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 2026-07-01.

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 6% 5% 5% 6% 5% 7% 9% 46%	146326
	Night	3 4 2 6% 5% 5% 5% 5% 7% 9% 50%	143390
Transparency	Day	29% 6% 15% 26% 18% 6%	141017
	Night	27% 6% 18% 28% 16% 4	139706
Seeing	Day	11% 17% 32% 25% 12% 3	17398
	Night	6% 11% 31% 34% 15% 2	26827

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	2 2 9% 7% 8% 9% 8% 11% 12% 33%	4401
	11th to 20th	Day	2 1 8% 6% 6% 6% 5% 7% 10% 49%	4740
	21st to end	Day	1 3 2 7% 5% 5% 6% 6% 9% 10% 47%	5382
February	1st to 10th	Day	1 8% 4 9% 7% 7% 8% 5% 8% 10% 35%	1890
	11th to 20th	Day	1 4 3 10% 7% 7% 7% 6% 9% 10% 36%	1866
	21st to end	Day	1 5% 3 11% 9% 7% 8% 6% 9% 10% 32%	1730
March	1st to 10th	Day	1 6% 3 7% 4 6% 6% 6% 8% 10% 42%	4742
	11th to 20th	Day	1 5% 3 8% 7% 8% 7% 6% 8% 9% 40%	5110
	21st to end	Day	2 9% 4 11% 8% 9% 9% 7% 9% 8% 23%	5607
April	1st to 10th	Day	3 9% 3 9% 7% 6% 7% 6% 9% 9% 32%	2457
	11th to 20th	Day	4 9% 3 11% 7% 6% 6% 4 6% 7% 35%	2697
	21st to end	Day	7% 11% 4 10% 6% 6% 6% 5% 7% 7% 31%	2730
May	1st to 10th	Day	5% 9% 2 7% 5% 5% 5% 5% 8% 9% 41%	7626
	11th to 20th	Day	6% 8% 3 8% 6% 6% 6% 5% 7% 8% 38%	7655
	21st to end	Day	8% 8% 2 6% 5% 5% 5% 5% 6% 8% 42%	9471
June	1st to 10th	Day	8% 8% 3 8% 6% 7% 6% 5% 8% 10% 31%	3150
	11th to 20th	Day	9% 6% 3 6% 6% 5% 5% 5% 7% 7% 41%	3150
	21st to end	Day	4 5% 2 5% 5% 5% 5% 5% 7% 6% 51%	3150
July	1st to 10th	Day	2 2 1 3 4 4 5% 4 6% 8% 60%	6780
	11th to 20th	Day	2 2 1 3 4 3 4 4 6% 8% 63%	6425
	21st to end	Day	2 2 1 4 4 4 5% 4 6% 9% 58%	6761
August	1st to 10th	Day	3 2 1 4 4 6% 7% 5% 8% 10% 49%	5779
	11th to 20th	Day	2 1 1 3 3 4 5% 4 6% 9% 61%	5798
	21st to end	Day	6% 3 1 5% 4 4 4 4 6% 6% 57%	6468
September	1st to 10th	Day	5% 3 1 4 4 3 3 3 6% 6% 61%	2354
	11th to 20th	Day	6% 6% 2 5% 4 4 4 5% 5% 7% 52%	2200
	21st to end	Day	7% 4 1 5% 4 4 4 3 4 6% 58%	2200
October	1st to 10th	Day	6% 5% 2 5% 4 3 3 2 4 5% 60%	3450
	11th to 20th	Day	3 6% 2 4 2 3 3 3 6% 7% 58%	3250
	21st to end	Day	2 5% 2 7% 5% 5% 5% 4 7% 8% 48%	3231
November	1st to 10th	Day	3 7% 2 7% 6% 6% 6% 5% 7% 10% 41%	1854
	11th to 20th	Day	1 6% 2 6% 6% 6% 6% 7% 9% 11% 39%	1722
	21st to end	Day	1 5% 2 7% 5% 5% 5% 5% 7% 11% 48%	1680
December	1st to 10th	Day	2 1 6% 6% 5% 5% 4 6% 8% 58%	2530
	11th to 20th	Day	3 2 7% 6% 7% 8% 8% 11% 13% 35%	2715
	21st to end	Day	1 5% 2 8% 7% 7% 7% 6% 8% 8% 40%	3575
January	1st to 10th	Night	3 2 8% 6% 7% 7% 7% 11% 12% 37%	8549
	11th to 20th	Night	1 1 5% 6% 6% 6% 5% 8% 10% 52%	8220
	21st to end	Night	3 2 5% 4 4 5% 5% 8% 10% 54%	8806
February	1st to 10th	Night	1 6% 4 9% 6% 6% 6% 5% 9% 10% 38%	2730
	11th to 20th	Night	1 4 3 9% 6% 5% 6% 6% 8% 9% 43%	2636
	21st to end	Night	4 4 8% 6% 7% 8% 5% 8% 10% 40%	2076
March	1st to 10th	Night	5% 3 8% 5% 5% 5% 5% 7% 8% 48%	5372
