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 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	13% 6% 2 6% 5% 4 5% 4 6% 7% 41%	114533
	Night	15% 7% 2 5% 4 3 3 3 5% 7% 46%	114922
Transparency	Day	40% 6% 11% 18% 17% 8%	118236
	Night	37% 5% 11% 19% 18% 10%	119348
Seeing	Day	14% 9% 22% 26% 22% 8%	15561
	Night	16% 8% 24% 32% 17% 2	25512

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	19% 10% 3 6% 3 3 3 3 4 4 43%	4077
	11th to 20th	Day	17% 6% 2 5% 3 4 3 3 3 4 50%	4080
	21st to end	Day	30% 8% 2 5% 2 2 2 2 4 6% 36%	4488
February	1st to 10th	Day	22% 8% 3 6% 4 3 4 3 4 5% 38%	1889
	11th to 20th	Day	22% 8% 3 8% 4 3 3 2 3 6% 38%	1953
	21st to end	Day	20% 9% 2 7% 4 3 3 3 3 5% 40%	1815
March	1st to 10th	Day	18% 8% 3 8% 5% 4 4 3 4 6% 37%	4936
	11th to 20th	Day	14% 7% 2 6% 4 3 4 3 5% 6% 46%	4980
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 5% 6% 40%	5462
April	1st to 10th	Day	15% 10% 3 8% 5% 4 4 3 5% 5% 38%	2200
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 4 5% 6% 39%	2329
	21st to end	Day	11% 8% 3 7% 5% 4 5% 4 6% 6% 40%	2400
May	1st to 10th	Day	19% 8% 3 7% 5% 6% 5% 4 6% 7% 29%	4014
	11th to 20th	Day	13% 6% 2 7% 4 4 5% 4 6% 7% 42%	4212
	21st to end	Day	9% 7% 2 7% 6% 6% 6% 5% 8% 11% 33%	4543
June	1st to 10th	Day	6% 6% 2 6% 6% 5% 6% 5% 8% 9% 41%	2470
	11th to 20th	Day	6% 4 1 5% 5% 6% 7% 5% 8% 11% 41%	2470
	21st to end	Day	6% 6% 2 9% 7% 8% 8% 6% 9% 10% 30%	2470
July	1st to 10th	Day	9% 4 2 6% 5% 6% 6% 5% 8% 12% 37%	4120
	11th to 20th	Day	7% 6% 3 8% 7% 6% 6% 6% 8% 11% 33%	4120
	21st to end	Day	15% 7% 3 8% 6% 5% 6% 5% 7% 10% 27%	4465
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 9% 63%	4104
	11th to 20th	Day	3 2 1 4 4 5% 7% 6% 9% 12% 47%	3914
	21st to end	Day	7% 4 2 7% 5% 6% 6% 5% 7% 10% 40%	4098
September	1st to 10th	Day	7% 6% 2 6% 6% 6% 6% 6% 7% 9% 41%	2090
	11th to 20th	Day	12% 7% 3 7% 5% 6% 6% 3 6% 8% 37%	2090
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 6% 8% 48%	2090
October	1st to 10th	Day	6% 3 2 5% 3 3 4 3 5% 6% 58%	1977
	11th to 20th	Day	9% 5% 3 5% 3 4 4 3 5% 5% 53%	1900
	21st to end	Day	13% 6% 2 4 3 3 3 3 4 5% 54%	2090
November	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 5% 7% 46%	1958
	11th to 20th	Day	10% 6% 3 6% 4 4 4 3 5% 5% 49%	2000
	21st to end	Day	16% 7% 3 6% 4 4 5% 3 4 5% 43%	1910
December	1st to 10th	Day	22% 7% 2 5% 4 3 3 3 4 5% 40%	2940
	11th to 20th	Day	16% 8% 3 6% 4 3 3 3 5% 6% 43%	3391
	21st to end	Day	17% 7% 2 6% 3 3 3 3 4 5% 46%	4488
January	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 4 6% 47%	5888
	11th to 20th	Night	15% 7% 2 4 3 3 3 2 3 5% 53%	5900
	21st to end	Night	30% 8% 2 5% 3 3 2 2 4 5% 36%	6490
February	1st to 10th	Night	20% 9% 2 7% 4 3 3 3 4 5% 40%	2528
	11th to 20th	Night	23% 8% 2 6% 3 3 3 2 3 5% 42%	2333
	21st to end	Night	21% 8% 3 6% 4 3 4 3 3 6% 39%	1815
