Astro Birdie Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro Birdie Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	148859
	Night	14% 6% 2 5% 4 3 4 3 5% 7% 46%	143224
Transparency	Day	38% 5% 13% 21% 16% 7%	152521
	Night	37% 5% 12% 20% 17% 9%	147629
Seeing	Day	13% 8% 21% 27% 22% 8%	17498
	Night	16% 8% 24% 33% 17% 2	27247

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	18% 9% 3 6% 3 3 3 3 4 5% 44%	5289
	11th to 20th	Day	14% 5% 2 6% 3 4 3 3 3 4 52%	5310
	21st to end	Day	29% 8% 2 8% 3 3 2 2 3 5% 34%	5812
February	1st to 10th	Day	21% 8% 3 7% 4 3 4 3 4 5% 38%	1984
	11th to 20th	Day	22% 8% 3 8% 4 3 3 3 3 6% 38%	2053
	21st to end	Day	19% 9% 2 6% 4 3 3 3 3 5% 42%	1903
March	1st to 10th	Day	18% 8% 2 8% 5% 4 4 3 4 6% 36%	5046
	11th to 20th	Day	14% 7% 2 6% 4 3 4 3 5% 6% 47%	5090
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 5% 6% 40%	5583
April	1st to 10th	Day	16% 9% 3 8% 5% 4 5% 3 5% 5% 37%	2310
	11th to 20th	Day	16% 8% 3 7% 5% 4 5% 4 5% 6% 37%	2447
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 6% 6% 39%	2520
May	1st to 10th	Day	23% 7% 3 7% 5% 5% 5% 4 5% 6% 32%	7257
	11th to 20th	Day	12% 6% 3 7% 5% 4 5% 4 6% 8% 41%	7627
	21st to end	Day	12% 7% 2 8% 5% 5% 6% 5% 8% 10% 32%	8392
June	1st to 10th	Day	6% 6% 2 7% 6% 5% 7% 6% 8% 8% 40%	2730
	11th to 20th	Day	7% 5% 1 5% 5% 6% 7% 5% 8% 11% 40%	2730
	21st to end	Day	6% 5% 2 9% 7% 7% 7% 6% 9% 9% 33%	2730
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 12% 41%	7680
	11th to 20th	Day	8% 6% 2 7% 6% 6% 6% 6% 8% 12% 33%	7680
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 6% 11% 45%	8337
August	1st to 10th	Day	1 1 1 3 3 4 4 4 6% 9% 63%	6014
	11th to 20th	Day	2 2 1 3 3 4 5% 5% 8% 13% 54%	5761
	21st to end	Day	5% 3 2 6% 5% 5% 6% 4 7% 11% 47%	5968
September	1st to 10th	Day	7% 5% 2 6% 6% 6% 6% 6% 7% 9% 41%	2310
	11th to 20th	Day	11% 6% 2 6% 5% 6% 6% 3 7% 8% 39%	2310
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 6% 9% 48%	2310
October	1st to 10th	Day	7% 3 2 4 3 3 5% 3 5% 6% 58%	2185
	11th to 20th	Day	9% 5% 3 5% 3 4 4 3 5% 5% 53%	2030
	21st to end	Day	13% 6% 2 5% 3 3 3 3 4 5% 54%	2200
November	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 48%	2058
	11th to 20th	Day	10% 6% 3 6% 4 4 4 3 5% 6% 50%	2100
	21st to end	Day	17% 7% 3 6% 4 4 5% 3 4 5% 42%	2005
December	1st to 10th	Day	22% 7% 2 5% 4 3 3 3 4 5% 41%	3030
	11th to 20th	Day	16% 8% 3 7% 4 4 3 3 5% 6% 43%	3481
	21st to end	Day	17% 7% 2 6% 3 3 3 3 4 5% 46%	4587
January	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 3 5% 48%	7634
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 6% 54%	7680
	21st to end	Night	30% 8% 2 6% 3 3 3 3 4 5% 34%	8402
February	1st to 10th	Night	20% 9% 2 7% 4 3 3 3 4 5% 41%	2654
	11th to 20th	Night	23% 8% 2 6% 3 3 3 2 4 5% 42%	2453
	21st to end	Night	20% 8% 2 6% 4 3 4 3 3 6% 41%	1903
