Astronight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astronight Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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	18% 7% 2 6% 4 4 4 4 5% 7% 38%	119183
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	119027
Transparency	Day	44% 6% 11% 21% 15% 3	122761
	Night	44% 6% 12% 21% 16% 3	123610
Seeing	Day	23% 17% 32% 18% 7% 2	15743
	Night	21% 14% 33% 24% 7% 1	24702

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	13% 6% 3 8% 5% 5% 5% 4 6% 6% 40%	3597
	11th to 20th	Day	14% 7% 2 6% 3 3 4 3 5% 6% 47%	3570
	21st to end	Day	17% 9% 3 4 4 4 4 3 4 5% 44%	3960
February	1st to 10th	Day	18% 8% 3 5% 4 4 5% 5% 6% 6% 37%	1853
	11th to 20th	Day	13% 9% 2 7% 4 4 5% 4 6% 7% 39%	1948
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 5% 37%	1650
March	1st to 10th	Day	21% 11% 2 6% 3 4 4 3 5% 5% 35%	4476
	11th to 20th	Day	16% 9% 3 6% 4 4 5% 4 5% 6% 39%	4783
	21st to end	Day	26% 9% 2 6% 4 4 4 4 5% 6% 31%	5776
April	1st to 10th	Day	19% 10% 3 7% 4 4 4 3 4 6% 36%	2400
	11th to 20th	Day	18% 7% 2 5% 3 5% 6% 3 5% 7% 39%	2394
	21st to end	Day	21% 6% 3 7% 5% 4 4 3 5% 6% 36%	2600
May	1st to 10th	Day	31% 10% 3 6% 4 4 4 3 5% 6% 25%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 5% 6% 32%	5988
	21st to end	Day	19% 6% 2 6% 4 4 4 4 6% 7% 36%	6949
June	1st to 10th	Day	19% 5% 2 6% 5% 5% 5% 4 6% 8% 34%	2786
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	2660
	21st to end	Day	18% 6% 3 6% 5% 5% 5% 4 6% 8% 35%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 7% 11% 40%	4440
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 5% 8% 36%	4814
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 33%	4153
	11th to 20th	Day	13% 8% 3 7% 5% 4 5% 4 6% 7% 38%	3998
	21st to end	Day	8% 4 2 6% 4 5% 6% 5% 7% 9% 45%	4180
September	1st to 10th	Day	14% 6% 2 4 5% 3 4 4 6% 6% 47%	2275
	11th to 20th	Day	16% 6% 2 5% 3 3 3 3 4 6% 51%	2090
	21st to end	Day	17% 6% 3 5% 4 4 3 3 5% 7% 44%	2090
October	1st to 10th	Day	22% 5% 3 6% 3 4 4 3 5% 6% 39%	2090
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 47%	1851
	21st to end	Day	21% 6% 3 6% 4 4 4 3 5% 5% 41%	1881
November	1st to 10th	Day	15% 6% 2 5% 4 4 3 3 4 5% 50%	1761
	11th to 20th	Day	18% 6% 2 5% 3 3 3 3 4 5% 48%	1800
	21st to end	Day	19% 7% 2 5% 3 3 3 3 5% 6% 43%	1800
December	1st to 10th	Day	15% 8% 2 5% 4 4 3 3 4 6% 47%	2638
	11th to 20th	Day	20% 5% 3 5% 3 3 3 3 4 5% 47%	2986
	21st to end	Day	20% 10% 2 6% 4 4 4 3 4 5% 38%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 6% 47%	6349
	11th to 20th	Night	13% 8% 3 9% 5% 4 4 3 4 5% 40%	6315
	21st to end	Night	15% 11% 3 7% 4 4 5% 4 5% 5% 38%	6996
February	1st to 10th	Night	16% 9% 2 6% 5% 4 5% 4 5% 6% 38%	2543
	11th to 20th	Night	15% 9% 3 6% 4 4 4 3 5% 7% 40%	2338
	21st to end	Night	17% 9% 2 6% 4 3 4 3 4 6% 41%	1980
