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 2024-10-20.

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	8% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	101734
	Night	6% 8% 3 9% 6% 5% 5% 5% 6% 8% 37%	97018
Transparency	Day	41% 8% 16% 24% 10% 1	101475
	Night	42% 8% 16% 23% 10% 1	96923
Seeing	Day	11% 9% 28% 30% 17% 5%	15571
	Night	11% 7% 31% 35% 14% 2	24689

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% 13% 6% 6% 5% 4 7% 7% 29%	2219
	11th to 20th	Day	3 8% 5% 11% 8% 6% 7% 10% 8% 9% 25%	2199
	21st to end	Day	4 12% 4 9% 6% 6% 7% 4 6% 8% 35%	2516
February	1st to 10th	Day	8% 15% 5% 11% 8% 6% 4 5% 5% 7% 26%	1676
	11th to 20th	Day	7% 12% 5% 10% 7% 6% 7% 4 6% 7% 27%	1665
	21st to end	Day	6% 11% 4 13% 9% 7% 6% 5% 6% 9% 24%	1413
March	1st to 10th	Day	5% 11% 4 11% 7% 7% 7% 5% 6% 6% 30%	3223
	11th to 20th	Day	4 12% 5% 9% 5% 4 5% 4 5% 11% 37%	3489
	21st to end	Day	7% 12% 5% 15% 7% 5% 6% 4 5% 7% 28%	3825
April	1st to 10th	Day	7% 10% 3 10% 6% 7% 6% 5% 6% 7% 33%	2323
	11th to 20th	Day	9% 13% 4 9% 6% 5% 6% 4 6% 8% 31%	2440
	21st to end	Day	12% 10% 3 9% 6% 5% 6% 3 6% 8% 33%	2501
May	1st to 10th	Day	17% 12% 4 8% 5% 5% 5% 4 6% 5% 29%	4430
	11th to 20th	Day	8% 8% 3 8% 6% 6% 7% 6% 7% 8% 33%	4539
	21st to end	Day	13% 8% 3 9% 6% 6% 5% 5% 7% 8% 29%	5161
June	1st to 10th	Day	15% 7% 2 7% 5% 4 5% 4 7% 8% 35%	2850
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 5% 6% 8% 30%	2850
	21st to end	Day	8% 5% 2 6% 6% 6% 5% 5% 8% 8% 41%	2850
July	1st to 10th	Day	5% 5% 2 6% 4 5% 5% 5% 8% 9% 47%	4740
	11th to 20th	Day	7% 4 2 5% 4 4 5% 5% 7% 8% 50%	4740
	21st to end	Day	4 3 1 3 3 3 3 3 6% 8% 63%	5138
August	1st to 10th	Day	7% 5% 2 6% 4 4 5% 5% 6% 7% 49%	4430
	11th to 20th	Day	5% 4 2 5% 5% 4 5% 3 6% 10% 52%	4028
	21st to end	Day	7% 4 2 4 3 3 4 4 7% 9% 53%	4521
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 7% 49%	2323
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 3 4 6% 44%	2187
	21st to end	Day	8% 5% 2 6% 3 3 4 3 5% 8% 52%	2064
October	1st to 10th	Day	11% 9% 3 6% 4 4 3 3 5% 7% 45%	1900
	11th to 20th	Day	6% 6% 3 8% 6% 5% 5% 5% 6% 7% 44%	1890
	21st to end	Day	6% 9% 4 10% 7% 5% 5% 5% 7% 9% 34%	1712
November	1st to 10th	Day	10% 14% 5% 11% 7% 6% 5% 4 6% 6% 27%	1488
	11th to 20th	Day	6% 11% 4 12% 6% 5% 5% 4 7% 8% 34%	1520
	21st to end	Day	6% 11% 5% 11% 7% 4 6% 5% 6% 8% 31%	1430
December	1st to 10th	Day	6% 11% 3 13% 7% 7% 7% 5% 9% 6% 25%	1330
	11th to 20th	Day	4 11% 4 13% 7% 7% 7% 4 6% 7% 30%	1682
	21st to end	Day	4 8% 4 7% 5% 5% 5% 4 6% 8% 44%	2442
January	1st to 10th	Night	3 8% 4 12% 8% 8% 7% 5% 8% 9% 27%	4726
	11th to 20th	Night	2 7% 4 12% 9% 8% 8% 8% 7% 7% 27%	4699
	21st to end	Night	4 10% 4 9% 6% 6% 6% 5% 7% 8% 36%	5168
February	1st to 10th	Night	6% 11% 4 11% 8% 6% 5% 6% 6% 8% 29%	2504
	11th to 20th	Night	5% 10% 5% 11% 7% 7% 6% 5% 7% 7% 28%	2402
	21st to end	Night	4 10% 3 9% 7% 6% 7% 5% 8% 9% 32%	2041
March	1st to 10th	Night	5% 9% 4 12% 9% 7% 6% 5% 7% 9% 27%	3645
	11th to 20th	Night	4 8% 4 8% 5% 5% 6% 5% 8% 9% 38%	3490
	21st to end	Night	6% 12% 6% 16% 8% 6% 5% 5% 6% 7% 24%	3815
April	1st to 10th	Night	7% 11% 3 7% 5% 5% 6% 4 7% 9% 35%	1855
