St. Mary Visitor Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Mary Visitor Center clear sky chart from 2005-11-06 to 2025-05-26.

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	5% 7% 3 8% 5% 5% 6% 6% 7% 8% 39%	117397
	Night	3 7% 3 8% 5% 5% 6% 6% 8% 9% 39%	120586
Transparency	Day	35% 12% 19% 20% 12% 3	117150
	Night	33% 13% 21% 20% 10% 3	120479
Seeing	Day	21% 28% 29% 16% 6% 1	15969
	Night	17% 28% 30% 18% 6% 1	25304

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	1 6% 5% 14% 8% 7% 7% 6% 10% 9% 27%	3179
	11th to 20th	Day	1 4 3 13% 9% 9% 7% 7% 9% 7% 32%	3450
	21st to end	Day	1 5% 3 8% 5% 6% 7% 8% 8% 10% 38%	4004
February	1st to 10th	Day	2 9% 4 12% 8% 7% 8% 7% 11% 9% 22%	1688
	11th to 20th	Day	1 10% 5% 12% 8% 6% 8% 8% 9% 10% 24%	1755
	21st to end	Day	1 7% 4 14% 9% 7% 7% 8% 8% 9% 25%	1630
March	1st to 10th	Day	1 6% 4 9% 6% 6% 7% 7% 9% 10% 36%	4535
	11th to 20th	Day	1 7% 3 8% 6% 6% 8% 8% 10% 9% 35%	4999
	21st to end	Day	2 15% 8% 13% 6% 7% 8% 7% 8% 8% 18%	5486
April	1st to 10th	Day	4 11% 5% 12% 7% 8% 8% 6% 9% 8% 22%	2432
	11th to 20th	Day	6% 15% 4 11% 6% 5% 7% 6% 7% 7% 26%	2570
	21st to end	Day	6% 15% 4 9% 7% 5% 6% 6% 7% 7% 27%	2600
May	1st to 10th	Day	9% 21% 5% 9% 5% 5% 5% 4 5% 5% 27%	4240
	11th to 20th	Day	8% 10% 3 7% 5% 6% 8% 6% 8% 10% 29%	4731
	21st to end	Day	13% 11% 3 7% 6% 6% 9% 8% 8% 9% 21%	5239
June	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 6% 7% 7% 29%	2850
	11th to 20th	Day	13% 9% 3 8% 7% 6% 7% 6% 6% 8% 27%	2850
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 7% 8% 44%	2850
July	1st to 10th	Day	3 3 1 4 3 4 4 4 7% 8% 59%	4740
	11th to 20th	Day	2 2 1 3 3 3 3 3 5% 7% 69%	4740
	21st to end	Day	2 2 1 2 2 3 3 3 5% 7% 70%	4857
August	1st to 10th	Day	8% 6% 2 6% 4 4 6% 5% 7% 7% 46%	4286
	11th to 20th	Day	5% 4 1 4 4 5% 5% 4 7% 8% 53%	4028
	21st to end	Day	5% 3 1 4 3 3 4 4 5% 8% 61%	4521
September	1st to 10th	Day	10% 6% 2 5% 3 4 5% 4 4 7% 51%	2311
	11th to 20th	Day	7% 6% 2 7% 5% 6% 5% 4 7% 9% 41%	2090
	21st to end	Day	10% 6% 2 5% 4 4 5% 4 5% 9% 46%	2090
October	1st to 10th	Day	7% 7% 2 5% 4 4 5% 3 6% 7% 50%	3343
	11th to 20th	Day	2 7% 2 6% 5% 5% 6% 5% 9% 11% 41%	3013
	21st to end	Day	2 7% 2 7% 5% 6% 7% 7% 9% 10% 38%	3101
November	1st to 10th	Day	5% 10% 3 7% 7% 7% 7% 8% 9% 10% 27%	1694
	11th to 20th	Day	1 6% 3 7% 6% 6% 6% 9% 9% 11% 35%	1600
	21st to end	Day	2 9% 3 10% 8% 7% 7% 7% 9% 10% 30%	1465
December	1st to 10th	Day	3 3 6% 5% 5% 5% 5% 8% 10% 50%	2290
	11th to 20th	Day	1 7% 4 10% 7% 6% 6% 6% 10% 11% 33%	2642
	21st to end	Day	1 7% 4 9% 7% 6% 7% 6% 8% 11% 33%	3498
January	1st to 10th	Night	1 6% 4 11% 8% 7% 8% 9% 11% 11% 26%	6766
	11th to 20th	Night	5% 2 10% 8% 8% 8% 6% 9% 10% 34%	6493
	21st to end	Night	1 7% 3 8% 5% 5% 7% 10% 9% 9% 36%	6952
February	1st to 10th	Night	2 10% 5% 11% 8% 6% 8% 8% 9% 9% 24%	2710
	11th to 20th	Night	1 8% 5% 12% 7% 8% 6% 9% 10% 9% 25%	2532
