Big Bird Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Bird Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	10% 3 1 3 2 2 2 2 3 5% 66%	135943
	Night	13% 4 1 3 2 2 3 2 4 5% 60%	132502
Transparency	Day	23% 3 7% 18% 25% 24%	130857
	Night	28% 4 7% 17% 22% 22%	128574
Seeing	Day	12% 8% 22% 32% 21% 5%	16525
	Night	14% 4 15% 35% 28% 4	25800

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	25% 6% 1 4 3 3 3 3 4 6% 42%	3639
	11th to 20th	Day	21% 6% 2 5% 3 2 2 2 2 4 54%	3976
	21st to end	Day	23% 5% 2 4 2 2 2 2 4 6% 47%	4510
February	1st to 10th	Day	23% 5% 1 5% 2 3 4 3 4 5% 46%	1800
	11th to 20th	Day	18% 8% 2 5% 4 3 3 2 4 6% 45%	1836
	21st to end	Day	19% 5% 1 3 2 3 3 3 6% 6% 47%	1650
March	1st to 10th	Day	24% 11% 3 6% 3 3 4 3 4 5% 36%	4711
	11th to 20th	Day	24% 6% 1 4 3 3 2 3 3 6% 46%	5000
	21st to end	Day	21% 9% 2 5% 4 4 3 3 5% 7% 36%	5656
April	1st to 10th	Day	16% 6% 2 5% 3 4 3 3 5% 7% 45%	2400
	11th to 20th	Day	14% 4 2 4 2 2 3 4 5% 6% 55%	2369
	21st to end	Day	7% 4 1 4 3 3 3 4 5% 7% 60%	2400
May	1st to 10th	Day	5% 2 1 2 1 2 2 2 3 6% 74%	5736
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 5% 76%	6277
	21st to end	Day	2 1 2 1 2 2 2 3 5% 78%	6902
June	1st to 10th	Day	6% 3 1 3 2 2 3 3 5% 8% 64%	2800
	11th to 20th	Day	2 1 2 2 2 2 2 4 5% 78%	2800
	21st to end	Day	2 1 1 1 1 1 1 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 2 92%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	6320
	21st to end	Day	1 1 2 1 1 2 2 3 4 84%	6451
August	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 82%	5777
	11th to 20th	Day	1 1 1 1 1 2 2 91%	5545
	21st to end	Day	1 1 1 2 1 1 1 1 2 4 86%	5984
September	1st to 10th	Day	1 1 1 1 1 1 2 1 2 4 86%	2400
	11th to 20th	Day	3 2 1 3 2 2 2 2 3 5% 76%	2400
	21st to end	Day	4 2 1 2 1 1 2 1 3 5% 79%	2140
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 4 76%	3280
	11th to 20th	Day	8% 2 1 3 2 2 3 2 4 6% 68%	3261
	21st to end	Day	8% 3 1 4 3 3 4 3 6% 7% 57%	3558
November	1st to 10th	Day	15% 4 1 4 3 2 3 2 5% 7% 53%	1899
	11th to 20th	Day	17% 4 1 4 3 4 3 4 5% 8% 47%	1879
	21st to end	Day	15% 4 1 5% 3 3 3 2 5% 5% 55%	1680
December	1st to 10th	Day	12% 3 1 3 2 3 3 3 5% 6% 59%	2700
	11th to 20th	Day	32% 6% 2 4 3 3 4 3 5% 8% 30%	3084
	21st to end	Day	34% 6% 2 4 3 3 4 3 4 6% 30%	4004
January	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 6% 7% 41%	6306
	11th to 20th	Night	20% 4 1 4 2 2 2 2 3 4 54%	5995
	21st to end	Night	22% 6% 2 5% 3 3 3 2 4 5% 46%	6468
February	1st to 10th	Night	21% 5% 2 4 3 2 3 2 4 6% 48%	2600
	11th to 20th	Night	18% 5% 2 5% 3 4 3 2 5% 6% 46%	2449
	21st to end	Night	18% 6% 2 4 2 4 4 2 4 5% 48%	1980
March	1st to 10th	Night	24% 10% 3 6% 4 3 3 2 4 6% 36%	5166
	11th to 20th	Night	26% 8% 1 3 2 2 2 2 4 4 46%	5000