March	1st to 10th	Night	22% 10% 2 6% 3 3 3 3 5% 6% 37%	5029
	11th to 20th	Night	17% 8% 2 6% 3 3 3 3 4 5% 46%	5090
	21st to end	Night	20% 9% 3 6% 4 4 3 2 3 4 42%	5588
April	1st to 10th	Night	16% 8% 3 6% 5% 4 4 3 6% 6% 41%	2310
	11th to 20th	Night	15% 8% 3 5% 4 4 3 3 6% 7% 43%	2120
	21st to end	Night	17% 7% 2 6% 4 3 4 3 4 6% 43%	2100
May	1st to 10th	Night	26% 10% 3 7% 4 3 4 3 5% 7% 28%	5590
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 4 5% 8% 39%	5265
	21st to end	Night	14% 8% 2 7% 5% 4 5% 4 7% 10% 33%	5793
June	1st to 10th	Night	7% 3 1 5% 3 4 5% 4 7% 10% 51%	1890
	11th to 20th	Night	6% 3 1 3 3 4 3 5% 7% 12% 54%	1890
	21st to end	Night	4 5% 2 6% 4 4 4 5% 8% 11% 46%	1890
July	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 8% 14% 47%	5308
	11th to 20th	Night	7% 5% 2 6% 5% 4 5% 5% 6% 11% 45%	5307
	21st to end	Night	7% 4 1 5% 4 3 4 3 6% 11% 52%	5770
August	1st to 10th	Night	2 1 1 2 2 2 2 2 5% 10% 72%	4156
	11th to 20th	Night	2 2 1 2 2 3 3 3 7% 11% 64%	4296
	21st to end	Night	4 2 1 4 3 4 5% 5% 7% 12% 52%	5139
September	1st to 10th	Night	5% 5% 2 6% 5% 4 4 4 7% 9% 48%	2310
	11th to 20th	Night	10% 5% 2 7% 4 4 5% 4 6% 8% 46%	2310
	21st to end	Night	7% 4 2 4 3 3 3 4 6% 9% 54%	2310
October	1st to 10th	Night	5% 4 2 4 3 3 4 3 5% 9% 59%	2412
	11th to 20th	Night	10% 4 1 3 2 2 3 3 4 6% 62%	2437
	21st to end	Night	12% 6% 2 4 3 3 3 2 3 4 59%	2640
November	1st to 10th	Night	11% 6% 2 4 3 3 4 3 4 6% 53%	2460
	11th to 20th	Night	12% 5% 2 5% 4 3 4 2 5% 5% 52%	2520
	21st to end	Night	17% 6% 3 6% 4 4 4 3 5% 5% 44%	2609
December	1st to 10th	Night	21% 9% 3 7% 4 4 3 3 3 5% 37%	4380
	11th to 20th	Night	18% 10% 2 6% 3 3 3 2 4 5% 44%	4946
	21st to end	Night	17% 7% 2 6% 3 2 3 2 4 4 50%	6633

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	43% 3 7% 22% 18% 7%	5290
	11th to 20th	Day	37% 3 4 17% 26% 13%	5310
	21st to end	Day	54% 2 3 13% 16% 11%	5812
February	1st to 10th	Day	48% 4 5% 14% 17% 12%	1984
	11th to 20th	Day	50% 3 7% 14% 16% 10%	2063
	21st to end	Day	46% 4 8% 15% 16% 11%	1903
March	1st to 10th	Day	48% 4 8% 12% 17% 10%	5046
	11th to 20th	Day	37% 5% 8% 11% 20% 19%	5090
	21st to end	Day	45% 5% 8% 20% 17% 6%	5585
April	1st to 10th	Day	49% 5% 6% 15% 15% 9%	2310
	11th to 20th	Day	46% 5% 10% 14% 16% 9%	2447
	21st to end	Day	43% 7% 9% 18% 18% 5%	2520
May	1st to 10th	Day	51% 6% 9% 21% 11% 2	7292
	11th to 20th	Day	40% 5% 14% 20% 19% 2	7631
	21st to end	Day	44% 7% 15% 25% 9% 1	8405
June	1st to 10th	Day	37% 8% 16% 21% 15% 1	2730
	11th to 20th	Day	35% 9% 17% 22% 14% 3	2730
	21st to end	Day	42% 11% 15% 21% 10%	2730
July	1st to 10th	Day	34% 6% 31% 26% 2	7616
	11th to 20th	Day	42% 6% 24% 20% 7%	7611
	21st to end	Day	34% 6% 15% 33% 11% 1	8155
August	1st to 10th	Day	17% 6% 19% 33% 20% 5%	6000
	11th to 20th	Day	21% 6% 30% 34% 8% 1	5800
	21st to end	Day	31% 9% 15% 30% 15% 1	5967
