How to Interpret Weather Charts

A brief guide on understanding weather images on KiteForum

Introduction

The weather charts below provide three main pieces of information. They are:

1. The wind speed
2. The wind direction
3. The amount of rainfall

(Click the images for a larger view)

The wind speed and direction are determined by the isobars and corresponding telltales (the white heads with a black tails), while the rainfall is shown by the colouring on the chart.

Wind speed and direction

The red lines that curve over the images are called isobars and indicate where the average sea-level air pressure is equal, the value of which is marked in hecto Pascals (a measure of pressue, where 1 hPa ~ 100 Pa). A high pressure system is surrounded by isobars of larger values while a low pressure system is surrounded by isobars of smaller values. In the southern hemisphere, wind moves counter-clockwise around a high and clockwise around a low.

Wind usually blows along the general path of the isobars but a more accurate prediction of its direction is given by the small telltales, with the white head pointing in the direction the wind blows.

Although it varies with latitude, the closer the isobars are together, the stronger the wind tends to be. The wind speed in knots can be calculated by looking at the small perpendicular black lines on the end of the telltales. A half-sized line indicates five knots while a full-sized one signifies ten knots – to calculate the wind speed at a particular spot, simply add the value of the lines on the telltales together.


Easy-breezy: Telltales and their corresponding wind speeds.
It is important to remember that isobars are only smoothed approximations and the telltales only estimates of wind flow. Just as you twist and turn when performing tricks on your board, actual wind speed increases down valleys, eddies behind mountains and contorts around the coastline. This means that while the weather chart provides a reasonable measure of the wind it is often affected by the local conditions. Unfortunately for snow kiters, the direction of isobars over mountains and high ground is often significantly distorted due to the pressure differences on either side of the raised terrain, making wind flow at these points difficult to interpret.

Rainfall

The weather charts also forecast rainfall, which is indicated by irregular shapes of colour spread over the image. Depending on the colour, it shows the predicted amount of rain to fall in that particular spot over the next six hours. This is known as six-hour accumulated rainfall, and is measured in millimetres.The guide below shows the amount of rain each colour represents.


Drizzle or downpour: The expected rainfall is indicated by different colours.
Note that if you are unable to see any coloured shapes in the weather charts at the top of that page it is because there is no rainfall predicted. Not to worry, New Zealand weather is likely to bring some within the next few days, so check back later.

Put it into practice

Once you feel you have the ability to accurately interpret weather charts, check out the national forecast page and take a look at the images there. You should be able to determine the predicted wind speed and direction where you live, and any rainfall coming your way. Keep in mind that the charts are only a forecast, however, and actual weather is subject to unexpected change. That said, it's still a good indication of when to grab your kite and board, and when to stay at home and browse around KiteForum instead.

Guide adapted from the MetService's article.