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 2024-11-02.

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% 8% 3 8% 6% 5% 6% 6% 7% 8% 37%	101675
	Night	4 7% 3 8% 6% 6% 7% 7% 9% 9% 36%	97374
Transparency	Day	36% 12% 19% 19% 11% 4	101428
	Night	34% 14% 21% 18% 9% 3	97267
Seeing	Day	20% 28% 29% 16% 6% 1	15321
	Night	16% 28% 30% 19% 7% 1	24476

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% 3 13% 9% 8% 9% 8% 11% 11% 22%	2219
	11th to 20th	Day	1 5% 3 15% 8% 8% 7% 8% 10% 8% 27%	2392
	21st to end	Day	1 7% 4 10% 7% 7% 10% 10% 10% 11% 23%	2794
February	1st to 10th	Day	2 9% 4 12% 8% 7% 8% 8% 11% 9% 22%	1603
	11th to 20th	Day	1 10% 5% 12% 8% 6% 8% 8% 9% 11% 22%	1665
	21st to end	Day	1 7% 4 14% 9% 7% 8% 8% 8% 9% 24%	1551
March	1st to 10th	Day	1 5% 3 10% 7% 7% 9% 10% 12% 10% 28%	3155
	11th to 20th	Day	1 9% 4 8% 7% 6% 8% 7% 7% 6% 38%	3489
	21st to end	Day	3 17% 7% 13% 6% 6% 8% 7% 7% 7% 18%	3825
April	1st to 10th	Day	4 11% 5% 12% 8% 8% 9% 6% 9% 8% 21%	2310
	11th to 20th	Day	6% 15% 5% 12% 6% 5% 7% 6% 7% 7% 25%	2440
	21st to end	Day	7% 15% 4 9% 7% 5% 6% 6% 7% 7% 27%	2470
May	1st to 10th	Day	9% 21% 5% 9% 5% 5% 5% 4 6% 5% 25%	4110
	11th to 20th	Day	8% 10% 3 7% 5% 6% 8% 6% 8% 10% 29%	4585
	21st to end	Day	13% 11% 3 7% 6% 6% 9% 8% 8% 9% 21%	5161
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	10% 11% 2 6% 5% 4 6% 4 7% 8% 37%	2008
	11th to 20th	Day	3 9% 3 8% 7% 6% 7% 5% 7% 9% 35%	1821
	21st to end	Day	3 9% 3 9% 6% 6% 6% 6% 9% 9% 34%	1881
November	1st to 10th	Day	5% 10% 3 7% 8% 7% 7% 8% 9% 10% 27%	1617
	11th to 20th	Day	1 6% 3 7% 6% 7% 6% 9% 9% 10% 34%	1520
	21st to end	Day	2 8% 3 9% 8% 7% 6% 7% 9% 10% 30%	1392
December	1st to 10th	Day	5% 4 9% 8% 7% 8% 8% 12% 13% 26%	1330
	11th to 20th	Day	1 7% 4 12% 8% 7% 6% 8% 10% 11% 24%	1682
	21st to end	Day	1 7% 4 8% 7% 6% 8% 7% 9% 11% 32%	2442
January	1st to 10th	Night	1 5% 3 11% 8% 7% 9% 11% 13% 12% 20%	4726
	11th to 20th	Night	6% 3 11% 9% 9% 8% 6% 10% 9% 28%	4537
	21st to end	Night	1 9% 3 8% 6% 6% 10% 13% 11% 10% 24%	4862
February	1st to 10th	Night	2 10% 5% 11% 8% 6% 8% 8% 10% 9% 24%	2575
	11th to 20th	Night	1 8% 5% 12% 7% 8% 6% 9% 10% 10% 24%	2402
	21st to end	Night	2 6% 4 13% 8% 8% 9% 8% 11% 8% 23%	1903
March	1st to 10th	Night	5% 3 10% 8% 8% 9% 9% 12% 9% 27%	3719
	11th to 20th	Night	1 5% 3 6% 5% 5% 7% 9% 9% 8% 40%	3490
	21st to end	Night	4 15% 6% 11% 8% 6% 7% 8% 9% 8% 18%	3828
April	1st to 10th	Night	4 10% 3 7% 5% 5% 8% 7% 9% 8% 32%	1867
	11th to 20th	Night	5% 13% 4 8% 6% 5% 5% 5% 7% 8% 34%	1691
	21st to end	Night	5% 11% 3 8% 6% 6% 6% 6% 8% 7% 34%	1710
