Canadian Ecology Centre Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Canadian Ecology Centre Observatory clear sky chart from 2005-11-06 to 2026-03-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	16% 12% 4 10% 5% 5% 4 3 5% 6% 29%	131643
	Night	15% 14% 5% 11% 6% 5% 4 3 4 5% 26%	129885
Transparency	Day	55% 5% 10% 20% 8% 1	135187
	Night	59% 5% 10% 17% 8% 1	134429
Seeing	Day	24% 16% 30% 20% 8% 2	16630
	Night	24% 13% 29% 24% 9% 1	25691

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 19% 9% 16% 10% 6% 4 3 4 4 14%	4094
	11th to 20th	Day	9% 20% 10% 20% 8% 4 4 4 4 5% 12%	4568
	21st to end	Day	9% 16% 8% 18% 7% 6% 5% 4 4 5% 18%	5756
February	1st to 10th	Day	9% 20% 8% 15% 9% 6% 5% 3 4 4 17%	1890
	11th to 20th	Day	10% 20% 6% 13% 7% 5% 5% 4 4 5% 21%	1843
	21st to end	Day	16% 17% 5% 13% 8% 5% 3 4 4 4 20%	1572
March	1st to 10th	Day	21% 14% 5% 11% 4 5% 5% 3 4 5% 23%	4968
	11th to 20th	Day	13% 15% 5% 13% 6% 5% 5% 3 5% 7% 23%	5057
	21st to end	Day	15% 15% 6% 10% 6% 5% 3 2 3 4 29%	5496
April	1st to 10th	Day	21% 16% 3 8% 5% 4 4 3 4 6% 26%	2400
	11th to 20th	Day	22% 13% 3 7% 3 3 4 3 4 5% 33%	2474
	21st to end	Day	22% 13% 3 7% 4 3 3 3 4 5% 33%	2600
May	1st to 10th	Day	21% 10% 3 8% 5% 4 4 4 5% 6% 31%	6164
	11th to 20th	Day	14% 10% 3 7% 4 5% 4 3 5% 7% 37%	6318
	21st to end	Day	21% 10% 3 7% 4 4 3 3 4 6% 34%	6949
June	1st to 10th	Day	20% 10% 3 8% 5% 5% 5% 3 5% 6% 31%	2867
	11th to 20th	Day	16% 5% 2 7% 5% 4 5% 4 6% 8% 36%	3000
	21st to end	Day	19% 8% 2 7% 5% 5% 5% 4 7% 8% 31%	3000
July	1st to 10th	Day	18% 7% 2 7% 5% 5% 5% 4 7% 9% 32%	6819
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 5% 8% 42%	6598
	21st to end	Day	10% 6% 2 6% 4 4 4 4 7% 10% 44%	6895
August	1st to 10th	Day	13% 7% 3 7% 6% 6% 6% 4 6% 9% 33%	4234
	11th to 20th	Day	18% 8% 2 6% 5% 5% 4 4 7% 8% 35%	4174
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 4 7% 9% 41%	4519
September	1st to 10th	Day	17% 7% 3 6% 4 4 5% 3 5% 8% 38%	2400
	11th to 20th	Day	17% 7% 2 5% 3 3 5% 3 5% 7% 42%	2337
	21st to end	Day	23% 8% 3 7% 5% 3 4 4 4 6% 32%	2126
October	1st to 10th	Day	18% 14% 3 7% 4 4 4 3 5% 6% 34%	2000
	11th to 20th	Day	25% 15% 4 8% 5% 4 4 3 5% 5% 23%	2000
	21st to end	Day	27% 17% 5% 9% 5% 4 4 3 4 4 19%	2170
November	1st to 10th	Day	20% 15% 5% 9% 4 4 3 3 4 6% 27%	1651
	11th to 20th	Day	17% 19% 6% 11% 6% 5% 5% 3 4 5% 19%	1680
	21st to end	Day	24% 23% 6% 12% 6% 4 2 3 3 3 14%	1680
December	1st to 10th	Day	16% 24% 8% 16% 7% 5% 4 3 3 4 10%	2680
	11th to 20th	Day	16% 19% 6% 16% 9% 4 3 2 3 3 19%	3066
	21st to end	Day	18% 22% 8% 17% 4 3 3 3 3 4 13%	3598
January	1st to 10th	Night	12% 19% 10% 17% 9% 7% 5% 3 4 3 11%	8831
	11th to 20th	Night	10% 17% 9% 20% 9% 7% 5% 3 4 3 14%	8316
	21st to end	Night	7% 19% 9% 15% 7% 9% 6% 4 5% 5% 15%	8424
February	1st to 10th	Night	10% 15% 6% 14% 10% 6% 4 4 6% 5% 18%	2730