	21st to end	Night	23% 10% 3 6% 3 4 4 3 5% 5% 34%	5319
April	1st to 10th	Night	12% 6% 2 5% 3 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	11% 5% 1 4 3 3 3 3 4 7% 56%	1979
	21st to end	Night	8% 4 1 3 3 2 3 2 4 6% 63%	2000
May	1st to 10th	Night	4 3 1 4 2 2 2 2 4 5% 70%	4246
	11th to 20th	Night	4 3 1 3 2 2 3 2 4 5% 71%	3616
	21st to end	Night	2 2 1 2 1 1 2 2 3 4 80%	3972
June	1st to 10th	Night	5% 3 1 4 3 3 3 3 5% 7% 64%	1600
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 6% 78%	1600
	21st to end	Night	2 2 1 2 1 2 2 2 3 3 81%	1600
July	1st to 10th	Night	1 1 1 1 2 2 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 1 89%	3640
	21st to end	Night	1 2 1 2 2 2 3 5% 82%	4416
August	1st to 10th	Night	2 2 1 2 1 2 2 2 2 3 81%	4035
	11th to 20th	Night	1 1 1 1 1 1 2 2 90%	4314
	21st to end	Night	2 2 2 1 2 1 2 2 3 82%	5015
September	1st to 10th	Night	2 1 1 1 1 1 1 3 87%	2000
	11th to 20th	Night	4 2 1 2 2 1 2 2 4 4 75%	2000
	21st to end	Night	3 1 2 1 1 2 1 3 5% 80%	2255
October	1st to 10th	Night	4 2 1 3 2 2 2 2 2 2 79%	3920
	11th to 20th	Night	5% 2 1 2 1 2 2 2 3 4 75%	3897
	21st to end	Night	10% 2 1 3 2 3 3 3 5% 7% 62%	4253
November	1st to 10th	Night	15% 4 1 3 2 3 3 3 6% 6% 53%	2618
	11th to 20th	Night	15% 4 1 4 3 3 3 3 6% 7% 51%	2737
	21st to end	Night	15% 4 1 4 3 2 3 3 4 6% 55%	2940
December	1st to 10th	Night	13% 3 1 3 2 2 3 2 5% 6% 61%	4700
	11th to 20th	Night	34% 6% 2 4 3 4 4 4 5% 8% 27%	5274
	21st to end	Night	34% 6% 2 6% 3 3 3 3 4 5% 31%	6952

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% 5% 10% 23% 11% 7%	3639
	11th to 20th	Day	39% 3 4 16% 23% 15%	3976
	21st to end	Day	40% 4 7% 13% 20% 16%	4510
February	1st to 10th	Day	41% 4 9% 18% 18% 10%	1800
	11th to 20th	Day	41% 3 9% 19% 21% 7%	1845
	21st to end	Day	36% 4 11% 23% 17% 8%	1650
March	1st to 10th	Day	51% 4 7% 19% 15% 4	4711
	11th to 20th	Day	42% 3 10% 25% 13% 8%	5000
	21st to end	Day	46% 6% 13% 23% 10% 2	5656
April	1st to 10th	Day	38% 4 11% 24% 18% 5%	2400
	11th to 20th	Day	30% 5% 12% 22% 22% 9%	2372
	21st to end	Day	24% 5% 14% 29% 24% 5%	2400
May	1st to 10th	Day	15% 2 7% 22% 33% 21%	5736
	11th to 20th	Day	13% 3 7% 26% 39% 14%	6280
	21st to end	Day	10% 3 9% 29% 39% 11%	6904
June	1st to 10th	Day	19% 6% 10% 26% 27% 13%	2800
	11th to 20th	Day	10% 3 8% 19% 34% 26%	2800
	21st to end	Day	7% 2 3 14% 35% 39%	2800
July	1st to 10th	Day	3 1 1 8% 21% 66%	6251
	11th to 20th	Day	5% 1 2 5% 15% 73%	6245
	21st to end	Day	6% 2 4 17% 28% 43%	6264
August	1st to 10th	Day	11% 2 3 8% 26% 51%	4206
	11th to 20th	Day	4 1 1 9% 42% 43%	4016
	21st to end	Day	6% 1 2 11% 38% 41%	4311
September	1st to 10th	Day	6% 1 2 10% 26% 55%	2400
	11th to 20th	Day	13% 2 6% 21% 34% 24%	2400
	21st to end	Day	11% 2 5% 18% 30% 33%	2140
