Birchwood
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birchwood clear sky chart from 2005-11-06 to 2025-10-19.

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	13% 7% 3 7% 5% 5% 5% 4 6% 8% 39%	126299
	Night	14% 7% 3 6% 4 4 4 4 6% 7% 41%	123426
Transparency	Day	40% 7% 12% 22% 16% 3	130036
	Night	39% 6% 11% 22% 17% 4	127757
Seeing	Day	16% 9% 21% 24% 20% 10%	15999
	Night	16% 7% 23% 31% 20% 3	25977

By Month

Percentage Cloud Cover SE (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	16% 9% 3 7% 4 3 3 3 4 5% 41%	4056
	11th to 20th	Day	15% 7% 4 7% 4 4 4 2 4 5% 43%	4060
	21st to end	Day	20% 9% 3 6% 3 3 4 3 5% 6% 38%	4664
February	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 5% 6% 37%	2000
	11th to 20th	Day	22% 8% 3 6% 5% 4 5% 3 5% 6% 34%	1948
	21st to end	Day	20% 7% 3 7% 5% 4 3 4 4 6% 38%	1650
March	1st to 10th	Day	18% 10% 2 8% 4 4 5% 4 5% 6% 34%	4498
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 5% 6% 49%	4980
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 6% 43%	5465
April	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 7% 8% 42%	2354
	11th to 20th	Day	13% 6% 2 6% 4 3 4 4 6% 7% 45%	2374
	21st to end	Day	12% 5% 2 7% 5% 5% 5% 4 6% 9% 42%	2542
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 7% 9% 30%	5862
	11th to 20th	Day	15% 9% 3 8% 6% 5% 6% 4 6% 7% 29%	5861
	21st to end	Day	14% 8% 3 6% 4 4 5% 4 6% 7% 38%	6433
June	1st to 10th	Day	8% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	2600
	11th to 20th	Day	5% 4 1 5% 5% 6% 6% 5% 9% 11% 42%	2600
	21st to end	Day	6% 4 2 6% 6% 6% 6% 4 8% 11% 40%	2600
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 6% 6% 8% 11% 38%	5900
	11th to 20th	Day	7% 5% 2 7% 5% 6% 7% 5% 9% 12% 34%	5900
	21st to end	Day	11% 8% 3 9% 7% 7% 7% 6% 8% 10% 23%	6424
August	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 6% 9% 11% 40%	4234
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 10% 41%	3885
	21st to end	Day	7% 4 2 6% 4 5% 6% 4 7% 8% 49%	4312
September	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 4 7% 7% 41%	2265
	11th to 20th	Day	14% 5% 1 6% 5% 5% 6% 4 6% 8% 41%	2200
	21st to end	Day	14% 6% 1 5% 3 4 4 3 5% 7% 47%	2200
October	1st to 10th	Day	10% 5% 2 6% 3 4 4 3 4 5% 54%	2200
	11th to 20th	Day	11% 5% 1 4 3 4 3 3 4 7% 56%	2095
	21st to end	Day	14% 4 1 4 3 3 4 3 5% 7% 50%	2006
November	1st to 10th	Day	11% 7% 2 5% 4 3 3 4 3 6% 52%	1762
	11th to 20th	Day	13% 5% 2 6% 5% 3 4 3 5% 7% 47%	1800
	21st to end	Day	16% 6% 2 5% 4 4 4 4 5% 6% 45%	1800
December	1st to 10th	Day	16% 7% 3 6% 4 3 4 3 4 4 47%	2930
	11th to 20th	Day	19% 10% 4 9% 5% 5% 5% 3 4 4 31%	3373
	21st to end	Day	20% 8% 3 6% 5% 3 4 4 5% 8% 34%	4466
January	1st to 10th	Night	15% 8% 4 8% 4 3 4 3 4 6% 41%	5889
	11th to 20th	Night	17% 7% 2 6% 4 3 4 4 5% 6% 41%	5900
	21st to end	Night	17% 10% 3 6% 4 3 4 4 5% 6% 38%	6288
February	1st to 10th	Night	16% 8% 3 6% 5% 4 4 4 5% 6% 40%	2400