	11th to 20th	Night	1 5% 3 6% 6% 6% 6% 5% 7% 9% 46%	5110
	21st to end	Night	2 8% 3 7% 5% 6% 7% 6% 10% 12% 34%	5610
April	1st to 10th	Night	2 8% 3 7% 6% 6% 5% 6% 7% 8% 42%	2158
	11th to 20th	Night	4 8% 4 9% 6% 6% 6% 4 7% 7% 38%	1871
	21st to end	Night	7% 10% 3 7% 5% 5% 4 4 8% 9% 38%	1890
May	1st to 10th	Night	7% 7% 2 6% 4 4 4 3 5% 7% 51%	5321
	11th to 20th	Night	8% 7% 3 6% 4 4 4 4 6% 10% 45%	5219
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 6% 7% 45%	4707
June	1st to 10th	Night	7% 6% 2 7% 6% 5% 5% 4 7% 9% 41%	1470
	11th to 20th	Night	11% 5% 2 4 4 4 4 3 7% 7% 48%	1470
	21st to end	Night	5% 3 2 4 3 3 4 4 5% 9% 57%	1470
July	1st to 10th	Night	1 2 1 3 3 4 4 4 6% 7% 66%	3180
	11th to 20th	Night	2 2 1 2 3 3 4 3 5% 6% 68%	3518
	21st to end	Night	2 2 1 4 4 4 5% 5% 7% 9% 57%	4092
August	1st to 10th	Night	2 2 1 4 4 4 4 4 7% 10% 57%	4033
	11th to 20th	Night	1 1 1 2 2 3 4 4 6% 8% 70%	4051
	21st to end	Night	7% 4 1 4 3 3 3 3 5% 7% 60%	4509
September	1st to 10th	Night	5% 4 1 4 3 3 3 3 4 6% 63%	2043
	11th to 20th	Night	6% 5% 2 5% 4 4 5% 4 6% 8% 53%	2200
	21st to end	Night	8% 4 2 4 3 4 3 2 5% 6% 60%	2200
October	1st to 10th	Night	4 4 1 3 2 3 3 2 5% 7% 64%	3745
	11th to 20th	Night	4 5% 2 6% 4 3 4 3 4 7% 58%	3898
	21st to end	Night	3 5% 3 5% 4 4 5% 4 7% 8% 52%	4566
November	1st to 10th	Night	2 6% 2 6% 6% 5% 5% 5% 7% 10% 46%	2665
	11th to 20th	Night	1 5% 2 5% 5% 6% 5% 6% 8% 10% 46%	2876
	21st to end	Night	1 4 2 6% 5% 5% 5% 5% 7% 10% 50%	2940
December	1st to 10th	Night	2 1 4 4 4 5% 5% 8% 9% 58%	4870
	11th to 20th	Night	4 2 6% 5% 6% 7% 6% 11% 12% 40%	5696
	21st to end	Night	5% 2 8% 6% 7% 7% 6% 10% 10% 40%	7623

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	30% 9% 26% 26% 8%	4402
	11th to 20th	Day	27% 6% 27% 26% 11% 3	4740
	21st to end	Day	27% 9% 22% 27% 14% 1	5382
February	1st to 10th	Day	38% 10% 24% 20% 8% 1	1890
	11th to 20th	Day	36% 7% 25% 23% 8% 1	1875
	21st to end	Day	37% 8% 19% 23% 11% 2	1730
March	1st to 10th	Day	29% 6% 17% 30% 15% 4	4742
	11th to 20th	Day	30% 7% 21% 32% 10% 1	5110
	21st to end	Day	42% 8% 21% 22% 5% 1	5607
April	1st to 10th	Day	40% 6% 20% 23% 10% 1	2457
	11th to 20th	Day	42% 5% 15% 26% 11% 1	2700
	21st to end	Day	46% 5% 14% 25% 9% 1	2730
May	1st to 10th	Day	32% 5% 13% 32% 16% 1	7626
	11th to 20th	Day	35% 6% 12% 24% 18% 5%	7658
	21st to end	Day	34% 5% 14% 32% 14% 2	9472
June	1st to 10th	Day	41% 5% 19% 23% 10% 1	3150
	11th to 20th	Day	36% 5% 12% 23% 17% 6%	3150
	21st to end	Day	26% 4 11% 26% 27% 6%	3150
July	1st to 10th	Day	17% 4 7% 22% 28% 23%	6707
	11th to 20th	Day	15% 3 6% 16% 31% 30%	6347
	21st to end	Day	19% 4 8% 22% 27% 19%	6564
August	1st to 10th	Day	24% 4 9% 26% 28% 10%	4208
	11th to 20th	Day	17% 4 8% 32% 30% 8%	4197
	21st to end	Day	20% 3 7% 25% 33% 11%	4663
September	1st to 10th	Day	22% 3 7% 25% 29% 15%	2354
