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 2024-02-20.

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	9% 8% 3 7% 6% 4 5% 6% 6% 8% 38%	77752
	Night	7% 8% 4 7% 7% 4 6% 6% 6% 8% 37%	81398
Transparency	Day	43% 8% 16% 23% 9% 1	77454
	Night	42% 8% 16% 22% 10% 1	81276
Seeing	Day	12% 8% 28% 30% 17% 5%	14532
	Night	11% 7% 31% 35% 14% 2	23967

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% 11% 6% 11% 8% 5% 5% 5% 7% 8% 29%	2219
	11th to 20th	Day	3 8% 5% 10% 9% 4 6% 12% 8% 10% 25%	2199
	21st to end	Day	4 11% 4 8% 7% 4 7% 6% 6% 8% 35%	2516
February	1st to 10th	Day	8% 13% 6% 10% 9% 5% 4 6% 5% 7% 26%	1676
	11th to 20th	Day	8% 11% 6% 9% 8% 5% 7% 6% 6% 7% 27%	1656
	21st to end	Day	6% 10% 5% 11% 10% 5% 7% 7% 6% 9% 25%	1332
March	1st to 10th	Day	7% 10% 4 11% 10% 5% 7% 5% 6% 8% 27%	1844
	11th to 20th	Day	6% 11% 6% 8% 7% 4 6% 5% 5% 9% 33%	1979
	21st to end	Day	9% 11% 6% 10% 8% 3 5% 5% 5% 7% 30%	2178
April	1st to 10th	Day	7% 9% 4 10% 6% 6% 7% 6% 5% 7% 33%	2200
	11th to 20th	Day	10% 11% 4 8% 7% 4 6% 6% 6% 7% 31%	2310
	21st to end	Day	12% 9% 3 7% 7% 3 6% 5% 5% 9% 34%	2369
May	1st to 10th	Day	14% 9% 3 8% 8% 4 6% 6% 6% 6% 32%	2520
	11th to 20th	Day	8% 6% 3 6% 5% 3 6% 6% 7% 8% 42%	2564
	21st to end	Day	17% 8% 3 6% 6% 3 5% 5% 5% 8% 33%	2917
June	1st to 10th	Day	16% 7% 2 6% 5% 3 5% 5% 6% 9% 35%	2700
	11th to 20th	Day	18% 8% 3 6% 6% 3 5% 5% 5% 9% 30%	2700
	21st to end	Day	8% 5% 2 5% 7% 4 6% 6% 7% 9% 41%	2700
July	1st to 10th	Day	6% 5% 2 5% 4 3 5% 7% 8% 10% 45%	2700
	11th to 20th	Day	9% 3 1 3 4 3 6% 7% 7% 9% 46%	2700
	21st to end	Day	4 3 1 3 4 3 4 5% 6% 9% 57%	2938
August	1st to 10th	Day	8% 4 2 4 5% 3 5% 6% 6% 8% 48%	2529
	11th to 20th	Day	8% 5% 2 5% 5% 3 4 5% 6% 9% 48%	2299
	21st to end	Day	8% 4 2 4 4 2 4 4 7% 10% 49%	2574
September	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 4 5% 7% 48%	2201
	11th to 20th	Day	14% 6% 2 5% 6% 3 4 4 4 6% 45%	2072
	21st to end	Day	9% 5% 2 5% 4 3 3 4 5% 9% 51%	1956
October	1st to 10th	Day	12% 9% 3 5% 5% 2 4 4 5% 7% 45%	1800
	11th to 20th	Day	7% 6% 4 8% 6% 3 4 6% 5% 7% 44%	1800
	21st to end	Day	6% 9% 4 8% 8% 4 5% 6% 7% 9% 34%	1712
November	1st to 10th	Day	10% 12% 6% 10% 8% 4 5% 6% 5% 6% 27%	1488
	11th to 20th	Day	6% 10% 5% 10% 7% 4 6% 4 7% 8% 34%	1520
	21st to end	Day	6% 9% 5% 11% 8% 3 6% 6% 5% 8% 31%	1430
December	1st to 10th	Day	7% 9% 4 12% 8% 5% 8% 6% 9% 6% 26%	1330
	11th to 20th	Day	4 10% 5% 11% 9% 6% 7% 6% 6% 7% 30%	1682
	21st to end	Day	4 8% 4 7% 6% 3 5% 5% 6% 8% 44%	2442
January	1st to 10th	Night	3 7% 5% 10% 10% 6% 7% 7% 8% 9% 27%	4726
	11th to 20th	Night	2 7% 5% 10% 11% 6% 8% 10% 7% 7% 27%	4699
	21st to end	Night	4 9% 4 8% 7% 4 6% 6% 7% 9% 36%	5168
February	1st to 10th	Night	7% 9% 5% 10% 9% 5% 5% 7% 6% 8% 29%	2504
	11th to 20th	Night	6% 10% 6% 9% 9% 5% 8% 7% 7% 7% 27%	2395
	21st to end	Night	5% 8% 4 8% 9% 5% 7% 7% 8% 10% 31%	1924