May	1st to 10th	Night	9% 20% 4 8% 5% 5% 5% 4 4 6% 31%	2860
	11th to 20th	Night	6% 6% 2 6% 5% 5% 8% 6% 6% 10% 39%	2292
	21st to end	Night	12% 12% 3 9% 6% 6% 7% 8% 8% 7% 22%	2414
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	8% 10% 2 5% 5% 4 4 4 7% 8% 44%	2171
	11th to 20th	Night	3 7% 3 6% 5% 5% 6% 7% 8% 11% 39%	2356
	21st to end	Night	3 7% 3 8% 7% 6% 6% 6% 8% 10% 36%	2717
November	1st to 10th	Night	4 9% 4 9% 7% 6% 7% 8% 9% 9% 29%	2464
	11th to 20th	Night	1 5% 3 7% 6% 8% 8% 10% 10% 11% 33%	2660
	21st to end	Night	2 7% 3 8% 6% 6% 6% 7% 10% 14% 31%	2786
December	1st to 10th	Night	1 7% 4 9% 7% 5% 8% 8% 13% 11% 27%	2850
	11th to 20th	Night	1 6% 4 11% 9% 9% 8% 7% 9% 11% 24%	3510
	21st to end	Night	1 6% 4 9% 6% 7% 9% 8% 9% 10% 32%	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	39% 16% 31% 12% 1 1	2219
	11th to 20th	Day	46% 14% 19% 7% 6% 8%	2416
	21st to end	Day	37% 21% 28% 11% 3	2794
February	1st to 10th	Day	47% 18% 22% 7% 4 2	1603
	11th to 20th	Day	45% 16% 24% 11% 4 1	1674
	21st to end	Day	45% 16% 21% 12% 4 2	1551
March	1st to 10th	Day	29% 19% 22% 22% 8%	3155
	11th to 20th	Day	32% 16% 30% 18% 2 1	3490
	21st to end	Day	54% 14% 21% 8% 3	3825
April	1st to 10th	Day	49% 15% 21% 13% 2	2310
	11th to 20th	Day	50% 12% 19% 15% 3	2440
	21st to end	Day	50% 12% 19% 14% 4 1	2470
May	1st to 10th	Day	54% 10% 15% 18% 3	4110
	11th to 20th	Day	38% 13% 21% 22% 5%	4589
	21st to end	Day	44% 15% 22% 17% 3	5162
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	39% 10% 20% 21% 9% 1	2008
	11th to 20th	Day	39% 11% 24% 17% 8% 1	1821
	21st to end	Day	39% 16% 19% 19% 6%	1881
November	1st to 10th	Day	43% 16% 22% 13% 4 1	1617
	11th to 20th	Day	34% 21% 28% 12% 5% 1	1520
	21st to end	Day	39% 19% 26% 8% 7% 1	1392
December	1st to 10th	Day	36% 23% 22% 13% 4 3	1330
	11th to 20th	Day	41% 22% 26% 9% 2 1	1684
	21st to end	Day	36% 15% 21% 14% 11% 4	2442
January	1st to 10th	Night	37% 20% 29% 10% 3 1	4726
	11th to 20th	Night	45% 13% 18% 10% 8% 7%	4575
	21st to end	Night	35% 23% 25% 13% 4	4862
February	1st to 10th	Night	44% 17% 21% 11% 6% 2	2575
	11th to 20th	Night	43% 18% 22% 11% 5%	2403
	21st to end	Night	43% 16% 21% 15% 4 2	1903
March	1st to 10th	Night	31% 19% 21% 22% 7%	3719
	11th to 20th	Night	25% 16% 34% 20% 4 1	3490
	21st to end	Night	50% 17% 22% 8% 3	3828
April	1st to 10th	Night	38% 15% 25% 18% 3	1867
	11th to 20th	Night	43% 13% 21% 17% 5% 1	1691
	21st to end	Night	43% 12% 20% 19% 6%	1710
May	1st to 10th	Night	52% 8% 17% 17% 5%	2860
	11th to 20th	Night	30% 12% 19% 31% 8%	2292
