Saint Mary's College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint Mary's College Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	6% 5% 2 5% 3 3 3 3 5% 7% 60%	139770
	Night	9% 7% 2 8% 5% 4 4 3 4 6% 50%	138873
Transparency	Day	24% 6% 8% 18% 25% 21%	134690
	Night	31% 7% 10% 16% 20% 16%	134942
Seeing	Day	11% 10% 28% 32% 15% 4	17033
	Night	13% 5% 13% 37% 27% 5%	26474

By Month

Percentage Cloud Cover SW (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	15% 8% 2 5% 3 4 4 4 6% 8% 41%	4999
	11th to 20th	Day	9% 6% 3 6% 3 3 2 2 3 6% 56%	5340
	21st to end	Day	14% 8% 3 8% 4 4 4 4 5% 7% 40%	5843
February	1st to 10th	Day	17% 7% 2 5% 3 3 3 3 6% 5% 45%	1890
	11th to 20th	Day	14% 7% 2 8% 5% 4 5% 3 6% 7% 39%	1940
	21st to end	Day	14% 7% 2 6% 4 3 4 2 6% 6% 46%	1730
March	1st to 10th	Day	12% 11% 3 10% 5% 5% 4 4 6% 6% 34%	4879
	11th to 20th	Day	11% 7% 3 5% 3 4 4 3 5% 8% 48%	5000
	21st to end	Day	11% 8% 2 7% 5% 5% 6% 4 7% 8% 36%	5571
April	1st to 10th	Day	13% 7% 2 5% 5% 4 3 3 6% 7% 45%	2400
	11th to 20th	Day	9% 5% 2 5% 3 3 3 4 5% 6% 55%	2369
	21st to end	Day	5% 6% 1 4 3 3 3 3 6% 9% 58%	2400
May	1st to 10th	Day	4 3 1 2 1 2 2 2 3 5% 76%	5615
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 6% 74%	6007
	21st to end	Day	2 2 1 4 2 2 2 2 4 6% 72%	6902
June	1st to 10th	Day	4 4 1 4 3 4 3 3 5% 7% 61%	2800
	11th to 20th	Day	1 2 1 2 2 2 2 3 5% 7% 73%	2800
	21st to end	Day	2 2 1 3 3 2 2 2 4 5% 74%	2800
July	1st to 10th	Day	1 1 3 2 2 2 1 3 5% 81%	6319
	11th to 20th	Day	1 1 1 3 2 2 2 2 3 6% 76%	5952
	21st to end	Day	3 3 5% 3 2 3 2 4 9% 66%	6402
August	1st to 10th	Day	1 1 1 3 2 3 3 3 5% 7% 71%	5777
	11th to 20th	Day	1 1 3 3 2 3 2 3 6% 75%	5517
	21st to end	Day	1 2 1 3 3 3 2 2 4 7% 72%	5984
September	1st to 10th	Day	1 2 2 3 1 2 2 2 4 5% 77%	2400
	11th to 20th	Day	3 4 2 5% 3 3 3 2 5% 7% 64%	2400
	21st to end	Day	3 3 1 2 2 2 3 1 4 6% 73%	2121
October	1st to 10th	Day	2 3 1 3 2 2 3 2 4 7% 71%	3280
	11th to 20th	Day	4 3 2 5% 3 3 3 2 4 6% 66%	3261
	21st to end	Day	5% 4 2 5% 3 3 3 3 5% 7% 59%	3558
November	1st to 10th	Day	8% 5% 1 4 4 2 4 3 5% 8% 56%	2007
	11th to 20th	Day	11% 6% 3 7% 4 4 4 3 4 7% 46%	1890
	21st to end	Day	11% 6% 2 4 3 3 3 4 6% 6% 53%	1717
December	1st to 10th	Day	8% 5% 1 4 2 3 3 3 6% 10% 54%	2700
	11th to 20th	Day	21% 9% 2 6% 5% 4 5% 3 7% 9% 28%	3108
	21st to end	Day	20% 14% 3 7% 4 4 5% 3 5% 7% 28%	4092
January	1st to 10th	Night	15% 8% 2 6% 3 3 3 3 5% 7% 45%	7904
	11th to 20th	Night	10% 5% 2 6% 4 3 3 2 4 6% 54%	7650
	21st to end	Night	13% 10% 3 9% 4 4 3 2 4 7% 40%	8371
February	1st to 10th	Night	16% 8% 2 5% 3 3 3 3 4 5% 48%	2730
	11th to 20th	Night	13% 8% 3 5% 5% 5% 4 3 4 7% 43%	2566
	21st to end	Night	12% 7% 2 4 4 4 4 2 5% 7% 50%	2076
March	1st to 10th	Night	14% 12% 4 9% 5% 3 4 3 4 6% 36%	5055
	11th to 20th	Night	15% 8% 2 6% 3 3 3 2 4 5% 48%	5000
