Birch Creek Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birch Creek Center clear sky chart from 2005-11-06 to 2025-02-25.

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	3 5% 2 7% 5% 5% 6% 5% 7% 8% 45%	112090
	Night	3 5% 2 6% 5% 5% 5% 5% 7% 9% 49%	115323
Transparency	Day	31% 5% 16% 25% 17% 6%	111827
	Night	29% 7% 18% 27% 15% 5%	115175
Seeing	Day	10% 15% 32% 27% 13% 3	15528
	Night	6% 10% 31% 35% 16% 2	25145

By Month

Percentage Cloud Cover SW (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	3 2 9% 7% 8% 8% 8% 10% 11% 33%	3386
	11th to 20th	Day	2 2 10% 8% 7% 7% 5% 8% 9% 40%	3640
	21st to end	Day	1 4 2 7% 5% 5% 5% 5% 7% 9% 50%	4157
February	1st to 10th	Day	1 8% 4 9% 7% 7% 8% 5% 8% 10% 34%	1800
	11th to 20th	Day	1 4 3 11% 8% 7% 7% 6% 8% 10% 36%	1775
	21st to end	Day	1 6% 3 12% 9% 8% 8% 7% 9% 10% 29%	1611
March	1st to 10th	Day	1 4 3 8% 5% 7% 7% 6% 9% 10% 40%	3221
	11th to 20th	Day	1 5% 3 7% 5% 6% 6% 5% 7% 9% 46%	3490
	21st to end	Day	2 12% 5% 14% 9% 8% 7% 6% 8% 7% 22%	3825
April	1st to 10th	Day	4 10% 3 9% 8% 6% 6% 6% 8% 9% 31%	2222
	11th to 20th	Day	4 10% 3 11% 7% 6% 6% 4 6% 7% 35%	2437
	21st to end	Day	8% 12% 5% 10% 5% 6% 5% 5% 6% 6% 32%	2470
May	1st to 10th	Day	7% 12% 3 10% 7% 7% 7% 5% 7% 7% 27%	4110
	11th to 20th	Day	7% 7% 3 7% 5% 5% 6% 5% 7% 8% 40%	4102
	21st to end	Day	9% 11% 3 8% 6% 5% 6% 4 6% 8% 33%	5062
June	1st to 10th	Day	8% 8% 3 8% 6% 7% 6% 5% 8% 10% 30%	2850
	11th to 20th	Day	10% 6% 3 7% 6% 5% 5% 5% 7% 7% 39%	2850
	21st to end	Day	3 4 2 5% 5% 5% 5% 5% 7% 6% 52%	2850
July	1st to 10th	Day	2 2 1 3 3 4 5% 4 7% 8% 61%	4740
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 9% 62%	4497
	21st to end	Day	3 3 1 4 4 4 4 4 5% 8% 61%	4710
August	1st to 10th	Day	4 3 2 5% 4 5% 6% 5% 7% 9% 50%	4092
	11th to 20th	Day	2 2 1 4 3 4 5% 5% 7% 10% 58%	4028
	21st to end	Day	3 2 1 4 4 4 4 4 6% 6% 61%	4521
September	1st to 10th	Day	5% 4 1 4 3 3 3 3 5% 6% 61%	2237
	11th to 20th	Day	6% 6% 2 5% 4 4 4 5% 5% 6% 53%	2090
	21st to end	Day	8% 5% 1 4 3 4 4 2 4 6% 58%	2090
October	1st to 10th	Day	6% 5% 1 5% 4 3 3 2 3 5% 60%	3344
	11th to 20th	Day	3 6% 2 4 2 3 3 3 6% 7% 59%	3150
	21st to end	Day	2 5% 3 8% 5% 6% 5% 5% 7% 8% 47%	3131
November	1st to 10th	Day	4 8% 2 8% 6% 6% 6% 5% 7% 9% 40%	1764
	11th to 20th	Day	1 6% 2 6% 6% 6% 6% 6% 9% 11% 40%	1640
