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 2025-11-03.

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	21% 9% 3 7% 5% 4 4 3 5% 6% 33%	131311
	Night	24% 10% 3 7% 4 4 4 3 4 5% 30%	127363
Transparency	Day	53% 4 14% 19% 8% 1	131027
	Night	57% 4 13% 18% 7% 1	127224
Seeing	Day	26% 8% 17% 26% 17% 6%	16339
	Night	32% 6% 11% 26% 21% 4	24645

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	38% 15% 4 9% 6% 5% 4 3 4 4 8%	3297
	11th to 20th	Day	20% 11% 3 9% 6% 5% 5% 3 5% 7% 25%	3616
	21st to end	Day	32% 11% 3 7% 4 3 3 3 3 4 28%	4078
February	1st to 10th	Day	31% 12% 4 10% 5% 4 5% 4 5% 5% 16%	1800
	11th to 20th	Day	30% 14% 4 8% 5% 4 3 3 4 5% 20%	1755
	21st to end	Day	29% 14% 4 7% 6% 4 4 3 4 4 20%	1560
March	1st to 10th	Day	22% 14% 4 9% 7% 5% 5% 4 5% 6% 19%	4600
	11th to 20th	Day	21% 14% 4 9% 5% 4 5% 4 4 5% 26%	5000
	21st to end	Day	27% 11% 3 10% 6% 6% 6% 5% 6% 7% 12%	5695
April	1st to 10th	Day	27% 13% 4 8% 6% 5% 4 4 6% 7% 18%	2400
	11th to 20th	Day	22% 10% 3 7% 5% 5% 5% 4 5% 6% 26%	2386
	21st to end	Day	22% 11% 3 8% 6% 5% 5% 5% 5% 6% 24%	2658
May	1st to 10th	Day	16% 7% 3 6% 5% 4 4 4 5% 7% 39%	6320
	11th to 20th	Day	23% 9% 3 7% 5% 4 4 4 6% 9% 26%	6277
	21st to end	Day	23% 8% 3 7% 4 4 4 3 5% 8% 30%	6903
June	1st to 10th	Day	22% 8% 3 6% 5% 5% 5% 3 6% 8% 30%	2831
	11th to 20th	Day	24% 10% 3 7% 6% 5% 5% 4 5% 6% 24%	3000
	21st to end	Day	14% 7% 2 7% 5% 4 5% 4 7% 7% 38%	3000
July	1st to 10th	Day	10% 4 2 4 2 2 2 3 4 6% 62%	6780
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 5% 70%	6443
	21st to end	Day	9% 4 2 4 3 3 3 3 4 6% 58%	6874
August	1st to 10th	Day	7% 3 1 4 2 3 3 3 5% 7% 62%	4488
	11th to 20th	Day	11% 6% 2 7% 5% 4 5% 4 6% 9% 41%	4156
	21st to end	Day	16% 8% 3 7% 4 3 3 3 4 7% 42%	4454
September	1st to 10th	Day	11% 6% 2 7% 4 4 4 2 4 8% 48%	2400
	11th to 20th	Day	16% 7% 3 8% 5% 4 4 4 5% 7% 38%	2400
	21st to end	Day	20% 7% 3 6% 4 4 5% 3 5% 6% 37%	2118
October	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 4 6% 10% 36%	3280
	11th to 20th	Day	31% 9% 3 8% 4 4 4 3 4 6% 24%	3261
	21st to end	Day	30% 11% 3 9% 5% 5% 4 3 6% 6% 18%	3336
November	1st to 10th	Day	38% 10% 3 6% 4 4 3 4 4 6% 17%	1636
	11th to 20th	Day	34% 12% 2 6% 4 4 4 3 4 4 21%	1600
	21st to end	Day	35% 10% 3 7% 5% 4 3 3 6% 5% 19%	1600
December	1st to 10th	Day	24% 9% 3 7% 6% 6% 5% 4 5% 6% 25%	2620
	11th to 20th	Day	35% 14% 4 8% 5% 5% 4 3 4 5% 12%	3025
	21st to end	Day	33% 13% 5% 10% 6% 5% 4 4 4 4 13%	3664
January	1st to 10th	Night	37% 13% 4 11% 6% 5% 5% 3 4 3 10%	6613
	11th to 20th	Night	21% 11% 4 6% 4 4 4 4 5% 8% 31%	6282
	21st to end	Night	29% 10% 3 7% 4 4 3 3 4 5% 30%	6864
February	1st to 10th	Night	32% 13% 4 8% 6% 4 3 3 4 5% 19%	2600
	11th to 20th	Night	28% 14% 3 7% 5% 4 4 3 4 6% 22%	2532
	21st to end	Night	31% 14% 4 7% 4 3 4 3 4 6% 21%	2070
