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 2026-09-29.

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 7% 5% 5% 6% 5% 7% 9% 43%	153730
	Night	3 6% 3 7% 5% 5% 6% 6% 8% 9% 42%	146426
Transparency	Day	32% 10% 18% 23% 13% 4	153442
	Night	31% 12% 21% 22% 11% 3	146297
Seeing	Day	22% 28% 28% 15% 6% 1	17849
	Night	18% 28% 29% 18% 6% 1	26924

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 5% 4 12% 7% 6% 7% 6% 11% 11% 32%	4125
	11th to 20th	Day	3 3 12% 7% 7% 5% 6% 8% 9% 39%	4494
	21st to end	Day	6% 4 10% 5% 6% 7% 7% 7% 9% 38%	5186
February	1st to 10th	Day	2 8% 4 12% 8% 7% 8% 7% 11% 9% 24%	1773
	11th to 20th	Day	10% 5% 12% 8% 6% 7% 7% 9% 11% 24%	1845
	21st to end	Day	1 7% 4 13% 9% 7% 7% 8% 9% 9% 26%	1709
March	1st to 10th	Day	1 6% 4 9% 6% 6% 7% 7% 9% 10% 36%	4635
	11th to 20th	Day	1 7% 3 8% 6% 6% 8% 7% 9% 9% 35%	5109
	21st to end	Day	2 15% 8% 13% 6% 7% 8% 7% 8% 8% 19%	5607
April	1st to 10th	Day	4 11% 5% 11% 7% 7% 8% 6% 9% 8% 23%	2553
	11th to 20th	Day	5% 14% 5% 11% 6% 5% 7% 6% 7% 7% 27%	2700
	21st to end	Day	6% 15% 4 9% 7% 5% 6% 6% 7% 7% 28%	2730
May	1st to 10th	Day	7% 14% 3 6% 4 4 4 3 6% 7% 42%	7626
	11th to 20th	Day	8% 10% 2 6% 4 5% 6% 5% 9% 11% 34%	8542
	21st to end	Day	11% 8% 2 5% 4 5% 7% 6% 8% 9% 35%	9649
June	1st to 10th	Day	13% 8% 3 7% 6% 6% 6% 6% 7% 7% 30%	3150
	11th to 20th	Day	13% 8% 3 8% 6% 6% 7% 6% 6% 8% 29%	3150
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 6% 7% 9% 43%	3150
July	1st to 10th	Day	4 3 1 3 3 4 4 4 6% 8% 60%	8820
	11th to 20th	Day	5% 2 1 3 3 3 4 3 5% 8% 63%	8820
	21st to end	Day	4 2 1 2 2 3 3 3 5% 6% 69%	9038
August	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 5% 6% 56%	6270
	11th to 20th	Day	9% 3 1 3 4 4 5% 4 7% 8% 50%	5925
	21st to end	Day	4 3 1 4 3 3 4 4 6% 8% 61%	6570
September	1st to 10th	Day	10% 6% 2 5% 3 4 5% 3 5% 7% 50%	2553
	11th to 20th	Day	8% 6% 2 6% 5% 6% 5% 4 7% 10% 41%	2310
	21st to end	Day	9% 6% 2 5% 4 4 5% 4 5% 9% 47%	2289
October	1st to 10th	Day	7% 7% 2 5% 4 4 5% 3 6% 7% 51%	3448
	11th to 20th	Day	2 7% 3 6% 5% 5% 6% 5% 9% 11% 41%	3109
	21st to end	Day	2 7% 2 7% 5% 5% 7% 6% 9% 10% 39%	3200
November	1st to 10th	Day	5% 10% 3 7% 7% 7% 7% 8% 9% 10% 27%	1780
	11th to 20th	Day	1 6% 3 7% 5% 6% 6% 9% 10% 11% 35%	1680
	21st to end	Day	2 9% 3 10% 8% 7% 6% 6% 9% 10% 31%	1538
December	1st to 10th	Day	4 3 6% 5% 5% 5% 5% 8% 10% 49%	2360
	11th to 20th	Day	1 7% 4 10% 7% 6% 6% 6% 10% 11% 33%	2712
	21st to end	Day	1 7% 4 9% 7% 6% 7% 6% 8% 11% 34%	3575
January	1st to 10th	Night	1 6% 3 10% 7% 6% 7% 8% 10% 11% 33%	8768
	11th to 20th	Night	4 2 8% 7% 7% 7% 6% 9% 10% 39%	8449
	21st to end	Night	1 6% 3 8% 5% 5% 7% 9% 9% 9% 37%	8994
February	1st to 10th	Night	2 10% 5% 11% 8% 6% 8% 8% 9% 9% 25%	2845
	11th to 20th	Night	1 8% 5% 11% 7% 8% 6% 9% 10% 9% 26%	2662
	21st to end	Night	1 6% 3 12% 7% 8% 9% 8% 11% 9% 26%	2097