	21st to end	Day	1 5% 2 7% 5% 5% 5% 5% 7% 11% 47%	1600
December	1st to 10th	Day	2 1 6% 6% 5% 5% 4 6% 7% 58%	2455
	11th to 20th	Day	3 2 7% 6% 7% 9% 8% 11% 13% 35%	2645
	21st to end	Day	1 5% 2 8% 7% 7% 7% 6% 8% 8% 40%	3498
January	1st to 10th	Night	2 2 8% 7% 7% 7% 7% 9% 10% 40%	6603
	11th to 20th	Night	2 1 7% 7% 7% 7% 6% 8% 10% 44%	6320
	21st to end	Night	4 2 6% 4 4 5% 4 7% 9% 54%	6803
February	1st to 10th	Night	1 6% 4 10% 6% 6% 6% 6% 9% 10% 37%	2600
	11th to 20th	Night	1 4 3 9% 7% 5% 6% 6% 8% 9% 43%	2507
	21st to end	Night	5% 4 8% 6% 7% 8% 5% 8% 10% 39%	1937
March	1st to 10th	Night	1 6% 2 7% 4 4 5% 5% 7% 9% 48%	3653
	11th to 20th	Night	1 4 2 5% 5% 5% 4 4 6% 8% 55%	3490
	21st to end	Night	3 8% 3 8% 6% 7% 7% 7% 9% 11% 31%	3828
April	1st to 10th	Night	2 9% 3 7% 6% 6% 5% 6% 7% 7% 41%	1953
	11th to 20th	Night	5% 8% 4 9% 7% 6% 6% 4 7% 7% 37%	1691
	21st to end	Night	7% 10% 3 7% 4 5% 4 4 8% 9% 39%	1710
May	1st to 10th	Night	10% 12% 4 8% 5% 4 4 4 6% 8% 34%	2860
	11th to 20th	Night	7% 6% 3 6% 4 4 4 4 6% 10% 46%	2785
	21st to end	Night	11% 10% 3 7% 5% 5% 6% 5% 6% 7% 35%	2512
June	1st to 10th	Night	8% 7% 3 7% 6% 5% 5% 4 7% 9% 39%	1330
	11th to 20th	Night	12% 5% 2 4 4 4 4 4 8% 7% 46%	1330
	21st to end	Night	5% 3 2 4 4 3 4 4 6% 9% 58%	1330
July	1st to 10th	Night	2 1 1 3 3 4 3 3 5% 7% 68%	2220
	11th to 20th	Night	2 2 1 3 3 3 4 3 6% 7% 66%	2460
	21st to end	Night	2 2 3 3 3 4 4 6% 7% 67%	2841
August	1st to 10th	Night	3 3 1 5% 4 3 4 3 7% 9% 57%	2847
	11th to 20th	Night	1 2 1 3 3 4 5% 4 7% 8% 63%	2811
	21st to end	Night	5% 3 1 4 3 3 3 3 5% 7% 64%	3146
September	1st to 10th	Night	5% 4 1 4 3 3 3 3 4 6% 63%	1940
	11th to 20th	Night	7% 5% 2 4 4 4 5% 4 6% 7% 54%	2090
	21st to end	Night	8% 4 2 4 3 4 3 2 5% 6% 59%	2090
October	1st to 10th	Night	4 4 1 3 2 3 3 2 4 7% 65%	3631
	11th to 20th	Night	4 5% 2 6% 4 3 4 3 4 7% 58%	3778
	21st to end	Night	4 5% 3 6% 4 4 5% 4 7% 8% 51%	4424
November	1st to 10th	Night	2 6% 2 6% 6% 5% 5% 5% 7% 10% 45%	2535
	11th to 20th	Night	1 6% 2 5% 5% 6% 5% 6% 8% 10% 46%	2739
	21st to end	Night	1 5% 2 6% 5% 4 5% 5% 7% 10% 50%	2800
December	1st to 10th	Night	2 1 4 4 4 5% 5% 7% 8% 59%	4725
	11th to 20th	Night	4 3 6% 5% 6% 7% 6% 11% 12% 40%	5546
	21st to end	Night	5% 2 8% 6% 7% 7% 5% 10% 10% 40%	7458