March	1st to 10th	Night	6% 8% 4 10% 9% 5% 6% 7% 6% 8% 31%	2113
	11th to 20th	Night	7% 8% 5% 8% 6% 3 6% 7% 7% 8% 36%	1980
	21st to end	Night	9% 11% 4 8% 9% 4 6% 5% 7% 9% 29%	2178
April	1st to 10th	Night	7% 9% 4 6% 6% 4 6% 6% 7% 10% 36%	1758
	11th to 20th	Night	8% 10% 4 8% 8% 5% 5% 6% 7% 7% 31%	1601
	21st to end	Night	12% 8% 3 5% 6% 3 6% 7% 6% 7% 35%	1589
May	1st to 10th	Night	12% 7% 3 5% 5% 4 6% 5% 7% 9% 38%	1440
	11th to 20th	Night	6% 5% 3 5% 6% 3 5% 6% 7% 8% 48%	1327
	21st to end	Night	15% 6% 4 6% 5% 3 4 6% 6% 7% 38%	1358
June	1st to 10th	Night	13% 8% 3 6% 6% 4 5% 7% 7% 8% 33%	1260
	11th to 20th	Night	16% 6% 3 5% 6% 3 4 5% 7% 8% 37%	1260
	21st to end	Night	6% 4 2 5% 5% 3 5% 6% 7% 10% 48%	1260
July	1st to 10th	Night	7% 5% 2 4 4 3 4 4 6% 10% 50%	1260
	11th to 20th	Night	7% 5% 2 4 3 3 4 5% 7% 11% 49%	1260
	21st to end	Night	2 2 1 4 4 2 3 4 6% 8% 65%	1379
August	1st to 10th	Night	6% 3 2 5% 5% 3 5% 5% 8% 10% 49%	1408
	11th to 20th	Night	8% 3 3 4 5% 3 4 5% 5% 10% 49%	1602
	21st to end	Night	7% 4 1 4 5% 3 4 5% 6% 8% 53%	1782
September	1st to 10th	Night	10% 4 2 6% 5% 3 4 4 6% 7% 50%	1759
	11th to 20th	Night	10% 6% 2 5% 5% 3 3 4 5% 8% 47%	1885
	21st to end	Night	7% 6% 2 4 4 3 5% 5% 7% 8% 49%	2004
October	1st to 10th	Night	10% 8% 3 6% 5% 3 4 6% 6% 7% 40%	2160
	11th to 20th	Night	6% 7% 3 7% 6% 4 4 5% 6% 7% 45%	2160
	21st to end	Night	7% 9% 4 8% 7% 3 6% 6% 6% 8% 36%	2626
November	1st to 10th	Night	10% 13% 5% 7% 6% 4 5% 5% 5% 8% 32%	2590
	11th to 20th	Night	6% 9% 4 8% 9% 4 6% 5% 6% 9% 35%	2660
	21st to end	Night	6% 11% 6% 9% 8% 4 6% 6% 5% 9% 32%	2749
December	1st to 10th	Night	6% 11% 5% 10% 10% 6% 7% 5% 6% 8% 27%	2850
	11th to 20th	Night	6% 10% 5% 10% 9% 5% 6% 5% 5% 8% 30%	3510
	21st to end	Night	3 6% 3 6% 6% 3 5% 6% 6% 7% 49%	5214

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	51% 8% 14% 17% 8% 1	2219
	11th to 20th	Day	51% 8% 14% 16% 9% 1	2220
	21st to end	Day	43% 10% 22% 20% 5%	2516
February	1st to 10th	Day	57% 6% 12% 18% 7% 1	1676
	11th to 20th	Day	53% 9% 13% 15% 10%	1665
	21st to end	Day	55% 6% 14% 16% 9%	1332
March	1st to 10th	Day	53% 10% 16% 16% 4	1844
	11th to 20th	Day	48% 8% 16% 20% 8% 1	1980
	21st to end	Day	53% 9% 18% 15% 5%	2178
April	1st to 10th	Day	48% 7% 16% 20% 9% 1	2200
	11th to 20th	Day	50% 7% 15% 20% 7% 1	2310
	21st to end	Day	47% 8% 14% 22% 7% 1	2369
May	1st to 10th	Day	48% 8% 14% 22% 8% 1	2520
	11th to 20th	Day	37% 9% 14% 27% 12% 1	2568
	21st to end	Day	48% 7% 13% 21% 10%	2918
June	1st to 10th	Day	44% 9% 14% 23% 9% 1	2700
	11th to 20th	Day	50% 9% 14% 19% 7% 1	2700
	21st to end	Day	36% 11% 21% 23% 8% 1	2700
July	1st to 10th	Day	29% 11% 23% 29% 8% 1	2626
	11th to 20th	Day	31% 10% 20% 28% 10% 1	2619
	21st to end	Day	22% 8% 21% 34% 14% 1	2775
August	1st to 10th	Day	31% 9% 22% 25% 11% 2	2511
	11th to 20th	Day	32% 8% 21% 28% 10% 1	2299
