Lab 2.5

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Lab 2.5 – Describing Wind Velocity

Fundamental concept: Describe wind or water current velocity as a speed and direction
Preparation for: Labs 8, 10, 11, 12
Estimated time to complete: Approximately 30 minutes
Materials needed: (none)

Oceanographers and mariners encounter wind and water currents all the time. Strong wind causes large waves and dangerous conditions for working at sea. Persistent wind influences ocean currents, which in turn carry material such as nutrients, sediment, and plankton around ocean habitats. Therefore, it is important to know how to talk about variability in wind and currents accurately. We’ll focus on describing wind in this lab, but similar principles apply to ocean currents.

Orientation

Speed

Wind is measured by its velocity, which is a measurement of speed and direction of motion. Speed is the rate at which the air moves. You might see wind speed reported in miles per hour ( mi/h or mph) or kilometers per hour (km/h or kmph) on local weather forecasts. The maritime industry most commonly uses units called knots (kts), which are nautical miles per hour. Finally, oceanographers often measure wind speed in meters per second (m/s).

Here are a few useful conversion factors

  • 1 mile (mi) = 1.61 kilometers (km)
  • 1 km = 1000 meters (m)
  • 1 nautical mile (nm) = 1.15 mi = 1.852 km
  • 1 hour (h) = 3600 seconds (s)

Take a few minutes to practice converting between units of wind speed. For example, let’s say an anemometer recorded a wind speed of 9.0 m/s and we want to know the speed in knots. We can make this conversion in multiple steps using the list of conversion factors above:

  1. Convert seconds to hours: 9.0 m/s*(3600 s)/(1 h)=32400 m/h
  2. Convert meters to kilometers: 32400 m/h*(1 km)/(1000 m)=32.4 km/h
  3. Convert kilometers to nautical miles: 32.4 km/h *(1 nautical mile)/(1.852 km)=17.5 kts

Or, if we know that 1 m/s equals 1.994 nm/h, we can directly convert: 9.0 m/s * (1.944 kts)/(1 m/s)=17.5 nm/h = 17.5 kts

Try it on your own:

Direction

Different scientific fields have their own conventions for describing the direction of wind movement.

Meteorologists and atmospheric scientists (and commonly in other industries such as shipping and fishing) focus on the direction of origin – that is, the source from which the wind originates:

  • A north wind or northerly wind originates in the north and blows toward the south
  • A west wind or westerly wind originates in the west and blows toward the east

In contrast, oceanographers often focus on the direction of motion – in other words, the destination toward which the wind is going:

  • A northward wind blows from the south toward the north
  • A westward wind blows from the east toward the west

It can become confusing if we are not clear about directional naming conventions. If an oceanographer mentions a westward wind and a boat captain interprets it as a westerly wind, they will be talking about wind moving in opposite directions! Notice the suffix -ly (or no suffix) when referring to the direction of origin (meteorology convention) and the suffix -ward when referring to direction of motion (oceanography convention).

Once we have selected a naming convention, we can use cardinal and intercardinal direction or degrees on the compass to report a specific direction. Review the compass rose below displaying the relationship between abbreviated directions and degrees measured clockwise from North.

Diagram of a compass rose with directions around a circle labeled with letter abbreviations and degrees. The compass rose is subdivided into four cardinal directions with bold arrows, intercardinal directions with solid lines, and secondary intercardinal directions with dashed lines.

Figure 2.5.1. Compass rose with letters indicating cardinal directions (N: North, E: East, etc.) and intercardinal directions (NE: Northeast, SW: Southwest, etc.) and numbers indicating degrees from 0 to 360.

For example, wind that is blowing from 225° toward 45° can be described as:

  • Northeastward wind (oceanography convention)
  • Southwesterly wind (meteorology convention)
  • Wind from 225° or SW on the compass

Practice by filling in the statements below.

Vectors and time series

An efficient way to display wind velocity is with a velocity vector, an arrow pointing in the direction of motion. The length of a vector is proportional to the speed.

Diagrams of a compass rose and four maps with examples of wind velocity vectors. Each map shows a black arrow of wind velocity, a blue arrow representing the eastward velocity component and a green arrow representing the northward velocity component.A wind vector pointing from southwest to northeast has an eastward component pointing toward east and a northward component pointing toward north, both of the same length. A wind vector pointing from east to west has an eastward component pointing toward west and zero northward component. A wind vector pointing from north-northeast to south-southwest has a smaller eastward component pointing toward west and a larger northward component pointing toward south.

Figure 2.5.2. Diagrams of wind velocity vectors with their eastward and northward velocity components. (A) Compass rose. (B) Wind blowing from the southwest toward northeast. (C) Wind blowing from the west toward east.(D) Wind blowing from the north-northeast toward south-southwest.

The diagrams above display examples of wind vectors on a hypothetical map.We can break the vector down into a component that is parallel to the east-west line on the compass rose (the “eastward component”) and a component that is parallel to the north-south line on the compass (the “northward component”). These components are given positive or negative signs to indicate direction. Velocity toward the north or east is positive. Velocity toward the south or west is negative.