March	1st to 10th	Night	6% 4 9% 6% 6% 7% 7% 10% 9% 35%	5479
	11th to 20th	Night	1 4 2 5% 5% 4 6% 8% 10% 11% 43%	5110
	21st to end	Night	3 12% 6% 12% 7% 6% 7% 8% 9% 10% 20%	5610
April	1st to 10th	Night	3 10% 3 7% 5% 5% 8% 7% 9% 8% 35%	2063
	11th to 20th	Night	4 12% 4 8% 6% 4 5% 5% 7% 9% 35%	1871
	21st to end	Night	5% 11% 3 8% 6% 6% 6% 6% 8% 8% 34%	1890
May	1st to 10th	Night	7% 13% 3 6% 4 4 4 4 5% 7% 44%	5322
	11th to 20th	Night	5% 7% 2 6% 5% 6% 7% 5% 7% 9% 40%	4292
	21st to end	Night	10% 7% 2 6% 5% 5% 5% 6% 7% 7% 40%	4522
June	1st to 10th	Night	12% 9% 2 7% 5% 5% 6% 5% 6% 7% 36%	1470
	11th to 20th	Night	11% 7% 2 5% 5% 5% 7% 5% 7% 10% 37%	1470
	21st to end	Night	6% 4 1 6% 3 4 5% 5% 8% 7% 50%	1470
July	1st to 10th	Night	4 2 1 2 2 3 4 3 6% 7% 67%	4140
	11th to 20th	Night	5% 3 1 3 3 2 3 3 5% 6% 66%	4140
	21st to end	Night	5% 3 1 4 3 2 2 2 4 5% 68%	5026
August	1st to 10th	Night	7% 4 1 4 3 3 3 3 4 6% 62%	3886
	11th to 20th	Night	9% 4 1 3 3 3 4 4 8% 11% 50%	4141
	21st to end	Night	3 3 1 3 2 3 3 3 6% 8% 65%	4578
September	1st to 10th	Night	9% 4 2 5% 3 3 4 3 5% 7% 54%	2063
	11th to 20th	Night	7% 7% 2 6% 4 4 5% 4 8% 7% 47%	2310
	21st to end	Night	7% 6% 2 4 3 4 5% 5% 7% 9% 49%	2288
October	1st to 10th	Night	5% 7% 2 3 3 3 3 3 7% 9% 55%	3746
	11th to 20th	Night	2 5% 2 5% 4 4 5% 6% 9% 12% 47%	4024
	21st to end	Night	2 4 2 6% 5% 5% 5% 5% 7% 10% 49%	4600
November	1st to 10th	Night	4 9% 4 8% 7% 6% 7% 8% 8% 9% 31%	2717
	11th to 20th	Night	1 5% 3 7% 6% 8% 8% 9% 9% 11% 33%	2940
	21st to end	Night	1 7% 4 8% 6% 6% 6% 6% 10% 14% 31%	3080
December	1st to 10th	Night	5% 3 7% 5% 4 6% 6% 10% 11% 42%	5040
	11th to 20th	Night	1 7% 4 10% 8% 8% 8% 7% 10% 11% 28%	5700
	21st to end	Night	1 8% 4 10% 6% 7% 8% 7% 9% 9% 31%	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	34% 12% 28% 19% 6% 1	4125
	11th to 20th	Day	38% 11% 24% 15% 6% 5%	4518
	21st to end	Day	34% 13% 22% 20% 11%	5187
February	1st to 10th	Day	46% 17% 22% 8% 6% 2	1773
	11th to 20th	Day	45% 14% 23% 12% 5% 1	1854
	21st to end	Day	43% 15% 22% 14% 5% 2	1709
March	1st to 10th	Day	29% 15% 21% 20% 13% 3	4635
	11th to 20th	Day	29% 14% 27% 21% 8% 1	5110
	21st to end	Day	53% 14% 19% 11% 3	5607
April	1st to 10th	Day	47% 14% 21% 15% 3	2553
	11th to 20th	Day	48% 12% 20% 16% 3	2700
	21st to end	Day	49% 11% 18% 16% 4 1	2730
May	1st to 10th	Day	40% 8% 17% 24% 12%	7626
	11th to 20th	Day	36% 11% 18% 25% 10%	8546
	21st to end	Day	36% 11% 23% 26% 4	9650
June	1st to 10th	Day	46% 12% 17% 18% 7%	3150
	11th to 20th	Day	46% 10% 16% 17% 10% 2	3150
	21st to end	Day	32% 10% 20% 26% 11% 1	3150
July	1st to 10th	Day	20% 5% 13% 29% 24% 8%	8746
	11th to 20th	Day	19% 5% 11% 25% 24% 16%	8739
	21st to end	Day	15% 5% 9% 24% 31% 16%	8842
August	1st to 10th	Day	25% 7% 9% 21% 25% 12%	6253
	11th to 20th	Day	28% 7% 15% 32% 17% 2	5964
