Sustained wind speeds, wind gusts, significant wave height and peak period. Those terms are just a small selection off the parameters used by a meteorologist in the weather forecast. Its from great value to know those terms well to better understand the forecast provided by our expert meteorologists. A variety of terms used in the weather forecast will be explained in this blog.
For clarity purposes the parameters from the weather forecast which are discussed in this blog will be divided into weather related parameters and wave related parameters. Although, they are split it is always good to bear in mind that all parameters go hand in hand with each other. For instance, when the sustained winds are strong, waves will be high as a result. Those links are common between many parameters.
Weather related parameters
Sustained wind speed and wind gusts might be the most used weather parameters in the offshore industry. The sustained wind is the average wind speed at a certain level for a certain period. Due to the averaging over for instance a 10-minute or an hour period of time, wind peaks and wind drops are filtered out. However, for certain operations it is valuable to understand the behavior of the wind gusts as well since they can be the limiting factor during certain operations. A wind gust is the wind averaged over a much shorter period. 3 or 10 seconds are a common period over which the wind is averaged to obtain one value for the wind gusts. The shorter the averaged time frame is, the higher the wind gusts are. All the parameters related to wind in our weather forecast are indicated by the red box in figure 1.
Another relevant weather parameter is the visibility. The visibility can be reduced due to a lot of different weather events. The most well-known event is possibly fog. However, severe snow or rain might occasionally reduce the visibility to very low values as well. As the term already suggests, the visibility is basically a number that shows how far you are able to see. This is the most hands-on definition of visibility. However, how far one can see depends for instance on light conditions, the height of the observer above sea/ground level and which object one is looking at. Therefore, the most exact definition of visibility is the following one: The visibility is from how far an observer can see a black object of sufficient size with a light sky as a background. You may already notice that this is still a subjective way of ‘guessing’ the visibility. Meteorological instruments can do this in an exact manner. In the Infoplaza forecasts the visibility is valid for a height of 2 meters above sea/ground level. On many days visibility will be better at a higher level, but during certain atmospheric conditions the opposite is the reality. For instance when low clouds are covering the platform in ‘fog’ while at sea level visibility is still more than 10 kilometers.
Besides visibility and wind, also lightning can play and important role in the decision whether an operation can be safely planned or not. The lightning risk will be assessed by our expert meteorologists. They evaluate instability parameters from numerical weather models and translate them to a low/medium or high lightning risk.
The lightning risk and visibility risk in our weather forecast are indicated by the yellow box in the upper left corner in figure 1. The yellow box on the right side of the forecast indicates the significant weather and the visibility.
Figure 1: An example of a weather forecast which is provided by Infoplaza. The red box indicates the wind parameters, the blue box the wave parameters and the yellow box the significant weather parameters.
Wave related parameters
The presence or absence of waves determine often whether a project can be safely executed or not. There are a lot of different wave parameters to consider when making the decision to proceed or not. The most common ones will be explained in this section.
The wave height is of course from great importance during offshore operations. When the waves exceed operational limits, the project cannot be safely executed. A common way to parameterize wave height is by the value Hs (significant wave height). The significant wave height is the average of the 1/3 highest waves measured or modelled at a certain location. This sounds as, and is, a very simplified definition. The reason everyone agreed to use the definition is that the resulting number comes very close to what a trained human observer would estimate as the significant wave height. The maximum wave height (Hmax) is parameterized the best by multiplying the Hs with roughly 1.8.
Furthermore, the wave period can determine if an operation can be safely executed as well. It depends on the vessel type if the wave period can be problematic. For instance, when the wave period is close to the length of the vessel, the wave period can be a limiting factor. The different wave periods, such as peak period (period which contains the most amount of energy in a certain wave system) are depicted in our weather forecast. All wave parameters are depicted with the blue box in figure 1.
All the tools that we provide to you as a customer help you in making the decision whether an operation can be executed or not within a certain time frame. It is from great importance to understand the basics of the weather and wave parameters to make a reliable decision. Our expert weather forecasters are always there to help you out and give advice on whether an operation can be executed. Moreover, we give special webinars in which we guide you through the weather forecast and learn you about the basics of the weather and the sea to make sure that you, as a customer, can read and understand the weather forecast well. By doing so, we provide you with the most precise and accurate information we can increase your workability. Understanding the basics of weather and wave parameters is from great importance in that case.
If any questions raised, please feel free to contact us.
Main picture source: Adobe Stock/Kim