March	1st to 10th	Night	19% 14% 4 9% 6% 5% 7% 5% 6% 7% 19%	5293
	11th to 20th	Night	26% 12% 3 7% 4 3 4 3 4 6% 29%	4999
	21st to end	Night	33% 13% 3 9% 5% 5% 5% 4 5% 5% 14%	5280
April	1st to 10th	Night	25% 11% 3 9% 6% 5% 6% 5% 7% 6% 18%	2000
	11th to 20th	Night	21% 10% 3 8% 5% 5% 4 4 5% 5% 30%	1961
	21st to end	Night	25% 11% 3 7% 5% 5% 3 4 5% 7% 26%	1740
May	1st to 10th	Night	16% 8% 3 8% 4 3 4 3 4 5% 42%	3640
	11th to 20th	Night	24% 9% 3 8% 5% 4 4 3 4 6% 30%	3616
	21st to end	Night	24% 10% 3 8% 6% 5% 5% 3 6% 6% 24%	3972
June	1st to 10th	Night	20% 7% 3 7% 6% 5% 5% 3 6% 5% 33%	1564
	11th to 20th	Night	24% 11% 4 8% 6% 4 4 3 6% 5% 27%	1400
	21st to end	Night	14% 5% 2 7% 5% 4 4 3 6% 7% 42%	1400
July	1st to 10th	Night	14% 5% 2 4 3 2 2 2 4 5% 58%	3180
	11th to 20th	Night	4 3 1 3 2 2 2 2 3 4 74%	3502
	21st to end	Night	9% 4 1 4 3 2 3 3 5% 6% 59%	3969
August	1st to 10th	Night	6% 2 1 3 3 2 3 2 4 5% 68%	2660
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 7% 50%	2902
	21st to end	Night	19% 6% 2 6% 4 3 3 2 4 5% 47%	3461
September	1st to 10th	Night	10% 5% 2 6% 3 4 3 3 4 6% 54%	2000
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 3 5% 5% 42%	2000
	21st to end	Night	19% 8% 2 6% 4 5% 4 3 4 5% 39%	2280
October	1st to 10th	Night	17% 7% 2 6% 5% 4 4 3 6% 8% 37%	3920
	11th to 20th	Night	29% 8% 2 7% 3 5% 4 3 4 6% 29%	3897
	21st to end	Night	31% 12% 3 8% 4 4 4 3 4 5% 22%	4466
November	1st to 10th	Night	35% 10% 3 7% 4 4 4 3 5% 5% 20%	2704
	11th to 20th	Night	36% 10% 3 8% 5% 4 4 2 4 3 22%	2800
	21st to end	Night	33% 10% 3 8% 5% 4 4 3 4 5% 22%	2800
December	1st to 10th	Night	25% 9% 3 7% 4 4 6% 6% 5% 5% 27%	4560
	11th to 20th	Night	35% 14% 4 9% 5% 5% 4 3 5% 3 13%	5180
	21st to end	Night	33% 17% 4 10% 5% 5% 4 3 3 4 12%	7256

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	79% 3 7% 9% 2	3297
	11th to 20th	Day	57% 4 11% 19% 7% 1	3640
	21st to end	Day	61% 2 9% 17% 11%	4078
February	1st to 10th	Day	70% 3 11% 10% 4 1	1800
	11th to 20th	Day	70% 3 9% 10% 8% 1	1764
	21st to end	Day	70% 3 10% 13% 5%	1560
March	1st to 10th	Day	66% 3 10% 15% 6%	4600
	11th to 20th	Day	63% 3 13% 19% 2	5000
	21st to end	Day	71% 4 13% 11% 1	5695
April	1st to 10th	Day	69% 4 14% 11% 1	2400
	11th to 20th	Day	62% 3 12% 15% 7% 1	2386
	21st to end	Day	64% 5% 12% 14% 4 1	2658
May	1st to 10th	Day	48% 3 14% 25% 11%	6320
	11th to 20th	Day	57% 5% 18% 18% 3	6280
	21st to end	Day	55% 4 24% 16% 1	6904
June	1st to 10th	Day	57% 4 17% 16% 5%	2831
	11th to 20th	Day	64% 4 15% 14% 4	3000
	21st to end	Day	47% 4 20% 20% 7% 1	3000
July	1st to 10th	Day	27% 4 17% 23% 20% 9%	6706
	11th to 20th	Day	20% 3 15% 32% 28% 2	6365
	21st to end	Day	28% 4 23% 32% 11% 1	6681
August	1st to 10th	Day	24% 4 13% 41% 16% 1	4471
	11th to 20th	Day	41% 7% 21% 22% 7% 2	4195
	21st to end	Day	44% 3 13% 28% 11% 1	4453
