BlueBirdsKnoll Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BlueBirdsKnoll Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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	17% 7% 2 6% 4 4 4 4 5% 7% 40%	113786
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 42%	115487
Transparency	Day	41% 7% 12% 21% 16% 3	117363
	Night	42% 5% 12% 22% 16% 3	120074
Seeing	Day	20% 16% 31% 21% 9% 2	15673
	Night	21% 14% 32% 25% 8% 1	24820

By Month

Percentage Cloud Cover NE (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% 7% 2 6% 4 4 4 4 5% 6% 45%	3597
	11th to 20th	Day	15% 8% 3 6% 4 4 5% 4 5% 5% 42%	3570
	21st to end	Day	18% 8% 3 8% 4 4 4 3 4 5% 39%	4115
February	1st to 10th	Day	17% 7% 2 6% 5% 5% 5% 3 5% 6% 38%	1860
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 7% 7% 40%	1948
	21st to end	Day	21% 8% 3 6% 3 3 3 4 5% 6% 38%	1650
March	1st to 10th	Day	23% 8% 2 5% 3 4 3 2 4 5% 40%	4476
	11th to 20th	Day	14% 8% 3 5% 4 4 5% 4 6% 8% 42%	4682
	21st to end	Day	21% 8% 2 5% 4 4 4 4 5% 6% 37%	5970
April	1st to 10th	Day	20% 9% 2 6% 4 4 4 3 6% 7% 37%	2400
	11th to 20th	Day	19% 7% 2 5% 4 4 3 3 5% 7% 41%	2374
	21st to end	Day	21% 6% 2 5% 4 4 4 4 5% 6% 41%	2500
May	1st to 10th	Day	29% 8% 2 5% 3 3 4 3 5% 7% 30%	4236
	11th to 20th	Day	22% 8% 2 6% 4 4 4 4 6% 7% 32%	4424
	21st to end	Day	16% 5% 2 5% 3 3 4 4 6% 8% 45%	4907
June	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 7% 8% 36%	2660
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 7% 10% 36%	2660
	21st to end	Day	13% 6% 2 7% 5% 4 5% 4 6% 9% 38%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 6% 6% 4 7% 8% 35%	4440
	11th to 20th	Day	14% 6% 2 5% 4 5% 5% 5% 7% 9% 38%	4440
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 5% 6% 8% 32%	4814
August	1st to 10th	Day	20% 7% 2 7% 5% 5% 5% 4 6% 9% 31%	4270
	11th to 20th	Day	12% 7% 3 6% 4 4 4 4 6% 9% 41%	3730
	21st to end	Day	8% 6% 2 5% 5% 4 5% 3 7% 10% 45%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 5% 3 5% 6% 45%	2280
	11th to 20th	Day	13% 5% 2 5% 4 3 4 3 4 6% 50%	2155
	21st to end	Day	20% 6% 2 6% 4 3 4 3 4 6% 40%	2090
October	1st to 10th	Day	21% 7% 2 6% 4 3 3 3 5% 6% 41%	2052
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 5% 7% 47%	1893
	21st to end	Day	20% 6% 2 6% 3 4 4 4 4 6% 41%	1881
November	1st to 10th	Day	13% 6% 2 6% 4 3 3 2 5% 5% 50%	1761
	11th to 20th	Day	18% 6% 1 5% 2 3 3 2 5% 7% 48%	1800
	21st to end	Day	18% 6% 2 5% 3 4 4 3 6% 7% 43%	1765
December	1st to 10th	Day	16% 6% 2 6% 4 4 2 4 5% 7% 45%	2600
	11th to 20th	Day	22% 7% 2 4 3 3 2 2 4 6% 45%	2986
	21st to end	Day	19% 9% 3 6% 4 4 3 2 4 6% 39%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 7% 48%	6349
	11th to 20th	Night	14% 8% 3 7% 4 4 3 3 4 5% 45%	6315
	21st to end	Night	18% 9% 3 6% 4 4 4 3 4 5% 39%	6840
February	1st to 10th	Night	17% 10% 3 5% 4 4 4 3 4 5% 40%	2538
