Birch Rd. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birch Rd. Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	14% 9% 4 8% 5% 5% 5% 4 6% 7% 35%	126968
	Night	15% 10% 4 8% 5% 4 4 3 5% 7% 35%	124595
Transparency	Day	47% 6% 12% 21% 13% 1	130536
	Night	48% 6% 11% 21% 13% 1	129101
Seeing	Day	17% 10% 28% 24% 15% 6%	17253
	Night	18% 10% 29% 29% 12% 1	27127

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	12% 14% 9% 17% 6% 4 4 4 5% 6% 19%	3625
	11th to 20th	Day	13% 15% 8% 13% 7% 7% 5% 6% 6% 4 16%	3620
	21st to end	Day	21% 18% 7% 12% 4 4 3 3 3 4 21%	4022
February	1st to 10th	Day	15% 16% 5% 11% 7% 5% 5% 4 4 4 23%	1949
	11th to 20th	Day	15% 13% 5% 12% 7% 5% 4 3 5% 6% 25%	1948
	21st to end	Day	16% 15% 6% 10% 5% 4 4 4 5% 5% 27%	1650
March	1st to 10th	Day	18% 13% 6% 10% 5% 4 4 2 3 4 30%	4480
	11th to 20th	Day	10% 10% 5% 7% 4 4 4 3 6% 7% 39%	4811
	21st to end	Day	18% 14% 5% 11% 6% 5% 5% 3 5% 6% 22%	5723
April	1st to 10th	Day	17% 13% 4 8% 5% 4 5% 4 4 6% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 5% 4 4 5% 4 6% 35%	2378
	21st to end	Day	18% 9% 3 7% 4 4 4 4 5% 8% 34%	2600
May	1st to 10th	Day	18% 9% 3 7% 4 5% 5% 4 7% 7% 30%	5862
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	5861
	21st to end	Day	14% 6% 2 7% 4 4 5% 4 6% 8% 40%	6785
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 8% 43%	2800
	11th to 20th	Day	9% 6% 2 5% 5% 5% 5% 4 7% 8% 44%	2800
	21st to end	Day	11% 6% 2 6% 6% 5% 5% 4 6% 9% 41%	2800
July	1st to 10th	Day	10% 5% 3 7% 5% 5% 5% 4 6% 10% 41%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	6360
	21st to end	Day	10% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	6865
August	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 6% 9% 47%	4234
	11th to 20th	Day	9% 4 2 6% 4 5% 5% 4 7% 8% 46%	4082
	21st to end	Day	6% 5% 2 5% 4 4 4 4 6% 11% 48%	4312
September	1st to 10th	Day	10% 6% 2 6% 4 4 5% 3 5% 8% 47%	2374
	11th to 20th	Day	10% 5% 2 5% 3 4 5% 4 7% 8% 49%	2200
	21st to end	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 43%	2200
October	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2200
	11th to 20th	Day	16% 7% 3 7% 4 4 4 4 6% 6% 39%	1997
	21st to end	Day	19% 10% 2 8% 6% 4 4 4 5% 7% 30%	1980
November	1st to 10th	Day	11% 8% 2 7% 4 4 4 3 4 7% 45%	1852
	11th to 20th	Day	16% 15% 4 8% 5% 4 4 2 4 7% 31%	1890
	21st to end	Day	19% 13% 5% 10% 5% 3 4 3 5% 6% 27%	1890
December	1st to 10th	Day	11% 14% 4 12% 6% 4 4 3 5% 6% 32%	2900
	11th to 20th	Day	15% 16% 8% 13% 6% 4 4 3 4 6% 22%	3088
	21st to end	Day	28% 19% 7% 12% 6% 4 4 3 3 4 11%	4070
January	1st to 10th	Night	15% 15% 7% 15% 6% 5% 4 3 5% 5% 19%	6363
	11th to 20th	Night	13% 20% 8% 15% 7% 5% 5% 3 5% 3 17%	6360
	21st to end	Night	20% 14% 7% 10% 5% 4 4 3 3 4 26%	6951
February	1st to 10th	Night	18% 14% 5% 10% 7% 4 3 3 5% 6% 26%	2449
	11th to 20th	Night	14% 13% 4 8% 6% 6% 4 3 4 5% 32%	2338
