Birdland Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Birdland Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	12% 9% 4 8% 5% 4 4 4 6% 7% 38%	131880
	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 38%	130258
Transparency	Day	46% 3 12% 26% 12% 1	135432
	Night	47% 3 12% 24% 13% 1	134783
Seeing	Day	18% 9% 30% 27% 12% 4	17827
	Night	17% 9% 32% 30% 11% 1	27865

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	8% 14% 7% 20% 8% 6% 4 3 4 6% 20%	4691
	11th to 20th	Day	5% 12% 7% 12% 7% 6% 7% 9% 9% 5% 21%	4728
	21st to end	Day	7% 12% 6% 10% 5% 5% 7% 4 6% 7% 30%	5812
February	1st to 10th	Day	8% 13% 5% 10% 7% 6% 5% 6% 6% 6% 28%	1890
	11th to 20th	Day	9% 12% 5% 10% 5% 5% 5% 4 6% 6% 34%	1844
	21st to end	Day	13% 11% 4 8% 5% 5% 5% 3 5% 6% 33%	1674
March	1st to 10th	Day	13% 7% 3 6% 4 3 3 3 5% 7% 48%	4851
	11th to 20th	Day	11% 9% 3 7% 4 3 3 4 6% 8% 42%	5090
	21st to end	Day	24% 11% 4 9% 5% 4 4 3 4 5% 28%	5718
April	1st to 10th	Day	17% 13% 3 8% 4 4 3 3 5% 7% 31%	2412
	11th to 20th	Day	20% 13% 4 8% 4 4 4 3 5% 5% 31%	2374
	21st to end	Day	21% 13% 3 7% 4 4 4 4 5% 5% 30%	2535
May	1st to 10th	Day	15% 6% 2 5% 4 3 4 3 5% 6% 46%	6155
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 5% 7% 42%	6319
	21st to end	Day	19% 8% 2 6% 4 4 4 3 5% 6% 38%	6935
June	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 46%	2800
	11th to 20th	Day	13% 7% 3 7% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	13% 6% 2 7% 4 5% 5% 4 7% 8% 39%	2800
July	1st to 10th	Day	9% 4 2 6% 5% 5% 5% 4 7% 9% 43%	6360
	11th to 20th	Day	7% 4 2 4 3 4 4 3 6% 8% 55%	6360
	21st to end	Day	8% 6% 2 7% 4 5% 5% 4 7% 8% 43%	6884
August	1st to 10th	Day	11% 7% 2 6% 4 5% 6% 4 6% 8% 41%	4234
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 42%	4171
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 8% 46%	4383
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 6% 7% 46%	2400
	11th to 20th	Day	11% 5% 3 6% 5% 4 4 3 6% 7% 46%	2400
	21st to end	Day	13% 7% 3 7% 3 4 4 3 6% 7% 43%	2094
October	1st to 10th	Day	14% 7% 2 5% 4 3 4 3 5% 5% 47%	2000
	11th to 20th	Day	15% 10% 3 6% 4 4 4 4 5% 7% 39%	2000
	21st to end	Day	18% 12% 3 7% 4 4 3 3 4 6% 36%	2153
November	1st to 10th	Day	12% 10% 4 7% 5% 3 4 3 5% 7% 40%	1805
	11th to 20th	Day	13% 14% 6% 10% 7% 4 4 3 5% 5% 30%	1680
	21st to end	Day	12% 13% 4 10% 5% 4 3 3 5% 6% 36%	1680
December	1st to 10th	Day	7% 9% 4 13% 10% 6% 6% 4 5% 6% 28%	2690
	11th to 20th	Day	12% 15% 6% 11% 9% 7% 4 4 5% 6% 23%	3088
	21st to end	Day	17% 19% 10% 19% 5% 4 3 2 3 3 14%	4070
January	1st to 10th	Night	9% 13% 5% 15% 8% 6% 5% 3 4 5% 26%	8232
	11th to 20th	Night	7% 13% 5% 14% 8% 7% 9% 8% 8% 4 19%	8247
	21st to end	Night	8% 12% 5% 10% 6% 5% 6% 4 6% 6% 33%	8402
February	1st to 10th	Night	7% 15% 5% 10% 6% 6% 4 4 5% 6% 33%	2730
	11th to 20th	Night	8% 11% 4 9% 6% 6% 5% 4 5% 6% 38%	2663
