Alsask Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Alsask Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	7% 8% 3 9% 5% 5% 5% 4 6% 8% 39%	142078
	Night	6% 8% 3 9% 6% 6% 5% 5% 7% 8% 36%	139327
Transparency	Day	41% 8% 16% 25% 10% 1	141775
	Night	43% 8% 16% 22% 11% 1	139216
Seeing	Day	11% 9% 28% 30% 17% 5%	17860
	Night	11% 7% 31% 35% 14% 2	26998

By Month

Percentage Cloud Cover NW (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	6% 12% 6% 15% 8% 7% 6% 4 6% 6% 24%	4125
	11th to 20th	Day	2 7% 5% 11% 7% 6% 6% 8% 7% 10% 32%	4119
	21st to end	Day	4 8% 4 9% 8% 7% 7% 4 6% 9% 34%	4688
February	1st to 10th	Day	7% 15% 5% 11% 8% 6% 4 5% 6% 8% 26%	1853
	11th to 20th	Day	7% 11% 5% 10% 7% 7% 7% 4 7% 7% 27%	1845
	21st to end	Day	5% 11% 4 12% 8% 7% 6% 5% 7% 10% 25%	1557
March	1st to 10th	Day	4 9% 3 12% 7% 6% 6% 4 6% 9% 33%	4742
	11th to 20th	Day	4 11% 6% 14% 7% 5% 5% 4 5% 9% 31%	5109
	21st to end	Day	6% 13% 6% 16% 6% 5% 5% 4 5% 7% 26%	5607
April	1st to 10th	Day	7% 10% 4 10% 6% 7% 6% 4 5% 8% 34%	2567
	11th to 20th	Day	9% 12% 3 9% 5% 5% 6% 4 6% 8% 32%	2700
	21st to end	Day	11% 10% 3 9% 6% 4 5% 3 5% 8% 35%	2765
May	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 4 7% 6% 40%	8224
	11th to 20th	Day	11% 9% 3 9% 6% 6% 7% 5% 7% 8% 30%	8472
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	9649
June	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	3150
	11th to 20th	Day	16% 9% 3 7% 5% 5% 6% 5% 6% 9% 30%	3150
	21st to end	Day	8% 6% 2 6% 6% 5% 5% 5% 8% 9% 40%	3150
July	1st to 10th	Day	6% 5% 2 5% 4 4 5% 5% 8% 9% 47%	6930
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 5% 7% 8% 45%	6856
	21st to end	Day	6% 3 1 4 3 4 4 4 6% 8% 56%	7376
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 5% 6% 7% 49%	4571
	11th to 20th	Day	5% 4 2 5% 4 4 5% 3 6% 10% 52%	4158
	21st to end	Day	7% 4 2 4 3 3 4 4 6% 9% 53%	4664
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 5% 7% 50%	2445
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 45%	2302
	21st to end	Day	8% 5% 2 5% 3 3 4 3 5% 8% 53%	2172
October	1st to 10th	Day	6% 6% 2 6% 5% 5% 4 3 6% 8% 50%	3270
	11th to 20th	Day	4 5% 2 7% 5% 5% 5% 4 7% 10% 47%	3250
	21st to end	Day	5% 6% 3 8% 5% 4 5% 5% 7% 8% 44%	3057
November	1st to 10th	Day	9% 13% 5% 11% 7% 6% 5% 4 6% 6% 27%	1648
	11th to 20th	Day	6% 10% 5% 11% 6% 5% 5% 4 7% 8% 33%	1680
	21st to end	Day	6% 12% 5% 12% 7% 4 6% 5% 6% 8% 30%	1580
December	1st to 10th	Day	6% 14% 4 14% 7% 6% 6% 4 7% 7% 25%	2360
	11th to 20th	Day	6% 13% 5% 13% 7% 8% 8% 4 5% 7% 23%	2712
	21st to end	Day	3 7% 3 12% 7% 6% 5% 4 5% 8% 41%	3575
January	1st to 10th	Night	4 8% 5% 14% 10% 8% 7% 5% 7% 8% 24%	8768
	11th to 20th	Night	2 7% 5% 12% 8% 7% 7% 6% 7% 8% 32%	8779
	21st to end	Night	3 9% 4 11% 8% 8% 7% 4 7% 9% 31%	9548
February	1st to 10th	Night	6% 10% 4 11% 7% 6% 5% 6% 6% 8% 29%	2767
	11th to 20th	Night	5% 10% 4 11% 7% 7% 6% 5% 7% 8% 29%	2662