	21st to end	Night	2 6% 4 12% 7% 8% 9% 8% 11% 9% 24%	2000
March	1st to 10th	Night	6% 4 9% 6% 6% 7% 7% 10% 9% 35%	5359
	11th to 20th	Night	1 4 3 5% 4 4 6% 8% 10% 11% 43%	5000
	21st to end	Night	3 13% 6% 12% 7% 6% 7% 8% 9% 10% 20%	5489
April	1st to 10th	Night	3 10% 3 7% 5% 5% 8% 7% 9% 8% 34%	1964
	11th to 20th	Night	5% 13% 4 8% 6% 4 5% 5% 7% 8% 35%	1781
	21st to end	Night	5% 11% 3 7% 6% 6% 6% 6% 8% 7% 35%	1800
May	1st to 10th	Night	9% 19% 4 8% 5% 5% 5% 4 4 6% 32%	2950
	11th to 20th	Night	6% 6% 2 6% 5% 5% 8% 6% 6% 10% 39%	2365
	21st to end	Night	12% 12% 3 8% 6% 6% 7% 8% 8% 7% 23%	2452
June	1st to 10th	Night	12% 9% 2 7% 5% 5% 6% 6% 7% 7% 35%	1330
	11th to 20th	Night	11% 7% 2 5% 6% 5% 7% 5% 6% 10% 35%	1330
	21st to end	Night	5% 4 1 6% 3 4 4 5% 7% 8% 53%	1330
July	1st to 10th	Night	3 2 1 2 2 3 5% 5% 7% 8% 62%	2220
	11th to 20th	Night	2 1 1 2 2 2 2 3 5% 6% 74%	2220
	21st to end	Night	2 1 1 3 3 2 2 2 4 6% 73%	2691
August	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 5% 7% 53%	2646
	11th to 20th	Night	4 3 1 3 4 4 5% 5% 8% 12% 52%	2811
	21st to end	Night	4 3 1 3 2 3 3 3 6% 8% 63%	3146
September	1st to 10th	Night	9% 4 2 6% 3 3 4 3 5% 8% 54%	1866
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 8% 7% 47%	2090
	21st to end	Night	8% 6% 2 4 3 4 5% 5% 7% 9% 48%	2090
October	1st to 10th	Night	5% 7% 2 3 3 3 3 3 7% 9% 55%	3631
	11th to 20th	Night	2 5% 2 4 4 4 5% 6% 9% 12% 47%	3900
	21st to end	Night	2 5% 2 6% 5% 5% 5% 5% 7% 10% 48%	4457
November	1st to 10th	Night	4 9% 4 9% 7% 6% 7% 8% 9% 9% 30%	2584
	11th to 20th	Night	1 5% 3 7% 6% 7% 8% 9% 10% 11% 33%	2800
	21st to end	Night	1 7% 4 9% 6% 6% 6% 7% 10% 14% 31%	2933
December	1st to 10th	Night	5% 3 7% 5% 4 6% 6% 10% 11% 42%	4890
	11th to 20th	Night	1 7% 4 10% 8% 8% 8% 7% 10% 11% 28%	5550
	21st to end	Night	1 8% 5% 10% 6% 7% 8% 7% 9% 9% 31%	7458

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	41% 13% 25% 17% 4 1	3179
	11th to 20th	Day	45% 12% 21% 11% 6% 5%	3474
	21st to end	Day	29% 16% 23% 20% 11%	4004
February	1st to 10th	Day	47% 18% 21% 7% 5% 2	1688
	11th to 20th	Day	45% 15% 23% 12% 4 1	1764
	21st to end	Day	44% 15% 21% 14% 4 2	1630
March	1st to 10th	Day	29% 15% 21% 20% 13% 3	4535
	11th to 20th	Day	29% 14% 26% 22% 8% 1	5000
	21st to end	Day	53% 14% 19% 10% 3	5486
April	1st to 10th	Day	48% 14% 20% 14% 3	2432
	11th to 20th	Day	49% 12% 20% 16% 3	2570
	21st to end	Day	49% 11% 19% 15% 5% 1	2600
May	1st to 10th	Day	53% 9% 15% 19% 4	4240
	11th to 20th	Day	38% 13% 21% 23% 5%	4735
	21st to end	Day	44% 15% 22% 17% 3	5240
June	1st to 10th	Day	48% 12% 18% 16% 6%	2850
	11th to 20th	Day	48% 10% 16% 16% 8% 1	2850
	21st to end	Day	31% 10% 21% 25% 11% 1	2850
July	1st to 10th	Day	20% 6% 13% 31% 22% 8%	4666
	11th to 20th	Day	13% 5% 10% 19% 31% 21%	4659
	21st to end	Day	13% 5% 8% 21% 35% 18%	4703
August	1st to 10th	Day	32% 8% 11% 18% 23% 8%	4269
