AstroDNA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AstroDNA Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	16% 7% 3 7% 5% 4 5% 4 6% 8% 37%	126236
	Night	16% 8% 3 7% 4 4 4 4 5% 7% 40%	122506
Transparency	Day	45% 4 11% 23% 15% 2	129750
	Night	44% 3 11% 24% 15% 2	127079
Seeing	Day	23% 16% 29% 20% 9% 3	15982
	Night	21% 15% 30% 25% 9% 1	25083

By Month

Percentage Cloud Cover NE (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	12% 8% 3 9% 5% 5% 5% 4 5% 6% 39%	3597
	11th to 20th	Day	14% 10% 4 8% 6% 5% 5% 3 5% 5% 36%	3570
	21st to end	Day	18% 10% 4 6% 4 5% 4 3 4 5% 38%	4307
February	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 6% 36%	1800
	11th to 20th	Day	14% 9% 3 7% 4 4 4 4 4 7% 39%	1923
	21st to end	Day	17% 10% 2 6% 4 4 4 4 4 6% 38%	1650
March	1st to 10th	Day	21% 8% 3 7% 4 4 4 2 4 5% 39%	4476
	11th to 20th	Day	11% 7% 3 7% 5% 4 4 3 5% 7% 42%	4830
	21st to end	Day	17% 6% 2 7% 4 4 4 4 5% 6% 40%	5896
April	1st to 10th	Day	19% 9% 2 6% 4 4 4 3 5% 6% 37%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 3 4 7% 40%	2374
	21st to end	Day	18% 7% 2 7% 5% 4 4 3 5% 6% 38%	2419
May	1st to 10th	Day	26% 9% 3 6% 5% 3 4 4 5% 8% 28%	5964
	11th to 20th	Day	19% 9% 2 7% 5% 5% 5% 4 5% 7% 33%	6318
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 7% 9% 32%	6949
June	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 4 6% 9% 36%	2800
	11th to 20th	Day	16% 6% 2 7% 4 5% 5% 5% 7% 8% 35%	2800
	21st to end	Day	12% 7% 2 7% 5% 5% 4 4 7% 9% 38%	2800
July	1st to 10th	Day	9% 7% 3 8% 5% 6% 6% 5% 7% 10% 34%	6360
	11th to 20th	Day	8% 6% 3 8% 6% 6% 6% 6% 8% 12% 32%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 4 4 6% 9% 41%	6926
August	1st to 10th	Day	19% 6% 3 8% 5% 4 6% 5% 7% 7% 32%	4352
	11th to 20th	Day	10% 7% 3 6% 4 4 5% 4 6% 9% 41%	3850
	21st to end	Day	12% 6% 2 5% 3 3 4 3 5% 10% 46%	4312
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	2316
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 7% 48%	2214
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 40%	2090
October	1st to 10th	Day	19% 6% 3 6% 4 4 4 3 4 7% 39%	1957
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 3 5% 7% 45%	1900
	21st to end	Day	20% 6% 2 6% 4 4 4 4 5% 6% 38%	1947
November	1st to 10th	Day	13% 7% 2 5% 4 3 3 2 4 6% 52%	1761
	11th to 20th	Day	16% 6% 2 4 5% 4 4 3 5% 7% 44%	1800
	21st to end	Day	17% 8% 2 6% 5% 4 4 4 6% 6% 39%	1672
December	1st to 10th	Day	14% 9% 3 7% 5% 4 5% 3 6% 7% 38%	2600
	11th to 20th	Day	19% 9% 2 5% 3 4 3 3 4 6% 42%	2986
	21st to end	Day	15% 9% 3 9% 5% 5% 5% 4 6% 7% 32%	3960
January	1st to 10th	Night	13% 7% 3 9% 5% 5% 4 4 5% 6% 39%	6349
	11th to 20th	Night	12% 8% 3 8% 5% 5% 4 4 5% 7% 38%	6315
	21st to end	Night	16% 9% 3 6% 5% 4 4 4 5% 6% 38%	6646
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 5% 7% 39%	2600
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 5% 5% 6% 38%	2361