	11th to 20th	Night	17% 8% 2 6% 4 4 3 3 5% 7% 41%	2338
	21st to end	Night	18% 9% 2 7% 4 4 3 3 3 6% 40%	1980
March	1st to 10th	Night	21% 9% 3 5% 3 3 3 3 4 5% 41%	5364
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 4 6% 41%	5271
	21st to end	Night	21% 9% 3 6% 3 3 3 3 5% 5% 38%	4962
April	1st to 10th	Night	20% 8% 2 6% 4 4 5% 4 5% 6% 37%	2000
	11th to 20th	Night	20% 6% 2 6% 4 3 4 3 6% 6% 41%	1975
	21st to end	Night	19% 7% 3 6% 4 3 4 3 4 5% 42%	1900
May	1st to 10th	Night	25% 11% 3 7% 4 4 3 3 5% 6% 28%	2924
	11th to 20th	Night	22% 9% 3 8% 4 4 4 4 4 6% 31%	2718
	21st to end	Night	19% 7% 2 5% 4 4 4 2 5% 7% 42%	2784
June	1st to 10th	Night	18% 7% 2 7% 4 4 4 4 5% 6% 40%	1520
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 6% 9% 41%	1520
	21st to end	Night	13% 7% 3 5% 4 4 4 3 6% 9% 42%	1520
July	1st to 10th	Night	14% 6% 3 7% 5% 5% 5% 5% 6% 9% 35%	2520
	11th to 20th	Night	17% 6% 2 6% 5% 4 5% 4 7% 8% 36%	2520
	21st to end	Night	21% 8% 2 7% 4 4 4 5% 6% 6% 33%	2739
August	1st to 10th	Night	23% 7% 2 8% 5% 5% 5% 4 6% 6% 29%	2626
	11th to 20th	Night	15% 7% 3 6% 5% 4 4 4 5% 6% 40%	3093
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 8% 54%	3476
September	1st to 10th	Night	14% 6% 3 6% 5% 3 4 3 4 7% 44%	1900
	11th to 20th	Night	14% 5% 2 4 4 2 4 3 4 7% 52%	2023
	21st to end	Night	17% 7% 3 5% 4 4 4 3 4 6% 43%	2090
October	1st to 10th	Night	19% 8% 2 6% 4 4 3 3 4 6% 40%	2126
	11th to 20th	Night	15% 5% 2 6% 3 3 4 3 5% 6% 49%	2285
	21st to end	Night	22% 7% 2 5% 3 3 3 3 5% 5% 42%	2717
November	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 50%	2537
	11th to 20th	Night	19% 6% 2 5% 4 3 3 2 4 6% 47%	2600
	21st to end	Night	16% 5% 2 5% 4 4 3 4 5% 6% 46%	2633
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 4 48%	4580
	11th to 20th	Night	25% 7% 2 4 3 3 4 3 4 4 42%	5208
	21st to end	Night	19% 9% 3 7% 4 3 3 2 4 4 41%	6996

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	37% 6% 8% 28% 17% 3	3598
	11th to 20th	Day	42% 5% 11% 21% 13% 8%	3600
	21st to end	Day	45% 8% 8% 22% 16% 2	4115
February	1st to 10th	Day	45% 5% 14% 20% 14% 2	1860
	11th to 20th	Day	40% 6% 12% 23% 17% 3	1957
	21st to end	Day	46% 6% 12% 20% 14% 3	1650
March	1st to 10th	Day	45% 5% 11% 16% 12% 10%	4479
	11th to 20th	Day	37% 7% 16% 27% 11% 2	4682
	21st to end	Day	46% 5% 9% 15% 20% 6%	5970
April	1st to 10th	Day	46% 5% 10% 20% 15% 4	2400
	11th to 20th	Day	42% 6% 11% 20% 18% 3	2374
	21st to end	Day	44% 6% 10% 20% 17% 4	2500
May	1st to 10th	Day	52% 8% 14% 14% 11% 1	4250
	11th to 20th	Day	49% 8% 14% 18% 10% 2	4428
	21st to end	Day	35% 8% 13% 24% 18% 1	4907
June	1st to 10th	Day	43% 8% 12% 24% 12% 1	2660
	11th to 20th	Day	41% 8% 15% 22% 12% 1	2660
	21st to end	Day	40% 8% 14% 23% 13% 1	2660
July	1st to 10th	Day	44% 10% 11% 20% 15% 1	4370
	11th to 20th	Day	40% 8% 16% 22% 13% 1	4365
	21st to end	Day	48% 8% 11% 17% 14% 1	4661
August	1st to 10th	Day	48% 8% 16% 20% 8%	4256