	21st to end	Night	3 10% 3 9% 7% 6% 7% 5% 8% 9% 32%	2249
March	1st to 10th	Night	4 8% 4 12% 8% 6% 6% 5% 7% 10% 31%	5363
	11th to 20th	Night	4 9% 4 9% 7% 6% 6% 5% 8% 10% 33%	5110
	21st to end	Night	8% 13% 6% 16% 7% 6% 5% 4 6% 7% 23%	5597
April	1st to 10th	Night	7% 11% 3 7% 5% 6% 6% 4 7% 9% 35%	2050
	11th to 20th	Night	9% 11% 4 9% 6% 5% 5% 5% 7% 7% 31%	1871
	21st to end	Night	12% 8% 4 6% 6% 5% 5% 5% 6% 7% 36%	1853
May	1st to 10th	Night	12% 9% 3 5% 4 4 4 3 5% 7% 43%	4724
	11th to 20th	Night	11% 7% 3 8% 6% 6% 6% 6% 8% 9% 32%	4387
	21st to end	Night	10% 5% 2 6% 4 5% 6% 4 7% 9% 41%	4522
June	1st to 10th	Night	13% 9% 3 7% 6% 5% 5% 6% 7% 8% 33%	1470
	11th to 20th	Night	14% 6% 3 7% 5% 4 5% 5% 6% 9% 36%	1470
	21st to end	Night	8% 5% 1 6% 4 4 5% 5% 6% 9% 46%	1470
July	1st to 10th	Night	5% 4 2 5% 4 5% 5% 4 6% 9% 52%	3250
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 4 7% 10% 46%	3215
	21st to end	Night	7% 3 1 4 3 3 3 4 5% 8% 57%	3467
August	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 48%	2570
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 6% 10% 51%	2901
	21st to end	Night	9% 3 1 4 4 4 4 4 6% 8% 53%	3245
September	1st to 10th	Night	9% 4 2 6% 4 4 3 3 5% 7% 53%	1955
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 46%	2095
	21st to end	Night	7% 6% 2 5% 3 4 4 4 7% 8% 50%	2228
October	1st to 10th	Night	6% 7% 2 6% 5% 4 4 4 6% 8% 47%	3920
	11th to 20th	Night	4 5% 2 5% 4 4 4 4 7% 8% 51%	3898
	21st to end	Night	6% 8% 3 8% 6% 5% 6% 4 7% 9% 40%	4711
November	1st to 10th	Night	10% 14% 4 9% 5% 5% 4 4 5% 8% 33%	2870
	11th to 20th	Night	6% 10% 4 9% 7% 5% 5% 4 7% 9% 35%	2940
	21st to end	Night	5% 12% 5% 11% 7% 5% 6% 5% 5% 8% 30%	3039
December	1st to 10th	Night	9% 13% 5% 13% 8% 7% 6% 4 6% 7% 22%	5040
	11th to 20th	Night	6% 13% 6% 13% 8% 6% 6% 5% 7% 8% 22%	5700
	21st to end	Night	3 6% 3 9% 8% 5% 5% 4 6% 7% 43%	7623

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	58% 6% 12% 16% 7%	4125
	11th to 20th	Day	45% 7% 15% 19% 14% 1	4140
	21st to end	Day	45% 8% 20% 20% 6%	4689
February	1st to 10th	Day	55% 6% 12% 17% 9%	1853
	11th to 20th	Day	52% 8% 13% 16% 11%	1854
	21st to end	Day	53% 7% 15% 17% 8%	1557
March	1st to 10th	Day	46% 7% 20% 21% 6%	4742
	11th to 20th	Day	49% 7% 18% 17% 9%	5110
	21st to end	Day	56% 9% 15% 16% 4	5607
April	1st to 10th	Day	47% 7% 16% 21% 9% 1	2567
	11th to 20th	Day	48% 7% 15% 21% 7% 1	2700
	21st to end	Day	46% 8% 13% 23% 9% 1	2765
May	1st to 10th	Day	40% 7% 12% 24% 14% 4	8224
	11th to 20th	Day	46% 9% 12% 22% 10%	8476
	21st to end	Day	36% 9% 14% 29% 11% 2	9650
June	1st to 10th	Day	43% 10% 14% 22% 10% 1	3150
	11th to 20th	Day	49% 9% 13% 20% 8% 1	3150
	21st to end	Day	36% 11% 21% 23% 8% 1	3150
July	1st to 10th	Day	28% 10% 21% 28% 13% 1	6856
	11th to 20th	Day	34% 8% 16% 29% 12%	6775
	21st to end	Day	23% 8% 20% 30% 16% 4	7169
August	1st to 10th	Day	32% 8% 17% 33% 9% 1	4553