	11th to 20th	Night	12% 17% 7% 13% 6% 6% 5% 3 4 4 22%	2663
	21st to end	Night	13% 19% 6% 12% 7% 6% 5% 3 4 5% 19%	2233
March	1st to 10th	Night	16% 18% 5% 10% 4 3 4 4 4 6% 26%	5074
	11th to 20th	Night	14% 17% 5% 10% 5% 4 5% 4 5% 7% 24%	5057
	21st to end	Night	17% 15% 5% 10% 5% 4 4 3 3 4 30%	5435
April	1st to 10th	Night	20% 16% 4 8% 4 4 4 3 5% 5% 28%	2000
	11th to 20th	Night	19% 12% 3 7% 4 4 4 3 4 5% 34%	1892
	21st to end	Night	22% 12% 3 7% 4 4 4 4 4 5% 30%	1800
May	1st to 10th	Night	20% 11% 2 7% 4 3 4 3 5% 6% 35%	3777
	11th to 20th	Night	13% 8% 2 6% 4 4 4 3 5% 8% 42%	3575
	21st to end	Night	17% 10% 3 8% 5% 4 3 3 5% 6% 36%	3926
June	1st to 10th	Night	16% 9% 4 8% 6% 5% 5% 3 5% 6% 34%	1529
	11th to 20th	Night	16% 7% 3 5% 4 4 5% 3 5% 8% 41%	1400
	21st to end	Night	16% 8% 3 7% 5% 5% 5% 5% 5% 7% 35%	1400
July	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 6% 7% 34%	3160
	11th to 20th	Night	15% 7% 3 6% 4 3 4 3 5% 7% 44%	3370
	21st to end	Night	12% 7% 2 5% 3 3 4 3 5% 8% 47%	3941
August	1st to 10th	Night	12% 6% 2 6% 3 4 5% 4 7% 8% 43%	2915
	11th to 20th	Night	17% 7% 3 6% 4 4 6% 3 5% 6% 40%	2875
	21st to end	Night	16% 6% 2 5% 3 4 4 4 5% 7% 44%	3345
September	1st to 10th	Night	15% 5% 2 5% 4 4 4 2 5% 7% 46%	2000
	11th to 20th	Night	15% 7% 2 6% 3 3 3 4 4 6% 46%	2060
	21st to end	Night	19% 8% 2 6% 4 4 3 3 4 7% 39%	2273
October	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 5% 6% 35%	2400
	11th to 20th	Night	25% 13% 4 7% 5% 4 4 3 4 4 28%	2400
	21st to end	Night	26% 15% 4 8% 5% 4 3 3 4 5% 22%	2669
November	1st to 10th	Night	20% 16% 4 8% 5% 4 4 3 4 5% 27%	2865
	11th to 20th	Night	18% 18% 4 11% 6% 5% 4 3 5% 5% 21%	2940
	21st to end	Night	23% 21% 6% 11% 5% 4 4 3 4 3 16%	2940
December	1st to 10th	Night	17% 24% 8% 16% 8% 5% 4 3 3 3 9%	4720
	11th to 20th	Night	17% 20% 6% 15% 6% 4 3 2 4 4 18%	5348
	21st to end	Night	19% 20% 7% 18% 6% 4 4 2 3 4 13%	7602

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	73% 3 7% 12% 3 1	4095
	11th to 20th	Day	75% 3 6% 9% 7%	4598
	21st to end	Day	69% 4 6% 13% 8%	5756
February	1st to 10th	Day	71% 3 6% 13% 6% 1	1890
	11th to 20th	Day	66% 5% 9% 12% 8% 1	1852
	21st to end	Day	68% 4 5% 15% 7% 1	1572
March	1st to 10th	Day	63% 5% 9% 12% 11% 1	4971
	11th to 20th	Day	60% 5% 13% 14% 7% 1	5057
	21st to end	Day	59% 4 8% 19% 9% 2	5496
April	1st to 10th	Day	59% 4 8% 19% 8% 2	2400
	11th to 20th	Day	54% 5% 7% 22% 12% 1	2474
	21st to end	Day	55% 4 7% 18% 13% 3	2600
May	1st to 10th	Day	52% 7% 7% 20% 9% 5%	6173
	11th to 20th	Day	46% 6% 10% 25% 13% 1	6322
	21st to end	Day	52% 5% 10% 19% 12% 2	6949
June	1st to 10th	Day	54% 5% 9% 20% 11% 1	2867
	11th to 20th	Day	45% 5% 11% 25% 12% 2	3000
	21st to end	Day	50% 7% 13% 19% 11% 1	3000
July	1st to 10th	Day	48% 5% 14% 26% 6%	6746
	11th to 20th	Day	39% 5% 15% 34% 7%	6517