	21st to end	Night	17% 8% 3 7% 5% 4 3 4 5% 5% 40%	1980
March	1st to 10th	Night	17% 10% 4 7% 4 3 3 3 4 6% 39%	5364
	11th to 20th	Night	12% 9% 3 6% 4 4 4 4 5% 6% 43%	5127
	21st to end	Night	16% 9% 3 8% 4 4 4 4 5% 6% 37%	5036
April	1st to 10th	Night	18% 7% 3 7% 5% 4 5% 4 6% 7% 34%	2000
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 5% 6% 38%	1975
	21st to end	Night	18% 7% 2 6% 4 3 3 4 5% 6% 41%	1980
May	1st to 10th	Night	29% 8% 3 7% 3 3 3 3 5% 8% 28%	4018
	11th to 20th	Night	21% 9% 3 7% 4 4 3 4 5% 6% 34%	3575
	21st to end	Night	19% 8% 2 7% 4 4 4 4 6% 7% 35%	3926
June	1st to 10th	Night	15% 6% 2 6% 5% 5% 4 3 5% 9% 41%	1600
	11th to 20th	Night	13% 6% 2 6% 4 5% 4 4 7% 9% 41%	1600
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 6% 10% 45%	1600
July	1st to 10th	Night	10% 7% 2 8% 5% 5% 5% 5% 6% 9% 39%	3600
	11th to 20th	Night	10% 6% 3 8% 5% 5% 6% 5% 7% 10% 35%	3600
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 6% 8% 45%	3926
August	1st to 10th	Night	18% 7% 2 7% 5% 5% 5% 4 6% 8% 33%	2772
	11th to 20th	Night	13% 7% 3 8% 5% 4 5% 3 5% 6% 41%	3188
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 7% 51%	3586
September	1st to 10th	Night	14% 6% 2 6% 3 4 3 3 5% 7% 47%	1931
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 6% 53%	1966
	21st to end	Night	16% 6% 3 6% 4 4 4 3 4 7% 45%	2090
October	1st to 10th	Night	20% 6% 2 6% 3 4 3 3 5% 6% 42%	2219
	11th to 20th	Night	14% 6% 1 6% 4 3 4 3 4 7% 48%	2280
	21st to end	Night	21% 6% 2 4 3 3 3 4 5% 7% 42%	2650
November	1st to 10th	Night	11% 6% 3 7% 4 3 4 3 6% 6% 48%	2537
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 4 6% 44%	2600
	21st to end	Night	14% 7% 2 5% 4 4 5% 3 5% 7% 42%	2725
December	1st to 10th	Night	14% 9% 3 7% 3 4 3 3 5% 6% 42%	4580
	11th to 20th	Night	22% 9% 3 5% 4 4 3 3 4 5% 38%	5208
	21st to end	Night	18% 10% 3 8% 5% 5% 4 3 5% 6% 33%	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	45% 4 7% 29% 13% 2	3598
	11th to 20th	Day	49% 3 8% 23% 13% 4	3600
	21st to end	Day	49% 3 8% 25% 14% 1	4307
February	1st to 10th	Day	50% 3 11% 21% 14% 1	1800
	11th to 20th	Day	47% 3 8% 23% 17% 3	1932
	21st to end	Day	48% 2 10% 24% 14% 2	1650
March	1st to 10th	Day	49% 2 9% 20% 12% 8%	4479
	11th to 20th	Day	42% 2 11% 31% 9% 4	4830
	21st to end	Day	46% 3 9% 13% 24% 5%	5896
April	1st to 10th	Day	49% 2 8% 22% 15% 4	2400
	11th to 20th	Day	46% 3 10% 21% 17% 3	2374
	21st to end	Day	47% 3 8% 23% 15% 4	2419
May	1st to 10th	Day	56% 3 13% 15% 11% 1	5975
	11th to 20th	Day	52% 3 10% 21% 13% 1	6322
	21st to end	Day	48% 3 11% 23% 13% 1	6949
June	1st to 10th	Day	45% 4 12% 23% 15% 1	2800
	11th to 20th	Day	44% 5% 12% 26% 11% 2	2800
	21st to end	Day	42% 4 12% 25% 15% 1	2800
July	1st to 10th	Day	45% 7% 11% 22% 14% 1	6290
	11th to 20th	Day	42% 6% 17% 24% 11%	6285
	21st to end	Day	41% 4 11% 23% 19% 1	6728
