Bluebird Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bluebird Observatory 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	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	126604
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	123117
Transparency	Day	39% 7% 12% 23% 16% 3	130198
	Night	39% 6% 11% 23% 17% 4	127593
Seeing	Day	19% 12% 28% 23% 13% 5%	16216
	Night	18% 10% 30% 29% 12% 1	25650

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	14% 5% 1 4 3 4 4 4 5% 6% 49%	3624
	11th to 20th	Day	13% 9% 3 8% 4 4 3 3 4 5% 45%	4060
	21st to end	Day	16% 8% 3 6% 4 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	14% 7% 2 7% 5% 4 4 5% 7% 5% 39%	1800
	11th to 20th	Day	18% 8% 2 6% 4 4 4 4 5% 6% 39%	1817
	21st to end	Day	17% 8% 3 7% 5% 4 4 2 4 5% 41%	1650
March	1st to 10th	Day	19% 5% 2 5% 3 4 4 3 5% 7% 44%	4800
	11th to 20th	Day	11% 6% 3 5% 4 4 4 4 5% 6% 49%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 7% 41%	5592
April	1st to 10th	Day	15% 7% 2 6% 5% 4 4 4 6% 8% 40%	2400
	11th to 20th	Day	14% 6% 2 7% 4 3 4 3 5% 6% 47%	2374
	21st to end	Day	18% 6% 2 5% 3 4 3 4 5% 7% 42%	2400
May	1st to 10th	Day	21% 7% 2 6% 4 4 5% 4 7% 9% 31%	5757
	11th to 20th	Day	23% 6% 2 6% 4 4 4 4 6% 8% 31%	6226
	21st to end	Day	16% 5% 2 7% 5% 4 5% 4 6% 8% 39%	6934
June	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 4 7% 9% 42%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 8% 40%	2800
	21st to end	Day	10% 4 1 5% 4 4 5% 4 6% 10% 45%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	6360
	11th to 20th	Day	9% 6% 3 7% 6% 6% 6% 5% 8% 11% 31%	6268
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 40%	6424
August	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 5% 7% 8% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 5% 6% 6% 4 7% 9% 38%	3986
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 5% 8% 46%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 47%	2400
	21st to end	Day	20% 8% 3 6% 4 3 5% 4 5% 6% 36%	2110
October	1st to 10th	Day	16% 6% 3 5% 3 3 3 3 5% 7% 47%	2000
	11th to 20th	Day	17% 5% 2 4 3 3 4 3 5% 5% 49%	1980
	21st to end	Day	13% 6% 2 6% 3 4 3 3 5% 6% 48%	2090
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 5% 7% 51%	1828
	11th to 20th	Day	14% 5% 2 4 3 4 4 4 5% 6% 50%	1782
	21st to end	Day	15% 5% 2 5% 4 4 5% 3 5% 5% 47%	1600
December	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 47%	2600
	11th to 20th	Day	18% 9% 3 6% 4 4 3 2 4 6% 41%	2990
	21st to end	Day	17% 8% 3 6% 4 4 5% 3 5% 8% 37%	3960
January	1st to 10th	Night	12% 7% 2 5% 4 4 3 3 5% 6% 48%	6307
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 6% 50%	5900
	21st to end	Night	16% 8% 3 5% 3 3 3 3 4 5% 48%	6490
February	1st to 10th	Night	15% 8% 2 6% 4 5% 4 4 5% 7% 41%	2600
	11th to 20th	Night	19% 7% 2 6% 3 4 3 3 4 6% 43%	2468
	21st to end	Night	16% 7% 2 7% 4 4 4 3 5% 5% 42%	1980