	21st to end	Day	35% 4 17% 33% 11%	6697
August	1st to 10th	Day	46% 6% 13% 27% 7%	4221
	11th to 20th	Day	46% 6% 15% 26% 7%	4213
	21st to end	Day	38% 6% 13% 33% 10% 1	4519
September	1st to 10th	Day	44% 6% 15% 24% 11%	2400
	11th to 20th	Day	42% 5% 13% 30% 10%	2337
	21st to end	Day	53% 5% 12% 23% 7%	2126
October	1st to 10th	Day	53% 5% 11% 24% 7%	3260
	11th to 20th	Day	56% 4 13% 19% 7%	3240
	21st to end	Day	69% 4 12% 11% 4	3484
November	1st to 10th	Day	59% 4 13% 18% 5%	1651
	11th to 20th	Day	67% 4 11% 11% 6% 1	1680
	21st to end	Day	77% 4 9% 8% 3	1680
December	1st to 10th	Day	78% 4 8% 8% 2	2680
	11th to 20th	Day	73% 3 5% 14% 5%	3066
	21st to end	Day	76% 3 4 15% 2	3598
January	1st to 10th	Night	77% 5% 7% 8% 3	8831
	11th to 20th	Night	75% 4 7% 8% 6%	8361
	21st to end	Night	71% 4 5% 10% 9%	8424
February	1st to 10th	Night	67% 4 9% 13% 6%	2730
	11th to 20th	Night	65% 5% 9% 15% 5% 1	2665
	21st to end	Night	68% 4 8% 14% 6% 1	2233
March	1st to 10th	Night	59% 5% 10% 13% 10% 4	5084
	11th to 20th	Night	58% 5% 14% 14% 7% 1	5058
	21st to end	Night	59% 4 7% 17% 12% 2	5435
April	1st to 10th	Night	58% 5% 7% 18% 11% 2	2000
	11th to 20th	Night	53% 4 7% 22% 13% 1	1892
	21st to end	Night	56% 4 8% 15% 14% 3	1800
May	1st to 10th	Night	48% 6% 9% 21% 13% 4	3784
	11th to 20th	Night	39% 7% 14% 21% 16% 3	3576
	21st to end	Night	49% 5% 10% 21% 13% 2	3927
June	1st to 10th	Night	50% 6% 9% 22% 12% 2	1529
	11th to 20th	Night	41% 6% 11% 25% 16% 1	1400
	21st to end	Night	46% 7% 13% 21% 12% 1	1400
July	1st to 10th	Night	48% 4 13% 26% 9% 1	3125
	11th to 20th	Night	41% 4 16% 29% 9% 1	3328
	21st to end	Night	36% 4 12% 35% 12%	3824
August	1st to 10th	Night	38% 5% 16% 29% 11% 1	2904
	11th to 20th	Night	43% 7% 17% 25% 8% 1	2902
	21st to end	Night	38% 4 16% 30% 10% 2	3345
September	1st to 10th	Night	39% 5% 16% 28% 11% 1	2000
	11th to 20th	Night	39% 4 16% 30% 10%	2060
	21st to end	Night	45% 5% 13% 27% 9%	2273
October	1st to 10th	Night	47% 4 12% 28% 9%	3920
	11th to 20th	Night	55% 3 13% 21% 8%	3899
	21st to end	Night	60% 5% 17% 13% 5%	4305
November	1st to 10th	Night	59% 5% 13% 17% 6%	2865
	11th to 20th	Night	65% 5% 12% 13% 5%	2940
	21st to end	Night	74% 5% 9% 9% 3	2940
December	1st to 10th	Night	82% 4 6% 6% 2	4720
	11th to 20th	Night	72% 3 5% 11% 9%	5348
	21st to end	Night	77% 3 5% 11% 3	7602

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	24% 25% 35% 11% 6%	220
	11th to 20th	Day	24% 19% 34% 15% 8%	220
	21st to end	Day	22% 21% 33% 17% 7% 1	358
February	1st to 10th	Day	29% 24% 32% 10% 5%	330
	11th to 20th	Day	25% 26% 39% 9% 1	330
	21st to end	Day	37% 18% 32% 12% 1	288
March	1st to 10th	Day	29% 21% 34% 14% 2 1	540
	11th to 20th	Day	27% 20% 33% 16% 4 1	528
	21st to end	Day	28% 19% 31% 17% 5%	550
April	1st to 10th	Day	29% 19% 29% 16% 5% 1	500
	11th to 20th	Day	36% 16% 23% 16% 7% 3	492