	11th to 20th	Night	8% 12% 3 10% 7% 6% 5% 5% 7% 7% 31%	1691
	21st to end	Night	12% 8% 3 6% 6% 6% 5% 6% 6% 7% 35%	1677
May	1st to 10th	Night	20% 13% 3 6% 4 4 4 3 5% 7% 31%	2540
	11th to 20th	Night	7% 6% 3 7% 6% 5% 5% 6% 9% 10% 36%	2343
	21st to end	Night	13% 7% 3 8% 5% 5% 6% 5% 7% 8% 34%	2414
June	1st to 10th	Night	13% 8% 3 7% 6% 5% 4 6% 7% 8% 33%	1330
	11th to 20th	Night	15% 6% 3 6% 5% 4 5% 5% 6% 9% 36%	1330
	21st to end	Night	7% 4 1 6% 5% 4 5% 5% 7% 9% 48%	1330
July	1st to 10th	Night	5% 4 2 5% 4 6% 6% 5% 7% 9% 47%	2220
	11th to 20th	Night	5% 3 1 4 3 3 4 4 7% 11% 54%	2220
	21st to end	Night	1 2 1 4 3 3 3 4 5% 8% 67%	2414
August	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 48%	2491
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 6% 10% 51%	2811
	21st to end	Night	9% 3 1 5% 4 4 4 4 6% 9% 52%	3146
September	1st to 10th	Night	9% 4 2 7% 4 4 3 3 6% 7% 52%	1857
	11th to 20th	Night	11% 6% 2 7% 4 4 4 3 5% 8% 45%	1990
	21st to end	Night	7% 6% 2 5% 3 4 4 4 7% 8% 49%	2116
October	1st to 10th	Night	10% 9% 3 7% 4 4 4 4 6% 8% 41%	2280
	11th to 20th	Night	6% 7% 2 7% 5% 5% 4 3 6% 7% 46%	2274
	21st to end	Night	7% 9% 3 10% 6% 4 6% 4 6% 8% 36%	2626
November	1st to 10th	Night	10% 14% 4 8% 6% 5% 5% 4 5% 8% 32%	2590
	11th to 20th	Night	5% 9% 3 10% 7% 5% 5% 4 6% 9% 35%	2660
	21st to end	Night	5% 12% 5% 10% 7% 5% 6% 4 6% 8% 32%	2749
December	1st to 10th	Night	6% 12% 5% 11% 9% 7% 6% 4 6% 7% 27%	2850
	11th to 20th	Night	5% 11% 5% 12% 8% 6% 5% 4 6% 8% 30%	3510
	21st to end	Night	3 7% 3 7% 5% 4 5% 4 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%	1674
	21st to end	Day	55% 7% 14% 15% 9%	1413
March	1st to 10th	Day	51% 8% 16% 18% 7%	3223
	11th to 20th	Day	41% 8% 23% 18% 10%	3490
	21st to end	Day	53% 11% 17% 15% 4	3825
April	1st to 10th	Day	47% 7% 16% 20% 9% 1	2323
	11th to 20th	Day	49% 7% 14% 21% 7% 1	2440
	21st to end	Day	46% 9% 14% 22% 8% 1	2501
May	1st to 10th	Day	52% 9% 13% 20% 5%	4430
	11th to 20th	Day	40% 11% 14% 23% 11% 1	4543
	21st to end	Day	45% 11% 17% 19% 7%	5162
June	1st to 10th	Day	43% 10% 14% 22% 9% 1	2850
	11th to 20th	Day	49% 9% 13% 19% 8% 1	2850
	21st to end	Day	35% 11% 20% 23% 9% 1	2850
July	1st to 10th	Day	29% 11% 21% 25% 15% 1	4666
	11th to 20th	Day	28% 9% 17% 31% 15% 1	4659
	21st to end	Day	18% 7% 15% 33% 22% 6%	4976
August	1st to 10th	Day	31% 9% 16% 34% 9% 1	4412
	11th to 20th	Day	27% 7% 18% 38% 9% 1	4067
	21st to end	Day	26% 7% 18% 35% 13% 1	4520
September	1st to 10th	Day	34% 6% 14% 25% 14% 7%	2323
	11th to 20th	Day	41% 5% 15% 28% 9% 2	2187
	21st to end	Day	30% 6% 16% 32% 13% 2	2064
October	1st to 10th	Day	40% 5% 17% 27% 10%	1900
	11th to 20th	Day	38% 7% 18% 26% 11%	1890
	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	51% 8% 14% 16% 10% 1	2403
	21st to end	Night	44% 8% 16% 22% 9% 1	2041
March	1st to 10th	Night	50% 8% 17% 18% 7%	3645
	11th to 20th	Night	37% 10% 24% 23% 6% 1	3490
	21st to end	Night	54% 11% 13% 16% 5% 1	3815
April	1st to 10th	Night	42% 8% 12% 23% 13% 1	1855
	11th to 20th	Night	50% 7% 11% 20% 11% 1	1691
	21st to end	Night	44% 9% 12% 22% 12% 2	1677
May	1st to 10th	Night	52% 7% 11% 22% 9%	2540
	11th to 20th	Night	34% 12% 16% 25% 12% 1	2343
