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 2024-03-06.

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% 7% 3 7% 7% 4 7% 7% 8% 9% 34%	78047
	Night	4 7% 3 7% 7% 4 7% 8% 9% 10% 35%	81427
Transparency	Day	39% 13% 20% 17% 9% 2	77761
	Night	36% 15% 22% 17% 8% 2	81293
Seeing	Day	19% 28% 29% 16% 6% 1	14313
	Night	16% 28% 30% 19% 7% 1	23751

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 10% 11% 6% 8% 10% 12% 11% 22%	2219
	11th to 20th	Day	1 5% 3 12% 10% 6% 8% 10% 10% 8% 27%	2392
	21st to end	Day	1 6% 5% 9% 8% 5% 10% 12% 10% 11% 23%	2794
February	1st to 10th	Day	2 8% 5% 10% 10% 5% 9% 9% 11% 9% 22%	1603
	11th to 20th	Day	1 10% 6% 11% 9% 4 7% 10% 9% 11% 22%	1665
	21st to end	Day	2 7% 5% 13% 10% 5% 8% 10% 9% 9% 24%	1551
March	1st to 10th	Day	1 6% 4 10% 11% 6% 8% 9% 11% 10% 25%	1855
	11th to 20th	Day	2 8% 5% 7% 9% 5% 9% 10% 10% 7% 28%	1979
	21st to end	Day	4 12% 5% 9% 8% 5% 8% 9% 10% 9% 21%	2178
April	1st to 10th	Day	5% 10% 6% 11% 9% 5% 9% 9% 9% 8% 21%	2187
	11th to 20th	Day	6% 14% 5% 10% 7% 4 7% 8% 7% 7% 24%	2310
	21st to end	Day	7% 15% 4 7% 7% 4 6% 7% 7% 8% 27%	2340
May	1st to 10th	Day	8% 13% 4 7% 6% 4 7% 6% 7% 8% 30%	2340
	11th to 20th	Day	13% 12% 3 7% 6% 4 5% 6% 6% 7% 30%	2585
	21st to end	Day	21% 11% 4 6% 6% 3 5% 7% 6% 8% 23%	2917
June	1st to 10th	Day	14% 8% 3 7% 6% 4 7% 7% 7% 7% 29%	2700
	11th to 20th	Day	14% 8% 4 8% 7% 5% 7% 8% 7% 8% 25%	2700
	21st to end	Day	6% 4 2 6% 6% 3 7% 7% 7% 9% 44%	2700
July	1st to 10th	Day	5% 4 2 4 5% 3 5% 6% 7% 9% 51%	2700
	11th to 20th	Day	3 3 2 3 5% 3 5% 5% 6% 8% 57%	2700
	21st to end	Day	3 3 1 2 3 3 4 4 5% 7% 64%	2780
August	1st to 10th	Day	6% 4 2 4 4 2 5% 5% 6% 7% 54%	2444
	11th to 20th	Day	7% 4 1 4 5% 3 4 4 6% 9% 53%	2299
	21st to end	Day	7% 4 2 4 5% 2 5% 5% 7% 8% 50%	2574
September	1st to 10th	Day	10% 6% 2 5% 4 3 5% 4 4 7% 49%	2190
	11th to 20th	Day	7% 6% 2 5% 6% 4 5% 6% 6% 10% 43%	1980
	21st to end	Day	11% 6% 2 4 5% 3 5% 6% 5% 9% 44%	1980
October	1st to 10th	Day	10% 11% 3 5% 6% 3 6% 5% 7% 9% 36%	1903
	11th to 20th	Day	4 9% 4 7% 8% 4 7% 6% 7% 9% 35%	1726
	21st to end	Day	3 9% 4 8% 7% 5% 6% 8% 8% 9% 33%	1782
November	1st to 10th	Day	5% 9% 4 6% 9% 4 8% 9% 10% 10% 26%	1608
	11th to 20th	Day	1 6% 4 6% 7% 3 8% 10% 10% 11% 35%	1520
	21st to end	Day	2 7% 4 8% 9% 5% 7% 8% 9% 10% 30%	1392
December	1st to 10th	Day	5% 4 8% 9% 5% 9% 9% 11% 14% 26%	1330
