Bear Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bear Creek Observatory clear sky chart from 2005-11-06 to 2024-03-18.

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	24% 9% 3 6% 6% 3 4 4 5% 6% 30%	78653
	Night	26% 10% 3 6% 5% 3 4 4 4 5% 28%	81032
Transparency	Day	58% 3 13% 18% 7% 1	78372
	Night	61% 3 12% 17% 7% 1	80893
Seeing	Day	27% 8% 17% 26% 17% 5%	14111
	Night	32% 6% 10% 26% 21% 4	22717

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	39% 16% 4 8% 5% 3 4 3 3 4 11%	2295
	11th to 20th	Day	27% 12% 4 7% 6% 3 4 4 5% 7% 21%	2516
	21st to end	Day	41% 14% 4 7% 5% 3 4 3 4 5% 11%	2840
February	1st to 10th	Day	33% 11% 5% 8% 6% 3 5% 4 4 5% 16%	1710
	11th to 20th	Day	29% 13% 4 7% 6% 3 4 4 4 5% 20%	1665
	21st to end	Day	29% 14% 4 6% 7% 3 4 5% 3 4 20%	1484
March	1st to 10th	Day	28% 13% 4 7% 6% 3 5% 4 5% 5% 20%	1932
	11th to 20th	Day	30% 12% 4 6% 5% 3 5% 5% 4 5% 21%	2059
	21st to end	Day	29% 12% 4 8% 7% 4 5% 4 5% 6% 15%	2258
April	1st to 10th	Day	27% 11% 4 7% 6% 4 4 5% 5% 7% 18%	2160
	11th to 20th	Day	24% 10% 4 7% 7% 4 6% 5% 5% 6% 23%	2144
	21st to end	Day	22% 9% 4 8% 7% 4 6% 5% 6% 6% 23%	2392
May	1st to 10th	Day	17% 9% 4 6% 7% 4 6% 5% 6% 7% 30%	2520
	11th to 20th	Day	19% 7% 4 6% 6% 4 5% 5% 6% 9% 29%	2477
	21st to end	Day	26% 8% 4 7% 6% 4 5% 4 5% 6% 25%	2723
June	1st to 10th	Day	23% 7% 3 6% 6% 3 6% 4 6% 7% 28%	2547
	11th to 20th	Day	25% 9% 4 6% 7% 4 5% 6% 5% 6% 23%	2700
	21st to end	Day	14% 6% 3 6% 6% 3 5% 5% 6% 7% 40%	2700
July	1st to 10th	Day	11% 6% 3 5% 4 3 4 5% 5% 6% 49%	2700
	11th to 20th	Day	11% 5% 2 4 4 3 4 4 6% 8% 51%	2573
	21st to end	Day	9% 4 2 3 3 1 3 3 4 5% 63%	2741
August	1st to 10th	Day	11% 5% 2 4 4 2 4 3 4 6% 55%	2482
	11th to 20th	Day	9% 4 1 4 3 2 3 4 4 9% 56%	2298
	21st to end	Day	14% 6% 3 6% 5% 3 3 4 4 7% 45%	2462
September	1st to 10th	Day	12% 5% 3 6% 5% 3 3 3 4 8% 49%	2160
	11th to 20th	Day	16% 6% 3 8% 6% 3 4 4 4 7% 38%	2160
	21st to end	Day	20% 7% 3 6% 5% 3 5% 4 5% 6% 37%	1908
October	1st to 10th	Day	19% 8% 4 7% 6% 3 5% 4 6% 7% 30%	1800
	11th to 20th	Day	27% 10% 3 6% 5% 3 4 4 5% 7% 26%	1800
	21st to end	Day	33% 8% 3 6% 5% 3 5% 4 5% 6% 22%	1860
November	1st to 10th	Day	39% 10% 3 5% 5% 2 4 5% 4 6% 18%	1536
	11th to 20th	Day	35% 11% 3 6% 5% 2 5% 4 3 4 22%	1520
	21st to end	Day	36% 9% 3 6% 5% 3 3 4 5% 5% 20%	1520
December	1st to 10th	Day	31% 11% 4 6% 6% 3 4 3 3 5% 23%	1520
	11th to 20th	Day	33% 13% 4 6% 6% 4 5% 4 4 5% 15%	1925
	21st to end	Day	29% 11% 6% 10% 7% 4 5% 5% 4 5% 14%	2566
January	1st to 10th	Night	39% 13% 4 8% 6% 3 4 4 4 3 11%	4629
	11th to 20th	Night	29% 12% 5% 6% 4 2 3 4 4 7% 23%	4382
	21st to end	Night	39% 12% 4 7% 6% 4 4 3 4 5% 13%	4800
February	1st to 10th	Night	34% 11% 4 7% 7% 3 4 4 4 5% 18%	2470
	11th to 20th	Night	28% 13% 4 6% 6% 4 4 4 4 6% 22%	2402
	21st to end	Night	31% 13% 4 6% 5% 2 4 4 4 6% 21%	1970