	21st to end	Night	12% 11% 4 9% 6% 5% 4 3 5% 6% 36%	2131
March	1st to 10th	Night	11% 8% 3 7% 3 3 4 3 4 6% 47%	5263
	11th to 20th	Night	9% 8% 2 7% 5% 4 4 4 5% 8% 42%	5090
	21st to end	Night	25% 12% 3 10% 4 3 3 2 4 5% 29%	5453
April	1st to 10th	Night	16% 12% 3 7% 4 4 4 3 5% 6% 37%	2010
	11th to 20th	Night	20% 10% 3 7% 4 3 3 3 5% 6% 36%	1975
	21st to end	Night	20% 10% 3 6% 4 4 4 2 4 6% 38%	1861
May	1st to 10th	Night	14% 6% 2 6% 3 4 3 3 5% 5% 48%	3746
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 7% 51%	3596
	21st to end	Night	17% 8% 3 8% 4 3 4 3 4 6% 40%	3942
June	1st to 10th	Night	10% 3 2 5% 5% 4 4 4 6% 9% 49%	1600
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 8% 43%	1600
	21st to end	Night	9% 5% 2 5% 4 3 4 4 5% 8% 53%	1600
July	1st to 10th	Night	8% 4 2 5% 4 4 4 4 7% 10% 48%	3618
	11th to 20th	Night	7% 4 1 4 3 3 4 4 6% 8% 55%	3617
	21st to end	Night	8% 4 2 6% 4 3 4 4 6% 8% 51%	3983
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 4 7% 48%	2926
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 6% 51%	2888
	21st to end	Night	10% 6% 2 6% 4 4 4 4 4 7% 50%	3517
September	1st to 10th	Night	9% 5% 2 4 4 3 4 3 4 7% 55%	2000
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 51%	2000
	21st to end	Night	12% 6% 2 5% 3 3 3 2 5% 6% 53%	2283
October	1st to 10th	Night	13% 7% 2 5% 3 3 3 3 4 6% 50%	2400
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	2400
	21st to end	Night	17% 10% 3 7% 4 4 4 3 5% 6% 38%	2686
November	1st to 10th	Night	12% 10% 3 7% 4 3 4 4 5% 6% 43%	2710
	11th to 20th	Night	12% 12% 5% 10% 5% 4 4 3 4 7% 35%	2940
	21st to end	Night	14% 12% 4 10% 5% 4 3 3 4 6% 36%	2940
December	1st to 10th	Night	7% 9% 4 15% 7% 7% 6% 4 5% 6% 30%	4720
	11th to 20th	Night	11% 16% 7% 14% 6% 5% 5% 3 5% 5% 24%	5339
	21st to end	Night	21% 16% 8% 16% 5% 4 4 3 3 3 16%	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	68% 2 11% 14% 5%	4692
	11th to 20th	Day	60% 3 7% 18% 12%	4728
	21st to end	Day	54% 3 12% 22% 9%	5812
February	1st to 10th	Day	54% 4 13% 18% 10% 1	1890
	11th to 20th	Day	51% 3 11% 26% 7% 1	1853
	21st to end	Day	53% 2 12% 21% 10% 1	1674
March	1st to 10th	Day	39% 2 16% 28% 13% 3	4851
	11th to 20th	Day	41% 3 14% 31% 11% 1	5090
	21st to end	Day	61% 2 10% 18% 8% 1	5720
April	1st to 10th	Day	54% 3 10% 21% 11% 1	2412
	11th to 20th	Day	56% 2 10% 19% 11% 1	2374
	21st to end	Day	56% 3 12% 20% 9% 1	2535
May	1st to 10th	Day	39% 2 10% 30% 17% 2	6194
	11th to 20th	Day	42% 4 9% 28% 15% 3	6323
	21st to end	Day	47% 3 9% 30% 11% 1	6949
June	1st to 10th	Day	36% 4 13% 32% 15% 1	2800
	11th to 20th	Day	43% 5% 14% 22% 15% 1	2800
	21st to end	Day	42% 4 15% 26% 12% 2	2800
July	1st to 10th	Day	36% 5% 16% 33% 9% 1	6291
	11th to 20th	Day	28% 4 14% 36% 17% 1	6285
	21st to end	Day	37% 6% 14% 28% 15% 1	6688
August	1st to 10th	Day	40% 4 12% 29% 13% 1	4220