March	1st to 10th	Night	21% 10% 2 6% 3 3 3 3 5% 6% 38%	4919
	11th to 20th	Night	17% 9% 2 6% 3 3 3 3 4 5% 46%	4980
	21st to end	Night	20% 9% 3 6% 4 4 3 2 3 4 41%	5467
April	1st to 10th	Night	15% 8% 2 6% 5% 4 4 3 6% 6% 42%	2200
	11th to 20th	Night	15% 7% 3 5% 4 4 3 3 6% 7% 44%	2018
	21st to end	Night	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2000
May	1st to 10th	Night	22% 10% 3 7% 5% 4 4 3 5% 7% 30%	3087
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 5% 7% 42%	2913
	21st to end	Night	12% 7% 2 6% 5% 4 4 5% 7% 10% 37%	3140
June	1st to 10th	Night	6% 3 1 4 3 4 5% 4 7% 10% 54%	1710
	11th to 20th	Night	4 3 1 3 3 4 3 5% 7% 12% 55%	1710
	21st to end	Night	4 5% 2 6% 4 4 4 6% 8% 11% 45%	1710
July	1st to 10th	Night	6% 4 2 5% 4 5% 5% 5% 8% 13% 44%	2850
	11th to 20th	Night	5% 4 1 6% 5% 4 5% 5% 7% 12% 47%	2850
	21st to end	Night	12% 7% 2 7% 5% 4 5% 4 6% 10% 38%	3098
August	1st to 10th	Night	2 1 1 2 2 2 3 2 5% 9% 71%	2836
	11th to 20th	Night	2 2 1 3 2 3 4 3 8% 13% 60%	2920
	21st to end	Night	6% 3 2 5% 3 4 6% 5% 8% 12% 47%	3539
September	1st to 10th	Night	5% 6% 2 6% 5% 4 4 4 7% 9% 48%	2090
	11th to 20th	Night	11% 6% 2 7% 5% 4 5% 4 6% 8% 43%	2090
	21st to end	Night	8% 4 2 4 3 3 3 3 6% 9% 55%	2090
October	1st to 10th	Night	5% 3 1 4 3 3 4 3 5% 9% 60%	2182
	11th to 20th	Night	10% 4 1 4 2 2 3 3 4 6% 60%	2280
	21st to end	Night	12% 6% 2 4 3 3 3 2 3 4 59%	2508
November	1st to 10th	Night	12% 6% 2 4 3 3 4 3 4 6% 51%	2340
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 6% 51%	2400
	21st to end	Night	16% 6% 2 6% 4 4 4 3 6% 5% 45%	2485
December	1st to 10th	Night	21% 9% 3 8% 4 4 3 3 4 5% 37%	4250
	11th to 20th	Night	18% 10% 2 6% 3 3 3 2 4 5% 44%	4816
	21st to end	Night	17% 7% 2 6% 3 2 3 2 4 4 50%	6490

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	45% 3 5% 21% 19% 7%	4078
	11th to 20th	Day	38% 3 3 17% 27% 11%	4080
	21st to end	Day	52% 2 3 13% 16% 14%	4488
February	1st to 10th	Day	49% 4 5% 14% 17% 12%	1889
	11th to 20th	Day	50% 4 6% 14% 16% 10%	1963
	21st to end	Day	48% 4 8% 15% 15% 11%	1815
March	1st to 10th	Day	47% 4 8% 12% 17% 11%	4936
	11th to 20th	Day	38% 5% 8% 10% 20% 19%	4980
	21st to end	Day	45% 5% 8% 19% 17% 6%	5464
April	1st to 10th	Day	49% 5% 6% 16% 16% 9%	2200
	11th to 20th	Day	45% 5% 9% 14% 17% 9%	2329
	21st to end	Day	41% 7% 9% 19% 19% 6%	2400
May	1st to 10th	Day	51% 8% 11% 15% 10% 4	4049
	11th to 20th	Day	40% 6% 11% 18% 23% 3	4216
	21st to end	Day	40% 9% 13% 27% 10% 1	4555
June	1st to 10th	Day	36% 9% 16% 22% 16% 1	2470
	11th to 20th	Day	33% 9% 17% 23% 15% 3	2470
	21st to end	Day	44% 11% 15% 19% 10%	2470
July	1st to 10th	Day	37% 8% 28% 23% 4	4056
	11th to 20th	Day	43% 8% 18% 21% 9% 1	4051
	21st to end	Day	50% 8% 17% 19% 4	4325
August	1st to 10th	Day	17% 7% 16% 37% 23%	4090
	11th to 20th	Day	26% 8% 30% 28% 6% 1	3953