	11th to 20th	Day	1 7% 5% 10% 10% 5% 7% 9% 11% 11% 24%	1682
	21st to end	Day	1 7% 4 7% 8% 4 8% 8% 9% 12% 32%	2442
January	1st to 10th	Night	1 5% 4 9% 9% 5% 9% 13% 14% 12% 20%	4726
	11th to 20th	Night	6% 3 9% 10% 7% 8% 8% 10% 10% 28%	4537
	21st to end	Night	1 9% 3 7% 6% 4 9% 15% 11% 10% 24%	4862
February	1st to 10th	Night	2 9% 5% 9% 9% 3 8% 10% 10% 10% 24%	2575
	11th to 20th	Night	1 7% 6% 11% 8% 6% 7% 10% 10% 10% 24%	2402
	21st to end	Night	2 5% 4 11% 9% 6% 10% 9% 11% 8% 24%	1903
March	1st to 10th	Night	1 6% 4 9% 7% 5% 9% 9% 12% 11% 27%	2221
	11th to 20th	Night	3 6% 4 6% 6% 4 7% 12% 10% 12% 31%	1980
	21st to end	Night	4 8% 4 9% 9% 4 7% 10% 10% 10% 25%	2178
April	1st to 10th	Night	3 8% 4 7% 6% 4 8% 9% 10% 9% 33%	1770
	11th to 20th	Night	5% 12% 5% 7% 7% 4 5% 5% 8% 8% 33%	1601
	21st to end	Night	5% 11% 3 7% 6% 4 6% 6% 7% 8% 35%	1620
May	1st to 10th	Night	7% 11% 4 6% 6% 5% 6% 7% 6% 8% 35%	1620
	11th to 20th	Night	10% 9% 2 5% 6% 4 6% 6% 6% 10% 36%	1304
	21st to end	Night	17% 9% 4 4 5% 3 4 6% 8% 7% 32%	1358
June	1st to 10th	Night	13% 8% 3 6% 7% 3 5% 7% 7% 7% 34%	1260
	11th to 20th	Night	12% 6% 2 4 6% 4 7% 7% 6% 10% 35%	1260
	21st to end	Night	5% 4 1 4 4 2 5% 6% 7% 9% 53%	1260
July	1st to 10th	Night	5% 3 1 2 3 2 3 5% 6% 6% 63%	1260
	11th to 20th	Night	3 2 1 2 4 2 4 4 5% 8% 63%	1260
	21st to end	Night	4 2 1 2 3 2 2 3 4 7% 70%	1533
August	1st to 10th	Night	5% 4 1 2 4 2 4 4 5% 8% 61%	1493
	11th to 20th	Night	5% 3 1 2 3 2 4 4 6% 11% 58%	1602
	21st to end	Night	7% 5% 2 4 3 2 3 4 6% 7% 56%	1782
September	1st to 10th	Night	9% 4 2 5% 4 2 3 5% 5% 8% 53%	1767
	11th to 20th	Night	6% 6% 2 5% 5% 2 5% 5% 7% 7% 48%	1980
	21st to end	Night	8% 6% 2 4 4 3 5% 6% 7% 9% 47%	1980
October	1st to 10th	Night	8% 10% 3 4 6% 3 4 5% 7% 8% 42%	2056
	11th to 20th	Night	4 7% 3 5% 6% 4 6% 9% 8% 11% 38%	2232
	21st to end	Night	3 7% 4 7% 8% 4 6% 8% 8% 11% 34%	2574
November	1st to 10th	Night	4 9% 4 8% 8% 4 7% 10% 9% 9% 29%	2451
	11th to 20th	Night	1 5% 4 6% 6% 5% 8% 10% 11% 11% 33%	2660
	21st to end	Night	2 5% 5% 8% 7% 4 6% 8% 9% 14% 32%	2786
December	1st to 10th	Night	1 6% 4 9% 8% 4 8% 10% 13% 12% 27%	2850
	11th to 20th	Night	1 6% 4 10% 10% 6% 9% 9% 9% 11% 25%	3510
	21st to end	Night	1 6% 4 7% 7% 5% 9% 10% 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	40% 20% 22% 14% 4	1855
	11th to 20th	Day	42% 18% 19% 16% 4 2	1980