March	1st to 10th	Night	24% 8% 2 6% 3 4 4 3 4 5% 37%	5364
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 3 4 5% 38%	5174
	21st to end	Night	26% 8% 2 6% 4 4 4 3 5% 5% 33%	5154
April	1st to 10th	Night	20% 7% 3 6% 5% 4 4 4 6% 6% 36%	2000
	11th to 20th	Night	21% 7% 2 5% 4 4 4 3 5% 5% 40%	1955
	21st to end	Night	20% 8% 2 6% 5% 3 4 3 5% 5% 39%	1800
May	1st to 10th	Night	32% 9% 3 6% 4 3 4 3 4 6% 25%	4054
	11th to 20th	Night	17% 9% 3 7% 5% 5% 5% 5% 7% 8% 29%	3948
	21st to end	Night	21% 8% 3 7% 4 5% 4 3 5% 6% 33%	3926
June	1st to 10th	Night	18% 9% 2 6% 5% 4 4 3 5% 5% 38%	1598
	11th to 20th	Night	19% 7% 3 5% 4 4 3 4 6% 7% 38%	1520
	21st to end	Night	16% 6% 3 5% 4 4 5% 4 6% 9% 38%	1520
July	1st to 10th	Night	12% 6% 3 7% 6% 5% 5% 5% 7% 10% 35%	2520
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 4 7% 9% 38%	2520
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 7% 37%	2739
August	1st to 10th	Night	19% 7% 3 8% 5% 4 5% 4 6% 7% 33%	2730
	11th to 20th	Night	10% 6% 3 8% 5% 6% 5% 5% 7% 8% 37%	2812
	21st to end	Night	9% 4 1 4 4 4 4 3 5% 8% 54%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 3 4 3 5% 6% 49%	1905
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 4 6% 52%	2090
	21st to end	Night	17% 6% 3 5% 4 3 3 4 5% 6% 44%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 4 5% 42%	2090
	11th to 20th	Night	16% 4 2 5% 3 3 4 4 5% 6% 47%	2326
	21st to end	Night	22% 7% 2 6% 4 4 4 3 5% 5% 39%	2717
November	1st to 10th	Night	15% 5% 2 5% 4 3 3 4 4 5% 49%	2537
	11th to 20th	Night	18% 6% 2 5% 3 3 4 3 4 5% 47%	2600
	21st to end	Night	17% 7% 2 5% 4 4 4 3 4 6% 44%	2600
December	1st to 10th	Night	18% 8% 2 5% 4 4 4 3 5% 5% 41%	4537
	11th to 20th	Night	22% 7% 3 6% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	17% 10% 3 6% 5% 4 4 2 4 5% 40%	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	42% 5% 9% 26% 16% 2	3598
	11th to 20th	Day	39% 6% 11% 23% 19% 3	3600
	21st to end	Day	42% 5% 9% 22% 19% 3	3960
February	1st to 10th	Day	46% 7% 12% 19% 13% 3	1853
	11th to 20th	Day	43% 7% 12% 20% 15% 3	1957
	21st to end	Day	48% 5% 10% 20% 14% 3	1650
March	1st to 10th	Day	51% 4 9% 15% 11% 10%	4479
	11th to 20th	Day	43% 7% 14% 24% 11% 2	4783
	21st to end	Day	53% 4 7% 15% 15% 7%	5776
April	1st to 10th	Day	49% 5% 9% 17% 16% 4	2400
	11th to 20th	Day	44% 7% 11% 19% 17% 3	2394
	21st to end	Day	47% 6% 8% 18% 17% 4	2600
May	1st to 10th	Day	59% 7% 12% 13% 8% 1	5897
	11th to 20th	Day	51% 7% 14% 17% 10% 1	5992
	21st to end	Day	45% 7% 12% 21% 14% 1	6949
June	1st to 10th	Day	47% 8% 12% 21% 12%	2786
	11th to 20th	Day	42% 8% 14% 18% 16% 3	2660
	21st to end	Day	45% 9% 12% 21% 12% 1	2660
July	1st to 10th	Day	42% 8% 12% 21% 15% 1	4370
	11th to 20th	Day	38% 7% 16% 25% 13% 1	4365
	21st to end	Day	47% 7% 12% 20% 13% 2	4661
August	1st to 10th	Day	45% 9% 15% 22% 8% 1	4139
	11th to 20th	Day	43% 8% 14% 26% 9% 1	4037