	21st to end	Day	38% 11% 17% 20% 13% 2	4097
September	1st to 10th	Day	38% 10% 13% 18% 14% 6%	2090
	11th to 20th	Day	44% 7% 13% 17% 15% 4	2090
	21st to end	Day	33% 8% 12% 18% 17% 13%	2090
October	1st to 10th	Day	17% 4 5% 14% 21% 38%	3289
	11th to 20th	Day	23% 4 5% 16% 34% 19%	3140
	21st to end	Day	27% 4 4 15% 37% 14%	3428
November	1st to 10th	Day	37% 6% 9% 16% 21% 10%	1958
	11th to 20th	Day	36% 5% 9% 14% 23% 14%	2000
	21st to end	Day	43% 5% 7% 16% 18% 11%	1910
December	1st to 10th	Day	46% 3 6% 16% 22% 5%	2940
	11th to 20th	Day	42% 5% 8% 13% 21% 11%	3389
	21st to end	Day	41% 3 5% 14% 26% 9%	4488
January	1st to 10th	Night	39% 4 6% 22% 20% 8%	5888
	11th to 20th	Night	36% 3 4 19% 27% 11%	5900
	21st to end	Night	52% 3 3 13% 17% 12%	6490
February	1st to 10th	Night	48% 4 5% 12% 16% 14%	2528
	11th to 20th	Night	46% 4 7% 15% 17% 11%	2333
	21st to end	Night	47% 4 6% 14% 15% 13%	1815
March	1st to 10th	Night	48% 5% 6% 13% 20% 9%	4919
	11th to 20th	Night	40% 5% 7% 14% 15% 19%	4980
	21st to end	Night	48% 4 7% 14% 19% 8%	5467
April	1st to 10th	Night	42% 6% 10% 16% 16% 10%	2200
	11th to 20th	Night	38% 5% 11% 17% 16% 12%	2018
	21st to end	Night	41% 6% 9% 19% 20% 6%	2000
May	1st to 10th	Night	52% 7% 11% 16% 10% 4	3117
	11th to 20th	Night	40% 6% 11% 18% 20% 4	2914
	21st to end	Night	39% 9% 17% 26% 8% 1	3149
June	1st to 10th	Night	24% 8% 19% 31% 18%	1710
	11th to 20th	Night	21% 8% 20% 29% 18% 4	1710
	21st to end	Night	29% 11% 20% 27% 12% 1	1710
July	1st to 10th	Night	31% 8% 33% 25% 4	2805
	11th to 20th	Night	30% 9% 24% 28% 10%	2796
	21st to end	Night	41% 10% 21% 23% 4	2999
August	1st to 10th	Night	11% 6% 18% 43% 22% 1	2826
	11th to 20th	Night	16% 7% 35% 34% 8% 1	2947
	21st to end	Night	29% 12% 21% 26% 10% 2	3539
September	1st to 10th	Night	31% 9% 22% 20% 12% 6%	2090
	11th to 20th	Night	36% 9% 15% 19% 15% 6%	2090
	21st to end	Night	26% 6% 15% 21% 19% 12%	2090
October	1st to 10th	Night	16% 4 7% 18% 16% 40%	3637
	11th to 20th	Night	16% 3 4 23% 31% 23%	3779
	21st to end	Night	24% 2 5% 16% 36% 16%	4120
November	1st to 10th	Night	34% 5% 11% 20% 20% 10%	2340
	11th to 20th	Night	35% 4 10% 17% 21% 14%	2400
	21st to end	Night	40% 5% 9% 16% 19% 11%	2485
December	1st to 10th	Night	51% 3 5% 16% 19% 6%	4250
	11th to 20th	Night	45% 3 8% 8% 21% 13%	4817
	21st to end	Night	38% 4 5% 14% 29% 9%	6490

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% 16% 36% 18% 3 1	300
	11th to 20th	Day	30% 13% 30% 21% 4 1	300
	21st to end	Day	28% 16% 35% 17% 4	330
February	1st to 10th	Day	30% 17% 30% 17% 5%	300
	11th to 20th	Day	30% 20% 31% 16% 2	300
	21st to end	Day	29% 23% 35% 10% 2	249
March	1st to 10th	Day	26% 21% 33% 16% 4	300
	11th to 20th	Day	20% 23% 39% 14% 4	300
	21st to end	Day	23% 16% 37% 19% 5%	330
April	1st to 10th	Day	19% 21% 36% 19% 4	300
	11th to 20th	Day	24% 19% 26% 21% 9%	447