	21st to end	Day	50% 14% 23% 10% 3 1	2178
April	1st to 10th	Day	50% 14% 20% 13% 2	2187
	11th to 20th	Day	51% 12% 20% 14% 3	2310
	21st to end	Day	51% 11% 19% 14% 4 1	2340
May	1st to 10th	Day	46% 11% 21% 17% 5%	2340
	11th to 20th	Day	50% 10% 16% 18% 6%	2589
	21st to end	Day	55% 11% 19% 12% 3	2918
June	1st to 10th	Day	49% 12% 18% 15% 6%	2700
	11th to 20th	Day	49% 11% 17% 16% 7% 1	2700
	21st to end	Day	31% 10% 22% 25% 11% 1	2700
July	1st to 10th	Day	27% 7% 16% 26% 18% 5%	2626
	11th to 20th	Day	21% 8% 15% 24% 24% 8%	2619
	21st to end	Day	18% 6% 11% 23% 34% 8%	2625
August	1st to 10th	Day	26% 8% 14% 24% 21% 7%	2427
	11th to 20th	Day	26% 8% 15% 31% 18% 3	2299
	21st to end	Day	28% 8% 15% 25% 20% 5%	2574
September	1st to 10th	Day	33% 8% 13% 23% 17% 6%	2190
	11th to 20th	Day	33% 10% 16% 23% 16% 3	1980
	21st to end	Day	34% 9% 21% 23% 10% 2	1980
October	1st to 10th	Day	40% 10% 20% 20% 8% 1	1903
	11th to 20th	Day	40% 11% 24% 17% 7% 1	1726
	21st to end	Day	40% 16% 19% 18% 6%	1782
November	1st to 10th	Day	44% 16% 22% 13% 4 1	1608
	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	37% 22% 23% 14% 3	2221
	11th to 20th	Night	33% 20% 25% 15% 6% 1	1980
	21st to end	Night	43% 17% 25% 10% 4	2178
April	1st to 10th	Night	38% 15% 25% 19% 3	1770
	11th to 20th	Night	44% 12% 20% 17% 5% 1	1601
	21st to end	Night	44% 11% 20% 18% 6%	1620
May	1st to 10th	Night	41% 11% 24% 18% 6%	1620
	11th to 20th	Night	40% 10% 19% 23% 8%	1304
	21st to end	Night	47% 11% 22% 16% 5%	1358
June	1st to 10th	Night	44% 9% 21% 20% 6%	1260
	11th to 20th	Night	39% 12% 21% 20% 7% 1	1260
	21st to end	Night	25% 9% 24% 28% 13% 1	1260
July	1st to 10th	Night	19% 5% 18% 31% 21% 6%	1225
	11th to 20th	Night	18% 6% 16% 27% 24% 8%	1218
	21st to end	Night	15% 3 11% 29% 30% 11%	1448
August	1st to 10th	Night	20% 6% 15% 30% 21% 8%	1485
	11th to 20th	Night	19% 6% 16% 37% 17% 4	1602
	21st to end	Night	25% 6% 14% 28% 23% 4	1782
September	1st to 10th	Night	28% 8% 17% 24% 15% 7%	1767
	11th to 20th	Night	30% 9% 17% 27% 14% 4	1980
	21st to end	Night	30% 10% 22% 22% 14% 2	1980
October	1st to 10th	Night	35% 11% 20% 21% 10% 2	2056
	11th to 20th	Night	32% 12% 25% 20% 9% 2	2232
	21st to end	Night	37% 17% 22% 17% 7% 1	2574
November	1st to 10th	Night	41% 16% 24% 12% 6% 1	2451
	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% 41% 26% 10% 3 1	441
	11th to 20th	Day	20% 32% 28% 13% 6% 1	420