August	1st to 10th	Day	50% 4 13% 22% 10% 1	4338
	11th to 20th	Day	40% 4 17% 29% 11% 1	3886
	21st to end	Day	35% 4 10% 33% 16% 2	4312
September	1st to 10th	Day	40% 4 11% 26% 18% 2	2316
	11th to 20th	Day	36% 3 8% 26% 25% 3	2214
	21st to end	Day	45% 4 9% 22% 18% 3	2090
October	1st to 10th	Day	41% 3 7% 21% 24% 4	3256
	11th to 20th	Day	31% 4 9% 30% 24% 3	3140
	21st to end	Day	30% 2 16% 27% 21% 4	3185
November	1st to 10th	Day	36% 3 12% 26% 19% 3	1761
	11th to 20th	Day	41% 2 9% 26% 20% 3	1800
	21st to end	Day	44% 3 13% 28% 10% 1	1672
December	1st to 10th	Day	46% 4 14% 25% 10% 1	2600
	11th to 20th	Day	46% 2 15% 22% 13% 2	2986
	21st to end	Day	50% 3 9% 22% 14% 1	3960
January	1st to 10th	Night	45% 3 9% 26% 15% 3	6349
	11th to 20th	Night	44% 4 13% 22% 13% 4	6360
	21st to end	Night	47% 2 10% 20% 17% 4	6646
February	1st to 10th	Night	45% 3 14% 24% 12% 2	2600
	11th to 20th	Night	46% 4 9% 20% 17% 4	2363
	21st to end	Night	47% 2 7% 25% 16% 4	1980
March	1st to 10th	Night	47% 2 10% 21% 15% 5%	5374
	11th to 20th	Night	43% 2 16% 25% 11% 3	5128
	21st to end	Night	48% 3 6% 16% 22% 4	5036
April	1st to 10th	Night	50% 3 9% 21% 14% 3	2000
	11th to 20th	Night	47% 3 9% 20% 19% 1	1975
	21st to end	Night	45% 3 8% 23% 17% 4	1980
May	1st to 10th	Night	57% 3 12% 17% 11% 1	4024
	11th to 20th	Night	52% 3 13% 17% 15% 1	3576
	21st to end	Night	48% 4 13% 24% 9% 1	3927
June	1st to 10th	Night	42% 4 10% 25% 19% 1	1600
	11th to 20th	Night	40% 5% 16% 29% 10% 1	1600
	21st to end	Night	35% 5% 16% 27% 16% 2	1600
July	1st to 10th	Night	40% 7% 15% 21% 15% 2	3560
	11th to 20th	Night	43% 6% 15% 23% 11% 1	3552
	21st to end	Night	35% 5% 12% 26% 19% 1	3809
August	1st to 10th	Night	49% 6% 16% 19% 9% 1	2762
	11th to 20th	Night	44% 4 14% 29% 9%	3218
	21st to end	Night	32% 4 12% 35% 16% 1	3586
September	1st to 10th	Night	39% 4 10% 28% 18% 1	1931
	11th to 20th	Night	33% 2 11% 26% 26% 2	1966
	21st to end	Night	41% 3 8% 24% 22% 1	2090
October	1st to 10th	Night	42% 4 10% 24% 19% 1	3700
	11th to 20th	Night	28% 2 10% 30% 27% 3	3779
	21st to end	Night	32% 3 15% 31% 19% 1	4362
November	1st to 10th	Night	37% 3 12% 23% 22% 2	2537
	11th to 20th	Night	43% 2 9% 24% 19% 4	2600
	21st to end	Night	42% 3 13% 30% 11% 1	2725
December	1st to 10th	Night	43% 3 12% 32% 9% 1	4580
	11th to 20th	Night	50% 2 11% 20% 15% 2	5208
	21st to end	Night	53% 2 10% 19% 14% 2	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	34% 23% 28% 14% 3	200
	11th to 20th	Day	37% 22% 29% 12% 1	200
	21st to end	Day	26% 29% 30% 11% 2 2	297
February	1st to 10th	Day	33% 25% 32% 8% 1	300
	11th to 20th	Day	33% 28% 30% 8% 1	471
	21st to end	Day	31% 21% 35% 11% 3	414
March	1st to 10th	Day	30% 23% 29% 14% 4	500
	11th to 20th	Day	26% 27% 30% 13% 4 1	500
	21st to end	Day	32% 21% 30% 11% 4 2	550
April	1st to 10th	Day	33% 20% 29% 11% 6% 1	500
	11th to 20th	Day	28% 23% 30% 11% 6% 1	492