	21st to end	Day	19% 5% 12% 43% 19% 2	6569
September	1st to 10th	Day	32% 8% 14% 25% 16% 6%	2553
	11th to 20th	Day	34% 9% 16% 23% 15% 3	2310
	21st to end	Day	31% 8% 21% 25% 13% 3	2289
October	1st to 10th	Day	29% 8% 20% 28% 15% 1	3448
	11th to 20th	Day	31% 11% 21% 20% 15% 1	3109
	21st to end	Day	31% 14% 21% 24% 10%	3200
November	1st to 10th	Day	43% 16% 23% 13% 4 1	1780
	11th to 20th	Day	32% 21% 27% 13% 6% 1	1680
	21st to end	Day	39% 18% 26% 9% 7% 1	1538
December	1st to 10th	Day	24% 17% 28% 23% 7% 2	2360
	11th to 20th	Day	35% 19% 26% 16% 3	2714
	21st to end	Day	36% 13% 19% 20% 10% 2	3575
January	1st to 10th	Night	33% 15% 24% 22% 6%	8769
	11th to 20th	Night	34% 10% 23% 18% 10% 5%	8487
	21st to end	Night	31% 15% 24% 20% 9% 1	8994
February	1st to 10th	Night	44% 16% 20% 11% 7% 2	2845
	11th to 20th	Night	43% 16% 21% 13% 7% 1	2663
	21st to end	Night	40% 15% 22% 17% 4 2	2097
March	1st to 10th	Night	30% 15% 21% 21% 10% 4	5479
	11th to 20th	Night	21% 13% 29% 25% 11%	5110
	21st to end	Night	47% 16% 20% 13% 4	5610
April	1st to 10th	Night	37% 14% 25% 20% 4	2063
	11th to 20th	Night	41% 12% 21% 19% 6% 1	1871
	21st to end	Night	42% 11% 19% 21% 7%	1890
May	1st to 10th	Night	38% 7% 19% 26% 9% 2	5319
	11th to 20th	Night	33% 10% 17% 31% 9%	4292
	21st to end	Night	35% 10% 20% 29% 6%	4526
June	1st to 10th	Night	42% 8% 20% 21% 8%	1470
	11th to 20th	Night	37% 11% 19% 21% 10% 1	1470
	21st to end	Night	27% 10% 22% 27% 13% 1	1470
July	1st to 10th	Night	15% 5% 15% 30% 23% 12%	4105
	11th to 20th	Night	19% 4 11% 26% 25% 15%	4098
	21st to end	Night	19% 4 11% 23% 23% 20%	4917
August	1st to 10th	Night	23% 5% 10% 20% 26% 17%	3878
	11th to 20th	Night	27% 6% 20% 34% 11% 2	4168
	21st to end	Night	16% 4 13% 40% 23% 3	4578
September	1st to 10th	Night	27% 7% 17% 27% 14% 7%	2063
	11th to 20th	Night	31% 8% 17% 28% 13% 3	2310
	21st to end	Night	27% 9% 21% 25% 16% 2	2288
October	1st to 10th	Night	23% 8% 19% 31% 19% 2	3746
	11th to 20th	Night	24% 10% 24% 23% 17% 1	4024
	21st to end	Night	25% 12% 20% 29% 13% 1	4600
November	1st to 10th	Night	40% 15% 24% 13% 6% 1	2717
	11th to 20th	Night	32% 19% 28% 14% 6% 1	2940
	21st to end	Night	35% 18% 27% 12% 8% 1	3080
December	1st to 10th	Night	26% 16% 29% 20% 8% 1	5040
	11th to 20th	Night	38% 21% 24% 13% 4 1	5697
	21st to end	Night	37% 14% 21% 17% 9% 3	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	28% 32% 22% 11% 6%	220
	11th to 20th	Day	24% 35% 28% 8% 5%	220
	21st to end	Day	28% 43% 20% 7% 2 1	242
February	1st to 10th	Day	29% 31% 30% 8% 3	270
	11th to 20th	Day	17% 40% 30% 11% 3	330
	21st to end	Day	30% 33% 26% 8% 1	425
March	1st to 10th	Day	24% 39% 24% 9% 3 1	540
	11th to 20th	Day	25% 31% 26% 12% 5% 1	540
	21st to end	Day	27% 33% 28% 8% 3 1	594
April	1st to 10th	Day	25% 36% 26% 10% 2	540
	11th to 20th	Day	25% 35% 27% 12% 2	532
	21st to end	Day	22% 30% 28% 14% 5% 1	540