March	1st to 10th	Night	27% 12% 4 6% 6% 3 4 5% 4 6% 24%	2247
	11th to 20th	Night	27% 10% 3 7% 5% 3 4 4 5% 6% 26%	2066
	21st to end	Night	29% 11% 4 7% 7% 4 5% 5% 5% 5% 18%	2098
April	1st to 10th	Night	26% 10% 3 8% 6% 4 5% 6% 7% 7% 19%	1800
	11th to 20th	Night	22% 10% 4 7% 5% 3 6% 5% 6% 6% 27%	1763
	21st to end	Night	25% 10% 4 7% 5% 4 5% 4 5% 6% 26%	1566
May	1st to 10th	Night	19% 8% 3 7% 5% 2 4 4 6% 6% 35%	1440
	11th to 20th	Night	20% 6% 3 6% 5% 3 4 6% 4 8% 34%	1416
	21st to end	Night	21% 8% 4 7% 7% 3 5% 5% 5% 6% 30%	1552
June	1st to 10th	Night	21% 6% 3 6% 7% 4 5% 4 5% 6% 32%	1408
	11th to 20th	Night	25% 9% 5% 7% 6% 3 5% 3 5% 6% 25%	1260
	21st to end	Night	13% 4 3 5% 7% 3 3 4 6% 7% 44%	1260
July	1st to 10th	Night	12% 5% 2 5% 5% 3 3 4 5% 7% 50%	1260
	11th to 20th	Night	9% 5% 2 4 4 2 3 4 4 6% 58%	1386
	21st to end	Night	8% 3 2 3 3 1 2 3 4 6% 64%	1576
August	1st to 10th	Night	10% 3 2 3 4 2 3 3 4 7% 59%	1453
	11th to 20th	Night	7% 4 1 3 3 1 2 3 5% 7% 62%	1602
	21st to end	Night	14% 5% 2 4 5% 2 2 4 4 5% 52%	1894
September	1st to 10th	Night	10% 5% 2 5% 4 2 3 3 4 5% 54%	1800
	11th to 20th	Night	17% 7% 2 6% 5% 2 4 4 5% 5% 43%	1800
	21st to end	Night	19% 7% 2 5% 5% 4 4 4 4 5% 40%	2050
October	1st to 10th	Night	20% 7% 3 6% 7% 4 4 4 4 6% 34%	2160
	11th to 20th	Night	25% 8% 3 6% 4 3 5% 5% 4 5% 32%	2160
	21st to end	Night	29% 9% 2 6% 4 3 4 3 5% 7% 29%	2494
November	1st to 10th	Night	35% 9% 3 6% 6% 3 4 4 4 5% 20%	2540
	11th to 20th	Night	35% 8% 3 7% 5% 3 5% 3 4 4 23%	2660
	21st to end	Night	34% 8% 3 7% 6% 3 4 3 4 5% 22%	2660
December	1st to 10th	Night	32% 9% 4 7% 5% 3 4 4 4 5% 25%	2660
	11th to 20th	Night	31% 13% 5% 8% 6% 4 5% 5% 5% 4 16%	3280
	21st to end	Night	30% 14% 4 9% 7% 4 4 4 4 5% 15%	5068

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	81% 1 6% 9% 2 1	2295
	11th to 20th	Day	65% 2 10% 15% 6% 1	2540
	21st to end	Day	78% 2 8% 9% 3	2840
February	1st to 10th	Day	71% 4 11% 10% 3 1	1710
	11th to 20th	Day	70% 3 9% 10% 7% 1	1674
	21st to end	Day	70% 2 10% 12% 5%	1484
March	1st to 10th	Day	69% 2 11% 15% 4	1932
	11th to 20th	Day	69% 2 10% 14% 3 1	2059
	21st to end	Day	73% 4 11% 11% 1	2258
April	1st to 10th	Day	70% 4 14% 11% 1	2160
	11th to 20th	Day	66% 3 11% 13% 6% 1	2144
	21st to end	Day	65% 5% 12% 13% 5% 1	2392
May	1st to 10th	Day	57% 4 16% 17% 5%	2520
	11th to 20th	Day	55% 5% 19% 16% 4	2480
	21st to end	Day	63% 4 16% 15% 2	2724
June	1st to 10th	Day	60% 4 17% 14% 5%	2547
	11th to 20th	Day	65% 3 15% 13% 3	2700
	21st to end	Day	46% 4 21% 20% 8% 1	2700
July	1st to 10th	Day	40% 4 14% 27% 13% 2	2626
	11th to 20th	Day	37% 4 12% 31% 15% 1	2495
	21st to end	Day	27% 2 14% 37% 17% 3	2589
August	1st to 10th	Day	34% 4 15% 32% 14% 2	2465
	11th to 20th	Day	29% 4 22% 30% 12% 3	2298
	21st to end	Day	42% 3 14% 32% 9% 1	2462