	11th to 20th	Day	25% 8% 14% 30% 21% 2	4067
	21st to end	Day	20% 6% 11% 42% 19% 3	4520
September	1st to 10th	Day	31% 7% 13% 23% 18% 7%	2311
	11th to 20th	Day	34% 10% 17% 22% 15% 3	2090
	21st to end	Day	32% 9% 21% 24% 11% 3	2090
October	1st to 10th	Day	29% 8% 20% 27% 15% 1	3343
	11th to 20th	Day	31% 11% 22% 20% 15% 1	3013
	21st to end	Day	31% 14% 21% 24% 9%	3101
November	1st to 10th	Day	44% 16% 22% 13% 4 1	1694
	11th to 20th	Day	33% 21% 27% 13% 6% 1	1600
	21st to end	Day	39% 19% 25% 9% 6% 1	1465
December	1st to 10th	Day	24% 17% 29% 22% 7% 2	2290
	11th to 20th	Day	35% 19% 27% 16% 3	2644
	21st to end	Day	36% 13% 19% 20% 10% 2	3498
January	1st to 10th	Night	38% 16% 25% 17% 4 1	6766
	11th to 20th	Night	39% 11% 20% 16% 9% 5%	6531
	21st to end	Night	29% 18% 23% 20% 9% 1	6952
February	1st to 10th	Night	44% 16% 20% 11% 6% 2	2710
	11th to 20th	Night	44% 17% 21% 11% 6% 1	2533
	21st to end	Night	41% 16% 22% 15% 4 2	2000
March	1st to 10th	Night	30% 15% 21% 21% 9% 4	5359
	11th to 20th	Night	21% 13% 29% 25% 11%	5000
	21st to end	Night	48% 16% 20% 12% 4	5489
April	1st to 10th	Night	38% 14% 24% 20% 4	1964
	11th to 20th	Night	42% 12% 21% 18% 6% 1	1781
	21st to end	Night	42% 11% 19% 20% 7%	1800
May	1st to 10th	Night	51% 8% 17% 18% 6%	2950
	11th to 20th	Night	31% 12% 19% 31% 8%	2365
	21st to end	Night	46% 14% 21% 15% 3	2452
June	1st to 10th	Night	43% 9% 21% 21% 6%	1330
	11th to 20th	Night	38% 12% 20% 20% 9% 1	1330
	21st to end	Night	24% 10% 23% 27% 14% 1	1330
July	1st to 10th	Night	17% 5% 15% 29% 25% 9%	2185
	11th to 20th	Night	12% 4 10% 23% 30% 21%	2178
	21st to end	Night	14% 3 10% 25% 28% 20%	2606
August	1st to 10th	Night	28% 6% 13% 22% 22% 9%	2638
	11th to 20th	Night	23% 7% 17% 35% 15% 3	2838
	21st to end	Night	18% 6% 13% 37% 23% 4	3146
September	1st to 10th	Night	27% 7% 17% 25% 16% 8%	1866
	11th to 20th	Night	31% 9% 17% 27% 13% 4	2090
	21st to end	Night	29% 9% 22% 23% 14% 2	2090
October	1st to 10th	Night	23% 8% 18% 31% 19% 2	3631
	11th to 20th	Night	24% 11% 24% 24% 17% 1	3900
	21st to end	Night	25% 12% 19% 29% 13% 1	4457
November	1st to 10th	Night	41% 16% 23% 12% 6% 1	2584
	11th to 20th	Night	32% 19% 28% 14% 6% 1	2800
	21st to end	Night	35% 18% 27% 11% 8% 1	2933
December	1st to 10th	Night	26% 16% 29% 20% 8% 1	4890
	11th to 20th	Night	38% 21% 24% 12% 3 1	5547
	21st to end	Night	38% 14% 20% 16% 9% 3	7458

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% 33% 24% 13% 7% 1	200
	11th to 20th	Day	22% 36% 29% 8% 6% 1	200
	21st to end	Day	25% 45% 20% 7% 2 1	220
February	1st to 10th	Day	27% 31% 32% 8% 3	245
	11th to 20th	Day	15% 39% 32% 11% 3	300
	21st to end	Day	27% 35% 28% 9% 1	394
March	1st to 10th	Day	22% 39% 25% 9% 3 1	500
	11th to 20th	Day	21% 32% 27% 13% 6% 1	500
	21st to end	Day	24% 34% 29% 8% 3 1	550
April	1st to 10th	Day	25% 37% 26% 10% 2	500
	11th to 20th	Day	24% 35% 26% 12% 2	492