	21st to end	Night	15% 12% 3 6% 3 3 3 2 4 7% 40%	5402
April	1st to 10th	Night	10% 8% 3 6% 4 3 3 3 5% 7% 47%	2000
	11th to 20th	Night	9% 7% 2 6% 3 3 3 2 5% 5% 53%	1979
	21st to end	Night	5% 5% 2 4 3 3 3 3 6% 8% 57%	2000
May	1st to 10th	Night	4 4 1 4 4 3 2 1 3 4 70%	4360
	11th to 20th	Night	2 5% 2 7% 6% 6% 4 3 4 6% 55%	3895
	21st to end	Night	2 3 2 12% 7% 4 2 2 3 5% 58%	3972
June	1st to 10th	Night	4 6% 2 6% 5% 4 4 4 7% 7% 53%	1600
	11th to 20th	Night	1 2 1 5% 4 3 3 3 4 7% 69%	1600
	21st to end	Night	2 4 2 6% 4 4 3 3 4 5% 64%	1600
July	1st to 10th	Night	1 1 6% 8% 5% 3 1 3 5% 67%	3640
	11th to 20th	Night	1 2 2 14% 8% 7% 4 2 4 6% 50%	4006
	21st to end	Night	1 10% 5% 23% 7% 5% 4 3 4 5% 34%	4472
August	1st to 10th	Night	2 3 1 13% 10% 12% 6% 3 4 5% 41%	4035
	11th to 20th	Night	2 1 14% 7% 6% 4 2 4 6% 54%	4343
	21st to end	Night	1 3 1 8% 8% 9% 5% 2 3 5% 53%	5015
September	1st to 10th	Night	1 3 2 7% 4 3 3 2 4 7% 65%	2000
	11th to 20th	Night	3 6% 3 9% 5% 4 3 3 5% 6% 53%	2000
	21st to end	Night	3 4 1 5% 4 3 2 1 4 6% 67%	2274
October	1st to 10th	Night	3 4 1 3 3 3 3 2 4 5% 69%	3920
	11th to 20th	Night	4 7% 2 12% 5% 4 3 2 4 4 53%	3897
	21st to end	Night	5% 6% 2 5% 3 4 4 3 5% 5% 58%	4253
November	1st to 10th	Night	8% 6% 2 5% 3 3 3 3 5% 8% 55%	2508
	11th to 20th	Night	11% 7% 2 5% 3 4 3 3 5% 7% 49%	2730
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 7% 54%	2898
December	1st to 10th	Night	8% 4 2 4 3 3 4 2 4 7% 58%	4700
	11th to 20th	Night	21% 12% 3 6% 4 4 4 3 7% 8% 29%	5316
	21st to end	Night	21% 11% 3 8% 4 5% 4 3 4 6% 31%	7106

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	39% 6% 16% 22% 13% 4	5000
	11th to 20th	Day	30% 4 6% 12% 27% 20%	5340
	21st to end	Day	43% 6% 5% 16% 16% 14%	5843
February	1st to 10th	Day	40% 4 8% 19% 20% 9%	1890
	11th to 20th	Day	42% 6% 9% 18% 20% 5%	1949
	21st to end	Day	38% 5% 8% 23% 19% 7%	1730
March	1st to 10th	Day	46% 8% 7% 18% 19% 2	4879
	11th to 20th	Day	34% 7% 13% 28% 10% 8%	5000
	21st to end	Day	39% 10% 15% 24% 10% 2	5571
April	1st to 10th	Day	37% 6% 12% 21% 20% 4	2400
	11th to 20th	Day	29% 6% 13% 24% 22% 6%	2372
	21st to end	Day	23% 6% 11% 30% 26% 4	2400
May	1st to 10th	Day	13% 4 6% 27% 32% 19%	5615
	11th to 20th	Day	11% 6% 5% 30% 40% 9%	6010
	21st to end	Day	11% 9% 5% 29% 42% 4	6904
June	1st to 10th	Day	20% 8% 9% 25% 28% 9%	2800
	11th to 20th	Day	10% 6% 7% 20% 37% 21%	2800
	21st to end	Day	12% 5% 4 11% 31% 37%	2800
July	1st to 10th	Day	6% 3 4 4 21% 61%	6251
	11th to 20th	Day	8% 2 8% 4 9% 69%	5881
	21st to end	Day	10% 6% 7% 13% 24% 39%	6217
August	1st to 10th	Day	13% 4 7% 7% 25% 44%	4206
	11th to 20th	Day	7% 5% 4 7% 39% 37%	3987
	21st to end	Day	10% 5% 5% 10% 34% 36%	4311
September	1st to 10th	Day	10% 4 4 8% 31% 43%	2400
	11th to 20th	Day	19% 7% 5% 20% 32% 17%	2400