	21st to end	Day	33% 7% 12% 29% 17% 2	4180
September	1st to 10th	Day	36% 6% 12% 24% 20% 2	2275
	11th to 20th	Day	36% 4 9% 26% 22% 2	2090
	21st to end	Day	40% 6% 9% 22% 21% 2	2090
October	1st to 10th	Day	44% 5% 9% 21% 19% 2	3480
	11th to 20th	Day	31% 5% 8% 28% 24% 3	3054
	21st to end	Day	34% 4 8% 31% 20% 3	3081
November	1st to 10th	Day	37% 5% 12% 27% 17% 2	1761
	11th to 20th	Day	39% 4 8% 23% 22% 3	1800
	21st to end	Day	42% 5% 15% 23% 13% 2	1800
December	1st to 10th	Day	40% 4 15% 29% 12% 1	2638
	11th to 20th	Day	41% 3 14% 18% 19% 5%	2986
	21st to end	Day	49% 5% 9% 19% 14% 5%	3960
January	1st to 10th	Night	37% 4 12% 31% 15% 2	6349
	11th to 20th	Night	46% 5% 13% 20% 13% 3	6360
	21st to end	Night	47% 6% 9% 16% 17% 5%	6996
February	1st to 10th	Night	46% 6% 13% 20% 13% 2	2543
	11th to 20th	Night	44% 5% 11% 21% 16% 4	2340
	21st to end	Night	44% 5% 9% 23% 15% 4	1980
March	1st to 10th	Night	49% 5% 9% 16% 14% 8%	5374
	11th to 20th	Night	46% 7% 16% 16% 12% 2	5175
	21st to end	Night	52% 4 6% 12% 20% 6%	5154
April	1st to 10th	Night	47% 6% 11% 17% 15% 3	2000
	11th to 20th	Night	46% 5% 9% 19% 18% 3	1955
	21st to end	Night	46% 5% 12% 16% 17% 4	1800
May	1st to 10th	Night	60% 6% 12% 12% 8% 2	4060
	11th to 20th	Night	51% 8% 16% 15% 9% 2	3949
	21st to end	Night	51% 7% 13% 18% 10% 1	3927
June	1st to 10th	Night	47% 6% 12% 22% 12% 1	1598
	11th to 20th	Night	44% 8% 13% 19% 14% 2	1520
	21st to end	Night	41% 9% 15% 21% 13% 1	1520
July	1st to 10th	Night	42% 9% 12% 21% 15%	2480
	11th to 20th	Night	41% 7% 16% 21% 14% 1	2472
	21st to end	Night	45% 7% 12% 20% 15% 1	2651
August	1st to 10th	Night	50% 8% 12% 19% 10% 1	2720
	11th to 20th	Night	42% 10% 14% 23% 10%	2839
	21st to end	Night	27% 6% 15% 28% 22% 1	3476
September	1st to 10th	Night	36% 6% 12% 25% 19% 1	1905
	11th to 20th	Night	33% 6% 11% 23% 26% 2	2090
	21st to end	Night	39% 7% 10% 24% 19% 2	2090
October	1st to 10th	Night	40% 5% 15% 23% 15% 2	3480
	11th to 20th	Night	27% 5% 11% 27% 27% 3	3862
	21st to end	Night	35% 4 12% 31% 17% 2	4467
November	1st to 10th	Night	37% 6% 11% 22% 20% 3	2537
	11th to 20th	Night	40% 4 10% 22% 20% 3	2600
	21st to end	Night	41% 5% 14% 24% 15% 2	2600
December	1st to 10th	Night	45% 4 11% 27% 12% 1	4537
	11th to 20th	Night	46% 5% 14% 14% 20% 1	5208
	21st to end	Night	46% 5% 8% 22% 15% 3	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	29% 26% 29% 14% 3	200
	11th to 20th	Day	35% 20% 32% 11% 2 1	200
	21st to end	Day	25% 25% 35% 10% 4	220
February	1st to 10th	Day	32% 19% 34% 13% 1	395
	11th to 20th	Day	30% 28% 30% 11% 2	496
	21st to end	Day	27% 20% 36% 13% 3	414
March	1st to 10th	Day	29% 21% 35% 11% 3	500
	11th to 20th	Day	28% 25% 31% 11% 3 1	500
	21st to end	Day	32% 23% 29% 10% 5% 1	550
April	1st to 10th	Day	31% 26% 25% 12% 5% 1	500
	11th to 20th	Day	31% 25% 28% 10% 5%	492