September	1st to 10th	Day	38% 10% 14% 17% 15% 6%	2310
	11th to 20th	Day	40% 7% 14% 21% 14% 4	2310
	21st to end	Day	32% 8% 12% 20% 16% 12%	2310
October	1st to 10th	Day	18% 4 5% 14% 22% 37%	3497
	11th to 20th	Day	23% 4 5% 16% 34% 19%	3270
	21st to end	Day	27% 4 5% 15% 36% 14%	3538
November	1st to 10th	Day	35% 6% 9% 16% 22% 11%	2058
	11th to 20th	Day	35% 5% 9% 15% 23% 13%	2100
	21st to end	Day	43% 5% 7% 16% 18% 10%	2005
December	1st to 10th	Day	46% 3 6% 16% 23% 5%	3030
	11th to 20th	Day	42% 5% 8% 13% 21% 11%	3479
	21st to end	Day	41% 4 5% 14% 26% 10%	4587
January	1st to 10th	Night	40% 3 8% 20% 19% 9%	7635
	11th to 20th	Night	34% 3 5% 19% 26% 14%	7680
	21st to end	Night	54% 3 3 14% 16% 10%	8402
February	1st to 10th	Night	46% 4 6% 12% 17% 14%	2654
	11th to 20th	Night	46% 4 7% 16% 17% 11%	2453
	21st to end	Night	46% 4 6% 14% 16% 14%	1903
March	1st to 10th	Night	48% 4 6% 13% 19% 9%	5029
	11th to 20th	Night	40% 5% 7% 14% 15% 18%	5090
	21st to end	Night	47% 4 7% 15% 19% 8%	5588
April	1st to 10th	Night	43% 5% 10% 16% 16% 10%	2310
	11th to 20th	Night	39% 5% 11% 17% 16% 12%	2120
	21st to end	Night	43% 6% 9% 18% 19% 6%	2100
May	1st to 10th	Night	55% 5% 10% 18% 9% 2	5620
	11th to 20th	Night	43% 5% 12% 19% 17% 3	5266
	21st to end	Night	46% 6% 17% 23% 7% 1	5805
June	1st to 10th	Night	26% 8% 18% 30% 17%	1890
	11th to 20th	Night	23% 8% 21% 28% 17% 4	1890
	21st to end	Night	29% 10% 20% 27% 12% 1	1890
July	1st to 10th	Night	28% 6% 37% 26% 2	5265
	11th to 20th	Night	34% 6% 28% 25% 7%	5256
	21st to end	Night	28% 7% 17% 36% 12%	5641
August	1st to 10th	Night	11% 4 23% 39% 19% 4	4146
	11th to 20th	Night	14% 6% 33% 37% 9% 1	4323
	21st to end	Night	24% 10% 19% 32% 13% 2	5139
September	1st to 10th	Night	31% 9% 22% 19% 12% 6%	2310
	11th to 20th	Night	34% 8% 17% 21% 15% 5%	2310
	21st to end	Night	26% 6% 16% 23% 18% 11%	2310
October	1st to 10th	Night	16% 4 7% 18% 17% 38%	3867
	11th to 20th	Night	15% 3 4 24% 31% 22%	3936
	21st to end	Night	24% 2 5% 16% 36% 16%	4252
November	1st to 10th	Night	33% 5% 11% 19% 22% 11%	2460
	11th to 20th	Night	34% 4 10% 18% 21% 14%	2520
	21st to end	Night	40% 5% 9% 16% 19% 11%	2609
December	1st to 10th	Night	51% 3 5% 15% 20% 6%	4380
	11th to 20th	Night	45% 3 8% 8% 22% 14%	4947
	21st to end	Night	39% 4 5% 14% 29% 9%	6633

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	26% 15% 35% 19% 3 1	330
	11th to 20th	Day	28% 14% 30% 22% 4 1	330
	21st to end	Day	29% 17% 33% 17% 4	363
February	1st to 10th	Day	28% 16% 33% 18% 5%	330
	11th to 20th	Day	29% 21% 31% 16% 2 1	330
	21st to end	Day	26% 24% 36% 11% 3	273
March	1st to 10th	Day	26% 21% 33% 16% 4	330
	11th to 20th	Day	19% 22% 39% 15% 4	330
	21st to end	Day	21% 15% 34% 22% 7% 1	363
April	1st to 10th	Day	19% 20% 37% 19% 5%	330
	11th to 20th	Day	24% 19% 28% 21% 8%	485
