Astor
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astor clear sky chart from 2005-11-06 to 2025-09-19.

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	8% 6% 2 7% 5% 5% 6% 5% 7% 10% 39%	126040
	Night	9% 6% 2 6% 4 4 5% 4 6% 9% 43%	123044
Transparency	Day	38% 9% 17% 19% 13% 5%	129750
	Night	36% 9% 16% 20% 14% 6%	127403
Seeing	Day	10% 7% 22% 25% 22% 14%	16730
	Night	10% 5% 21% 32% 25% 6%	26770

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	9% 5% 1 5% 3 3 3 3 4 7% 55%	4056
	11th to 20th	Day	21% 9% 3 8% 5% 4 4 3 4 5% 33%	4060
	21st to end	Day	14% 5% 2 6% 3 3 4 3 4 8% 48%	4534
February	1st to 10th	Day	13% 7% 3 5% 5% 4 5% 5% 6% 7% 41%	2000
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 3 4 6% 52%	1948
	21st to end	Day	10% 6% 2 6% 4 5% 4 4 5% 7% 46%	1757
March	1st to 10th	Day	9% 6% 2 7% 5% 4 5% 4 6% 10% 41%	4935
	11th to 20th	Day	5% 4 2 5% 3 3 3 3 5% 8% 60%	4980
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 9% 46%	5465
April	1st to 10th	Day	7% 3 2 6% 5% 5% 6% 4 6% 10% 48%	2200
	11th to 20th	Day	8% 5% 2 6% 6% 5% 5% 6% 6% 7% 43%	2233
	21st to end	Day	5% 4 1 5% 5% 4 4 4 7% 9% 51%	2400
May	1st to 10th	Day	4 4 2 5% 4 5% 6% 5% 8% 11% 46%	5806
	11th to 20th	Day	10% 5% 2 7% 4 4 5% 4 6% 8% 44%	5861
	21st to end	Day	4 3 2 5% 4 5% 6% 5% 8% 12% 45%	6433
June	1st to 10th	Day	11% 7% 3 10% 8% 7% 7% 6% 7% 10% 26%	2600
	11th to 20th	Day	6% 6% 3 9% 8% 9% 8% 7% 9% 10% 27%	2600
	21st to end	Day	9% 6% 3 10% 8% 8% 8% 6% 7% 9% 27%	2600
July	1st to 10th	Day	7% 7% 2 9% 7% 8% 9% 7% 12% 14% 17%	5900
	11th to 20th	Day	4 5% 2 8% 7% 7% 8% 7% 10% 15% 28%	5900
	21st to end	Day	5% 5% 2 7% 7% 7% 7% 7% 10% 14% 30%	6424
August	1st to 10th	Day	7% 7% 3 10% 7% 7% 7% 6% 9% 12% 26%	4234
	11th to 20th	Day	5% 4 2 7% 6% 7% 7% 6% 9% 13% 34%	4056
	21st to end	Day	8% 6% 3 10% 8% 8% 9% 6% 8% 12% 22%	3949
September	1st to 10th	Day	8% 7% 3 10% 9% 9% 10% 6% 9% 10% 20%	2200
	11th to 20th	Day	10% 8% 3 11% 8% 7% 7% 6% 8% 10% 22%	2178
	21st to end	Day	11% 7% 2 9% 6% 7% 7% 5% 7% 8% 31%	2090
October	1st to 10th	Day	13% 7% 3 9% 7% 5% 6% 4 6% 8% 32%	2055
	11th to 20th	Day	7% 5% 2 6% 6% 4 5% 4 6% 9% 46%	1900
	21st to end	Day	9% 4 2 7% 4 4 5% 4 6% 8% 47%	2090
November	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 8% 44%	1957
	11th to 20th	Day	11% 6% 3 7% 5% 6% 5% 5% 5% 8% 41%	2000
	21st to end	Day	11% 6% 2 6% 5% 5% 4 4 5% 7% 44%	1870
December	1st to 10th	Day	6% 4 1 5% 4 4 4 4 6% 8% 55%	2930
	11th to 20th	Day	11% 8% 3 6% 5% 6% 6% 5% 7% 7% 37%	3373
	21st to end	Day	12% 7% 3 7% 4 4 5% 4 4 7% 42%	4466