	11th to 20th	Night	22% 8% 4 6% 5% 3 4 3 4 4 37%	2338
	21st to end	Night	22% 9% 2 6% 4 4 4 3 4 6% 36%	1980
March	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 33%	5340
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 6% 7% 49%	4980
	21st to end	Night	14% 8% 3 7% 4 5% 4 3 5% 6% 41%	5467
April	1st to 10th	Night	12% 5% 2 5% 4 3 4 4 5% 8% 48%	2061
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 5% 7% 49%	1975
	21st to end	Night	11% 5% 2 6% 4 3 5% 4 5% 8% 47%	1855
May	1st to 10th	Night	11% 8% 3 9% 6% 5% 6% 4 7% 9% 31%	4033
	11th to 20th	Night	18% 9% 3 9% 5% 5% 5% 4 5% 7% 29%	4032
	21st to end	Night	14% 7% 3 7% 4 5% 5% 4 7% 8% 38%	4421
June	1st to 10th	Night	7% 6% 3 8% 5% 6% 6% 5% 6% 10% 40%	1800
	11th to 20th	Night	4 3 2 4 4 4 5% 5% 9% 11% 49%	1800
	21st to end	Night	5% 4 1 6% 5% 5% 6% 5% 8% 11% 44%	1800
July	1st to 10th	Night	7% 5% 2 7% 6% 6% 6% 5% 9% 11% 37%	4058
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 5% 8% 13% 38%	4057
	21st to end	Night	11% 8% 4 9% 6% 7% 6% 5% 9% 11% 25%	4427
August	1st to 10th	Night	4 4 2 6% 5% 5% 6% 5% 8% 12% 43%	2915
	11th to 20th	Night	6% 5% 2 6% 5% 5% 4 4 6% 9% 49%	3160
	21st to end	Night	6% 4 2 4 4 5% 5% 4 7% 9% 51%	3586
September	1st to 10th	Night	9% 6% 3 7% 6% 5% 5% 4 6% 8% 41%	2115
	11th to 20th	Night	12% 5% 2 7% 5% 4 5% 4 6% 8% 41%	2200
	21st to end	Night	11% 6% 3 5% 4 5% 4 4 5% 7% 48%	2200
October	1st to 10th	Night	11% 4 2 4 3 3 3 2 5% 7% 56%	2200
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 4 5% 55%	2262
	21st to end	Night	13% 5% 2 4 4 3 3 3 4 5% 53%	2589
November	1st to 10th	Night	12% 5% 2 6% 4 3 3 3 4 6% 51%	2536
	11th to 20th	Night	13% 7% 2 5% 4 3 3 3 5% 6% 50%	2600
	21st to end	Night	16% 8% 2 6% 4 4 3 4 4 6% 44%	2600
December	1st to 10th	Night	17% 8% 3 6% 4 4 5% 4 5% 5% 40%	4250
	11th to 20th	Night	22% 8% 3 6% 4 3 4 3 4 6% 37%	4822
	21st to end	Night	19% 8% 3 7% 5% 5% 5% 4 5% 6% 32%	6490

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	44% 4 8% 24% 19% 1	4057
	11th to 20th	Day	43% 4 6% 19% 26% 2	4060
	21st to end	Day	46% 4 8% 24% 15% 3	4664
February	1st to 10th	Day	48% 4 9% 20% 16% 3	2000
	11th to 20th	Day	51% 5% 8% 18% 16% 3	1958
	21st to end	Day	48% 4 8% 15% 18% 6%	1650
March	1st to 10th	Day	48% 7% 7% 13% 17% 8%	4498
	11th to 20th	Day	36% 5% 6% 19% 24% 11%	4980
	21st to end	Day	41% 5% 8% 29% 16% 1	5465
April	1st to 10th	Day	38% 7% 11% 22% 18% 3	2354
	11th to 20th	Day	37% 6% 8% 20% 23% 6%	2374
	21st to end	Day	39% 7% 11% 26% 14% 2	2542
May	1st to 10th	Day	48% 8% 13% 20% 10% 1	5897
	11th to 20th	Day	51% 8% 14% 17% 8% 1	5865
	21st to end	Day	43% 7% 10% 21% 16% 3	6447
June	1st to 10th	Day	36% 10% 16% 22% 13% 2	2600
	11th to 20th	Day	30% 10% 20% 23% 15% 2	2600
	21st to end	Day	33% 12% 19% 18% 14% 3	2600
July	1st to 10th	Day	36% 9% 19% 22% 13% 1	5835
	11th to 20th	Day	39% 9% 27% 19% 5% 1	5831
	21st to end	Day	52% 10% 17% 16% 4	6240