	21st to end	Day	29% 18% 31% 13% 8% 1	500
May	1st to 10th	Day	35% 17% 25% 15% 7% 2	503
	11th to 20th	Day	22% 17% 34% 17% 9% 2	527
	21st to end	Day	21% 12% 26% 28% 11% 2	573
June	1st to 10th	Day	19% 13% 31% 24% 10% 2	530
	11th to 20th	Day	19% 12% 31% 26% 12% 1	530
	21st to end	Day	17% 12% 30% 29% 11% 2	530
July	1st to 10th	Day	15% 8% 29% 29% 15% 4	525
	11th to 20th	Day	12% 5% 30% 31% 18% 4	524
	21st to end	Day	12% 8% 33% 32% 12% 2	576
August	1st to 10th	Day	13% 7% 29% 30% 15% 7%	526
	11th to 20th	Day	15% 5% 26% 32% 15% 7%	519
	21st to end	Day	14% 9% 27% 27% 16% 6%	583
September	1st to 10th	Day	18% 12% 24% 23% 17% 7%	502
	11th to 20th	Day	15% 7% 29% 23% 14% 12%	490
	21st to end	Day	23% 10% 27% 24% 11% 5%	490
October	1st to 10th	Day	27% 10% 26% 18% 14% 6%	490
	11th to 20th	Day	18% 17% 31% 21% 12% 2	490
	21st to end	Day	25% 14% 32% 17% 10% 1	463
November	1st to 10th	Day	18% 17% 31% 22% 10% 2	395
	11th to 20th	Day	24% 24% 26% 20% 5% 1	400
	21st to end	Day	28% 22% 28% 16% 4 1	272
December	1st to 10th	Day	29% 28% 27% 11% 6%	200
	11th to 20th	Day	37% 23% 27% 11% 3	200
	21st to end	Day	25% 22% 34% 12% 6% 1	220
January	1st to 10th	Night	22% 26% 34% 16% 3	800
	11th to 20th	Night	25% 21% 31% 20% 3	800
	21st to end	Night	20% 23% 36% 19% 2	880
February	1st to 10th	Night	24% 19% 36% 20% 1	800
	11th to 20th	Night	26% 27% 33% 13% 1	783
	21st to end	Night	28% 21% 33% 15% 3	660
March	1st to 10th	Night	26% 22% 31% 18% 4	800
	11th to 20th	Night	24% 23% 31% 18% 3 1	800
	21st to end	Night	24% 22% 33% 17% 4 1	880
April	1st to 10th	Night	25% 21% 33% 18% 4	800
	11th to 20th	Night	24% 18% 33% 20% 5%	791
	21st to end	Night	23% 17% 30% 24% 5%	780
May	1st to 10th	Night	27% 15% 30% 23% 4	590
	11th to 20th	Night	23% 11% 34% 24% 7% 1	394
	21st to end	Night	17% 7% 26% 38% 10% 2	433
June	1st to 10th	Night	18% 5% 29% 37% 11% 1	400
	11th to 20th	Night	15% 7% 31% 34% 14% 1	400
	21st to end	Night	12% 5% 31% 40% 12%	400
July	1st to 10th	Night	14% 3 20% 43% 19% 2	390
	11th to 20th	Night	13% 2 26% 42% 16% 1	388
	21st to end	Night	11% 4 26% 42% 16% 1	438
August	1st to 10th	Night	12% 1 22% 44% 19% 1	506
	11th to 20th	Night	14% 3 20% 37% 24% 1	781
	21st to end	Night	16% 3 21% 37% 21% 2	880
September	1st to 10th	Night	16% 5% 24% 35% 18% 2	773
	11th to 20th	Night	16% 5% 24% 29% 21% 4	760
	21st to end	Night	19% 6% 25% 31% 17% 2	760
October	1st to 10th	Night	22% 10% 26% 26% 15% 2	760
	11th to 20th	Night	18% 12% 33% 28% 9%	760
	21st to end	Night	25% 12% 33% 22% 7%	836
November	1st to 10th	Night	17% 14% 34% 25% 9% 1	780
	11th to 20th	Night	22% 19% 30% 21% 7% 1	800
	21st to end	Night	21% 20% 34% 19% 6% 1	800
December	1st to 10th	Night	27% 19% 30% 19% 6%	800
	11th to 20th	Night	25% 21% 31% 20% 4	800
	21st to end	Night	26% 19% 34% 17% 3	880

Page generated at 2025-09-01 01:51:03. Version ac07

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