January	1st to 10th	Night	10% 5% 2 4 3 3 3 3 4 8% 56%	5889
	11th to 20th	Night	20% 10% 3 6% 4 4 4 4 6% 7% 32%	5900
	21st to end	Night	16% 6% 2 5% 3 3 4 3 6% 8% 42%	6419
February	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 48%	2338
	21st to end	Night	8% 6% 2 4 5% 4 4 4 6% 6% 52%	1870
March	1st to 10th	Night	15% 7% 3 7% 4 3 3 3 5% 6% 44%	4918
	11th to 20th	Night	6% 5% 1 4 3 4 4 3 6% 9% 55%	4980
	21st to end	Night	9% 6% 2 6% 4 3 4 3 5% 7% 50%	5467
April	1st to 10th	Night	8% 4 1 4 3 4 4 4 7% 9% 51%	2200
	11th to 20th	Night	8% 4 2 6% 3 4 4 2 7% 7% 52%	2115
	21st to end	Night	6% 3 1 4 4 4 4 3 7% 9% 55%	2000
May	1st to 10th	Night	5% 4 2 6% 4 4 4 4 6% 9% 52%	4130
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 6% 10% 47%	4032
	21st to end	Night	4 3 2 5% 4 4 4 4 7% 11% 53%	4421
June	1st to 10th	Night	10% 6% 2 7% 5% 5% 6% 4 8% 11% 36%	1800
	11th to 20th	Night	6% 6% 3 8% 6% 7% 7% 6% 9% 12% 31%	1800
	21st to end	Night	6% 6% 3 8% 6% 7% 7% 5% 9% 10% 33%	1800
July	1st to 10th	Night	6% 7% 4 9% 7% 8% 8% 6% 10% 11% 23%	4058
	11th to 20th	Night	3 4 2 8% 6% 6% 7% 7% 11% 13% 32%	4057
	21st to end	Night	5% 7% 3 9% 6% 6% 6% 5% 9% 12% 32%	4427
August	1st to 10th	Night	5% 6% 3 8% 6% 6% 7% 5% 8% 13% 33%	2915
	11th to 20th	Night	4 4 2 6% 5% 6% 6% 5% 8% 15% 39%	2951
	21st to end	Night	8% 6% 3 8% 5% 5% 6% 5% 9% 12% 33%	3949
September	1st to 10th	Night	9% 6% 3 8% 7% 5% 7% 5% 9% 12% 29%	2200
	11th to 20th	Night	10% 7% 3 8% 7% 6% 7% 5% 8% 11% 28%	2179
	21st to end	Night	11% 7% 2 7% 4 5% 6% 5% 7% 9% 36%	2090
October	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 4 7% 8% 38%	2123
	11th to 20th	Night	5% 5% 1 5% 5% 4 4 4 7% 9% 52%	2280
	21st to end	Night	9% 5% 2 5% 3 5% 4 4 5% 7% 52%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 4 5% 3 5% 8% 48%	2341
	11th to 20th	Night	11% 7% 2 6% 5% 6% 4 4 6% 7% 44%	2400
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 6% 8% 45%	2525
December	1st to 10th	Night	6% 4 2 6% 4 4 4 3 4 7% 58%	4250
	11th to 20th	Night	11% 7% 3 7% 5% 4 4 3 4 6% 47%	4822
	21st to end	Night	14% 8% 3 7% 4 4 5% 4 5% 6% 39%	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	30% 5% 10% 14% 28% 14%	4057
	11th to 20th	Day	53% 4 5% 14% 16% 8%	4060
	21st to end	Day	35% 6% 8% 25% 16% 9%	4534
February	1st to 10th	Day	40% 7% 9% 14% 20% 9%	2000
	11th to 20th	Day	33% 5% 10% 19% 22% 11%	1958
	21st to end	Day	37% 5% 11% 17% 21% 10%	1757
March	1st to 10th	Day	37% 8% 12% 17% 20% 7%	4935
	11th to 20th	Day	24% 6% 10% 21% 23% 17%	4980
	21st to end	Day	32% 6% 12% 25% 16% 9%	5465
April	1st to 10th	Day	31% 7% 13% 20% 19% 9%	2200