October	1st to 10th	Day	14% 2 4 12% 25% 44%	3280
	11th to 20th	Day	18% 3 4 22% 31% 22%	3261
	21st to end	Day	25% 5% 6% 20% 28% 16%	3558
November	1st to 10th	Day	31% 4 9% 22% 20% 15%	1899
	11th to 20th	Day	36% 5% 12% 20% 18% 9%	1879
	21st to end	Day	32% 3 8% 19% 23% 14%	1680
December	1st to 10th	Day	27% 3 6% 23% 28% 13%	2700
	11th to 20th	Day	54% 4 10% 16% 13% 4	3084
	21st to end	Day	55% 5% 12% 16% 9% 2	4004
January	1st to 10th	Night	40% 7% 15% 18% 14% 7%	6306
	11th to 20th	Night	35% 5% 8% 13% 24% 16%	5995
	21st to end	Night	43% 4 7% 9% 16% 21%	6468
February	1st to 10th	Night	39% 3 8% 19% 19% 13%	2600
	11th to 20th	Night	39% 3 9% 19% 21% 9%	2451
	21st to end	Night	38% 4 9% 21% 19% 9%	1980
March	1st to 10th	Night	50% 4 7% 18% 14% 7%	5166
	11th to 20th	Night	43% 4 7% 20% 18% 8%	5000
	21st to end	Night	50% 5% 10% 21% 11% 2	5319
April	1st to 10th	Night	35% 6% 11% 21% 20% 7%	2000
	11th to 20th	Night	28% 5% 12% 22% 20% 12%	1979
	21st to end	Night	23% 5% 12% 28% 26% 6%	2000
May	1st to 10th	Night	17% 4 9% 19% 31% 21%	4243
	11th to 20th	Night	16% 4 6% 21% 35% 18%	3616
	21st to end	Night	10% 3 8% 28% 37% 14%	3972
June	1st to 10th	Night	20% 6% 11% 27% 26% 11%	1600
	11th to 20th	Night	10% 4 8% 17% 32% 29%	1600
	21st to end	Night	11% 2 3 14% 30% 40%	1600
July	1st to 10th	Night	3 1 1 8% 21% 66%	3600
	11th to 20th	Night	6% 1 1 5% 13% 74%	3592
	21st to end	Night	7% 2 3 19% 26% 43%	4291
August	1st to 10th	Night	15% 2 2 8% 24% 49%	2929
	11th to 20th	Night	4 1 1 9% 35% 49%	3113
	21st to end	Night	8% 1 2 11% 34% 44%	3608
September	1st to 10th	Night	7% 2 2 8% 27% 55%	2000
	11th to 20th	Night	14% 3 4 17% 37% 25%	2000
	21st to end	Night	10% 2 5% 16% 31% 37%	2255
October	1st to 10th	Night	14% 2 3 11% 18% 52%	3920
	11th to 20th	Night	15% 3 6% 24% 29% 23%	3897
	21st to end	Night	23% 3 6% 20% 29% 20%	4253
November	1st to 10th	Night	31% 4 9% 23% 17% 16%	2618
	11th to 20th	Night	32% 5% 12% 23% 18% 10%	2737
	21st to end	Night	32% 3 8% 21% 22% 14%	2940
December	1st to 10th	Night	24% 4 6% 21% 25% 21%	4700
	11th to 20th	Night	55% 4 7% 14% 16% 5%	5274
	21st to end	Night	56% 5% 10% 15% 10% 3	6952

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	44% 4 22% 18% 13% 1	200
	11th to 20th	Day	26% 7% 22% 28% 14% 4	341
	21st to end	Day	23% 7% 20% 29% 18% 4	440
February	1st to 10th	Day	27% 9% 21% 27% 16% 1	400
	11th to 20th	Day	24% 11% 28% 22% 13% 1	436
	21st to end	Day	18% 14% 26% 31% 11% 1	414
March	1st to 10th	Day	29% 17% 26% 20% 8% 1	500
	11th to 20th	Day	29% 11% 24% 19% 15% 2	500
	21st to end	Day	25% 13% 28% 23% 10% 1	550
April	1st to 10th	Day	21% 13% 28% 23% 13% 2	500
	11th to 20th	Day	16% 15% 27% 26% 12% 3	492
	21st to end	Day	7% 14% 26% 31% 17% 4	500