	21st to end	Day	27% 19% 34% 15% 4 1	500
May	1st to 10th	Day	32% 21% 29% 11% 4 2	503
	11th to 20th	Day	23% 17% 38% 16% 4 1	527
	21st to end	Day	20% 14% 38% 21% 6% 1	573
June	1st to 10th	Day	21% 15% 37% 18% 8% 2	526
	11th to 20th	Day	15% 18% 36% 22% 7% 1	490
	21st to end	Day	19% 16% 36% 19% 8% 2	490
July	1st to 10th	Day	17% 10% 31% 29% 11% 3	485
	11th to 20th	Day	15% 7% 34% 29% 13% 2	484
	21st to end	Day	13% 9% 37% 30% 9% 1	532
August	1st to 10th	Day	12% 10% 34% 30% 13% 2	486
	11th to 20th	Day	17% 10% 27% 28% 14% 4	479
	21st to end	Day	12% 9% 34% 26% 14% 4	539
September	1st to 10th	Day	18% 11% 29% 22% 15% 5%	490
	11th to 20th	Day	20% 14% 26% 19% 12% 9%	490
	21st to end	Day	19% 13% 32% 22% 9% 5%	490
October	1st to 10th	Day	27% 13% 24% 22% 10% 4	490
	11th to 20th	Day	20% 18% 28% 23% 10% 2	433
	21st to end	Day	28% 14% 32% 18% 7% 1	429
November	1st to 10th	Day	22% 15% 33% 19% 9% 2	395
	11th to 20th	Day	31% 20% 28% 16% 5% 1	400
	21st to end	Day	30% 18% 30% 16% 6% 1	400
December	1st to 10th	Day	24% 25% 33% 15% 4	225
	11th to 20th	Day	33% 26% 31% 9% 2	200
	21st to end	Day	22% 27% 30% 15% 4 2	220
January	1st to 10th	Night	22% 21% 38% 17% 2	800
	11th to 20th	Night	27% 19% 34% 18% 3	800
	21st to end	Night	20% 20% 37% 20% 3	880
February	1st to 10th	Night	23% 17% 38% 21% 2	800
	11th to 20th	Night	22% 20% 39% 17% 2	783
	21st to end	Night	25% 18% 34% 19% 3	660
March	1st to 10th	Night	26% 18% 34% 19% 3	800
	11th to 20th	Night	22% 20% 37% 17% 4	800
	21st to end	Night	27% 18% 32% 19% 4	880
April	1st to 10th	Night	25% 19% 33% 20% 4	800
	11th to 20th	Night	26% 20% 33% 17% 5%	771
	21st to end	Night	23% 16% 37% 21% 2 1	600
May	1st to 10th	Night	28% 14% 34% 20% 5%	600
	11th to 20th	Night	21% 14% 37% 22% 5%	559
	21st to end	Night	19% 10% 34% 30% 6%	433
June	1st to 10th	Night	21% 8% 35% 30% 7% 1	400
	11th to 20th	Night	18% 9% 34% 30% 7% 1	380
	21st to end	Night	19% 5% 37% 32% 7%	380
July	1st to 10th	Night	15% 5% 29% 41% 9% 1	370
	11th to 20th	Night	13% 4 27% 42% 13% 1	368
	21st to end	Night	13% 5% 29% 43% 10%	416
August	1st to 10th	Night	12% 5% 29% 41% 12% 1	511
	11th to 20th	Night	13% 6% 30% 38% 13% 1	579
	21st to end	Night	14% 5% 25% 39% 16% 2	836
September	1st to 10th	Night	16% 4 34% 32% 14% 1	760
	11th to 20th	Night	16% 8% 26% 28% 19% 3	760
	21st to end	Night	20% 7% 27% 30% 13% 2	760
October	1st to 10th	Night	24% 10% 26% 26% 12% 2	760
	11th to 20th	Night	19% 12% 31% 25% 11% 1	760
	21st to end	Night	25% 13% 32% 23% 5%	836
November	1st to 10th	Night	18% 13% 34% 26% 8% 1	780
	11th to 20th	Night	22% 18% 32% 21% 6% 1	800
	21st to end	Night	24% 19% 33% 20% 4 1	800
December	1st to 10th	Night	25% 17% 36% 17% 4	800
	11th to 20th	Night	25% 17% 32% 23% 3 1	800
	21st to end	Night	24% 18% 35% 20% 3	880

Page generated at 2025-06-07 10:28:12. Version ac07

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