	11th to 20th	Day	26% 7% 19% 38% 9% 1	4197
	21st to end	Day	26% 6% 18% 35% 14% 1	4663
September	1st to 10th	Day	34% 6% 14% 27% 13% 6%	2445
	11th to 20th	Day	39% 6% 15% 29% 9% 2	2302
	21st to end	Day	29% 6% 16% 33% 14% 2	2172
October	1st to 10th	Day	33% 5% 17% 35% 10%	3270
	11th to 20th	Day	31% 6% 19% 32% 11%	3250
	21st to end	Day	35% 7% 14% 36% 8%	3057
November	1st to 10th	Day	56% 6% 12% 17% 8% 1	1648
	11th to 20th	Day	49% 9% 17% 20% 5%	1680
	21st to end	Day	52% 8% 17% 19% 5%	1580
December	1st to 10th	Day	57% 10% 18% 11% 4 1	2360
	11th to 20th	Day	60% 7% 17% 13% 3	2714
	21st to end	Day	42% 5% 10% 26% 15% 2	3575
January	1st to 10th	Night	55% 8% 14% 14% 9%	8769
	11th to 20th	Night	47% 6% 17% 15% 13% 1	8820
	21st to end	Night	47% 8% 17% 21% 7%	9548
February	1st to 10th	Night	50% 7% 14% 17% 11% 1	2767
	11th to 20th	Night	50% 8% 13% 17% 12% 1	2663
	21st to end	Night	43% 8% 17% 22% 9% 1	2249
March	1st to 10th	Night	45% 7% 22% 19% 7%	5363
	11th to 20th	Night	41% 9% 20% 24% 5%	5110
	21st to end	Night	56% 10% 11% 17% 6% 1	5597
April	1st to 10th	Night	43% 8% 12% 23% 12% 1	2050
	11th to 20th	Night	49% 7% 12% 20% 11% 1	1871
	21st to end	Night	44% 8% 12% 23% 12% 1	1853
May	1st to 10th	Night	39% 6% 8% 26% 19% 3	4724
	11th to 20th	Night	42% 10% 13% 21% 12% 1	4387
	21st to end	Night	33% 10% 17% 28% 11% 1	4526
June	1st to 10th	Night	45% 9% 14% 20% 11% 2	1470
	11th to 20th	Night	42% 10% 16% 20% 11% 1	1470
	21st to end	Night	31% 10% 20% 28% 9% 1	1470
July	1st to 10th	Night	26% 9% 22% 27% 15% 1	3215
	11th to 20th	Night	32% 8% 16% 32% 12% 1	3173
	21st to end	Night	24% 7% 17% 31% 18% 3	3369
August	1st to 10th	Night	32% 8% 16% 32% 11% 1	2563
	11th to 20th	Night	27% 6% 17% 39% 9% 1	2928
	21st to end	Night	28% 6% 16% 33% 16% 2	3245
September	1st to 10th	Night	30% 7% 14% 25% 16% 8%	1955
	11th to 20th	Night	36% 7% 13% 28% 14% 2	2095
	21st to end	Night	28% 7% 15% 29% 18% 2	2228
October	1st to 10th	Night	34% 6% 19% 27% 13% 1	3920
	11th to 20th	Night	29% 6% 16% 35% 14% 1	3898
	21st to end	Night	40% 6% 14% 34% 5%	4711
November	1st to 10th	Night	51% 7% 14% 20% 8% 1	2870
	11th to 20th	Night	45% 8% 19% 20% 7%	2940
	21st to end	Night	51% 7% 17% 18% 7%	3039
December	1st to 10th	Night	60% 7% 16% 11% 5%	5040
	11th to 20th	Night	57% 9% 19% 12% 3	5697
	21st to end	Night	39% 6% 13% 23% 16% 2	7623

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	9% 8% 30% 30% 18% 5%	220
	11th to 20th	Day	7% 10% 31% 32% 17% 3	220
	21st to end	Day	10% 10% 36% 30% 11% 3	242
February	1st to 10th	Day	16% 10% 31% 30% 11% 2	311
	11th to 20th	Day	9% 6% 28% 37% 17% 4	330
	21st to end	Day	5% 7% 34% 33% 15% 6%	273
March	1st to 10th	Day	8% 14% 32% 32% 14% 2	472
	11th to 20th	Day	8% 14% 38% 26% 11% 3	540
	21st to end	Day	12% 9% 33% 32% 11% 2	594
April	1st to 10th	Day	10% 12% 36% 28% 11% 3	540
	11th to 20th	Day	14% 14% 30% 30% 10% 2	532
	21st to end	Day	15% 10% 29% 27% 16% 4	575