May	1st to 10th	Day	9% 13% 33% 28% 14% 3	503
	11th to 20th	Day	10% 12% 23% 29% 20% 6%	528
	21st to end	Day	5% 9% 25% 30% 25% 6%	573
June	1st to 10th	Day	5% 11% 21% 31% 26% 6%	530
	11th to 20th	Day	4 8% 29% 37% 17% 5%	530
	21st to end	Day	2 5% 21% 39% 26% 7%	530
July	1st to 10th	Day	2 2 21% 45% 25% 7%	526
	11th to 20th	Day	1 1 16% 44% 30% 8%	524
	21st to end	Day	1 1 15% 46% 30% 8%	575
August	1st to 10th	Day	2 2 17% 44% 28% 8%	527
	11th to 20th	Day	1 3 21% 37% 30% 9%	517
	21st to end	Day	2 2 16% 41% 33% 7%	583
September	1st to 10th	Day	2 4 18% 45% 28% 5%	530
	11th to 20th	Day	5% 6% 20% 32% 31% 5%	530
	21st to end	Day	3 7% 23% 37% 28% 3	530
October	1st to 10th	Day	6% 8% 24% 28% 29% 5%	530
	11th to 20th	Day	8% 8% 20% 30% 29% 6%	530
	21st to end	Day	13% 7% 19% 32% 24% 5%	583
November	1st to 10th	Day	17% 6% 19% 28% 23% 6%	375
	11th to 20th	Day	18% 4 21% 30% 17% 10%	324
	21st to end	Day	15% 6% 21% 30% 21% 6%	220
December	1st to 10th	Day	25% 4 19% 25% 18% 9%	220
	11th to 20th	Day	28% 5% 18% 25% 15% 10%	214
	21st to end	Day	26% 10% 22% 21% 18% 3	220
January	1st to 10th	Night	25% 4 18% 34% 15% 3	800
	11th to 20th	Night	23% 4 18% 38% 16% 2	800
	21st to end	Night	20% 3 20% 35% 19% 2	880
February	1st to 10th	Night	23% 4 18% 35% 19% 2	800
	11th to 20th	Night	20% 7% 17% 33% 21% 2	784
	21st to end	Night	21% 6% 20% 39% 12% 2	660
March	1st to 10th	Night	28% 8% 16% 29% 17% 2	800
	11th to 20th	Night	25% 6% 15% 29% 24% 2	800
	21st to end	Night	23% 7% 22% 30% 17% 1	880
April	1st to 10th	Night	16% 6% 22% 35% 19% 2	800
	11th to 20th	Night	13% 7% 19% 36% 21% 4	791
	21st to end	Night	10% 5% 19% 37% 25% 4	800
May	1st to 10th	Night	8% 5% 27% 31% 25% 3	668
	11th to 20th	Night	7% 3 20% 37% 29% 4	394
	21st to end	Night	6% 4 18% 32% 36% 3	433
June	1st to 10th	Night	6% 3 17% 36% 36% 4	400
	11th to 20th	Night	2 2 21% 38% 34% 4	400
	21st to end	Night	3 2 12% 40% 39% 5%	400
July	1st to 10th	Night	1 1 8% 36% 51% 4	390
	11th to 20th	Night	1 7% 43% 46% 4	388
	21st to end	Night	5% 33% 54% 7%	627
August	1st to 10th	Night	3 1 5% 35% 50% 6%	597
	11th to 20th	Night	1 1 6% 39% 45% 8%	700
	21st to end	Night	1 1 6% 36% 52% 5%	880
September	1st to 10th	Night	2 1 9% 38% 45% 6%	800
	11th to 20th	Night	4 1 10% 34% 41% 9%	800
	21st to end	Night	3 3 14% 37% 34% 9%	800
October	1st to 10th	Night	6% 4 15% 37% 31% 6%	800
	11th to 20th	Night	7% 5% 13% 36% 33% 6%	800
	21st to end	Night	11% 4 14% 35% 30% 7%	880
November	1st to 10th	Night	17% 4 15% 30% 28% 5%	820
	11th to 20th	Night	17% 4 17% 38% 21% 3	840
	21st to end	Night	16% 3 18% 37% 24% 3	840
December	1st to 10th	Night	21% 3 16% 35% 22% 3	840
	11th to 20th	Night	29% 4 12% 35% 18% 2	828
	21st to end	Night	28% 4 20% 33% 14% 1	880

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