August	1st to 10th	Day	33% 10% 16% 31% 9% 1	4221
	11th to 20th	Day	33% 9% 17% 25% 15% 2	3922
	21st to end	Day	32% 7% 12% 31% 16% 2	4311
September	1st to 10th	Day	38% 10% 12% 18% 16% 6%	2265
	11th to 20th	Day	39% 7% 13% 21% 19% 2	2200
	21st to end	Day	36% 6% 13% 20% 20% 6%	2200
October	1st to 10th	Day	26% 4 7% 19% 33% 11%	3590
	11th to 20th	Day	21% 4 6% 25% 32% 12%	3400
	21st to end	Day	29% 4 9% 24% 26% 8%	3282
November	1st to 10th	Day	33% 5% 8% 21% 27% 6%	1762
	11th to 20th	Day	37% 4 6% 23% 25% 5%	1800
	21st to end	Day	40% 4 8% 24% 20% 4	1800
December	1st to 10th	Day	41% 4 6% 25% 22% 2	2930
	11th to 20th	Day	55% 5% 7% 19% 12% 3	3370
	21st to end	Day	47% 5% 14% 24% 9% 1	4466
January	1st to 10th	Night	42% 5% 9% 24% 18% 2	5889
	11th to 20th	Night	42% 6% 6% 19% 23% 4	5900
	21st to end	Night	45% 4 7% 21% 18% 5%	6288
February	1st to 10th	Night	44% 4 11% 20% 17% 3	2400
	11th to 20th	Night	52% 4 7% 17% 15% 5%	2339
	21st to end	Night	49% 5% 7% 17% 17% 5%	1980
March	1st to 10th	Night	51% 5% 7% 15% 19% 4	5340
	11th to 20th	Night	33% 5% 7% 20% 22% 13%	4980
	21st to end	Night	44% 5% 7% 25% 16% 3	5467
April	1st to 10th	Night	33% 5% 12% 23% 22% 4	2061
	11th to 20th	Night	34% 4 9% 24% 22% 7%	1975
	21st to end	Night	35% 6% 14% 27% 17% 2	1855
May	1st to 10th	Night	47% 8% 15% 19% 10% 1	4060
	11th to 20th	Night	52% 7% 15% 17% 9%	4033
	21st to end	Night	41% 9% 13% 22% 14% 1	4429
June	1st to 10th	Night	36% 11% 15% 22% 14% 2	1800
	11th to 20th	Night	25% 9% 21% 28% 16% 2	1800
	21st to end	Night	31% 11% 18% 22% 14% 4	1800
July	1st to 10th	Night	38% 10% 20% 22% 9% 1	4015
	11th to 20th	Night	35% 8% 29% 21% 4 2	4006
	21st to end	Night	49% 11% 20% 15% 4 1	4297
August	1st to 10th	Night	30% 8% 16% 32% 12% 1	2904
	11th to 20th	Night	30% 8% 21% 25% 14% 1	3189
	21st to end	Night	27% 8% 15% 32% 17% 1	3586
September	1st to 10th	Night	38% 8% 16% 19% 14% 5%	2115
	11th to 20th	Night	38% 9% 12% 19% 21% 1	2200
	21st to end	Night	34% 6% 11% 22% 21% 6%	2200
October	1st to 10th	Night	25% 3 7% 21% 30% 14%	3590
	11th to 20th	Night	20% 4 5% 21% 35% 16%	3696
	21st to end	Night	25% 4 5% 27% 29% 10%	4263
November	1st to 10th	Night	34% 4 8% 22% 24% 8%	2536
	11th to 20th	Night	35% 4 6% 21% 26% 8%	2600
	21st to end	Night	41% 4 7% 23% 21% 4	2600
December	1st to 10th	Night	44% 7% 6% 20% 22% 2	4250
	11th to 20th	Night	47% 5% 8% 20% 18% 2	4824
	21st to end	Night	48% 5% 13% 22% 11% 1	6490

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	24% 21% 30% 20% 3 1	300
	11th to 20th	Day	24% 24% 30% 14% 7% 1	300
	21st to end	Day	21% 16% 43% 15% 5%	330
February	1st to 10th	Day	24% 20% 30% 20% 6%	300
	11th to 20th	Day	38% 15% 30% 14% 2 1	300
	21st to end	Day	31% 20% 32% 12% 4	249
March	1st to 10th	Day	24% 19% 34% 15% 6% 1	305
	11th to 20th	Day	22% 17% 31% 23% 7% 1	500
	21st to end	Day	21% 13% 34% 23% 7% 1	550
April	1st to 10th	Day	17% 16% 28% 26% 11% 2	500