	21st to end	Night	46% 15% 21% 15% 3	2414
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	34% 11% 20% 21% 12% 2	2171
	11th to 20th	Night	31% 12% 25% 21% 10% 2	2356
	21st to end	Night	35% 17% 22% 18% 7% 1	2717
November	1st to 10th	Night	41% 16% 24% 12% 6% 1	2464
	11th to 20th	Night	33% 20% 28% 13% 5% 1	2660
	21st to end	Night	35% 18% 27% 11% 8% 1	2786
December	1st to 10th	Night	35% 21% 23% 13% 6% 2	2850
	11th to 20th	Night	40% 23% 24% 10% 3 1	3507
	21st to end	Night	33% 16% 23% 15% 8% 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	22% 33% 23% 13% 8% 1	180
	11th to 20th	Day	19% 37% 29% 8% 6% 1	180
	21st to end	Day	23% 45% 21% 8% 2 1	198
February	1st to 10th	Day	27% 29% 33% 9% 2	220
	11th to 20th	Day	15% 40% 30% 11% 3	270
	21st to end	Day	22% 37% 30% 9% 1	363
March	1st to 10th	Day	20% 40% 27% 10% 3 1	460
	11th to 20th	Day	19% 32% 29% 13% 6% 1	460
	21st to end	Day	22% 36% 29% 9% 3 1	506
April	1st to 10th	Day	25% 38% 26% 10% 2	460
	11th to 20th	Day	22% 35% 27% 13% 2	452
	21st to end	Day	23% 32% 26% 13% 4 1	460
May	1st to 10th	Day	22% 24% 29% 17% 6% 2	463
	11th to 20th	Day	27% 18% 25% 18% 12% 2	642
	21st to end	Day	26% 19% 24% 17% 12% 2	734
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% 32% 24% 9% 3 1	330
November	1st to 10th	Day	32% 34% 26% 7% 1	273
	11th to 20th	Day	28% 39% 25% 6% 1	270
	21st to end	Day	27% 37% 20% 10% 6% 1	210
December	1st to 10th	Day	28% 34% 25% 8% 4 1	180
	11th to 20th	Day	30% 37% 27% 5% 2	180
	21st to end	Day	17% 36% 28% 11% 7% 1	198
January	1st to 10th	Night	23% 37% 28% 10% 2	950
	11th to 20th	Night	17% 36% 32% 11% 4	850
	21st to end	Night	15% 40% 32% 9% 3 1	836
February	1st to 10th	Night	17% 31% 33% 14% 4 1	760
	11th to 20th	Night	16% 30% 37% 14% 3	744
	21st to end	Night	17% 29% 30% 19% 5%	628
March	1st to 10th	Night	14% 32% 34% 16% 4 1	760
	11th to 20th	Night	17% 28% 29% 21% 4 1	760
	21st to end	Night	15% 29% 29% 20% 6% 1	836
April	1st to 10th	Night	14% 29% 31% 20% 5%	715
	11th to 20th	Night	16% 30% 31% 17% 6% 1	563
	21st to end	Night	15% 25% 34% 19% 7% 1	570
May	1st to 10th	Night	16% 21% 25% 25% 10% 1	570
	11th to 20th	Night	17% 13% 27% 27% 14% 2	410
	21st to end	Night	23% 17% 21% 23% 15% 1	411
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% 34% 31% 10% 1	790
	11th to 20th	Night	23% 39% 28% 9% 1	950
	21st to end	Night	19% 37% 29% 10% 4	950
December	1st to 10th	Night	17% 36% 30% 13% 4	950
	11th to 20th	Night	16% 39% 33% 11% 2	950
	21st to end	Night	19% 33% 34% 11% 3	1045

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