March	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 4 5% 48%	5038
	11th to 20th	Night	10% 5% 3 5% 3 3 4 3 5% 6% 51%	4980
	21st to end	Night	13% 8% 3 6% 4 3 4 4 5% 6% 45%	5340
April	1st to 10th	Night	16% 7% 2 5% 5% 4 4 4 6% 6% 40%	2000
	11th to 20th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 44%	1975
	21st to end	Night	16% 6% 2 5% 4 3 4 4 6% 7% 43%	2000
May	1st to 10th	Night	23% 9% 2 6% 5% 4 4 4 5% 7% 30%	4179
	11th to 20th	Night	24% 8% 3 6% 3 3 4 4 6% 8% 31%	3667
	21st to end	Night	19% 6% 2 6% 4 4 5% 4 6% 9% 36%	3920
June	1st to 10th	Night	11% 5% 2 5% 4 5% 4 4 8% 9% 44%	1600
	11th to 20th	Night	12% 6% 3 6% 4 5% 5% 4 6% 9% 41%	1600
	21st to end	Night	10% 5% 2 5% 5% 5% 5% 4 7% 10% 43%	1600
July	1st to 10th	Night	8% 6% 3 7% 6% 6% 6% 5% 7% 9% 40%	3598
	11th to 20th	Night	9% 8% 2 9% 6% 6% 6% 6% 8% 10% 30%	3684
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 7% 9% 43%	4427
August	1st to 10th	Night	18% 6% 2 7% 4 4 5% 5% 6% 10% 33%	2915
	11th to 20th	Night	10% 8% 3 8% 6% 5% 6% 4 7% 7% 37%	3029
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 9% 47%	3586
September	1st to 10th	Night	14% 6% 2 6% 3 3 4 4 5% 7% 47%	2000
	11th to 20th	Night	11% 6% 2 6% 4 3 3 3 4 7% 51%	2000
	21st to end	Night	20% 7% 2 7% 5% 4 4 3 5% 6% 38%	2283
October	1st to 10th	Night	19% 7% 2 5% 3 4 3 3 4 5% 47%	2400
	11th to 20th	Night	16% 5% 1 4 3 3 3 3 4 6% 53%	2377
	21st to end	Night	15% 6% 2 4 4 3 4 3 5% 7% 49%	2508
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 5% 6% 53%	2470
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 5% 5% 49%	2615
	21st to end	Night	14% 4 2 5% 5% 4 4 4 6% 7% 47%	2800
December	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 4 7% 50%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 6% 40%	5205
	21st to end	Night	17% 7% 2 6% 4 4 4 4 6% 8% 37%	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	34% 4 7% 28% 22% 5%	3625
	11th to 20th	Day	42% 4 8% 21% 17% 8%	4060
	21st to end	Day	42% 5% 11% 26% 14% 3	4466
February	1st to 10th	Day	43% 6% 12% 25% 11% 2	1800
	11th to 20th	Day	45% 4 9% 24% 15% 3	1826
	21st to end	Day	46% 4 11% 20% 15% 4	1650
March	1st to 10th	Day	38% 5% 10% 22% 17% 7%	4800
	11th to 20th	Day	34% 5% 10% 26% 19% 6%	4980
	21st to end	Day	43% 5% 10% 18% 19% 5%	5592
April	1st to 10th	Day	41% 7% 11% 20% 17% 5%	2400
	11th to 20th	Day	39% 5% 10% 22% 19% 5%	2374
	21st to end	Day	41% 5% 10% 21% 18% 4	2400
May	1st to 10th	Day	48% 8% 14% 18% 10% 2	5791
	11th to 20th	Day	48% 9% 11% 22% 9% 1	6230
	21st to end	Day	40% 7% 13% 22% 16% 2	6949
June	1st to 10th	Day	36% 8% 14% 24% 16% 1	2800
	11th to 20th	Day	39% 10% 12% 23% 14% 2	2800
	21st to end	Day	32% 9% 15% 24% 18% 2	2800
July	1st to 10th	Day	35% 10% 16% 19% 16% 4	6290
	11th to 20th	Day	41% 10% 17% 22% 8% 1	6193
	21st to end	Day	37% 8% 12% 21% 22% 1	6240
August	1st to 10th	Day	45% 9% 13% 24% 9%	4221