	21st to end	Day	13% 5% 4 19% 34% 25%	2121
October	1st to 10th	Day	15% 3 4 12% 27% 40%	3280
	11th to 20th	Day	18% 7% 4 22% 31% 18%	3261
	21st to end	Day	23% 5% 6% 21% 31% 14%	3558
November	1st to 10th	Day	26% 5% 10% 26% 22% 11%	2007
	11th to 20th	Day	38% 5% 8% 23% 21% 6%	1890
	21st to end	Day	31% 4 9% 19% 25% 10%	1717
December	1st to 10th	Day	26% 4 6% 28% 29% 6%	2700
	11th to 20th	Day	51% 4 9% 16% 16% 5%	3108
	21st to end	Day	54% 7% 11% 17% 10% 2	4092
January	1st to 10th	Night	38% 7% 16% 21% 15% 3	7904
	11th to 20th	Night	31% 5% 7% 11% 27% 20%	7650
	21st to end	Night	43% 6% 5% 14% 19% 13%	8371
February	1st to 10th	Night	39% 4 8% 18% 20% 12%	2730
	11th to 20th	Night	40% 6% 9% 18% 19% 7%	2568
	21st to end	Night	33% 6% 10% 27% 18% 6%	2076
March	1st to 10th	Night	46% 9% 9% 17% 17% 3	5055
	11th to 20th	Night	36% 7% 10% 23% 15% 8%	5000
	21st to end	Night	40% 11% 12% 23% 12% 2	5402
April	1st to 10th	Night	33% 8% 13% 22% 20% 5%	2000
	11th to 20th	Night	29% 8% 10% 25% 19% 9%	1979
	21st to end	Night	23% 8% 11% 29% 23% 6%	2000
May	1st to 10th	Night	15% 9% 6% 24% 28% 18%	4357
	11th to 20th	Night	20% 10% 12% 22% 26% 10%	3896
	21st to end	Night	16% 17% 8% 22% 33% 5%	3972
June	1st to 10th	Night	23% 11% 10% 20% 26% 10%	1600
	11th to 20th	Night	14% 7% 9% 15% 33% 23%	1600
	21st to end	Night	15% 7% 11% 9% 25% 33%	1600
July	1st to 10th	Night	11% 4 14% 3 16% 53%	3600
	11th to 20th	Night	20% 2 22% 3 6% 46%	3955
	21st to end	Night	32% 10% 18% 3 14% 22%	4345
August	1st to 10th	Night	21% 7% 31% 4 16% 21%	2929
	11th to 20th	Night	15% 7% 14% 4 29% 30%	3143
	21st to end	Night	19% 6% 16% 6% 24% 29%	3608
September	1st to 10th	Night	16% 6% 8% 5% 22% 43%	2000
	11th to 20th	Night	26% 9% 9% 16% 23% 18%	2000
	21st to end	Night	15% 7% 5% 18% 28% 27%	2274
October	1st to 10th	Night	15% 4 6% 10% 18% 46%	3920
	11th to 20th	Night	29% 8% 5% 17% 19% 22%	3897
	21st to end	Night	24% 7% 4 19% 24% 21%	4253
November	1st to 10th	Night	27% 7% 10% 24% 19% 14%	2508
	11th to 20th	Night	34% 5% 9% 25% 20% 7%	2730
	21st to end	Night	29% 5% 9% 20% 25% 13%	2898
December	1st to 10th	Night	27% 4 6% 22% 29% 12%	4700
	11th to 20th	Night	52% 4 8% 14% 17% 4	5316
	21st to end	Night	53% 8% 11% 16% 9% 3	7106

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	27% 10% 30% 21% 11% 2	315
	11th to 20th	Day	18% 10% 23% 33% 14% 1	430
	21st to end	Day	18% 8% 29% 28% 15% 2	473
February	1st to 10th	Day	23% 9% 24% 26% 15% 3	430
	11th to 20th	Day	23% 17% 25% 23% 11% 1	475
	21st to end	Day	17% 12% 32% 28% 10% 2	446
March	1st to 10th	Day	26% 14% 30% 19% 9% 2	509
	11th to 20th	Day	24% 14% 25% 20% 13% 4	500
	21st to end	Day	19% 18% 31% 22% 9% 1	550
April	1st to 10th	Day	18% 16% 31% 22% 10% 3	500
	11th to 20th	Day	16% 17% 32% 23% 8% 3	492
	21st to end	Day	8% 20% 30% 26% 10% 5%	500
May	1st to 10th	Day	12% 13% 35% 27% 11% 2	503