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	33% 9% 24% 27% 6%	3386
	11th to 20th	Day	34% 7% 23% 26% 9% 1	3640
	21st to end	Day	28% 9% 21% 26% 14% 1	4157
February	1st to 10th	Day	40% 11% 24% 19% 7% 1	1800
	11th to 20th	Day	37% 7% 26% 22% 8% 1	1784
	21st to end	Day	40% 8% 20% 22% 9% 1	1611
March	1st to 10th	Day	30% 6% 20% 30% 14%	3221
	11th to 20th	Day	29% 7% 25% 34% 5%	3490
	21st to end	Day	53% 5% 21% 17% 3	3825
April	1st to 10th	Day	42% 6% 21% 22% 8% 1	2222
	11th to 20th	Day	44% 5% 16% 24% 10% 1	2440
	21st to end	Day	47% 4 14% 25% 9% 1	2470
May	1st to 10th	Day	47% 5% 13% 25% 9% 1	4110
	11th to 20th	Day	34% 7% 14% 27% 14% 4	4105
	21st to end	Day	44% 5% 16% 25% 9% 1	5062
June	1st to 10th	Day	43% 5% 21% 21% 9% 1	2850
	11th to 20th	Day	39% 5% 12% 23% 16% 6%	2850
	21st to end	Day	26% 4 11% 26% 27% 7%	2850
July	1st to 10th	Day	16% 4 7% 18% 30% 25%	4666
	11th to 20th	Day	14% 3 5% 14% 25% 38%	4419
	21st to end	Day	18% 3 4 17% 31% 26%	4561
August	1st to 10th	Day	24% 4 8% 25% 28% 10%	4078
	11th to 20th	Day	18% 4 8% 32% 31% 8%	4067
	21st to end	Day	20% 3 7% 25% 34% 11%	4520
September	1st to 10th	Day	22% 3 7% 23% 29% 16%	2237
	11th to 20th	Day	29% 4 12% 24% 24% 7%	2090
	21st to end	Day	27% 2 12% 30% 22% 6%	2090
October	1st to 10th	Day	25% 3 9% 26% 33% 3	3344
	11th to 20th	Day	23% 3 22% 29% 22% 1	3150
	21st to end	Day	29% 6% 16% 29% 19% 1	3131
November	1st to 10th	Day	37% 6% 18% 27% 11% 2	1764
	11th to 20th	Day	33% 7% 26% 24% 9% 1	1640
	21st to end	Day	30% 8% 22% 30% 10% 1	1600
December	1st to 10th	Day	24% 5% 25% 31% 15% 1	2455
	11th to 20th	Day	30% 9% 30% 25% 5%	2644
	21st to end	Day	33% 11% 20% 24% 10% 2	3498
January	1st to 10th	Night	32% 12% 17% 32% 7%	6603
	11th to 20th	Night	31% 8% 20% 28% 11% 2	6320
	21st to end	Night	26% 9% 22% 27% 13% 3	6803
February	1st to 10th	Night	39% 8% 26% 19% 7%	2600
	11th to 20th	Night	35% 8% 26% 21% 10%	2508
	21st to end	Night	36% 10% 20% 23% 11% 1	1937
March	1st to 10th	Night	28% 10% 17% 25% 18% 2	3653
	11th to 20th	Night	27% 8% 29% 28% 8%	3490
	21st to end	Night	43% 7% 23% 24% 3	3828
April	1st to 10th	Night	37% 8% 20% 24% 10% 1	1953
	11th to 20th	Night	41% 6% 16% 25% 11% 1	1691
	21st to end	Night	41% 5% 20% 26% 8%	1710
May	1st to 10th	Night	44% 6% 14% 26% 8% 1	2860
	11th to 20th	Night	30% 8% 12% 28% 16% 5%	2786
	21st to end	Night	43% 7% 17% 24% 9% 1	2512
June	1st to 10th	Night	39% 8% 19% 25% 8% 1	1330
	11th to 20th	Night	34% 5% 17% 24% 15% 5%	1330
	21st to end	Night	22% 5% 14% 27% 25% 8%	1330
July	1st to 10th	Night	15% 3 10% 22% 23% 26%	2185
	11th to 20th	Night	13% 3 5% 19% 21% 38%	2415
	21st to end	Night	13% 3 6% 25% 28% 25%	2752
August	1st to 10th	Night	21% 4 9% 28% 26% 13%	2838
	11th to 20th	Night	15% 3 9% 36% 28% 7%	2838
	21st to end	Night	18% 2 7% 28% 33% 11%	3146
September	1st to 10th	Night	23% 4 8% 25% 25% 15%	1940
	11th to 20th	Night	28% 4 11% 28% 22% 7%	2090
	21st to end	Night	26% 4 9% 30% 25% 6%	2090
October	1st to 10th	Night	20% 3 9% 25% 36% 7%	3631
	11th to 20th	Night	26% 4 18% 29% 20% 3	3778
	21st to end	Night	27% 4 17% 26% 25% 1	4424
November	1st to 10th	Night	33% 7% 18% 28% 13% 2	2535
	11th to 20th	Night	29% 10% 24% 28% 8% 1	2739
	21st to end	Night	27% 8% 22% 29% 13% 1	2800
December	1st to 10th	Night	18% 8% 28% 31% 14% 2	4725
	11th to 20th	Night	30% 9% 30% 26% 5%	5547
	21st to end	Night	32% 10% 22% 23% 12% 1	7458