	11th to 20th	Day	38% 4 13% 33% 11%	4210
	21st to end	Day	35% 5% 15% 30% 14% 1	4382
September	1st to 10th	Day	37% 4 13% 31% 15% 1	2400
	11th to 20th	Day	36% 4 18% 31% 10% 1	2400
	21st to end	Day	41% 3 10% 33% 12% 1	2094
October	1st to 10th	Day	28% 2 7% 39% 24%	3260
	11th to 20th	Day	40% 2 13% 24% 20% 2	3240
	21st to end	Day	44% 2 12% 32% 10%	3455
November	1st to 10th	Day	44% 3 14% 22% 15% 2	1805
	11th to 20th	Day	57% 3 11% 18% 10% 1	1680
	21st to end	Day	51% 3 17% 20% 8% 1	1680
December	1st to 10th	Day	57% 4 15% 16% 7% 1	2690
	11th to 20th	Day	63% 3 14% 15% 5%	3085
	21st to end	Day	77% 2 6% 11% 3	4070
January	1st to 10th	Night	62% 2 12% 19% 5%	8233
	11th to 20th	Night	64% 3 8% 14% 10%	8247
	21st to end	Night	51% 3 11% 23% 12%	8402
February	1st to 10th	Night	53% 3 12% 22% 9% 1	2730
	11th to 20th	Night	48% 4 12% 26% 10% 1	2664
	21st to end	Night	50% 3 12% 23% 11% 2	2131
March	1st to 10th	Night	40% 2 11% 30% 14% 3	5263
	11th to 20th	Night	40% 3 11% 28% 16% 1	5090
	21st to end	Night	60% 3 10% 17% 10% 1	5453
April	1st to 10th	Night	49% 3 10% 22% 14% 2	2010
	11th to 20th	Night	50% 2 9% 20% 16% 2	1975
	21st to end	Night	50% 2 10% 22% 15% 2	1861
May	1st to 10th	Night	38% 3 9% 26% 23% 1	3772
	11th to 20th	Night	35% 3 11% 25% 20% 6%	3597
	21st to end	Night	45% 3 9% 31% 12%	3950
June	1st to 10th	Night	32% 5% 14% 31% 16% 3	1600
	11th to 20th	Night	39% 4 13% 24% 18% 3	1600
	21st to end	Night	30% 4 16% 31% 16% 3	1600
July	1st to 10th	Night	31% 4 21% 31% 12% 1	3580
	11th to 20th	Night	26% 5% 17% 37% 13% 2	3572
	21st to end	Night	31% 5% 17% 29% 17% 2	3868
August	1st to 10th	Night	37% 4 11% 30% 16% 3	2916
	11th to 20th	Night	34% 4 14% 31% 15% 1	2915
	21st to end	Night	35% 4 16% 28% 15% 2	3517
September	1st to 10th	Night	29% 4 11% 36% 19% 1	2000
	11th to 20th	Night	33% 3 15% 33% 16% 1	2000
	21st to end	Night	33% 3 13% 30% 20% 1	2283
October	1st to 10th	Night	26% 2 6% 35% 27% 4	3920
	11th to 20th	Night	31% 3 12% 29% 22% 3	3899
	21st to end	Night	45% 2 10% 27% 15% 1	4334
November	1st to 10th	Night	42% 3 13% 26% 14% 2	2710
	11th to 20th	Night	51% 4 11% 21% 12% 2	2940
	21st to end	Night	52% 3 15% 23% 8% 1	2940
December	1st to 10th	Night	55% 3 17% 19% 6%	4720
	11th to 20th	Night	64% 3 11% 16% 6% 1	5341
	21st to end	Night	74% 3 7% 12% 5%	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	15% 12% 38% 22% 9% 3	330
	11th to 20th	Day	19% 11% 43% 22% 4 2	330
	21st to end	Day	16% 12% 38% 25% 8% 1	363
February	1st to 10th	Day	18% 15% 43% 18% 4 2	330
	11th to 20th	Day	16% 19% 37% 22% 5% 1	330
	21st to end	Day	21% 14% 38% 25% 2	273
March	1st to 10th	Day	19% 10% 43% 21% 6% 1	330
	11th to 20th	Day	25% 13% 34% 20% 7% 2	330
	21st to end	Day	29% 14% 29% 19% 6% 3	421
April	1st to 10th	Day	27% 14% 34% 18% 6%	504
	11th to 20th	Day	30% 15% 25% 21% 8% 1	492
	21st to end	Day	30% 11% 27% 23% 7% 2	635
