read here to learn more about:

The four tools mentioned above rely on weather forecasts. However, when severe weather or a storm warning is imminent, you’ll need additional tools to monitor conditions in real time. Under the Observations menu, the Rain Radar displays rainfall intensity in dBZ. By activating the animation mode, you can track the movement of rain cells over the past two hours, providing an early indication of their behavior and potential impact.

A cloud line over the sea is a visual indicator that the wind may differ on both sides of the line. In the previous section, we saw two different types of cloud lines: Convergence and Confluent. The wind will change differently depending on those two types:

Convergence:

Confluent:

The diagram below recaps the above information.

To learn more, read on! In the next article, Marine Meteorology 3: Mid-latitude Weather, we explore Ferrel Cells (30° to 60° North or South). These areas are mid-latitudes and correspond to where most of the human population lives, making them very important weather-wise.