May	1st to 10th	Day	15% 8% 23% 34% 14% 5%	753
	11th to 20th	Day	12% 9% 26% 31% 18% 4	775
	21st to end	Day	15% 7% 22% 28% 21% 8%	844
June	1st to 10th	Day	14% 12% 27% 28% 15% 4	780
	11th to 20th	Day	15% 12% 31% 26% 13% 3	780
	21st to end	Day	11% 7% 23% 31% 22% 6%	780
July	1st to 10th	Day	9% 4 19% 38% 23% 6%	771
	11th to 20th	Day	12% 5% 25% 29% 21% 7%	744
	21st to end	Day	6% 5% 23% 30% 27% 10%	794
August	1st to 10th	Day	8% 7% 19% 36% 23% 8%	685
	11th to 20th	Day	8% 7% 22% 32% 25% 7%	517
	21st to end	Day	8% 7% 23% 32% 22% 8%	583
September	1st to 10th	Day	12% 9% 28% 28% 18% 5%	530
	11th to 20th	Day	15% 9% 21% 32% 18% 4	530
	21st to end	Day	12% 11% 33% 28% 13% 3	502
October	1st to 10th	Day	16% 6% 29% 27% 18% 4	330
	11th to 20th	Day	8% 12% 35% 29% 13% 3	330
	21st to end	Day	9% 13% 39% 25% 11% 1	363
November	1st to 10th	Day	14% 11% 34% 29% 11% 1	330
	11th to 20th	Day	10% 10% 40% 27% 11% 1	330
	21st to end	Day	8% 9% 33% 31% 13% 5%	278
December	1st to 10th	Day	7% 9% 35% 37% 10% 2	220
	11th to 20th	Day	10% 14% 37% 33% 7%	220
	21st to end	Day	11% 6% 29% 34% 17% 4	242
January	1st to 10th	Night	8% 9% 36% 34% 13% 1	1050
	11th to 20th	Night	8% 10% 37% 33% 11% 1	1050
	21st to end	Night	9% 12% 36% 31% 10% 1	1118
February	1st to 10th	Night	9% 9% 27% 38% 15% 2	840
	11th to 20th	Night	9% 7% 32% 37% 13% 1	824
	21st to end	Night	7% 5% 26% 36% 22% 4	692
March	1st to 10th	Night	9% 6% 32% 38% 12% 2	840
	11th to 20th	Night	8% 7% 35% 36% 12% 2	840
	21st to end	Night	14% 10% 28% 32% 14% 2	923
April	1st to 10th	Night	10% 8% 37% 32% 11% 2	630
	11th to 20th	Night	15% 8% 35% 31% 11% 1	623
	21st to end	Night	15% 8% 25% 37% 14% 1	630
May	1st to 10th	Night	14% 7% 25% 39% 15% 1	630
	11th to 20th	Night	10% 7% 27% 38% 18% 1	516
	21st to end	Night	15% 6% 21% 35% 21% 2	455
June	1st to 10th	Night	13% 9% 31% 29% 16% 2	420
	11th to 20th	Night	18% 6% 26% 34% 15% 1	420
	21st to end	Night	11% 5% 22% 39% 22% 1	420
July	1st to 10th	Night	9% 5% 23% 41% 21%	410
	11th to 20th	Night	9% 3 27% 41% 19% 1	398
	21st to end	Night	3 5% 25% 41% 24% 3	438
August	1st to 10th	Night	8% 4 23% 36% 24% 4	532
	11th to 20th	Night	8% 5% 20% 43% 22% 2	594
	21st to end	Night	8% 5% 25% 39% 22% 2	660
September	1st to 10th	Night	11% 4 24% 43% 17% 2	600
	11th to 20th	Night	12% 5% 27% 36% 19% 2	695
	21st to end	Night	9% 6% 34% 37% 13% 2	800
October	1st to 10th	Night	13% 7% 29% 35% 15% 1	800
	11th to 20th	Night	10% 7% 33% 36% 13% 2	800
	21st to end	Night	12% 11% 36% 32% 10%	970
November	1st to 10th	Night	16% 8% 36% 31% 7% 1	1025
	11th to 20th	Night	11% 10% 35% 32% 10% 2	1050
	21st to end	Night	9% 8% 35% 37% 9% 1	1050
December	1st to 10th	Night	12% 8% 32% 33% 13% 2	1050
	11th to 20th	Night	10% 9% 37% 33% 10% 1	1050
	21st to end	Night	11% 5% 33% 35% 14% 2	1155

Page generated at 2026-07-13 01:14:49. Version ac07

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