	11th to 20th	Day	39% 7% 13% 26% 14% 1	4025
	21st to end	Day	35% 6% 13% 27% 18% 2	4312
September	1st to 10th	Day	37% 7% 11% 21% 19% 4	2400
	11th to 20th	Day	35% 5% 10% 25% 21% 4	2400
	21st to end	Day	48% 6% 8% 19% 17% 2	2110
October	1st to 10th	Day	38% 4 8% 17% 28% 5%	3260
	11th to 20th	Day	26% 4 10% 27% 24% 10%	3220
	21st to end	Day	24% 3 10% 30% 29% 5%	3428
November	1st to 10th	Day	32% 5% 13% 26% 21% 4	1828
	11th to 20th	Day	35% 5% 10% 23% 23% 4	1782
	21st to end	Day	38% 5% 11% 27% 17% 2	1600
December	1st to 10th	Day	37% 5% 14% 28% 12% 4	2600
	11th to 20th	Day	44% 4 12% 28% 10% 2	2986
	21st to end	Day	44% 6% 16% 19% 15% 1	3960
January	1st to 10th	Night	37% 4 8% 24% 23% 4	6307
	11th to 20th	Night	36% 4 10% 24% 20% 7%	5900
	21st to end	Night	39% 4 8% 27% 19% 2	6490
February	1st to 10th	Night	42% 6% 13% 24% 13% 1	2600
	11th to 20th	Night	43% 4 9% 22% 18% 4	2470
	21st to end	Night	44% 4 10% 21% 16% 5%	1980
March	1st to 10th	Night	39% 5% 8% 26% 13% 10%	5038
	11th to 20th	Night	32% 6% 11% 28% 16% 6%	4980
	21st to end	Night	40% 4 10% 19% 22% 5%	5340
April	1st to 10th	Night	42% 6% 11% 19% 19% 4	2000
	11th to 20th	Night	38% 6% 12% 19% 19% 5%	1975
	21st to end	Night	38% 7% 10% 23% 17% 5%	2000
May	1st to 10th	Night	53% 6% 11% 17% 10% 3	4208
	11th to 20th	Night	49% 8% 11% 21% 11% 1	3668
	21st to end	Night	44% 8% 14% 22% 11% 2	3927
June	1st to 10th	Night	34% 8% 11% 27% 18% 3	1600
	11th to 20th	Night	39% 9% 12% 24% 14% 2	1600
	21st to end	Night	35% 8% 13% 23% 18% 3	1600
July	1st to 10th	Night	40% 9% 16% 17% 14% 3	3560
	11th to 20th	Night	45% 11% 17% 18% 8% 1	3639
	21st to end	Night	35% 7% 14% 22% 20% 1	4297
August	1st to 10th	Night	44% 9% 17% 21% 9% 1	2904
	11th to 20th	Night	44% 8% 11% 24% 12% 1	3056
	21st to end	Night	34% 6% 12% 29% 16% 2	3586
September	1st to 10th	Night	37% 7% 11% 21% 19% 5%	2000
	11th to 20th	Night	35% 5% 9% 26% 23% 3	2000
	21st to end	Night	47% 7% 9% 18% 17% 3	2283
October	1st to 10th	Night	41% 5% 10% 20% 20% 5%	3920
	11th to 20th	Night	25% 4 10% 24% 30% 7%	3876
	21st to end	Night	23% 4 11% 32% 24% 7%	4120
November	1st to 10th	Night	33% 5% 10% 25% 22% 6%	2470
	11th to 20th	Night	37% 5% 9% 23% 21% 5%	2615
	21st to end	Night	36% 6% 12% 24% 20% 3	2800
December	1st to 10th	Night	34% 4 13% 29% 17% 2	4580
	11th to 20th	Night	45% 4 12% 18% 17% 3	5208
	21st to end	Night	42% 6% 15% 20% 16% 1	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	32% 20% 31% 13% 3	207
	11th to 20th	Day	30% 24% 32% 12% 2	300
	21st to end	Day	23% 24% 35% 14% 3 1	330
February	1st to 10th	Day	26% 22% 38% 10% 3	300
	11th to 20th	Day	35% 22% 36% 6% 1	364
	21st to end	Day	29% 21% 30% 18% 2	414
March	1st to 10th	Day	24% 23% 31% 18% 4	500
	11th to 20th	Day	27% 19% 32% 17% 4 1	500
	21st to end	Day	24% 20% 32% 17% 5% 1	550
April	1st to 10th	Day	23% 21% 33% 17% 4 2	500
	11th to 20th	Day	21% 17% 32% 17% 11% 3	492