	21st to end	Day	28% 8% 16% 32% 15% 1	2574
September	1st to 10th	Day	35% 6% 15% 25% 13% 6%	2201
	11th to 20th	Day	40% 6% 14% 28% 10% 2	2072
	21st to end	Day	31% 6% 16% 33% 13% 2	1956
October	1st to 10th	Day	40% 5% 18% 27% 10%	1800
	11th to 20th	Day	38% 7% 18% 26% 11%	1800
	21st to end	Day	44% 10% 15% 22% 8% 1	1712
November	1st to 10th	Day	56% 7% 12% 16% 7% 1	1488
	11th to 20th	Day	49% 8% 17% 20% 6%	1520
	21st to end	Day	50% 8% 17% 20% 4	1430
December	1st to 10th	Day	55% 9% 15% 14% 6% 1	1330
	11th to 20th	Day	53% 7% 17% 18% 5%	1684
	21st to end	Day	37% 6% 12% 26% 16% 3	2442
January	1st to 10th	Night	49% 9% 18% 19% 4 1	4726
	11th to 20th	Night	53% 7% 14% 15% 10% 1	4740
	21st to end	Night	42% 10% 17% 25% 5%	5168
February	1st to 10th	Night	51% 7% 14% 17% 10%	2504
	11th to 20th	Night	52% 8% 14% 16% 10%	2396
	21st to end	Night	44% 8% 16% 21% 9% 1	1924
March	1st to 10th	Night	49% 8% 18% 18% 7%	2113
	11th to 20th	Night	43% 8% 16% 23% 9% 1	1980
	21st to end	Night	50% 9% 16% 17% 7% 1	2178
April	1st to 10th	Night	41% 9% 12% 23% 14% 1	1758
	11th to 20th	Night	50% 7% 11% 21% 11% 1	1601
	21st to end	Night	44% 9% 12% 22% 11% 2	1589
May	1st to 10th	Night	41% 8% 11% 25% 15% 1	1440
	11th to 20th	Night	31% 8% 12% 30% 18% 1	1327
	21st to end	Night	42% 8% 15% 22% 13% 1	1358
June	1st to 10th	Night	45% 9% 15% 22% 8% 1	1260
	11th to 20th	Night	42% 10% 16% 18% 12% 1	1260
	21st to end	Night	29% 12% 21% 28% 10% 1	1260
July	1st to 10th	Night	27% 9% 23% 29% 11% 1	1225
	11th to 20th	Night	28% 10% 21% 28% 12% 2	1218
	21st to end	Night	16% 7% 20% 37% 17% 3	1302
August	1st to 10th	Night	27% 9% 22% 30% 11% 1	1401
	11th to 20th	Night	31% 7% 18% 29% 13% 2	1602
	21st to end	Night	27% 6% 16% 30% 19% 3	1782
September	1st to 10th	Night	32% 8% 15% 22% 15% 8%	1759
	11th to 20th	Night	36% 7% 12% 28% 15% 3	1885
	21st to end	Night	29% 8% 15% 28% 17% 2	2004
October	1st to 10th	Night	40% 7% 15% 24% 12% 1	2160
	11th to 20th	Night	37% 6% 14% 30% 12% 1	2160
	21st to end	Night	45% 7% 15% 24% 7% 1	2626
November	1st to 10th	Night	51% 7% 14% 19% 9% 1	2590
	11th to 20th	Night	45% 9% 19% 20% 7%	2660
	21st to end	Night	49% 8% 18% 18% 7%	2749
December	1st to 10th	Night	54% 7% 14% 17% 8%	2850
	11th to 20th	Night	51% 9% 20% 14% 6%	3507
	21st to end	Night	33% 7% 15% 23% 19% 4	5214

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	8% 8% 29% 30% 19% 6%	180
	11th to 20th	Day	7% 8% 30% 33% 18% 3	180
	21st to end	Day	11% 10% 33% 30% 13% 4	198
February	1st to 10th	Day	18% 10% 32% 29% 10% 1	254
	11th to 20th	Day	10% 6% 31% 35% 14% 4	267
	21st to end	Day	6% 5% 32% 34% 16% 8%	198
March	1st to 10th	Day	8% 14% 34% 30% 12% 1	361
	11th to 20th	Day	10% 13% 35% 27% 10% 4	420
	21st to end	Day	12% 8% 34% 31% 12% 2	462
April	1st to 10th	Day	10% 11% 38% 27% 11% 3	420
	11th to 20th	Day	13% 11% 34% 30% 10% 2	412