	21st to end	Day	16% 15% 37% 23% 8% 1	500
May	1st to 10th	Day	15% 15% 36% 23% 10% 1	580
	11th to 20th	Day	14% 8% 32% 30% 14% 2	725
	21st to end	Day	11% 6% 29% 34% 16% 3	760
June	1st to 10th	Day	6% 3 16% 35% 30% 10%	680
	11th to 20th	Day	6% 2 18% 37% 28% 8%	680
	21st to end	Day	8% 2 13% 31% 33% 13%	680
July	1st to 10th	Day	8% 2 10% 28% 35% 17%	671
	11th to 20th	Day	4 1 4 32% 42% 17%	668
	21st to end	Day	6% 1 5% 26% 46% 16%	739
August	1st to 10th	Day	3 1 4 26% 49% 18%	676
	11th to 20th	Day	4 1 2 21% 50% 22%	590
	21st to end	Day	7% 3 8% 21% 33% 28%	496
September	1st to 10th	Day	8% 1 9% 26% 37% 19%	300
	11th to 20th	Day	10% 2 10% 29% 36% 13%	300
	21st to end	Day	9% 2 9% 32% 35% 12%	300
October	1st to 10th	Day	5% 3 16% 41% 30% 5%	300
	11th to 20th	Day	10% 6% 21% 38% 19% 6%	300
	21st to end	Day	21% 11% 24% 23% 18% 4	330
November	1st to 10th	Day	15% 8% 23% 36% 17% 1	300
	11th to 20th	Day	15% 11% 31% 32% 9% 2	300
	21st to end	Day	21% 20% 31% 17% 8% 3	300
December	1st to 10th	Day	20% 12% 35% 22% 8% 4	300
	11th to 20th	Day	23% 17% 34% 21% 5% 1	300
	21st to end	Day	25% 15% 32% 21% 6% 1	330
January	1st to 10th	Night	23% 12% 32% 26% 6%	1000
	11th to 20th	Night	23% 15% 30% 26% 6%	1000
	21st to end	Night	24% 13% 31% 27% 5%	1100
February	1st to 10th	Night	27% 14% 31% 25% 4	928
	11th to 20th	Night	29% 13% 30% 24% 4	779
	21st to end	Night	27% 15% 34% 21% 3	495
March	1st to 10th	Night	25% 13% 33% 25% 4	600
	11th to 20th	Night	21% 11% 36% 27% 5%	600
	21st to end	Night	19% 12% 39% 24% 6%	660
April	1st to 10th	Night	17% 12% 39% 28% 5%	600
	11th to 20th	Night	20% 11% 32% 29% 7%	593
	21st to end	Night	18% 12% 36% 25% 9%	600
May	1st to 10th	Night	18% 11% 34% 31% 6%	600
	11th to 20th	Night	15% 7% 31% 33% 12% 2	591
	21st to end	Night	10% 5% 31% 35% 18% 1	634
June	1st to 10th	Night	7% 2 14% 45% 28% 3	570
	11th to 20th	Night	4 1 20% 48% 24% 3	570
	21st to end	Night	5% 3 12% 43% 32% 5%	570
July	1st to 10th	Night	4 2 10% 42% 35% 6%	555
	11th to 20th	Night	4 1 5% 45% 39% 6%	552
	21st to end	Night	5% 1 7% 45% 38% 5%	624
August	1st to 10th	Night	2 1 4 50% 39% 5%	567
	11th to 20th	Night	5% 4 46% 39% 6%	564
	21st to end	Night	5% 2 6% 34% 47% 6%	627
September	1st to 10th	Night	7% 2 8% 33% 43% 8%	570
	11th to 20th	Night	12% 2 9% 32% 40% 6%	570
	21st to end	Night	9% 2 14% 35% 35% 6%	570
October	1st to 10th	Night	5% 3 13% 41% 33% 5%	662
	11th to 20th	Night	12% 4 23% 41% 20% 2	760
	21st to end	Night	15% 8% 26% 31% 18% 2	836
November	1st to 10th	Night	12% 5% 26% 35% 20% 1	780
	11th to 20th	Night	13% 10% 27% 34% 16%	800
	21st to end	Night	21% 10% 29% 26% 13% 1	885
December	1st to 10th	Night	22% 11% 28% 28% 11% 1	1000
	11th to 20th	Night	26% 11% 29% 28% 6%	1000
	21st to end	Night	23% 11% 29% 29% 7%	1100

Page generated at 2025-05-23 08:21:02. Version ac07

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