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	3 19% 30% 30% 17% 2	200
	11th to 20th	Day	2 16% 44% 25% 10% 6%	200
	21st to end	Day	4 19% 37% 29% 9% 1	252
February	1st to 10th	Day	8% 23% 49% 16% 3 1	300
	11th to 20th	Day	2 22% 42% 25% 6% 3	320
	21st to end	Day	6% 27% 34% 24% 8% 1	399
March	1st to 10th	Day	4 24% 39% 27% 6%	460
	11th to 20th	Day	7% 17% 36% 28% 11% 2	460
	21st to end	Day	9% 22% 36% 23% 8% 1	506
April	1st to 10th	Day	13% 22% 36% 22% 7% 1	460
	11th to 20th	Day	8% 23% 41% 19% 7% 1	452
	21st to end	Day	16% 22% 32% 18% 10% 3	460
May	1st to 10th	Day	13% 19% 25% 27% 13% 2	463
	11th to 20th	Day	24% 14% 27% 20% 13% 1	506
	21st to end	Day	18% 11% 24% 24% 18% 5%	734
June	1st to 10th	Day	11% 12% 26% 30% 16% 5%	680
	11th to 20th	Day	17% 18% 30% 23% 11% 2	680
	21st to end	Day	8% 11% 28% 33% 18% 3	680
July	1st to 10th	Day	4 9% 28% 35% 20% 4	671
	11th to 20th	Day	5% 12% 34% 32% 17% 2	532
	21st to end	Day	7% 10% 31% 32% 17% 3	531
August	1st to 10th	Day	9% 8% 34% 26% 19% 3	487
	11th to 20th	Day	5% 9% 27% 35% 19% 5%	477
	21st to end	Day	9% 9% 29% 32% 17% 5%	539
September	1st to 10th	Day	7% 7% 33% 30% 17% 6%	490
	11th to 20th	Day	12% 12% 29% 27% 16% 4	490
	21st to end	Day	14% 15% 26% 26% 17% 3	490
October	1st to 10th	Day	17% 12% 27% 22% 17% 4	409
	11th to 20th	Day	11% 18% 23% 28% 14% 6%	300
	21st to end	Day	10% 18% 39% 22% 8% 2	330
November	1st to 10th	Day	9% 20% 36% 23% 11% 1	300
	11th to 20th	Day	10% 22% 32% 27% 8% 2	300
	21st to end	Day	7% 18% 35% 30% 10%	300
December	1st to 10th	Day	5% 17% 38% 25% 12% 3	250
	11th to 20th	Day	9% 17% 42% 27% 5% 2	200
	21st to end	Day	9% 17% 38% 23% 11% 2	220
January	1st to 10th	Night	2 14% 36% 38% 10% 1	928
	11th to 20th	Night	2 12% 41% 32% 10% 3	800
	21st to end	Night	3 9% 41% 35% 11% 1	880
February	1st to 10th	Night	3 17% 44% 27% 8% 1	800
	11th to 20th	Night	2 11% 42% 33% 8% 2	784
	21st to end	Night	3 13% 38% 35% 11% 1	646
March	1st to 10th	Night	4 9% 41% 35% 10% 1	760
	11th to 20th	Night	4 11% 33% 35% 17% 1	760
	21st to end	Night	7% 11% 32% 36% 12% 1	836
April	1st to 10th	Night	6% 11% 34% 34% 13% 2	732
	11th to 20th	Night	8% 10% 37% 34% 9% 1	563
	21st to end	Night	13% 10% 27% 34% 15% 1	570
May	1st to 10th	Night	12% 12% 25% 34% 17% 1	570
	11th to 20th	Night	16% 7% 24% 29% 23% 1	561
	21st to end	Night	20% 5% 21% 32% 20% 2	471
June	1st to 10th	Night	11% 7% 22% 35% 23% 2	380
	11th to 20th	Night	13% 9% 28% 37% 13% 1	380
	21st to end	Night	6% 4 21% 44% 25% 1	380
July	1st to 10th	Night	3 3 15% 46% 30% 3	370
	11th to 20th	Night	3 2 17% 52% 26%	368
	21st to end	Night	3 2 15% 47% 30% 2	460
August	1st to 10th	Night	5% 2 20% 44% 26% 1	567
	11th to 20th	Night	2 2 18% 47% 27% 3	564
	21st to end	Night	9% 6% 17% 40% 28% 1	627
September	1st to 10th	Night	6% 3 21% 40% 25% 5%	693
	11th to 20th	Night	9% 4 14% 37% 30% 5%	760
	21st to end	Night	9% 7% 23% 34% 22% 5%	760
October	1st to 10th	Night	11% 6% 23% 39% 18% 2	760
	11th to 20th	Night	7% 10% 29% 33% 18% 4	760
	21st to end	Night	7% 13% 33% 31% 13% 4	836
November	1st to 10th	Night	6% 13% 34% 34% 12% 1	780
	11th to 20th	Night	5% 16% 37% 33% 8% 1	939
	21st to end	Night	3 13% 38% 31% 14% 1	1000
December	1st to 10th	Night	3 14% 39% 32% 10% 2	1000
	11th to 20th	Night	4 12% 41% 34% 9% 1	1000
	21st to end	Night	5% 13% 43% 28% 10% 1	1100

Page generated at 2025-02-22 04:51:37. Version ac07

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