May	1st to 10th	Day	23% 24% 31% 15% 6% 1	543
	11th to 20th	Day	29% 19% 24% 16% 10% 2	738
	21st to end	Day	25% 19% 26% 18% 11% 2	844
June	1st to 10th	Day	22% 27% 25% 18% 7% 2	780
	11th to 20th	Day	29% 27% 23% 15% 4 1	780
	21st to end	Day	13% 21% 34% 20% 10% 2	780
July	1st to 10th	Day	12% 20% 30% 26% 11% 2	771
	11th to 20th	Day	9% 23% 35% 24% 8% 1	768
	21st to end	Day	9% 21% 37% 24% 7% 2	664
August	1st to 10th	Day	12% 20% 37% 22% 8% 1	567
	11th to 20th	Day	11% 19% 38% 23% 8% 2	557
	21st to end	Day	13% 23% 37% 19% 6% 2	627
September	1st to 10th	Day	20% 25% 28% 18% 8% 1	570
	11th to 20th	Day	17% 25% 31% 19% 7% 1	570
	21st to end	Day	26% 33% 26% 11% 3 1	563
October	1st to 10th	Day	25% 27% 27% 15% 6%	443
	11th to 20th	Day	27% 31% 20% 15% 6% 2	330
	21st to end	Day	34% 31% 23% 9% 2 1	363
November	1st to 10th	Day	34% 35% 24% 6% 1	330
	11th to 20th	Day	33% 35% 25% 5% 1	330
	21st to end	Day	32% 36% 18% 8% 5% 1	256
December	1st to 10th	Day	34% 33% 23% 7% 3	220
	11th to 20th	Day	37% 35% 23% 4 1	220
	21st to end	Day	25% 34% 25% 10% 5% 1	242
January	1st to 10th	Night	25% 36% 27% 10% 2	1050
	11th to 20th	Night	19% 36% 31% 11% 3	938
	21st to end	Night	17% 39% 31% 9% 3 1	924
February	1st to 10th	Night	19% 32% 32% 13% 3 1	840
	11th to 20th	Night	16% 29% 37% 15% 2	824
	21st to end	Night	22% 28% 27% 18% 5%	692
March	1st to 10th	Night	16% 32% 32% 15% 3 1	840
	11th to 20th	Night	21% 28% 27% 19% 4 1	840
	21st to end	Night	17% 29% 29% 18% 6% 1	924
April	1st to 10th	Night	14% 28% 33% 20% 5%	790
	11th to 20th	Night	16% 32% 30% 16% 5%	623
	21st to end	Night	15% 24% 33% 20% 7% 1	630
May	1st to 10th	Night	16% 22% 27% 24% 9% 1	630
	11th to 20th	Night	18% 15% 27% 26% 13% 2	454
	21st to end	Night	22% 16% 23% 24% 14% 1	455
June	1st to 10th	Night	22% 19% 24% 23% 11% 1	420
	11th to 20th	Night	20% 23% 30% 19% 8%	420
	21st to end	Night	10% 15% 31% 28% 14%	420
July	1st to 10th	Night	9% 14% 25% 37% 15% 1	410
	11th to 20th	Night	6% 12% 38% 34% 9%	408
	21st to end	Night	7% 12% 28% 40% 12%	460
August	1st to 10th	Night	10% 11% 28% 36% 15%	491
	11th to 20th	Night	7% 13% 30% 33% 15% 2	624
	21st to end	Night	12% 19% 29% 30% 10% 1	693
September	1st to 10th	Night	12% 18% 28% 25% 15% 2	745
	11th to 20th	Night	13% 17% 26% 30% 11% 3	840
	21st to end	Night	18% 22% 30% 20% 8% 2	832
October	1st to 10th	Night	20% 23% 27% 23% 7% 1	800
	11th to 20th	Night	19% 27% 26% 21% 6% 1	800
	21st to end	Night	22% 33% 30% 11% 3 1	880
November	1st to 10th	Night	26% 34% 29% 9% 1	872
	11th to 20th	Night	26% 37% 27% 9% 1	1050
	21st to end	Night	21% 36% 29% 10% 4	1050
December	1st to 10th	Night	20% 36% 28% 12% 4	1050
	11th to 20th	Night	20% 38% 31% 10% 2	1050
	21st to end	Night	23% 31% 32% 10% 3	1155

Page generated at 2026-09-26 02:18:41. Version ac07

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