Errata and background info for the Islander's Almanac.
Contact the author: johnohanley85@gmail.com
V1:
- Inside front cover: the daily tide is shown as a grey curve. It should be black.
The explanation of how to understand the weekly and daily graphics is presented on the inside front cover. You'll need to read that explanation closely in order to make sense of the data in the almanac.
The Islander's Almanac uses the following algorithm for calculating weather normals, described here using an example.
What is the normal high temperature for Charlottetown for March 10?
- use daily-high temperature data for the Charlottetown A weather station (the airport) over a long period; let's say there are 51 years of data
- use plus-minus 3 days on either side of March 10, that is, March 7 to March 13 inclusive
- gather together all the daily highs for those 7 days, across all 51 years (7 x 51 = 357 readings)
- sort/list the 357 readings from lowest to highest
- select the reading in the middle of the list (the median value) as the normal high temperature for March 10 for Charlottetown
- to find the 80%-range for the daily high at Charlottetown on March 10, drop the top and bottom 10% of the values in the sorted list. This is a measure of the spread in the data. The daily high temperature value is in this range 80% of the time (in the given data set).
Note :
- this technique uses median values, not average/mean values. This avoids some minor skew from extreme values.
- data for leap days February 29 are coerced into data for March 1.
- at the start and end of the year, the data is wrapped around: for January 1, plus-minus 3 days gives the date range December 29 to January 4 inclusive.
- the idea of using plus-minus 3 days of data is used to smooth the data.
- often the median value is not centered in the corresponding 80%-range. That means there's some skew in the data (which is useful to know).
This technique is not the same as the technique used by Environment Canada, so you will see small differences with their weather normals.
The almanac makes use in several places of the 80%-range of data. This is used to indicate how much spread there is in the numbers.
For example, if there are 100 data-points (numbers) in a data-set, then you get the 80%-range of the data-set by:
- sorting the data-points into a list, from low to high
- dropping the first 10 data-points in the list (at the low-end), and the last 10 data-points in the list (at the high-end)
- selecting the data-points at the start and end of the new list
For the daily entries in the almanac, the daily high temperature is shown as a kind of vertical "thermometer" on the right hand side. The black dash indicates the normal high for that day, while the enclosing grey bar indicates the 80%-range for the daily high temperature (also for that day).
In the list of daily events, some cryptic abbreviations are used to make the best use of limited space.
Abbreviations used for occultations (when the Moon eclipses a star or planet):
DDdisappearance at the dark limb of the MoonDBdisappearance at the bright limb of the MoonRDreappearance at the dark limb of the MoonRBreappearance at the bright limb of the Moon
Abbreviations for some astronomical events:
ICinferior conjunction of a planet with the SunSCsuperior conjunction of a planet with the SunGEgreatest elongation of a planet from the Sun
The hour angle of an object in the sky is the number of hours since it was due south. An hour angle of 2h means an object was due south 2 hours ago. An hour angle of 22h means an object will be due south in 2 hours.
In many places (such as Charlottetown, on the south shore), the time of high or low tide is correlated fairly well with the hour angle of the Moon. In some places, however, the correlation is poor (such as Stanhope, on the north shore).