	21st to end	Day	15% 11% 30% 28% 15% 2	449
May	1st to 10th	Day	16% 7% 21% 34% 16% 5%	603
	11th to 20th	Day	11% 7% 24% 34% 19% 5%	625
	21st to end	Day	16% 7% 22% 27% 22% 7%	679
June	1st to 10th	Day	13% 9% 27% 30% 17% 4	630
	11th to 20th	Day	17% 13% 30% 24% 13% 3	630
	21st to end	Day	10% 6% 24% 31% 22% 7%	630
July	1st to 10th	Day	9% 5% 20% 39% 21% 7%	621
	11th to 20th	Day	12% 5% 26% 31% 21% 6%	618
	21st to end	Day	6% 4 25% 29% 27% 9%	684
August	1st to 10th	Day	8% 6% 19% 36% 23% 8%	590
	11th to 20th	Day	8% 7% 20% 31% 26% 7%	437
	21st to end	Day	8% 7% 22% 32% 23% 9%	495
September	1st to 10th	Day	13% 8% 30% 29% 15% 4	450
	11th to 20th	Day	16% 10% 21% 31% 18% 4	450
	21st to end	Day	13% 10% 33% 27% 14% 4	426
October	1st to 10th	Day	19% 6% 29% 25% 17% 4	270
	11th to 20th	Day	8% 13% 34% 30% 11% 4	270
	21st to end	Day	9% 13% 38% 27% 11% 2	297
November	1st to 10th	Day	16% 11% 30% 28% 13% 1	270
	11th to 20th	Day	11% 10% 40% 27% 10% 1	270
	21st to end	Day	7% 10% 34% 32% 12% 5%	228
December	1st to 10th	Day	7% 7% 37% 34% 12% 3	180
	11th to 20th	Day	8% 14% 38% 33% 7%	180
	21st to end	Day	11% 7% 29% 33% 16% 4	198
January	1st to 10th	Night	8% 9% 35% 33% 13% 1	950
	11th to 20th	Night	8% 9% 36% 34% 12% 1	950
	21st to end	Night	10% 11% 36% 31% 10% 1	1012
February	1st to 10th	Night	9% 9% 26% 37% 16% 2	760
	11th to 20th	Night	10% 8% 34% 35% 12% 1	742
	21st to end	Night	8% 5% 24% 35% 24% 4	592
March	1st to 10th	Night	10% 5% 33% 38% 13% 2	720
	11th to 20th	Night	9% 6% 35% 36% 12% 2	720
	21st to end	Night	14% 9% 27% 33% 15% 3	792
April	1st to 10th	Night	9% 7% 37% 32% 12% 2	540
	11th to 20th	Night	14% 8% 36% 31% 11% 1	533
	21st to end	Night	15% 8% 26% 37% 13% 1	540
May	1st to 10th	Night	14% 6% 25% 37% 16% 1	540
	11th to 20th	Night	9% 5% 25% 40% 21% 1	442
	21st to end	Night	16% 6% 21% 34% 21% 3	389
June	1st to 10th	Night	14% 8% 29% 30% 17% 2	360
	11th to 20th	Night	20% 6% 23% 34% 17% 1	360
	21st to end	Night	7% 4 23% 40% 24% 2	360
July	1st to 10th	Night	9% 5% 23% 41% 20% 1	350
	11th to 20th	Night	9% 3 28% 42% 18% 1	348
	21st to end	Night	2 5% 25% 41% 24% 3	394
August	1st to 10th	Night	8% 3 23% 38% 24% 5%	478
	11th to 20th	Night	9% 5% 19% 43% 22% 2	534
	21st to end	Night	8% 4 25% 39% 23% 2	594
September	1st to 10th	Night	12% 4 25% 41% 15% 2	540
	11th to 20th	Night	12% 5% 28% 36% 16% 2	625
	21st to end	Night	9% 5% 35% 36% 13% 3	720
October	1st to 10th	Night	14% 7% 29% 34% 16% 1	720
	11th to 20th	Night	10% 6% 33% 36% 13% 2	720
	21st to end	Night	12% 10% 36% 31% 10%	872
November	1st to 10th	Night	17% 8% 36% 31% 8% 1	925
	11th to 20th	Night	10% 10% 35% 32% 11% 3	950
	21st to end	Night	9% 9% 36% 36% 9% 1	950
December	1st to 10th	Night	12% 7% 32% 33% 14% 3	950
	11th to 20th	Night	10% 9% 37% 33% 11% 1	950
	21st to end	Night	11% 5% 33% 34% 14% 2	1045

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