May	1st to 10th	Day	21% 10% 32% 25% 10% 2	703
	11th to 20th	Day	21% 9% 28% 26% 11% 4	725
	21st to end	Day	20% 7% 26% 28% 16% 3	789
June	1st to 10th	Day	11% 8% 28% 31% 16% 5%	730
	11th to 20th	Day	16% 7% 32% 31% 11% 3	730
	21st to end	Day	17% 8% 29% 27% 15% 5%	730
July	1st to 10th	Day	8% 4 22% 33% 23% 10%	721
	11th to 20th	Day	10% 4 27% 31% 19% 8%	718
	21st to end	Day	8% 3 25% 40% 18% 6%	794
August	1st to 10th	Day	10% 3 21% 35% 23% 8%	726
	11th to 20th	Day	16% 3 19% 33% 23% 7%	715
	21st to end	Day	11% 5% 25% 32% 21% 6%	628
September	1st to 10th	Day	16% 5% 26% 30% 16% 7%	530
	11th to 20th	Day	14% 4 27% 31% 20% 4	530
	21st to end	Day	15% 11% 27% 27% 16% 4	354
October	1st to 10th	Day	15% 12% 27% 26% 16% 3	330
	11th to 20th	Day	20% 17% 35% 21% 6%	330
	21st to end	Day	27% 13% 35% 16% 7% 2	363
November	1st to 10th	Day	21% 16% 35% 20% 7% 1	330
	11th to 20th	Day	20% 20% 38% 17% 5%	330
	21st to end	Day	22% 11% 43% 21% 4	330
December	1st to 10th	Day	15% 12% 40% 27% 5% 2	330
	11th to 20th	Day	22% 14% 38% 22% 4	330
	21st to end	Day	29% 11% 38% 18% 3 1	363
January	1st to 10th	Night	14% 13% 38% 26% 7% 1	1050
	11th to 20th	Night	19% 12% 38% 25% 6%	1050
	21st to end	Night	17% 12% 36% 27% 7% 1	1155
February	1st to 10th	Night	14% 14% 40% 26% 5% 1	1050
	11th to 20th	Night	15% 13% 41% 24% 6% 1	1029
	21st to end	Night	18% 15% 35% 25% 6%	747
March	1st to 10th	Night	19% 12% 35% 25% 7% 1	740
	11th to 20th	Night	18% 12% 35% 28% 6%	630
	21st to end	Night	22% 8% 32% 26% 10% 2	693
April	1st to 10th	Night	21% 13% 33% 28% 5%	603
	11th to 20th	Night	28% 8% 30% 28% 6%	593
	21st to end	Night	27% 6% 33% 25% 8% 1	600
May	1st to 10th	Night	21% 8% 32% 31% 7% 1	600
	11th to 20th	Night	20% 5% 30% 33% 11% 1	591
	21st to end	Night	17% 6% 25% 39% 13% 1	650
June	1st to 10th	Night	12% 4 28% 41% 15% 1	600
	11th to 20th	Night	14% 4 34% 37% 12%	600
	21st to end	Night	13% 3 25% 42% 17% 1	600
July	1st to 10th	Night	7% 1 21% 40% 28% 3	585
	11th to 20th	Night	7% 4 23% 42% 23% 2	582
	21st to end	Night	7% 2 17% 52% 19% 3	657
August	1st to 10th	Night	8% 1 16% 47% 25% 2	597
	11th to 20th	Night	13% 3 16% 36% 28% 4	594
	21st to end	Night	10% 3 24% 38% 23% 2	660
September	1st to 10th	Night	12% 2 24% 39% 18% 5%	600
	11th to 20th	Night	14% 3 28% 35% 18% 3	600
	21st to end	Night	13% 8% 29% 34% 16% 1	710
October	1st to 10th	Night	17% 8% 29% 34% 12% 1	800
	11th to 20th	Night	19% 11% 33% 30% 7% 1	800
	21st to end	Night	22% 9% 33% 29% 7%	880
November	1st to 10th	Night	18% 11% 36% 26% 8% 1	864
	11th to 20th	Night	18% 15% 36% 24% 6%	1050
	21st to end	Night	20% 14% 37% 22% 7%	1050
December	1st to 10th	Night	18% 10% 39% 25% 7% 1	1050
	11th to 20th	Night	21% 12% 36% 23% 7% 1	1050
	21st to end	Night	26% 12% 32% 25% 6%	1155

Page generated at 2026-03-30 01:18:40. Version ac07

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