	11th to 20th	Day	38% 7% 16% 28% 11% 1	3766
	21st to end	Day	33% 7% 15% 27% 17% 2	4180
September	1st to 10th	Day	39% 8% 11% 22% 19% 2	2280
	11th to 20th	Day	34% 6% 11% 23% 24% 2	2155
	21st to end	Day	44% 6% 10% 20% 18% 3	2090
October	1st to 10th	Day	36% 7% 10% 18% 23% 6%	3416
	11th to 20th	Day	27% 5% 10% 24% 30% 4	3121
	21st to end	Day	31% 5% 14% 24% 23% 3	3081
November	1st to 10th	Day	36% 5% 12% 23% 21% 3	1761
	11th to 20th	Day	37% 4 11% 22% 22% 4	1800
	21st to end	Day	41% 4 14% 27% 12% 3	1765
December	1st to 10th	Day	39% 5% 16% 27% 12% 1	2600
	11th to 20th	Day	41% 3 15% 19% 18% 3	2986
	21st to end	Day	47% 5% 8% 20% 15% 4	3960
January	1st to 10th	Night	36% 5% 10% 27% 20% 3	6349
	11th to 20th	Night	41% 4 14% 20% 15% 6%	6360
	21st to end	Night	47% 4 10% 19% 16% 5%	6840
February	1st to 10th	Night	46% 5% 12% 23% 12% 2	2538
	11th to 20th	Night	43% 4 11% 20% 18% 4	2340
	21st to end	Night	46% 4 8% 22% 15% 5%	1980
March	1st to 10th	Night	45% 5% 11% 18% 14% 7%	5374
	11th to 20th	Night	43% 6% 15% 21% 11% 4	5272
	21st to end	Night	47% 4 7% 13% 24% 5%	4962
April	1st to 10th	Night	46% 6% 11% 19% 14% 4	2000
	11th to 20th	Night	44% 5% 11% 19% 20% 1	1975
	21st to end	Night	44% 5% 9% 22% 16% 4	1900
May	1st to 10th	Night	55% 8% 12% 14% 10% 1	2930
	11th to 20th	Night	52% 9% 13% 14% 10% 2	2719
	21st to end	Night	42% 6% 14% 24% 12% 1	2785
June	1st to 10th	Night	45% 7% 11% 19% 17% 2	1520
	11th to 20th	Night	39% 8% 16% 25% 11% 1	1520
	21st to end	Night	39% 8% 13% 25% 15% 1	1520
July	1st to 10th	Night	44% 10% 14% 18% 13% 1	2480
	11th to 20th	Night	43% 8% 16% 22% 10% 1	2472
	21st to end	Night	48% 7% 14% 14% 15% 2	2651
August	1st to 10th	Night	52% 9% 13% 16% 9% 1	2616
	11th to 20th	Night	42% 7% 13% 29% 8% 1	3123
	21st to end	Night	27% 6% 14% 33% 18% 1	3476
September	1st to 10th	Night	40% 6% 10% 25% 18% 2	1900
	11th to 20th	Night	33% 6% 10% 23% 25% 3	2023
	21st to end	Night	43% 7% 8% 20% 20% 2	2090
October	1st to 10th	Night	38% 6% 12% 22% 19% 2	3542
	11th to 20th	Night	25% 5% 9% 27% 29% 4	3796
	21st to end	Night	32% 4 14% 28% 20% 3	4467
November	1st to 10th	Night	35% 5% 12% 22% 21% 4	2537
	11th to 20th	Night	40% 3 10% 23% 20% 4	2600
	21st to end	Night	39% 4 13% 29% 13% 2	2633
December	1st to 10th	Night	40% 4 12% 30% 13% 1	4580
	11th to 20th	Night	47% 4 11% 18% 17% 2	5208
	21st to end	Night	46% 4 10% 19% 16% 5%	6996

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	27% 29% 26% 17% 2	200
	11th to 20th	Day	20% 36% 30% 14% 1 1	200
	21st to end	Day	21% 30% 34% 10% 5% 1	253
February	1st to 10th	Day	27% 25% 35% 12% 1	360
	11th to 20th	Day	24% 27% 35% 12% 1	496
	21st to end	Day	26% 26% 32% 13% 3	414
March	1st to 10th	Day	23% 24% 36% 14% 3	500
	11th to 20th	Day	20% 27% 35% 14% 3 1	500
	21st to end	Day	26% 23% 36% 11% 3 1	550
April	1st to 10th	Day	26% 26% 30% 12% 6% 1	500
	11th to 20th	Day	27% 22% 32% 14% 5% 1	492