	21st to end	Night	20% 13% 3 9% 5% 4 4 3 4 5% 30%	1980
March	1st to 10th	Night	16% 11% 5% 7% 5% 4 4 3 5% 6% 35%	5373
	11th to 20th	Night	12% 8% 2 6% 4 5% 4 4 6% 7% 42%	5147
	21st to end	Night	18% 10% 4 8% 4 4 4 4 5% 6% 33%	5248
April	1st to 10th	Night	15% 9% 3 6% 5% 4 4 4 5% 7% 36%	2000
	11th to 20th	Night	19% 7% 3 6% 4 4 3 3 6% 7% 38%	1970
	21st to end	Night	18% 9% 3 7% 4 4 4 4 6% 6% 37%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 4 5% 3 5% 6% 31%	4033
	11th to 20th	Night	16% 7% 3 6% 4 4 3 3 6% 8% 39%	4032
	21st to end	Night	15% 8% 3 7% 4 4 4 3 5% 7% 40%	4082
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 9% 49%	1600
	11th to 20th	Night	9% 6% 2 7% 4 4 4 4 5% 10% 45%	1600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 10% 44%	1600
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 4 4 6% 9% 46%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 6% 11% 46%	3617
	21st to end	Night	9% 6% 3 6% 5% 5% 5% 4 7% 8% 42%	3990
August	1st to 10th	Night	10% 5% 3 5% 4 4 4 5% 6% 9% 45%	2915
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 49%	2964
	21st to end	Night	8% 5% 2 6% 4 4 4 4 6% 8% 51%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 4 7% 53%	2005
	11th to 20th	Night	9% 4 2 4 3 3 3 3 4 8% 57%	2200
	21st to end	Night	13% 7% 2 5% 4 3 4 3 5% 6% 48%	2200
October	1st to 10th	Night	11% 5% 2 5% 4 4 4 2 6% 6% 51%	2200
	11th to 20th	Night	16% 8% 3 6% 3 4 4 3 4 6% 43%	2399
	21st to end	Night	19% 8% 3 6% 5% 4 4 3 5% 7% 37%	2860
November	1st to 10th	Night	13% 9% 2 7% 4 4 3 3 6% 6% 43%	2666
	11th to 20th	Night	14% 12% 3 7% 5% 4 4 4 5% 8% 34%	2730
	21st to end	Night	18% 14% 4 8% 5% 4 4 3 5% 6% 28%	2730
December	1st to 10th	Night	13% 10% 4 8% 6% 5% 4 3 6% 7% 34%	4500
	11th to 20th	Night	17% 16% 8% 12% 5% 5% 4 3 4 4 21%	5339
	21st to end	Night	26% 18% 6% 11% 5% 5% 4 3 4 5% 12%	7150

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	66% 5% 11% 14% 4	3626
	11th to 20th	Day	68% 6% 9% 8% 8% 2	3620
	21st to end	Day	69% 3 9% 13% 6%	4022
February	1st to 10th	Day	64% 5% 10% 12% 7% 1	1949
	11th to 20th	Day	59% 6% 10% 17% 7%	1958
	21st to end	Day	57% 6% 12% 16% 7% 1	1650
March	1st to 10th	Day	58% 5% 7% 21% 9%	4480
	11th to 20th	Day	43% 5% 12% 32% 8% 1	4811
	21st to end	Day	62% 7% 12% 14% 5% 1	5725
April	1st to 10th	Day	53% 6% 11% 18% 11%	2400
	11th to 20th	Day	48% 6% 11% 21% 11% 2	2378
	21st to end	Day	48% 8% 13% 19% 11% 2	2600
May	1st to 10th	Day	50% 8% 12% 18% 12% 1	5897
	11th to 20th	Day	43% 7% 13% 23% 12% 1	5865
	21st to end	Day	40% 7% 11% 26% 16% 1	6799
June	1st to 10th	Day	37% 8% 12% 24% 16% 2	2800
	11th to 20th	Day	36% 7% 12% 24% 18% 3	2800
	21st to end	Day	38% 9% 15% 19% 16% 3	2800
July	1st to 10th	Day	38% 5% 11% 26% 17% 2	6291
	11th to 20th	Day	34% 7% 16% 27% 15% 1	6285
	21st to end	Day	41% 6% 13% 22% 16% 2	6670
August	1st to 10th	Day	33% 7% 16% 28% 15% 1	4221