	11th to 20th	Day	17% 14% 30% 23% 13% 2	492
	21st to end	Day	16% 10% 30% 28% 12% 4	500
May	1st to 10th	Day	15% 13% 27% 26% 13% 6%	503
	11th to 20th	Day	13% 6% 27% 26% 20% 9%	527
	21st to end	Day	13% 7% 20% 23% 20% 17%	573
June	1st to 10th	Day	10% 5% 17% 28% 26% 14%	530
	11th to 20th	Day	6% 3 14% 27% 29% 22%	530
	21st to end	Day	9% 2 12% 28% 29% 20%	530
July	1st to 10th	Day	8% 3 11% 22% 32% 24%	525
	11th to 20th	Day	9% 2 9% 23% 31% 26%	524
	21st to end	Day	11% 2 7% 23% 33% 25%	576
August	1st to 10th	Day	12% 4 9% 21% 35% 20%	526
	11th to 20th	Day	12% 3 12% 26% 31% 16%	519
	21st to end	Day	11% 2 10% 21% 34% 22%	583
September	1st to 10th	Day	11% 2 9% 27% 32% 19%	530
	11th to 20th	Day	15% 2 11% 21% 33% 18%	530
	21st to end	Day	17% 4 13% 24% 32% 10%	530
October	1st to 10th	Day	12% 4 18% 26% 27% 13%	530
	11th to 20th	Day	11% 9% 23% 31% 21% 5%	522
	21st to end	Day	17% 8% 20% 31% 20% 4	455
November	1st to 10th	Day	19% 7% 24% 29% 16% 5%	300
	11th to 20th	Day	12% 14% 31% 33% 9% 1	300
	21st to end	Day	13% 13% 35% 25% 13% 2	300
December	1st to 10th	Day	30% 10% 31% 23% 5% 1	300
	11th to 20th	Day	22% 14% 30% 24% 10% 1	300
	21st to end	Day	29% 15% 26% 20% 9% 1	330
January	1st to 10th	Night	23% 13% 38% 21% 5%	800
	11th to 20th	Night	21% 16% 32% 26% 6%	800
	21st to end	Night	20% 12% 36% 30% 2	880
February	1st to 10th	Night	19% 12% 35% 30% 5%	800
	11th to 20th	Night	26% 15% 34% 23% 2	783
	21st to end	Night	27% 13% 36% 22% 2	660
March	1st to 10th	Night	19% 13% 31% 32% 5%	800
	11th to 20th	Night	21% 10% 34% 27% 7% 1	800
	21st to end	Night	18% 11% 32% 32% 7%	880
April	1st to 10th	Night	14% 11% 31% 35% 9%	661
	11th to 20th	Night	16% 8% 33% 31% 13%	593
	21st to end	Night	14% 6% 27% 39% 14% 1	600
May	1st to 10th	Night	11% 6% 28% 35% 20% 1	600
	11th to 20th	Night	15% 4 22% 31% 24% 5%	591
	21st to end	Night	11% 4 14% 33% 34% 5%	650
June	1st to 10th	Night	8% 3 14% 36% 34% 5%	600
	11th to 20th	Night	6% 1 11% 35% 37% 11%	600
	21st to end	Night	7% 2 7% 32% 41% 11%	600
July	1st to 10th	Night	11% 2 8% 26% 43% 10%	585
	11th to 20th	Night	9% 2 8% 25% 43% 14%	582
	21st to end	Night	9% 2 9% 25% 44% 12%	657
August	1st to 10th	Night	7% 2 6% 28% 47% 11%	597
	11th to 20th	Night	8% 3 11% 31% 40% 7%	594
	21st to end	Night	10% 2 9% 25% 44% 10%	660
September	1st to 10th	Night	10% 3 12% 28% 38% 8%	715
	11th to 20th	Night	15% 2 10% 29% 40% 5%	800
	21st to end	Night	14% 3 16% 30% 35% 3	800
October	1st to 10th	Night	12% 5% 16% 32% 29% 7%	800
	11th to 20th	Night	14% 6% 22% 35% 22% 1	793
	21st to end	Night	14% 7% 20% 39% 17% 2	836
November	1st to 10th	Night	14% 7% 21% 36% 19% 2	780
	11th to 20th	Night	15% 9% 31% 33% 12% 1	800
	21st to end	Night	20% 9% 27% 34% 10% 1	800
December	1st to 10th	Night	25% 13% 25% 30% 8%	800
	11th to 20th	Night	21% 8% 31% 33% 8%	800
	21st to end	Night	25% 11% 25% 31% 7%	880

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