	21st to end	Day	24% 14% 34% 18% 8% 2	500
May	1st to 10th	Day	26% 15% 30% 18% 8% 3	503
	11th to 20th	Day	22% 12% 28% 24% 11% 2	527
	21st to end	Day	19% 7% 24% 25% 21% 4	573
June	1st to 10th	Day	14% 10% 30% 20% 19% 7%	530
	11th to 20th	Day	15% 7% 26% 28% 18% 5%	530
	21st to end	Day	10% 4 27% 33% 20% 6%	530
July	1st to 10th	Day	11% 4 19% 33% 22% 11%	525
	11th to 20th	Day	5% 3 19% 34% 29% 10%	524
	21st to end	Day	12% 2 21% 32% 25% 7%	576
August	1st to 10th	Day	14% 3 19% 29% 24% 10%	526
	11th to 20th	Day	13% 4 20% 31% 21% 11%	519
	21st to end	Day	10% 4 23% 29% 23% 11%	583
September	1st to 10th	Day	15% 6% 20% 27% 20% 12%	530
	11th to 20th	Day	14% 3 21% 32% 20% 11%	530
	21st to end	Day	25% 9% 21% 28% 12% 4	530
October	1st to 10th	Day	21% 7% 23% 24% 19% 7%	530
	11th to 20th	Day	18% 14% 27% 25% 13% 3	522
	21st to end	Day	15% 12% 35% 26% 10% 2	539
November	1st to 10th	Day	15% 12% 33% 26% 12% 2	430
	11th to 20th	Day	18% 14% 36% 22% 10% 1	382
	21st to end	Day	25% 17% 34% 18% 7%	200
December	1st to 10th	Day	22% 22% 39% 14% 4 2	200
	11th to 20th	Day	28% 18% 30% 19% 7%	200
	21st to end	Day	25% 14% 37% 19% 5%	220
January	1st to 10th	Night	20% 20% 39% 19% 3	800
	11th to 20th	Night	22% 17% 35% 23% 4	800
	21st to end	Night	19% 18% 40% 22% 1	880
February	1st to 10th	Night	20% 17% 36% 26% 2	800
	11th to 20th	Night	28% 20% 35% 16% 1	783
	21st to end	Night	23% 17% 38% 18% 3	660
March	1st to 10th	Night	19% 17% 38% 22% 4	800
	11th to 20th	Night	21% 17% 35% 21% 5% 1	800
	21st to end	Night	19% 14% 43% 20% 4 1	880
April	1st to 10th	Night	22% 17% 38% 20% 4	800
	11th to 20th	Night	16% 15% 33% 29% 7% 1	791
	21st to end	Night	21% 10% 34% 28% 8%	800
May	1st to 10th	Night	21% 8% 30% 30% 10%	670
	11th to 20th	Night	23% 8% 28% 32% 10%	434
	21st to end	Night	17% 4 23% 33% 20% 3	433
June	1st to 10th	Night	13% 3 25% 38% 21%	400
	11th to 20th	Night	14% 4 23% 38% 21% 1	400
	21st to end	Night	12% 2 23% 46% 18% 1	400
July	1st to 10th	Night	9% 3 18% 41% 26% 3	390
	11th to 20th	Night	8% 1 13% 41% 35% 3	423
	21st to end	Night	9% 1 15% 43% 28% 4	657
August	1st to 10th	Night	12% 2 11% 43% 29% 3	597
	11th to 20th	Night	13% 3 16% 40% 26% 3	683
	21st to end	Night	12% 3 16% 38% 26% 5%	880
September	1st to 10th	Night	15% 3 19% 34% 25% 4	800
	11th to 20th	Night	13% 3 16% 37% 27% 4	800
	21st to end	Night	23% 4 21% 34% 16% 2	800
October	1st to 10th	Night	21% 5% 22% 28% 20% 4	800
	11th to 20th	Night	18% 8% 29% 32% 13% 1	793
	21st to end	Night	19% 10% 30% 29% 12%	836
November	1st to 10th	Night	15% 8% 33% 34% 9% 2	780
	11th to 20th	Night	18% 16% 31% 28% 7% 1	800
	21st to end	Night	17% 15% 32% 28% 7% 1	800
December	1st to 10th	Night	23% 13% 37% 20% 7% 1	800
	11th to 20th	Night	18% 12% 42% 24% 5%	800
	21st to end	Night	19% 13% 36% 29% 3	880

Page generated at 2025-10-16 01:50:28. Version ac07

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