	11th to 20th	Day	36% 9% 10% 18% 20% 7%	2233
	21st to end	Day	26% 8% 13% 24% 21% 8%	2400
May	1st to 10th	Day	26% 9% 23% 25% 12% 4	5840
	11th to 20th	Day	33% 10% 15% 31% 10% 1	5865
	21st to end	Day	27% 9% 20% 30% 13% 1	6447
June	1st to 10th	Day	49% 13% 17% 15% 6%	2600
	11th to 20th	Day	46% 15% 18% 18% 2	2600
	21st to end	Day	48% 13% 22% 13% 3	2600
July	1st to 10th	Day	49% 14% 28% 8% 1	5835
	11th to 20th	Day	40% 13% 35% 11% 1	5831
	21st to end	Day	39% 12% 29% 19% 1	6240
August	1st to 10th	Day	46% 13% 29% 10% 2	4221
	11th to 20th	Day	36% 14% 25% 21% 5%	4095
	21st to end	Day	50% 12% 22% 14% 1	3949
September	1st to 10th	Day	52% 19% 16% 9% 3 1	2200
	11th to 20th	Day	53% 14% 18% 12% 3	2178
	21st to end	Day	47% 12% 14% 15% 10% 3	2090
October	1st to 10th	Day	56% 8% 13% 11% 7% 5%	3419
	11th to 20th	Day	29% 6% 7% 22% 27% 8%	3140
	21st to end	Day	27% 6% 7% 22% 32% 6%	3428
November	1st to 10th	Day	35% 8% 13% 17% 18% 9%	1957
	11th to 20th	Day	40% 7% 9% 16% 19% 8%	2000
	21st to end	Day	38% 7% 9% 21% 19% 7%	1870
December	1st to 10th	Day	26% 5% 16% 24% 19% 9%	2930
	11th to 20th	Day	42% 7% 10% 15% 17% 8%	3370
	21st to end	Day	40% 6% 11% 21% 17% 5%	4466
January	1st to 10th	Night	28% 5% 8% 17% 29% 13%	5889
	11th to 20th	Night	50% 5% 8% 16% 16% 4	5900
	21st to end	Night	38% 7% 10% 21% 15% 9%	6419
February	1st to 10th	Night	38% 6% 11% 16% 18% 12%	2400
	11th to 20th	Night	33% 7% 13% 18% 18% 10%	2339
	21st to end	Night	31% 6% 14% 19% 21% 9%	1870
March	1st to 10th	Night	39% 7% 11% 16% 19% 8%	4918
	11th to 20th	Night	24% 7% 14% 21% 17% 17%	4980
	21st to end	Night	32% 7% 11% 23% 19% 7%	5467
April	1st to 10th	Night	28% 8% 17% 24% 14% 9%	2200
	11th to 20th	Night	29% 7% 11% 25% 19% 8%	2115
	21st to end	Night	24% 7% 15% 27% 20% 7%	2000
May	1st to 10th	Night	25% 11% 22% 27% 12% 3	4158
	11th to 20th	Night	30% 11% 20% 27% 12% 1	4033
	21st to end	Night	24% 8% 21% 31% 15% 1	4429
June	1st to 10th	Night	39% 13% 22% 21% 5%	1800
	11th to 20th	Night	39% 16% 23% 20% 3	1800
	21st to end	Night	41% 15% 23% 16% 5% 1	1800
July	1st to 10th	Night	49% 13% 27% 10% 1	4015
	11th to 20th	Night	36% 16% 37% 11% 1	4006
	21st to end	Night	41% 11% 27% 20% 1	4297
August	1st to 10th	Night	41% 13% 34% 11% 1	2904
	11th to 20th	Night	31% 13% 30% 21% 5%	2978
	21st to end	Night	40% 15% 27% 16% 2	3949
September	1st to 10th	Night	44% 19% 20% 13% 5%	2200
	11th to 20th	Night	46% 16% 20% 14% 4	2179
	21st to end	Night	40% 12% 18% 19% 9% 2	2090
October	1st to 10th	Night	53% 10% 13% 13% 7% 4	3539
	11th to 20th	Night	27% 7% 8% 21% 31% 5%	3779