September	1st to 10th	Day	38% 2 15% 28% 12% 5%	2160
	11th to 20th	Day	48% 3 13% 20% 14% 2	2160
	21st to end	Day	49% 4 16% 20% 10% 2	1908
October	1st to 10th	Day	54% 3 12% 24% 7% 1	1800
	11th to 20th	Day	59% 3 15% 18% 5% 1	1800
	21st to end	Day	64% 3 10% 14% 9%	1860
November	1st to 10th	Day	71% 3 11% 11% 4	1536
	11th to 20th	Day	69% 3 9% 11% 7% 2	1520
	21st to end	Day	68% 3 7% 12% 8% 2	1520
December	1st to 10th	Day	66% 5% 8% 12% 8% 1	1520
	11th to 20th	Day	71% 3 10% 13% 3 1	1928
	21st to end	Day	73% 2 11% 11% 3	2566
January	1st to 10th	Night	80% 2 6% 8% 2 1	4629
	11th to 20th	Night	64% 2 8% 19% 6% 1	4420
	21st to end	Night	76% 1 9% 9% 4 1	4800
February	1st to 10th	Night	71% 3 10% 10% 5% 1	2470
	11th to 20th	Night	65% 3 12% 11% 7% 1	2404
	21st to end	Night	68% 3 10% 13% 5% 1	1970
March	1st to 10th	Night	63% 3 13% 16% 5%	2247
	11th to 20th	Night	62% 4 12% 17% 4 1	2067
	21st to end	Night	70% 4 15% 11% 1	2098
April	1st to 10th	Night	65% 5% 14% 13% 3	1800
	11th to 20th	Night	61% 3 14% 14% 8% 1	1763
	21st to end	Night	63% 4 12% 15% 5% 1	1566
May	1st to 10th	Night	52% 3 18% 20% 7% 1	1440
	11th to 20th	Night	52% 5% 20% 17% 6% 1	1416
	21st to end	Night	59% 6% 17% 16% 2	1552
June	1st to 10th	Night	58% 3 16% 18% 5%	1408
	11th to 20th	Night	64% 4 15% 14% 2	1260
	21st to end	Night	42% 4 24% 21% 7% 2	1260
July	1st to 10th	Night	39% 3 15% 25% 14% 4	1225
	11th to 20th	Night	33% 3 14% 33% 16% 1	1341
	21st to end	Night	24% 3 11% 37% 21% 4	1488
August	1st to 10th	Night	29% 4 14% 34% 17% 3	1445
	11th to 20th	Night	26% 4 18% 35% 14% 3	1602
	21st to end	Night	37% 2 16% 34% 10% 1	1894
September	1st to 10th	Night	35% 2 16% 30% 13% 4	1800
	11th to 20th	Night	47% 3 11% 21% 14% 4	1800
	21st to end	Night	47% 4 14% 22% 11% 2	2050
October	1st to 10th	Night	52% 3 14% 24% 7% 1	2160
	11th to 20th	Night	56% 3 15% 19% 7% 1	2160
	21st to end	Night	60% 4 11% 16% 9%	2494
November	1st to 10th	Night	68% 3 13% 13% 4 1	2540
	11th to 20th	Night	68% 3 10% 11% 5% 2	2660
	21st to end	Night	68% 3 8% 12% 9% 1	2660
December	1st to 10th	Night	65% 4 10% 12% 8% 1	2660
	11th to 20th	Night	72% 2 11% 11% 3 1	3276
	21st to end	Night	75% 2 10% 11% 3	5068

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	40% 11% 10% 17% 12% 10%	180
	11th to 20th	Day	38% 8% 13% 17% 20% 4	180
	21st to end	Day	47% 12% 8% 16% 11% 6%	231
February	1st to 10th	Day	44% 7% 16% 19% 9% 4	370
	11th to 20th	Day	43% 11% 19% 16% 10% 2	365
	21st to end	Day	38% 12% 21% 19% 9% 2	341
March	1st to 10th	Day	36% 12% 20% 18% 11% 2	460
	11th to 20th	Day	33% 11% 17% 20% 14% 4	449
	21st to end	Day	40% 10% 21% 17% 11% 1	462
April	1st to 10th	Day	36% 10% 20% 18% 12% 3	420
	11th to 20th	Day	31% 11% 24% 17% 10% 6%	412
	21st to end	Day	28% 10% 23% 24% 13% 2	420
May	1st to 10th	Day	20% 8% 21% 35% 14% 2	423
	11th to 20th	Day	29% 7% 16% 25% 17% 7%	448