	21st to end	Day	23% 30% 27% 14% 5% 1	500
May	1st to 10th	Day	23% 23% 30% 16% 6% 2	503
	11th to 20th	Day	27% 18% 24% 17% 11% 2	690
	21st to end	Day	25% 19% 24% 18% 13% 2	760
June	1st to 10th	Day	20% 27% 26% 18% 7% 2	680
	11th to 20th	Day	29% 27% 23% 16% 5% 1	680
	21st to end	Day	12% 20% 34% 21% 11% 2	680
July	1st to 10th	Day	10% 17% 30% 29% 12% 2	671
	11th to 20th	Day	7% 23% 37% 25% 8% 1	668
	21st to end	Day	7% 20% 37% 26% 8% 2	572
August	1st to 10th	Day	11% 21% 37% 23% 8% 1	487
	11th to 20th	Day	9% 16% 39% 25% 8% 2	477
	21st to end	Day	13% 23% 36% 20% 6% 2	539
September	1st to 10th	Day	18% 26% 29% 18% 7% 1	490
	11th to 20th	Day	17% 27% 31% 19% 5% 1	490
	21st to end	Day	26% 33% 26% 11% 3 1	490
October	1st to 10th	Day	24% 27% 27% 15% 6%	408
	11th to 20th	Day	26% 31% 20% 15% 7% 2	300
	21st to end	Day	32% 31% 24% 9% 3 1	330
November	1st to 10th	Day	31% 37% 25% 6% 1	300
	11th to 20th	Day	32% 37% 25% 6% 1	300
	21st to end	Day	28% 37% 19% 9% 6% 1	233
December	1st to 10th	Day	31% 34% 24% 8% 4 1	200
	11th to 20th	Day	32% 37% 26% 5% 2	200
	21st to end	Day	22% 35% 26% 10% 6% 1	220
January	1st to 10th	Night	22% 37% 28% 10% 3	1000
	11th to 20th	Night	17% 36% 32% 11% 4	894
	21st to end	Night	16% 40% 31% 8% 3 1	880
February	1st to 10th	Night	17% 32% 32% 14% 4 1	800
	11th to 20th	Night	15% 30% 37% 15% 3	784
	21st to end	Night	20% 29% 28% 18% 5%	660
March	1st to 10th	Night	15% 31% 33% 16% 4 1	800
	11th to 20th	Night	18% 29% 28% 20% 4 1	800
	21st to end	Night	16% 29% 29% 19% 6% 1	880
April	1st to 10th	Night	14% 28% 32% 20% 5%	752
	11th to 20th	Night	16% 32% 31% 16% 5% 1	593
	21st to end	Night	16% 24% 34% 19% 7% 1	600
May	1st to 10th	Night	16% 22% 26% 25% 10% 1	600
	11th to 20th	Night	17% 15% 27% 26% 13% 2	432
	21st to end	Night	22% 17% 21% 24% 15% 1	422
June	1st to 10th	Night	20% 19% 23% 24% 12% 2	380
	11th to 20th	Night	20% 24% 28% 18% 8% 1	380
	21st to end	Night	9% 15% 31% 29% 15% 1	380
July	1st to 10th	Night	7% 14% 25% 37% 16% 1	370
	11th to 20th	Night	5% 13% 38% 34% 10%	368
	21st to end	Night	7% 11% 29% 40% 12%	416
August	1st to 10th	Night	9% 9% 27% 38% 16%	443
	11th to 20th	Night	6% 13% 29% 34% 16% 2	564
	21st to end	Night	12% 19% 29% 29% 10% 1	627
September	1st to 10th	Night	11% 18% 30% 23% 15% 3	674
	11th to 20th	Night	12% 18% 27% 28% 11% 4	760
	21st to end	Night	17% 22% 30% 20% 9% 2	760
October	1st to 10th	Night	19% 23% 27% 24% 7% 1	760
	11th to 20th	Night	18% 26% 27% 21% 7% 1	760
	21st to end	Night	20% 33% 31% 12% 3 1	836
November	1st to 10th	Night	24% 35% 30% 9% 1	829
	11th to 20th	Night	25% 38% 27% 9% 1	1000
	21st to end	Night	20% 37% 29% 10% 4	1000
December	1st to 10th	Night	18% 36% 29% 13% 4	1000
	11th to 20th	Night	17% 39% 32% 10% 2	1000
	21st to end	Night	21% 32% 33% 10% 3	1100

Page generated at 2025-05-23 07:21:01. Version ac07

Copyright © 2025 Attilla Danko.