Understanding the Weather

"IF IN DOUBT, DON'T GO OUT!"

That's the message from Maritime NZ, concerned that too many people are taking a risk on the weather when they go out, or sometimes not even checking it at all.

New Zealand weather is so varied, from hour to hour, and all over the country. A change in expected weather, or a turn for the worse, could make all the difference between a lovely day out and an emergency situation.

1: Check the marine forecast, not land weather

  Image from Maritime NZ

Image from Maritime NZ

Marine weather forecasts state what the weather is expected to do. Land and general forecasts do not take into account win speed over water (which is double that over land) or the waves or swell.

If a land forecast does give wind speed, it is in km/hr rather than knots, which is a good indicator that you are listening to the wrong forecast.

To get the latest marine forecast:

To get forecasts when you're out:

  • The MNZ maritime radio service provides forecasts at scheduled times. These are announced on Channel 16 at 0133, 0533, 0733, 1333, 1733 and 2133 hours.
    For the Chatham Islands, forecasts are broadcast at 0603, 1403, 1803, and 2200 on Channels 60 and 62.
  • On your local Coastguard channel on VHF radio, including NowCasting continuous broadcasts on Channels 20, 21, 22 and 23, in many recreational boating adverts.
  • Or you can phone MetPhone by dialling 0900 999 + your map area number.

2: Get local information

Seek local knowledge from those who live in or are familiar with the area - harbourmasters are an excellent source of local information.

While marine forecasts are almost always accurate when predicting major weather events, such as gales, they can be less accurate when predicting local changes of conditions, so you should always be prepared for the unexpected.

3: Keep an eye on the weather

The weather can change quickly and without warning. At the first sight of deteriorating conditions, head for shelter.

Look ahead If you're planning ahead or intend to be away for a day or two, get a long-range weather forecast. The outlook will tell you what weather is predicted up to 5 days ahead.

Having information in advance leaves plenty of time to alter plans ro decide to wait for better conditions.

Prepare for the unexpected Remember that the weather can change suddenly and without warning. Keep an eye on the weather while you’re out, listen to the Maritime Radio and NowCasts in your area so you get a warning of increasing winds before they arrive, and head for shelter at the first sight of worsening weather. When the wind starts to blow, the water becomes very rough, very quickly, especially on lakes and rivers.

Remember that forecasts are only the best prediction available at any given time.

If in doubt, don't go out! A large proportion of accidents involving small vessels are weather related. Bad weather makes the environment onboard a vessel extremely hazardous. It also places a lot of strain on the vessel’s structure and equipment and the people on board.

It is important to respect the weather at sea. Skippers should make sure they understand the different parts of a weather forecast and the best way to find up-to-date local information.

  Screen shot from MetService website showing Significant Wave Height. Arrows show direction of wind.

Screen shot from MetService website showing Significant Wave Height. Arrows show direction of wind.

4: Understand the marine weather forecast

It is incredibly important to not just check the weather forecast, but to understand what you're seeing when you do.

Wind
Forecasts usually give a direction (such as 'north-west') as the direction the wind is expected to come from.

In a coastal or marine forecast, this wind speed will be given in knots, where 1 knot is approximately 2km/hr. This is an average speed, so always expect that gusts may be 50% higher.

Also allow for funnelling between headlands, causing the wind speed to double.

On maps like the one opposite from the MetService website, the arrows show the direction of the wind, where the key below indicates the severity of the measure (wave height, for example).

Sea
'Sea' is a description of the waves formed by the local wind.

Swell
A 'swell' comes from either a distance disturbance, such as a cyclone or depression, or develops from wind waves that have been blowing from the same direction for a length of time. Swells increase in height and get steeper when they reach shallow water.

The measures used for swells are:

  • low - under 2.0 metres
  • moderate - 2-4 metres
  • heavy - over 4 metres
  Screen shot from MetService website showing current rainfall. Wind here is shown in knots, and there is the option to view a 3-day forecast and a 5-day forecast.

Screen shot from MetService website showing current rainfall. Wind here is shown in knots, and there is the option to view a 3-day forecast and a 5-day forecast.

Visibility
The average visibility in New Zealand is about 15 nautical miles.

This information is typically given when visibility is expected to be less than 6 miles (10km).

The visibility distances are:

  • fog – less than 1.0 nautical mile
  • poor – 1–3 nautical miles
  • fair – 3–6 nautical miles
  • good – over 6 nautical miles

Outlook
All marine forecasts are for up to 48 hours, with the outlook for a further 3 days.

The MetService website has the option of a 3-day forecast or a 5-day forecast, where you can play a video to watch the progression of rainfall across New Zealand, and keep an eye on changes in wind speed.

Wave height
This refers to the size of of significant waves that are generated by the wind in the area.

The approximate wind wave height measures used are:

  • calm – up to 0.1 metre
  • smooth – up to 0.5 metre
  • slight – up to 1.0 metre
  • moderate – up to 2.0 metres
  • rough – up to 3.0 metres
  • very rough – up to 4.5 metres
  • high – up to 6.5 metres
  • very high – up to 8.5 metres
  • phenomenal – up to 11.0 metres (or more)

Recreational areas
Most boating areas are covered by the recreational marine forecast.

Situation
A situation is a description of the position and movements of highs, lows and frontal systems expected to affect the New Zealand coast within the next 36 hours. It also names those areas affected by warnings.

Warnings
These are issued for gales, storms or squalls anywhere on the New Zealand coast. A strong wind advisory is issued in recreational areas if the wind is expected to be over 25 knots (about 40km/hr).

If the winds are associated with a cyclone from the tropics this will be mentioned in the warning, but tropical cyclone warnings are not issued in New Zealand.

Winds can be reported as:

  • strong – the wind is expected to exceed 25 knots as a steady wind or 33 knots in gusts
  • gale – expect 33 to 46 knots as a steady wind – gusts can be 50 percent higher
  • storm – 47 knots or more as a steady wind gusts can be 50 percent higher

Wind and tide
When the wind is opposing the tide, expect a much rougher sea.

When the wind is with the tide, expect a calmer sea.

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