	21st to end	Day	29% 7% 17% 25% 16% 5%	485
June	1st to 10th	Day	25% 9% 21% 26% 15% 5%	477
	11th to 20th	Day	26% 11% 19% 27% 13% 3	630
	21st to end	Day	14% 5% 19% 37% 21% 5%	630
July	1st to 10th	Day	14% 5% 17% 34% 27% 4	621
	11th to 20th	Day	10% 5% 18% 34% 28% 5%	494
	21st to end	Day	11% 2 12% 39% 28% 8%	487
August	1st to 10th	Day	13% 4 12% 34% 24% 12%	447
	11th to 20th	Day	8% 2 15% 34% 31% 11%	437
	21st to end	Day	17% 5% 19% 32% 19% 8%	495
September	1st to 10th	Day	15% 8% 20% 35% 18% 4	450
	11th to 20th	Day	20% 8% 14% 28% 25% 6%	450
	21st to end	Day	22% 7% 17% 29% 19% 6%	450
October	1st to 10th	Day	26% 7% 15% 25% 18% 8%	450
	11th to 20th	Day	34% 8% 13% 23% 15% 7%	450
	21st to end	Day	37% 10% 13% 22% 13% 5%	375
November	1st to 10th	Day	42% 12% 10% 19% 16% 2	204
	11th to 20th	Day	42% 12% 10% 8% 19% 9%	180
	21st to end	Day	37% 8% 12% 19% 13% 11%	180
December	1st to 10th	Day	46% 7% 8% 13% 13% 14%	180
	11th to 20th	Day	42% 12% 11% 22% 12% 2	180
	21st to end	Day	41% 9% 11% 20% 16% 4	198
January	1st to 10th	Night	48% 9% 10% 16% 13% 3	822
	11th to 20th	Night	42% 8% 9% 20% 18% 4	760
	21st to end	Night	41% 6% 10% 25% 16% 3	836
February	1st to 10th	Night	43% 6% 10% 26% 12% 2	760
	11th to 20th	Night	39% 7% 11% 27% 15% 1	744
	21st to end	Night	41% 8% 11% 25% 12% 3	628
March	1st to 10th	Night	36% 7% 13% 26% 16% 2	760
	11th to 20th	Night	34% 9% 13% 26% 15% 3	752
	21st to end	Night	38% 7% 13% 28% 11% 2	792
April	1st to 10th	Night	35% 6% 15% 26% 16% 3	720
	11th to 20th	Night	30% 7% 15% 28% 16% 3	695
	21st to end	Night	32% 7% 12% 29% 16% 3	486
May	1st to 10th	Night	23% 9% 11% 31% 23% 2	360
	11th to 20th	Night	21% 6% 9% 27% 31% 6%	354
	21st to end	Night	27% 5% 12% 21% 29% 6%	389
June	1st to 10th	Night	24% 5% 11% 31% 25% 3	360
	11th to 20th	Night	31% 8% 9% 29% 20% 3	360
	21st to end	Night	16% 4 12% 31% 32% 6%	360
July	1st to 10th	Night	15% 4 8% 25% 41% 6%	350
	11th to 20th	Night	11% 3 6% 33% 43% 5%	348
	21st to end	Night	11% 1 6% 28% 47% 7%	394
August	1st to 10th	Night	11% 2 5% 24% 46% 11%	379
	11th to 20th	Night	10% 2 4 24% 44% 16%	534
	21st to end	Night	18% 4 6% 28% 37% 7%	706
September	1st to 10th	Night	13% 4 7% 33% 36% 7%	720
	11th to 20th	Night	20% 4 5% 23% 36% 10%	720
	21st to end	Night	23% 4 8% 28% 29% 7%	720
October	1st to 10th	Night	25% 4 8% 27% 30% 5%	720
	11th to 20th	Night	31% 7% 10% 24% 22% 7%	720
	21st to end	Night	38% 6% 11% 26% 16% 4	792
November	1st to 10th	Night	41% 6% 14% 26% 12% 1	740
	11th to 20th	Night	40% 8% 11% 23% 14% 3	760
	21st to end	Night	41% 7% 11% 22% 14% 5%	760
December	1st to 10th	Night	38% 6% 10% 26% 18% 3	760
	11th to 20th	Night	45% 8% 12% 23% 10% 2	760
	21st to end	Night	50% 8% 11% 20% 10% 1	896

Page generated at 2024-03-15 11:01:42. Version ac07

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