	21st to end	Night	40% 12% 17% 23% 8% 1	2414
June	1st to 10th	Night	45% 9% 15% 21% 9% 2	1330
	11th to 20th	Night	42% 10% 16% 19% 12% 1	1330
	21st to end	Night	29% 11% 20% 28% 10% 1	1330
July	1st to 10th	Night	29% 9% 21% 24% 16% 1	2185
	11th to 20th	Night	23% 8% 16% 37% 15% 1	2178
	21st to end	Night	15% 6% 15% 36% 24% 4	2337
August	1st to 10th	Night	31% 8% 16% 32% 11% 1	2484
	11th to 20th	Night	27% 6% 17% 40% 9% 1	2838
	21st to end	Night	28% 6% 16% 33% 16% 2	3146
September	1st to 10th	Night	31% 7% 14% 23% 16% 8%	1857
	11th to 20th	Night	37% 7% 12% 27% 14% 3	1990
	21st to end	Night	30% 8% 15% 28% 17% 2	2116
October	1st to 10th	Night	40% 7% 15% 24% 13% 1	2280
	11th to 20th	Night	36% 6% 14% 31% 12% 1	2274
	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% 5%	270
	21st to end	Day	6% 6% 32% 34% 16% 7%	225
March	1st to 10th	Day	8% 15% 32% 30% 13% 2	398
	11th to 20th	Day	9% 13% 36% 27% 11% 4	460
	21st to end	Day	12% 9% 33% 31% 12% 2	506
April	1st to 10th	Day	11% 13% 37% 27% 10% 2	460
	11th to 20th	Day	14% 12% 34% 29% 10% 2	452
	21st to end	Day	14% 11% 29% 28% 15% 2	491
May	1st to 10th	Day	17% 8% 22% 33% 15% 5%	653
	11th to 20th	Day	11% 8% 25% 33% 19% 5%	675
	21st to end	Day	15% 8% 22% 27% 22% 7%	734
June	1st to 10th	Day	14% 11% 26% 29% 16% 4	680
	11th to 20th	Day	16% 13% 30% 24% 13% 3	680
	21st to end	Day	10% 7% 25% 30% 22% 6%	680
July	1st to 10th	Day	9% 4 19% 38% 23% 7%	671
	11th to 20th	Day	11% 5% 24% 30% 22% 7%	668
	21st to end	Day	6% 4 24% 30% 27% 10%	739
August	1st to 10th	Day	8% 6% 19% 37% 23% 8%	637
	11th to 20th	Day	8% 7% 20% 32% 26% 7%	477
	21st to end	Day	8% 7% 24% 31% 22% 8%	539
September	1st to 10th	Day	12% 8% 29% 28% 17% 5%	490
	11th to 20th	Day	17% 10% 21% 31% 18% 3	490
	21st to end	Day	13% 11% 33% 27% 14% 3	464
October	1st to 10th	Day	18% 7% 29% 26% 18% 4	300
	11th to 20th	Day	7% 13% 34% 30% 13% 4	297
	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	744
	21st to end	Night	7% 5% 25% 35% 24% 4	628
March	1st to 10th	Night	10% 7% 32% 38% 13% 2	760
	11th to 20th	Night	9% 6% 36% 35% 12% 2	760
	21st to end	Night	13% 9% 28% 33% 14% 3	835
April	1st to 10th	Night	9% 8% 37% 32% 11% 2	570
	11th to 20th	Night	14% 8% 35% 31% 11% 1	563
	21st to end	Night	15% 8% 26% 37% 13% 1	570
May	1st to 10th	Night	15% 7% 24% 37% 15% 1	570
	11th to 20th	Night	8% 6% 25% 40% 20% 1	466
	21st to end	Night	15% 7% 21% 34% 21% 3	411
June	1st to 10th	Night	13% 8% 31% 29% 16% 2	380
	11th to 20th	Night	19% 6% 24% 33% 17% 1	380
	21st to end	Night	8% 5% 23% 39% 23% 2	380
July	1st to 10th	Night	9% 5% 22% 42% 22% 1	370
	11th to 20th	Night	9% 3 26% 42% 20% 1	368
	21st to end	Night	2 5% 25% 41% 24% 3	416
August	1st to 10th	Night	7% 4 23% 37% 24% 5%	505
	11th to 20th	Night	9% 5% 19% 43% 22% 2	564
	21st to end	Night	8% 5% 25% 39% 22% 2	627
September	1st to 10th	Night	11% 4 25% 41% 16% 2	570
	11th to 20th	Night	12% 6% 28% 36% 17% 2	660
	21st to end	Night	9% 5% 34% 36% 13% 2	760
October	1st to 10th	Night	14% 7% 29% 34% 15% 1	760
	11th to 20th	Night	9% 6% 33% 36% 13% 2	758
	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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