	21st to end	Day	22% 36% 29% 8% 3 1	462
April	1st to 10th	Day	23% 37% 27% 11% 2	420
	11th to 20th	Day	23% 35% 27% 12% 3	412
	21st to end	Day	23% 31% 25% 14% 5% 1	420
May	1st to 10th	Day	21% 25% 29% 17% 7% 2	423
	11th to 20th	Day	27% 17% 24% 18% 12% 2	593
	21st to end	Day	26% 17% 24% 18% 13% 2	679
June	1st to 10th	Day	19% 27% 25% 19% 8% 2	630
	11th to 20th	Day	27% 28% 23% 16% 5% 1	630
	21st to end	Day	10% 20% 33% 23% 11% 2	630
July	1st to 10th	Day	9% 17% 30% 29% 13% 2	621
	11th to 20th	Day	8% 23% 37% 24% 8%	618
	21st to end	Day	6% 19% 38% 27% 8% 2	526
August	1st to 10th	Day	9% 21% 37% 23% 9% 1	447
	11th to 20th	Day	10% 16% 39% 24% 8% 2	437
	21st to end	Day	11% 23% 37% 21% 6% 1	495
September	1st to 10th	Day	20% 27% 30% 16% 7% 1	450
	11th to 20th	Day	15% 27% 32% 20% 6% 1	450
	21st to end	Day	23% 34% 27% 12% 3 1	450
October	1st to 10th	Day	21% 27% 28% 16% 7% 1	373
	11th to 20th	Day	23% 32% 21% 15% 7% 2	270
	21st to end	Day	28% 33% 25% 10% 3 1	297
November	1st to 10th	Day	32% 33% 27% 7% 1	270
	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	742
	11th to 20th	Night	17% 28% 29% 21% 4 1	720
	21st to end	Night	15% 28% 29% 19% 6% 1	792
April	1st to 10th	Night	12% 28% 32% 21% 6%	678
	11th to 20th	Night	16% 30% 30% 17% 6% 1	533
	21st to end	Night	15% 24% 33% 19% 7% 1	540
May	1st to 10th	Night	16% 21% 26% 25% 11% 1	540
	11th to 20th	Night	17% 13% 25% 28% 15% 2	389
	21st to end	Night	23% 15% 21% 24% 16% 2	389
June	1st to 10th	Night	19% 18% 23% 25% 13% 2	360
	11th to 20th	Night	20% 24% 28% 19% 9% 1	360
	21st to end	Night	8% 15% 31% 29% 16% 1	360
July	1st to 10th	Night	7% 13% 24% 38% 17% 1	350
	11th to 20th	Night	5% 14% 38% 34% 9%	348
	21st to end	Night	6% 10% 29% 41% 13% 1	394
August	1st to 10th	Night	9% 9% 26% 38% 17%	419
	11th to 20th	Night	6% 12% 29% 34% 16% 2	534
	21st to end	Night	11% 18% 29% 30% 10% 1	594
September	1st to 10th	Night	12% 19% 29% 22% 16% 3	638
	11th to 20th	Night	11% 18% 28% 28% 12% 4	720
	21st to end	Night	16% 22% 31% 21% 9% 2	720
October	1st to 10th	Night	18% 21% 28% 25% 7% 1	720
	11th to 20th	Night	17% 27% 27% 21% 7% 1	720
	21st to end	Night	19% 33% 32% 13% 3 1	792
November	1st to 10th	Night	24% 34% 31% 10% 1	786
	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

Page generated at 2024-03-03 04:28:55. Version ac07

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