	11th to 20th	Day	29% 4 12% 24% 25% 6%	2200
	21st to end	Day	26% 3 12% 30% 24% 6%	2200
October	1st to 10th	Day	25% 3 9% 27% 33% 3	3450
	11th to 20th	Day	23% 3 22% 29% 21% 1	3250
	21st to end	Day	28% 5% 16% 30% 19% 1	3231
November	1st to 10th	Day	35% 6% 19% 27% 12% 2	1854
	11th to 20th	Day	32% 7% 26% 25% 9% 1	1722
	21st to end	Day	29% 8% 23% 30% 10% 1	1680
December	1st to 10th	Day	24% 5% 25% 31% 14% 1	2530
	11th to 20th	Day	30% 9% 30% 25% 6% 1	2714
	21st to end	Day	33% 11% 20% 24% 10% 2	3575
January	1st to 10th	Night	29% 11% 21% 30% 9%	8550
	11th to 20th	Night	24% 6% 22% 32% 12% 4	8220
	21st to end	Night	23% 8% 23% 28% 16% 3	8806
February	1st to 10th	Night	38% 8% 26% 19% 8% 1	2730
	11th to 20th	Night	34% 8% 26% 21% 11% 1	2637
	21st to end	Night	34% 10% 19% 24% 12% 1	2076
March	1st to 10th	Night	29% 8% 13% 28% 17% 5%	5372
	11th to 20th	Night	28% 6% 25% 29% 12%	5110
	21st to end	Night	36% 6% 23% 28% 7%	5610
April	1st to 10th	Night	36% 7% 18% 26% 11% 1	2158
	11th to 20th	Night	39% 6% 16% 26% 12% 1	1871
	21st to end	Night	40% 5% 19% 26% 9%	1890
May	1st to 10th	Night	30% 4 10% 37% 16% 2	5319
	11th to 20th	Night	30% 6% 11% 27% 21% 5%	5220
	21st to end	Night	33% 5% 17% 30% 14% 1	4708
June	1st to 10th	Night	38% 7% 18% 25% 11% 1	1470
	11th to 20th	Night	32% 5% 17% 23% 18% 5%	1470
	21st to end	Night	23% 5% 13% 28% 25% 7%	1470
July	1st to 10th	Night	15% 3 9% 29% 23% 21%	3145
	11th to 20th	Night	13% 3 5% 22% 26% 31%	3473
	21st to end	Night	17% 4 12% 25% 25% 17%	3977
August	1st to 10th	Night	20% 4 9% 29% 26% 12%	2928
	11th to 20th	Night	15% 3 9% 36% 28% 8%	2928
	21st to end	Night	19% 2 7% 28% 33% 11%	3245
September	1st to 10th	Night	22% 4 9% 26% 25% 14%	2043
	11th to 20th	Night	27% 4 11% 29% 22% 7%	2200
	21st to end	Night	26% 4 9% 29% 27% 6%	2200
October	1st to 10th	Night	20% 3 9% 25% 35% 7%	3745
	11th to 20th	Night	26% 4 18% 29% 20% 3	3898
	21st to end	Night	26% 4 16% 27% 25% 1	4566
November	1st to 10th	Night	32% 7% 17% 29% 14% 2	2665
	11th to 20th	Night	29% 10% 24% 29% 8% 1	2876
	21st to end	Night	27% 8% 21% 29% 13% 1	2940
December	1st to 10th	Night	18% 8% 28% 31% 14% 2	4870
	11th to 20th	Night	29% 9% 30% 27% 6%	5697
	21st to end	Night	31% 10% 22% 23% 12% 1	7623

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	6% 21% 28% 27% 15% 2	220
	11th to 20th	Day	3 18% 43% 22% 9% 5%	220
	21st to end	Day	5% 22% 37% 27% 8% 1	277
February	1st to 10th	Day	8% 25% 47% 15% 4 1	330
	11th to 20th	Day	4 22% 40% 25% 6% 2	351
	21st to end	Day	11% 28% 32% 22% 7% 1	446
March	1st to 10th	Day	7% 25% 37% 25% 5%	540
	11th to 20th	Day	15% 18% 32% 24% 9% 1	540
	21st to end	Day	13% 25% 34% 21% 7% 1	594
April	1st to 10th	Day	14% 22% 35% 21% 7% 1	540
	11th to 20th	Day	10% 25% 41% 18% 6% 1	532
	21st to end	Day	16% 23% 32% 18% 9% 2	540
May	1st to 10th	Day	13% 20% 27% 25% 13% 2	543