	21st to end	Day	18% 15% 36% 22% 8% 1	540
May	1st to 10th	Day	16% 15% 36% 22% 9% 1	624
	11th to 20th	Day	14% 9% 32% 29% 13% 3	775
	21st to end	Day	11% 6% 30% 35% 16% 3	844
June	1st to 10th	Day	7% 4 17% 35% 28% 9%	780
	11th to 20th	Day	8% 2 18% 37% 26% 8%	780
	21st to end	Day	7% 2 13% 33% 31% 13%	780
July	1st to 10th	Day	7% 2 10% 28% 35% 18%	771
	11th to 20th	Day	5% 1 5% 32% 41% 16%	768
	21st to end	Day	6% 1 5% 27% 47% 15%	849
August	1st to 10th	Day	3 1 4 27% 47% 18%	776
	11th to 20th	Day	4 1 2 22% 49% 21%	682
	21st to end	Day	7% 3 7% 22% 35% 27%	580
September	1st to 10th	Day	8% 1 9% 28% 34% 19%	360
	11th to 20th	Day	8% 2 8% 31% 37% 14%	360
	21st to end	Day	8% 1 12% 35% 33% 11%	360
October	1st to 10th	Day	6% 3 17% 41% 28% 5%	360
	11th to 20th	Day	9% 7% 20% 38% 21% 5%	339
	21st to end	Day	20% 11% 24% 24% 18% 4	363
November	1st to 10th	Day	14% 8% 23% 36% 18% 2	330
	11th to 20th	Day	13% 11% 31% 34% 9% 2	330
	21st to end	Day	21% 20% 32% 18% 8% 2	330
December	1st to 10th	Day	21% 12% 35% 21% 7% 4	330
	11th to 20th	Day	23% 16% 32% 22% 7% 1	330
	21st to end	Day	23% 15% 32% 22% 7% 1	363
January	1st to 10th	Night	23% 12% 33% 26% 6%	1050
	11th to 20th	Night	22% 15% 30% 26% 6%	1050
	21st to end	Night	24% 13% 31% 27% 5%	1155
February	1st to 10th	Night	26% 14% 31% 25% 4	974
	11th to 20th	Night	29% 13% 30% 24% 4	819
	21st to end	Night	26% 16% 34% 21% 4	519
March	1st to 10th	Night	27% 13% 32% 25% 4	630
	11th to 20th	Night	22% 11% 35% 27% 5%	630
	21st to end	Night	18% 11% 39% 25% 6%	693
April	1st to 10th	Night	18% 12% 38% 28% 4	630
	11th to 20th	Night	21% 12% 33% 28% 7%	623
	21st to end	Night	20% 12% 35% 24% 8%	630
May	1st to 10th	Night	19% 11% 34% 30% 6%	630
	11th to 20th	Night	15% 7% 31% 32% 13% 2	621
	21st to end	Night	10% 5% 30% 35% 18% 1	683
June	1st to 10th	Night	8% 3 15% 45% 27% 3	630
	11th to 20th	Night	6% 2 20% 47% 23% 2	630
	21st to end	Night	5% 3 13% 43% 31% 5%	630
July	1st to 10th	Night	4 2 9% 42% 36% 6%	615
	11th to 20th	Night	4 1 5% 46% 38% 5%	612
	21st to end	Night	4 1 7% 45% 38% 4	690
August	1st to 10th	Night	2 1 5% 49% 39% 5%	627
	11th to 20th	Night	4 4 47% 39% 5%	624
	21st to end	Night	5% 2 6% 35% 46% 6%	693
September	1st to 10th	Night	7% 2 8% 33% 42% 8%	630
	11th to 20th	Night	11% 2 8% 32% 42% 6%	630
	21st to end	Night	8% 2 13% 36% 36% 5%	630
October	1st to 10th	Night	5% 3 13% 42% 32% 5%	732
	11th to 20th	Night	11% 4 23% 40% 21% 2	813
	21st to end	Night	15% 8% 26% 31% 18% 2	880
November	1st to 10th	Night	12% 5% 25% 36% 21% 1	820
	11th to 20th	Night	13% 10% 27% 35% 15%	840
	21st to end	Night	21% 10% 29% 27% 12% 1	929
December	1st to 10th	Night	23% 11% 27% 28% 10%	1050
	11th to 20th	Night	25% 11% 29% 28% 6%	1050
	21st to end	Night	24% 11% 28% 29% 7%	1155

Page generated at 2026-10-11 04:06:27. Version ac07

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