	11th to 20th	Day	33% 7% 14% 27% 17% 1	4121
	21st to end	Day	30% 7% 14% 26% 20% 3	4311
September	1st to 10th	Day	35% 6% 15% 23% 18% 3	2374
	11th to 20th	Day	31% 7% 11% 27% 22% 2	2200
	21st to end	Day	39% 6% 11% 23% 20% 2	2200
October	1st to 10th	Day	28% 4 11% 32% 24% 1	3590
	11th to 20th	Day	37% 6% 10% 21% 26%	3226
	21st to end	Day	45% 4 10% 27% 13% 2	3180
November	1st to 10th	Day	40% 6% 12% 25% 16% 1	1852
	11th to 20th	Day	54% 4 10% 19% 10% 2	1890
	21st to end	Day	58% 5% 13% 18% 6% 1	1890
December	1st to 10th	Day	54% 5% 15% 21% 5%	2900
	11th to 20th	Day	64% 5% 11% 15% 4	3085
	21st to end	Day	79% 4 6% 8% 3	4070
January	1st to 10th	Night	66% 6% 9% 15% 5%	6363
	11th to 20th	Night	72% 3 8% 8% 9%	6360
	21st to end	Night	63% 4 11% 15% 8%	6951
February	1st to 10th	Night	59% 6% 12% 14% 8% 1	2449
	11th to 20th	Night	54% 6% 11% 19% 10% 1	2339
	21st to end	Night	55% 5% 12% 16% 10% 2	1980
March	1st to 10th	Night	50% 6% 9% 26% 8% 1	5373
	11th to 20th	Night	39% 7% 13% 28% 12% 2	5147
	21st to end	Night	50% 6% 11% 22% 10% 1	5248
April	1st to 10th	Night	45% 8% 12% 20% 14% 1	2000
	11th to 20th	Night	44% 6% 11% 19% 17% 2	1970
	21st to end	Night	46% 6% 11% 22% 13% 2	1800
May	1st to 10th	Night	53% 7% 8% 17% 13% 2	4060
	11th to 20th	Night	41% 7% 11% 25% 14% 1	4033
	21st to end	Night	42% 7% 10% 21% 17% 2	4090
June	1st to 10th	Night	32% 6% 13% 27% 19% 3	1600
	11th to 20th	Night	35% 7% 12% 25% 18% 3	1600
	21st to end	Night	34% 9% 17% 22% 16% 3	1600
July	1st to 10th	Night	35% 5% 11% 28% 16% 4	3580
	11th to 20th	Night	33% 6% 17% 23% 20% 2	3572
	21st to end	Night	38% 6% 16% 20% 18% 3	3874
August	1st to 10th	Night	34% 8% 14% 29% 14% 1	2904
	11th to 20th	Night	32% 6% 12% 30% 18% 1	2991
	21st to end	Night	31% 7% 11% 27% 21% 3	3586
September	1st to 10th	Night	31% 7% 13% 26% 19% 5%	2005
	11th to 20th	Night	25% 6% 8% 31% 27% 3	2200
	21st to end	Night	37% 6% 10% 23% 21% 2	2200
October	1st to 10th	Night	23% 4 8% 33% 30% 1	3590
	11th to 20th	Night	36% 3 8% 23% 29% 1	3909
	21st to end	Night	37% 4 10% 28% 20% 1	4610
November	1st to 10th	Night	40% 5% 12% 24% 18% 1	2666
	11th to 20th	Night	48% 5% 12% 21% 12% 2	2730
	21st to end	Night	57% 5% 12% 19% 7%	2730
December	1st to 10th	Night	50% 5% 14% 25% 6%	4500
	11th to 20th	Night	66% 5% 11% 10% 7% 1	5341
	21st to end	Night	75% 5% 9% 8% 3	7150

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	19% 19% 38% 17% 5% 1	303
	11th to 20th	Day	22% 16% 42% 16% 3 1	300
	21st to end	Day	29% 16% 33% 18% 3	330
February	1st to 10th	Day	25% 18% 37% 15% 4 1	300
	11th to 20th	Day	27% 18% 35% 16% 3	300
	21st to end	Day	24% 21% 40% 11% 4	249
March	1st to 10th	Day	22% 20% 39% 16% 2	300
	11th to 20th	Day	24% 15% 33% 21% 6%	422
	21st to end	Day	31% 17% 27% 17% 6% 1	550
April	1st to 10th	Day	22% 15% 34% 20% 8% 1	500
	11th to 20th	Day	19% 19% 31% 19% 9% 3	492