	11th to 20th	Day	26% 16% 27% 18% 12% 1	588
	21st to end	Day	18% 12% 26% 23% 17% 5%	844
June	1st to 10th	Day	11% 13% 27% 29% 15% 4	780
	11th to 20th	Day	17% 20% 31% 22% 9% 2	780
	21st to end	Day	9% 12% 29% 31% 16% 2	780
July	1st to 10th	Day	4 10% 28% 35% 19% 3	722
	11th to 20th	Day	5% 13% 34% 31% 16% 2	574
	21st to end	Day	7% 10% 32% 32% 16% 3	575
August	1st to 10th	Day	9% 9% 34% 26% 18% 3	527
	11th to 20th	Day	5% 10% 29% 34% 18% 5%	517
	21st to end	Day	9% 9% 31% 31% 16% 5%	583
September	1st to 10th	Day	7% 8% 33% 29% 18% 6%	530
	11th to 20th	Day	11% 11% 29% 28% 17% 4	530
	21st to end	Day	13% 15% 27% 25% 17% 4	530
October	1st to 10th	Day	17% 15% 26% 22% 16% 4	445
	11th to 20th	Day	12% 20% 22% 26% 13% 6%	330
	21st to end	Day	11% 19% 39% 22% 8% 1	363
November	1st to 10th	Day	11% 22% 35% 22% 10% 1	330
	11th to 20th	Day	10% 23% 31% 27% 7% 2	330
	21st to end	Day	8% 20% 35% 28% 9%	330
December	1st to 10th	Day	8% 20% 35% 23% 11% 3	275
	11th to 20th	Day	12% 19% 39% 24% 5% 2	220
	21st to end	Day	12% 19% 36% 21% 10% 2	242
January	1st to 10th	Night	3 15% 35% 36% 10% 1	974
	11th to 20th	Night	2 14% 41% 31% 9% 3	840
	21st to end	Night	3 11% 41% 34% 10% 1	924
February	1st to 10th	Night	3 18% 44% 26% 8% 1	840
	11th to 20th	Night	3 13% 42% 33% 8% 2	824
	21st to end	Night	5% 15% 37% 32% 10% 1	692
March	1st to 10th	Night	5% 12% 40% 33% 10% 1	840
	11th to 20th	Night	6% 15% 31% 32% 15% 1	840
	21st to end	Night	8% 15% 31% 33% 11% 1	924
April	1st to 10th	Night	6% 13% 34% 33% 13% 2	809
	11th to 20th	Night	9% 13% 37% 32% 9% 1	623
	21st to end	Night	13% 11% 27% 33% 14% 1	630
May	1st to 10th	Night	12% 13% 25% 33% 17% 1	630
	11th to 20th	Night	16% 10% 24% 28% 21% 1	621
	21st to end	Night	18% 6% 22% 32% 19% 2	521
June	1st to 10th	Night	10% 8% 24% 34% 22% 2	420
	11th to 20th	Night	12% 11% 29% 36% 12% 1	420
	21st to end	Night	7% 5% 22% 42% 24%	420
July	1st to 10th	Night	3 4 15% 46% 29% 3	390
	11th to 20th	Night	3 3 16% 53% 24%	388
	21st to end	Night	3 2 15% 48% 30% 2	485
August	1st to 10th	Night	5% 4 22% 43% 25% 1	597
	11th to 20th	Night	2 3 19% 47% 26% 3	594
	21st to end	Night	9% 6% 17% 40% 27% 1	660
September	1st to 10th	Night	6% 3 21% 39% 26% 5%	730
	11th to 20th	Night	8% 4 15% 38% 30% 5%	800
	21st to end	Night	9% 7% 23% 34% 22% 5%	800
October	1st to 10th	Night	11% 7% 24% 39% 18% 2	800
	11th to 20th	Night	7% 12% 28% 32% 17% 4	800
	21st to end	Night	7% 15% 33% 30% 12% 4	880
November	1st to 10th	Night	7% 14% 34% 33% 11% 1	820
	11th to 20th	Night	5% 17% 37% 33% 8% 1	986
	21st to end	Night	3 14% 38% 30% 14% 1	1050
December	1st to 10th	Night	4 16% 37% 31% 10% 2	1050
	11th to 20th	Night	6% 13% 40% 32% 8% 1	1050
	21st to end	Night	5% 16% 41% 27% 9% 1	1155

Page generated at 2026-06-28 00:10:43. Version ac07

Copyright © 2026 Attilla Danko.