September	1st to 10th	Day	39% 3 15% 27% 11% 5%	2400
	11th to 20th	Day	48% 3 13% 20% 14% 2	2400
	21st to end	Day	48% 4 17% 20% 9% 2	2118
October	1st to 10th	Day	43% 4 16% 31% 6%	3280
	11th to 20th	Day	62% 3 11% 16% 8%	3261
	21st to end	Day	67% 4 11% 9% 7% 1	3336
November	1st to 10th	Day	71% 3 11% 11% 4	1636
	11th to 20th	Day	69% 3 9% 10% 7% 2	1600
	21st to end	Day	68% 4 7% 12% 8% 2	1600
December	1st to 10th	Day	55% 5% 9% 18% 12%	2620
	11th to 20th	Day	74% 5% 9% 10% 2	3028
	21st to end	Day	76% 3 10% 9% 2	3664
January	1st to 10th	Night	79% 3 7% 8% 3	6613
	11th to 20th	Night	52% 4 15% 22% 6% 1	6320
	21st to end	Night	59% 2 11% 18% 9%	6864
February	1st to 10th	Night	70% 3 10% 10% 6% 1	2600
	11th to 20th	Night	66% 3 11% 11% 7% 1	2534
	21st to end	Night	67% 3 10% 14% 4	2070
March	1st to 10th	Night	63% 3 12% 16% 5%	5293
	11th to 20th	Night	59% 4 15% 19% 2	5000
	21st to end	Night	74% 4 13% 9% 1	5280
April	1st to 10th	Night	66% 5% 14% 12% 2	2000
	11th to 20th	Night	58% 3 15% 15% 9% 1	1961
	21st to end	Night	63% 4 13% 15% 5% 1	1740
May	1st to 10th	Night	48% 2 14% 27% 8%	3637
	11th to 20th	Night	59% 3 17% 17% 3	3616
	21st to end	Night	62% 6% 19% 12% 1	3972
June	1st to 10th	Night	56% 3 17% 20% 5%	1564
	11th to 20th	Night	62% 4 14% 17% 2	1400
	21st to end	Night	44% 4 23% 21% 6% 2	1400
July	1st to 10th	Night	32% 3 15% 20% 18% 12%	3145
	11th to 20th	Night	17% 3 14% 30% 34% 1	3457
	21st to end	Night	27% 5% 20% 32% 14% 2	3857
August	1st to 10th	Night	20% 3 14% 42% 20% 2	2652
	11th to 20th	Night	40% 6% 20% 24% 8% 2	2929
	21st to end	Night	42% 3 16% 30% 9% 1	3461
September	1st to 10th	Night	35% 3 17% 28% 13% 4	2000
	11th to 20th	Night	47% 3 12% 21% 13% 4	2000
	21st to end	Night	48% 4 14% 22% 10% 2	2280
October	1st to 10th	Night	44% 5% 19% 28% 4	3920
	11th to 20th	Night	57% 3 12% 19% 7%	3897
	21st to end	Night	66% 5% 10% 12% 7%	4466
November	1st to 10th	Night	67% 3 13% 12% 3 1	2704
	11th to 20th	Night	69% 3 10% 11% 5% 2	2800
	21st to end	Night	68% 3 8% 12% 9% 1	2800
December	1st to 10th	Night	55% 5% 9% 20% 9% 1	4560
	11th to 20th	Night	75% 3 9% 10% 2	5176
	21st to end	Night	77% 3 8% 9% 2	7256

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	42% 10% 10% 19% 12% 9%	200
	11th to 20th	Day	36% 8% 14% 19% 19% 6%	200
	21st to end	Day	44% 11% 9% 17% 12% 8%	255
February	1st to 10th	Day	43% 8% 17% 20% 10% 4	400
	11th to 20th	Day	42% 10% 20% 15% 10% 2	395
	21st to end	Day	38% 11% 20% 18% 9% 2	369
March	1st to 10th	Day	36% 12% 20% 18% 11% 2	500
	11th to 20th	Day	32% 13% 17% 19% 14% 4	500
	21st to end	Day	38% 10% 22% 19% 9% 1	550
April	1st to 10th	Day	35% 10% 21% 20% 11% 3	500
	11th to 20th	Day	28% 12% 25% 20% 10% 6%	492
	21st to end	Day	27% 10% 22% 23% 14% 3	500
May	1st to 10th	Day	19% 9% 22% 33% 14% 2	503
	11th to 20th	Day	28% 9% 16% 24% 16% 7%	528