	21st to end	Day	32% 19% 28% 14% 6% 1	500
May	1st to 10th	Day	27% 17% 29% 18% 7% 2	503
	11th to 20th	Day	22% 15% 29% 21% 9% 2	527
	21st to end	Day	24% 14% 32% 19% 8% 2	573
June	1st to 10th	Day	25% 13% 33% 23% 6% 1	597
	11th to 20th	Day	18% 18% 31% 24% 7% 2	730
	21st to end	Day	22% 15% 33% 20% 8% 2	730
July	1st to 10th	Day	15% 12% 32% 32% 9% 1	721
	11th to 20th	Day	16% 14% 30% 26% 11% 3	626
	21st to end	Day	13% 12% 32% 29% 11% 3	576
August	1st to 10th	Day	16% 9% 28% 30% 13% 4	526
	11th to 20th	Day	18% 7% 26% 28% 14% 8%	519
	21st to end	Day	17% 13% 31% 21% 14% 5%	583
September	1st to 10th	Day	22% 12% 29% 20% 12% 5%	530
	11th to 20th	Day	17% 10% 25% 25% 14% 9%	530
	21st to end	Day	24% 14% 25% 21% 13% 3	530
October	1st to 10th	Day	25% 15% 25% 19% 12% 2	530
	11th to 20th	Day	28% 22% 30% 16% 4 1	530
	21st to end	Day	34% 17% 29% 17% 3	566
November	1st to 10th	Day	34% 22% 28% 14% 2	225
	11th to 20th	Day	29% 23% 31% 13% 5%	220
	21st to end	Day	35% 21% 32% 10% 2	220
December	1st to 10th	Day	28% 20% 33% 17% 2	220
	11th to 20th	Day	32% 22% 31% 12% 2	220
	21st to end	Day	34% 21% 28% 13% 3 1	242
January	1st to 10th	Night	26% 16% 34% 19% 4	1050
	11th to 20th	Night	27% 15% 33% 20% 5%	869
	21st to end	Night	18% 13% 34% 25% 9% 1	924
February	1st to 10th	Night	16% 20% 37% 22% 4	840
	11th to 20th	Night	22% 17% 36% 20% 5%	823
	21st to end	Night	23% 19% 33% 22% 3	692
March	1st to 10th	Night	23% 20% 33% 21% 4	840
	11th to 20th	Night	24% 20% 32% 18% 5%	829
	21st to end	Night	23% 13% 32% 25% 6% 1	880
April	1st to 10th	Night	29% 16% 29% 23% 5%	800
	11th to 20th	Night	26% 17% 25% 23% 8% 1	708
	21st to end	Night	27% 12% 28% 25% 8% 1	600
May	1st to 10th	Night	24% 10% 26% 30% 9%	480
	11th to 20th	Night	23% 9% 32% 27% 9%	394
	21st to end	Night	19% 11% 29% 31% 9%	433
June	1st to 10th	Night	18% 9% 33% 33% 8%	400
	11th to 20th	Night	20% 8% 27% 34% 11%	400
	21st to end	Night	18% 8% 29% 31% 13% 1	400
July	1st to 10th	Night	13% 4 33% 37% 12% 1	390
	11th to 20th	Night	13% 5% 30% 34% 16% 1	388
	21st to end	Night	12% 7% 29% 37% 15% 1	441
August	1st to 10th	Night	15% 6% 22% 38% 18% 1	597
	11th to 20th	Night	12% 7% 22% 36% 22% 2	594
	21st to end	Night	19% 8% 24% 31% 15% 3	730
September	1st to 10th	Night	20% 7% 25% 30% 18% 1	800
	11th to 20th	Night	19% 7% 24% 26% 21% 3	800
	21st to end	Night	22% 9% 23% 28% 16% 2	800
October	1st to 10th	Night	22% 12% 28% 25% 11% 1	800
	11th to 20th	Night	30% 13% 29% 21% 6% 1	800
	21st to end	Night	35% 13% 26% 23% 4	880
November	1st to 10th	Night	31% 14% 27% 21% 7%	820
	11th to 20th	Night	27% 20% 29% 18% 7%	840
	21st to end	Night	34% 16% 26% 19% 6%	840
December	1st to 10th	Night	32% 15% 30% 17% 5% 1	840
	11th to 20th	Night	27% 17% 32% 18% 4 1	840
	21st to end	Night	32% 16% 28% 19% 5%	1129

Page generated at 2026-03-15 02:23:50. Version ac07

Copyright © 2026 Attilla Danko.