	21st to end	Day	26% 12% 31% 20% 9% 2	500
May	1st to 10th	Day	24% 13% 30% 20% 11% 2	503
	11th to 20th	Day	21% 8% 26% 24% 17% 4	527
	21st to end	Day	13% 6% 29% 26% 18% 8%	717
June	1st to 10th	Day	12% 9% 27% 28% 18% 5%	730
	11th to 20th	Day	11% 7% 28% 28% 19% 6%	730
	21st to end	Day	13% 6% 26% 32% 17% 7%	730
July	1st to 10th	Day	10% 3 19% 29% 24% 15%	721
	11th to 20th	Day	8% 3 25% 30% 23% 12%	718
	21st to end	Day	9% 3 20% 32% 26% 9%	772
August	1st to 10th	Day	7% 2 13% 30% 32% 15%	526
	11th to 20th	Day	12% 3 18% 29% 25% 12%	519
	21st to end	Day	9% 3 15% 30% 31% 13%	583
September	1st to 10th	Day	12% 6% 19% 26% 25% 12%	530
	11th to 20th	Day	8% 5% 14% 26% 30% 18%	530
	21st to end	Day	15% 9% 20% 25% 22% 8%	530
October	1st to 10th	Day	13% 5% 27% 30% 18% 8%	530
	11th to 20th	Day	17% 18% 28% 26% 9% 1	435
	21st to end	Day	27% 14% 33% 22% 3 1	363
November	1st to 10th	Day	12% 15% 36% 25% 9% 3	330
	11th to 20th	Day	24% 20% 32% 17% 7%	330
	21st to end	Day	31% 15% 34% 16% 4	330
December	1st to 10th	Day	20% 17% 40% 15% 8%	330
	11th to 20th	Day	21% 20% 38% 15% 5%	330
	21st to end	Day	28% 17% 39% 13% 2	363
January	1st to 10th	Night	23% 15% 36% 20% 5%	1006
	11th to 20th	Night	24% 14% 38% 20% 3 1	1000
	21st to end	Night	22% 14% 37% 23% 4	1081
February	1st to 10th	Night	26% 12% 36% 23% 3	800
	11th to 20th	Night	21% 13% 39% 23% 3	783
	21st to end	Night	27% 15% 38% 18% 3	660
March	1st to 10th	Night	20% 12% 36% 26% 5% 1	800
	11th to 20th	Night	18% 14% 37% 26% 6%	800
	21st to end	Night	25% 12% 31% 27% 5%	790
April	1st to 10th	Night	18% 13% 37% 25% 6% 1	600
	11th to 20th	Night	20% 12% 35% 25% 7% 1	593
	21st to end	Night	21% 10% 35% 26% 9% 1	600
May	1st to 10th	Night	21% 7% 33% 30% 8%	600
	11th to 20th	Night	19% 6% 29% 31% 15% 1	591
	21st to end	Night	13% 4 25% 40% 17% 1	650
June	1st to 10th	Night	12% 4 26% 42% 17% 1	600
	11th to 20th	Night	11% 3 26% 41% 19% 1	600
	21st to end	Night	11% 3 24% 38% 24% 1	600
July	1st to 10th	Night	8% 2 15% 42% 29% 4	585
	11th to 20th	Night	10% 3 17% 39% 28% 3	582
	21st to end	Night	7% 2 15% 43% 32% 2	657
August	1st to 10th	Night	8% 2 11% 44% 32% 4	597
	11th to 20th	Night	9% 3 14% 41% 29% 4	594
	21st to end	Night	9% 2 15% 36% 35% 3	660
September	1st to 10th	Night	11% 4 16% 37% 29% 3	605
	11th to 20th	Night	10% 4 15% 35% 32% 4	800
	21st to end	Night	16% 6% 23% 32% 21% 3	800
October	1st to 10th	Night	13% 6% 24% 36% 19% 3	800
	11th to 20th	Night	19% 13% 29% 29% 10%	800
	21st to end	Night	23% 13% 31% 26% 6%	880
November	1st to 10th	Night	17% 12% 35% 26% 10% 1	820
	11th to 20th	Night	21% 16% 33% 24% 6% 1	840
	21st to end	Night	25% 14% 32% 23% 6%	840
December	1st to 10th	Night	24% 14% 37% 21% 4	908
	11th to 20th	Night	23% 14% 36% 22% 4	1050
	21st to end	Night	31% 14% 33% 20% 2	1155

Page generated at 2025-12-30 01:14:05. Version ac07

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