	21st to end	Day	29% 8% 18% 24% 16% 6%	573
June	1st to 10th	Day	24% 8% 19% 28% 15% 5%	561
	11th to 20th	Day	25% 11% 20% 28% 14% 3	730
	21st to end	Day	15% 5% 19% 35% 21% 5%	730
July	1st to 10th	Day	13% 4 17% 34% 27% 4	721
	11th to 20th	Day	9% 4 17% 35% 28% 6%	579
	21st to end	Day	11% 3 13% 38% 28% 8%	575
August	1st to 10th	Day	13% 5% 12% 35% 24% 11%	527
	11th to 20th	Day	10% 3 15% 34% 29% 10%	517
	21st to end	Day	16% 5% 19% 32% 19% 9%	583
September	1st to 10th	Day	14% 8% 18% 33% 21% 6%	530
	11th to 20th	Day	19% 7% 15% 28% 25% 6%	530
	21st to end	Day	20% 8% 17% 29% 19% 6%	530
October	1st to 10th	Day	24% 8% 18% 25% 18% 8%	530
	11th to 20th	Day	34% 9% 13% 22% 15% 7%	530
	21st to end	Day	37% 11% 15% 21% 12% 4	449
November	1st to 10th	Day	41% 12% 12% 18% 15% 3	232
	11th to 20th	Day	43% 13% 11% 9% 17% 9%	200
	21st to end	Day	36% 9% 12% 19% 15% 10%	200
December	1st to 10th	Day	43% 6% 8% 13% 14% 18%	200
	11th to 20th	Day	44% 13% 11% 21% 11% 2	200
	21st to end	Day	44% 9% 10% 19% 15% 4	220
January	1st to 10th	Night	47% 10% 10% 17% 13% 3	868
	11th to 20th	Night	40% 7% 9% 23% 18% 4	800
	21st to end	Night	39% 5% 10% 26% 16% 3	880
February	1st to 10th	Night	42% 7% 11% 26% 12% 2	800
	11th to 20th	Night	39% 7% 11% 28% 15% 1	784
	21st to end	Night	42% 8% 11% 26% 12% 3	660
March	1st to 10th	Night	35% 7% 14% 27% 16% 2	800
	11th to 20th	Night	34% 10% 13% 26% 14% 3	800
	21st to end	Night	38% 8% 14% 28% 11% 2	880
April	1st to 10th	Night	34% 7% 15% 25% 16% 2	800
	11th to 20th	Night	29% 7% 16% 30% 15% 3	773
	21st to end	Night	31% 8% 11% 29% 17% 4	540
May	1st to 10th	Night	22% 10% 12% 33% 22% 2	400
	11th to 20th	Night	22% 8% 10% 26% 29% 5%	394
	21st to end	Night	27% 6% 12% 21% 29% 5%	433
June	1st to 10th	Night	23% 6% 12% 31% 26% 3	400
	11th to 20th	Night	30% 7% 12% 29% 21% 3	400
	21st to end	Night	16% 5% 12% 31% 31% 5%	400
July	1st to 10th	Night	15% 4 9% 27% 39% 5%	390
	11th to 20th	Night	10% 3 6% 34% 42% 5%	388
	21st to end	Night	11% 1 6% 28% 47% 7%	438
August	1st to 10th	Night	11% 2 5% 24% 47% 10%	423
	11th to 20th	Night	12% 2 5% 24% 42% 14%	594
	21st to end	Night	18% 4 7% 27% 37% 8%	785
September	1st to 10th	Night	13% 4 7% 32% 37% 8%	800
	11th to 20th	Night	20% 4 7% 25% 35% 10%	800
	21st to end	Night	23% 5% 9% 29% 28% 6%	800
October	1st to 10th	Night	25% 4 10% 27% 29% 5%	800
	11th to 20th	Night	32% 7% 11% 25% 20% 6%	800
	21st to end	Night	37% 7% 12% 25% 15% 4	880
November	1st to 10th	Night	41% 6% 14% 26% 12% 1	788
	11th to 20th	Night	41% 9% 11% 23% 13% 3	800
	21st to end	Night	40% 7% 11% 22% 15% 5%	800
December	1st to 10th	Night	37% 6% 9% 26% 19% 4	800
	11th to 20th	Night	45% 7% 12% 23% 10% 2	800
	21st to end	Night	50% 9% 12% 19% 10% 1	947

Page generated at 2025-10-30 23:02:37. Version ac07

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