	11th to 20th	Day	11% 13% 34% 30% 11% 1	528
	21st to end	Day	10% 13% 32% 31% 12% 2	573
June	1st to 10th	Day	8% 9% 34% 35% 11% 3	530
	11th to 20th	Day	4 12% 38% 35% 9% 2	530
	21st to end	Day	2 9% 32% 40% 13% 3	530
July	1st to 10th	Day	3 5% 31% 43% 15% 2	526
	11th to 20th	Day	3 27% 50% 17% 3	524
	21st to end	Day	2 3 28% 45% 16% 6%	575
August	1st to 10th	Day	2 8% 28% 41% 16% 4	527
	11th to 20th	Day	2 5% 30% 40% 17% 7%	517
	21st to end	Day	1 4 24% 45% 21% 5%	583
September	1st to 10th	Day	4 6% 23% 42% 19% 7%	530
	11th to 20th	Day	6% 12% 26% 35% 15% 6%	530
	21st to end	Day	5% 8% 25% 34% 24% 5%	530
October	1st to 10th	Day	5% 9% 28% 31% 18% 8%	530
	11th to 20th	Day	8% 9% 22% 33% 21% 7%	530
	21st to end	Day	11% 8% 25% 29% 21% 5%	583
November	1st to 10th	Day	12% 10% 19% 27% 24% 7%	483
	11th to 20th	Day	12% 9% 19% 34% 19% 6%	330
	21st to end	Day	13% 8% 28% 31% 18% 1	239
December	1st to 10th	Day	17% 10% 22% 30% 17% 4	220
	11th to 20th	Day	20% 11% 22% 24% 21% 2	220
	21st to end	Day	29% 11% 26% 22% 12% 1	242
January	1st to 10th	Night	21% 5% 13% 34% 21% 6%	840
	11th to 20th	Night	15% 5% 12% 39% 25% 5%	840
	21st to end	Night	19% 4 11% 38% 23% 4	924
February	1st to 10th	Night	21% 6% 12% 36% 22% 4	840
	11th to 20th	Night	17% 9% 16% 35% 19% 5%	824
	21st to end	Night	15% 7% 18% 38% 20% 2	692
March	1st to 10th	Night	23% 7% 14% 31% 21% 3	808
	11th to 20th	Night	22% 6% 14% 33% 23% 3	800
	21st to end	Night	18% 8% 18% 36% 19% 2	880
April	1st to 10th	Night	13% 9% 17% 38% 20% 2	800
	11th to 20th	Night	14% 10% 18% 34% 21% 3	791
	21st to end	Night	9% 7% 21% 35% 22% 5%	800
May	1st to 10th	Night	9% 5% 19% 41% 23% 3	717
	11th to 20th	Night	8% 6% 12% 42% 27% 5%	517
	21st to end	Night	9% 11% 16% 39% 21% 4	433
June	1st to 10th	Night	7% 6% 17% 43% 25% 2	400
	11th to 20th	Night	2 4 16% 56% 22% 2	400
	21st to end	Night	5% 4 13% 44% 33% 2	400
July	1st to 10th	Night	5% 2 10% 43% 37% 3	390
	11th to 20th	Night	3 2 7% 45% 35% 8%	543
	21st to end	Night	4 4 9% 34% 41% 8%	657
August	1st to 10th	Night	8% 6% 10% 35% 33% 8%	597
	11th to 20th	Night	2 6% 11% 38% 36% 7%	717
	21st to end	Night	3 3 9% 39% 38% 8%	880
September	1st to 10th	Night	4 3 8% 36% 40% 9%	800
	11th to 20th	Night	10% 3 10% 36% 35% 7%	800
	21st to end	Night	5% 4 10% 34% 38% 10%	800
October	1st to 10th	Night	8% 3 9% 38% 34% 8%	800
	11th to 20th	Night	10% 6% 9% 33% 35% 8%	800
	21st to end	Night	11% 4 10% 32% 32% 11%	880
November	1st to 10th	Night	10% 4 11% 34% 33% 8%	820
	11th to 20th	Night	15% 4 12% 35% 27% 7%	840
	21st to end	Night	13% 4 12% 38% 27% 6%	840
December	1st to 10th	Night	17% 4 12% 38% 24% 6%	840
	11th to 20th	Night	25% 5% 10% 36% 22% 4	840
	21st to end	Night	23% 7% 18% 31% 18% 2	924

Page generated at 2026-02-27 22:40:21. Version ac07

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