	21st to end	Day	27% 18% 34% 15% 5% 1	500
May	1st to 10th	Day	29% 19% 32% 13% 5% 1	503
	11th to 20th	Day	21% 17% 36% 20% 6% 1	527
	21st to end	Day	19% 10% 30% 28% 11% 3	549
June	1st to 10th	Day	18% 12% 30% 26% 11% 2	490
	11th to 20th	Day	15% 11% 32% 28% 12% 1	490
	21st to end	Day	14% 9% 36% 27% 12% 2	490
July	1st to 10th	Day	16% 5% 30% 29% 17% 3	485
	11th to 20th	Day	13% 5% 29% 31% 19% 3	484
	21st to end	Day	14% 8% 32% 34% 11% 1	532
August	1st to 10th	Day	15% 6% 26% 37% 11% 5%	486
	11th to 20th	Day	15% 6% 25% 32% 16% 5%	479
	21st to end	Day	11% 9% 31% 27% 16% 6%	539
September	1st to 10th	Day	16% 11% 28% 23% 19% 4	490
	11th to 20th	Day	15% 7% 29% 27% 13% 8%	490
	21st to end	Day	21% 11% 31% 23% 10% 3	490
October	1st to 10th	Day	25% 10% 26% 21% 13% 4	490
	11th to 20th	Day	15% 20% 30% 24% 10% 2	485
	21st to end	Day	24% 16% 31% 20% 7% 1	429
November	1st to 10th	Day	16% 16% 31% 23% 11% 2	395
	11th to 20th	Day	22% 24% 29% 20% 5%	400
	21st to end	Day	22% 22% 30% 20% 5%	365
December	1st to 10th	Day	26% 19% 34% 18% 5%	200
	11th to 20th	Day	31% 26% 28% 14% 3	200
	21st to end	Day	20% 22% 35% 17% 5% 1	220
January	1st to 10th	Night	22% 22% 36% 18% 3	800
	11th to 20th	Night	25% 20% 33% 20% 3	800
	21st to end	Night	21% 19% 39% 18% 2	880
February	1st to 10th	Night	22% 19% 36% 21% 2	800
	11th to 20th	Night	25% 21% 37% 15% 2	783
	21st to end	Night	23% 21% 36% 17% 3	660
March	1st to 10th	Night	24% 19% 34% 20% 3	800
	11th to 20th	Night	21% 17% 38% 20% 3 1	800
	21st to end	Night	22% 19% 38% 17% 4	880
April	1st to 10th	Night	24% 18% 35% 20% 3	800
	11th to 20th	Night	23% 16% 36% 22% 4	791
	21st to end	Night	21% 16% 36% 21% 5% 1	700
May	1st to 10th	Night	27% 11% 34% 22% 6%	600
	11th to 20th	Night	23% 12% 37% 23% 5%	485
	21st to end	Night	22% 6% 23% 38% 9% 1	423
June	1st to 10th	Night	19% 6% 30% 33% 10% 1	380
	11th to 20th	Night	17% 6% 28% 36% 13%	380
	21st to end	Night	17% 6% 25% 42% 10%	380
July	1st to 10th	Night	14% 4 23% 40% 18% 1	370
	11th to 20th	Night	15% 3 20% 45% 17% 2	368
	21st to end	Night	14% 4 23% 44% 14% 1	416
August	1st to 10th	Night	11% 3 29% 40% 15% 1	446
	11th to 20th	Night	16% 4 21% 38% 19% 2	746
	21st to end	Night	14% 5% 22% 37% 19% 2	836
September	1st to 10th	Night	17% 5% 27% 33% 15% 2	760
	11th to 20th	Night	17% 6% 23% 32% 18% 4	760
	21st to end	Night	21% 7% 27% 30% 15% 1	760
October	1st to 10th	Night	22% 10% 29% 25% 13% 2	760
	11th to 20th	Night	17% 13% 32% 28% 10% 1	760
	21st to end	Night	26% 12% 34% 21% 7%	836
November	1st to 10th	Night	16% 13% 34% 26% 10% 1	780
	11th to 20th	Night	22% 16% 35% 19% 6% 1	800
	21st to end	Night	21% 18% 32% 23% 5% 1	800
December	1st to 10th	Night	26% 16% 32% 20% 5% 1	800
	11th to 20th	Night	25% 19% 33% 20% 3	800
	21st to end	Night	25% 18% 34% 20% 2	880

Page generated at 2025-05-23 09:21:01. Version ac07

Copyright © 2025 Attilla Danko.