In Example B, a northeastward wind is represented by a wind vector pointing from southwest toward northeast.The eastward velocity component points toward east and is therefore positive. The northward velocity component points toward north, also positive. Note that the eastward and northward velocity vectors have the same length, meaning that they are equal in speed.

In Example C, the wind vector points westward. Because the wind itself is parallel to the east-west direction, the eastward velocity component has the same magnitude as the total wind vector and is negative. The northward component is zero, meaning no portion of the wind is blowing north or south.

In Example D, the wind vector points south-southwestward. Here, the eastward velocity vector is shorter (slower speed) than the northward vector. Both the eastward and northward velocities are negative since they point toward the west and south, respectively.

Why do we bother to break down the vector components? Oceanographers are often interested in wind blowing in a certain direction, such as into a harbor or parallel to the coastline, due to the way the ocean responds to wind forcing. Also, extracting the eastward and northward components make it easier to represent wind velocity in a time series graph. Here’s an example:

Graph of eastward wind speed in meters per second over time. Speeds vary from negative 10 to positive 3 meters per second between September 7 and September 9. Arrows show that positive values indicate wind out of the west (toward east) and negative values indicate wind out of the east (toward west).

Figure 2.5.3. Eastward wind velocity measured at the OOI Endurance Array Oregon Shelf Surface Mooring in September 2020.

The y-axis of Figure 2.5.3 displays eastward wind velocity. Remember that positive values indicate wind blowing from the west to the east, while negative values indicate wind blowing from the east toward the west. For example:

  • +2 m/s on Figure 2.5.3 means wind is moving from west to east at 2 meters per second
  • -5 m/s on Figure 2.5.3 means wind is moving from east to west at 5 meters per second

Look at September 8 at 12:00 on Figure 2.5.3. The eastward wind had a value of negative 7.5 m/s. We could describe this wind velocity as:

  • “wind blowing from east to west at 5 m/s”
  • “5 m/s westward wind”
  • “easterly wind at 5 m/s”

When viewing time series of a wind velocity component such as eastward wind, it is important to remember that the wind may also have strength in the perpendicular direction. In the case of September 8 at 00:00 in Figure 2.5.3, the eastward wind was only 0.5 m/s. What we can’t see is that the northward wind speed was -5.0 m/s at the same time. This means that by compass direction the wind was blowing southward with a slight eastward tilt.

Quick check

Interpretation

Figure 2.5.4 contains a short time series of wind velocity at the Pioneer Mid-Atlantic Bight (MAB) Array from 9:00 in the morning to 18:00 (6 pm) in the evening of one day.

Time series graphs of wind direction, eastward wind speed in meters per second, and northward wind speed in meters per second from 9:00 in the morning to 18:00 in the evening. Wind direction changes from 90 degrees to near zero degrees at 12:00, then up to 360 degrees at 14:00. Eastward wind speed changes from negative 3 at 9:00 to positive 2 at 16:00. Northward wind speed changes from near zero at 9:00 to negative 4.5 at 16:00

Figure 2.5.4. Wind velocity at the Pioneer MAB Central Surface Mooring on April 20, 2024. Top: Wind direction in compass degrees. Middle: Eastward wind speed. Bottom: Northward wind speed.

  1. How many hours do the data in Figure 2.5.4 cover?
  2. What was the wind direction in degrees at 10:00 on April 20?
  3. List the eastward and northward wind speeds at 10:00:
  4. Describe the wind velocity at 10:00 in the meteorological convention (focusing on source of the wind):
  5. Describe wind velocity at 10:00 in the oceanographic convention (focusing on the direction of motion):
  6. At what hour was the strongest positive eastward wind recorded? What was the northward wind speed at the same time?

The OOI Pioneer Mid-Atlantic Bight (MAB) Array experienced a wind storm on April 12, 2024. View time series of wind velocity before, during, and after the storm in Figure 2.5.5.

Time series graphs of wind velocity between April 10 and April 14 at the Pioneer MAB Central Surface Mooring. From top to bottom, line graphs show the wind speed in meters per second, wind direction in degrees, eastward wind speed in meters per second, and northward wind speed in meters per second. Wind speed ranges from 2.5 m/s at the start of April 10 to 14 m/s just after midnight on April 12 and then decreases to 5 m/s. Wind direction changes from 220 degrees on April 10 to 180 degrees on April 12 up to 270 degrees on April 13. At the time of maximum wind speed, the eastward wind speed was 0 m/s and the northward wind speed was 14 m/s.

Figure 2.5.5. Time series of wind velocity at the Pioneer MAB Central Surface Mooring on April 10-14, 2024.

  1. Over the four days shown in Figure 2.5.5, what was the range in wind speed? (Use the top panel that shows total wind speed)
  2. Give an example of a time when the wind was blowing from the southwest toward the northeast:
  3. On what date and approximate time did the maximum wind speed occur?
  4. At the time of maximum wind speed, what was the
    1. wind direction in degrees?
    2. eastward wind speed?
    3. northward wind speed?
  5. Describe the wind direction at the time of maximum wind using the oceanographic convention:
  6. Describe the wind direction at the time of maximum wind using the meteorological convention:

Reflection

  1. In what ways does wind velocity affect your work or activities that you enjoy doing outside?
  2. Before completing this lab, what terms did you typically use to describe wind?
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