	21st to end	Night	29% 6% 8% 21% 29% 7%	4120
November	1st to 10th	Night	34% 7% 13% 19% 18% 9%	2341
	11th to 20th	Night	36% 7% 11% 17% 20% 9%	2400
	21st to end	Night	35% 8% 12% 19% 19% 8%	2525
December	1st to 10th	Night	28% 4 15% 28% 12% 13%	4250
	11th to 20th	Night	38% 6% 12% 21% 15% 9%	4824
	21st to end	Night	43% 7% 13% 19% 13% 5%	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	12% 15% 47% 19% 5% 2	300
	11th to 20th	Day	11% 16% 42% 20% 8% 3	300
	21st to end	Day	18% 20% 45% 12% 5%	330
February	1st to 10th	Day	17% 14% 35% 23% 10% 2	300
	11th to 20th	Day	13% 13% 43% 22% 8% 2	300
	21st to end	Day	9% 15% 34% 26% 15% 1	356
March	1st to 10th	Day	8% 16% 37% 27% 11% 1	500
	11th to 20th	Day	13% 13% 30% 27% 15% 3	500
	21st to end	Day	10% 11% 31% 29% 17% 2	550
April	1st to 10th	Day	9% 13% 32% 30% 13% 2	500
	11th to 20th	Day	9% 12% 33% 28% 15% 4	492
	21st to end	Day	7% 6% 28% 29% 19% 11%	500
May	1st to 10th	Day	6% 8% 23% 33% 21% 10%	503
	11th to 20th	Day	9% 4 23% 29% 24% 12%	527
	21st to end	Day	7% 3 12% 28% 33% 18%	573
June	1st to 10th	Day	11% 4 13% 28% 27% 17%	530
	11th to 20th	Day	7% 3 12% 22% 31% 25%	530
	21st to end	Day	6% 3 7% 21% 33% 29%	530
July	1st to 10th	Day	7% 1 5% 17% 30% 39%	525
	11th to 20th	Day	4 1 3 18% 32% 42%	524
	21st to end	Day	8% 1 5% 21% 30% 35%	576
August	1st to 10th	Day	7% 3 8% 18% 32% 31%	526
	11th to 20th	Day	5% 1 4 17% 33% 40%	519
	21st to end	Day	12% 1 7% 20% 31% 29%	583
September	1st to 10th	Day	9% 2 10% 21% 31% 27%	530
	11th to 20th	Day	10% 2 12% 29% 28% 19%	522
	21st to end	Day	11% 3 11% 26% 29% 20%	490
October	1st to 10th	Day	16% 7% 24% 29% 19% 6%	490
	11th to 20th	Day	7% 7% 20% 32% 27% 7%	490
	21st to end	Day	8% 4 27% 35% 22% 4	539
November	1st to 10th	Day	11% 8% 25% 30% 22% 4	495
	11th to 20th	Day	13% 7% 30% 30% 16% 3	500
	21st to end	Day	13% 11% 33% 30% 11% 3	370
December	1st to 10th	Day	7% 14% 38% 24% 15% 2	300
	11th to 20th	Day	14% 14% 33% 28% 9% 2	300
	21st to end	Day	11% 11% 34% 34% 9% 2	330
January	1st to 10th	Night	13% 13% 33% 30% 11%	800
	11th to 20th	Night	12% 10% 29% 34% 14% 1	800
	21st to end	Night	17% 13% 35% 29% 7%	880
February	1st to 10th	Night	15% 8% 32% 32% 13% 1	800
	11th to 20th	Night	11% 9% 32% 37% 9% 1	783
	21st to end	Night	11% 9% 35% 38% 7%	660
March	1st to 10th	Night	9% 9% 36% 36% 10%	800
	11th to 20th	Night	10% 10% 31% 35% 13% 1	800
	21st to end	Night	9% 9% 33% 38% 11%	880
April	1st to 10th	Night	9% 8% 33% 36% 14%	800
	11th to 20th	Night	9% 9% 33% 32% 16% 1	791
	21st to end	Night	7% 5% 27% 43% 17% 1	800
May	1st to 10th	Night	5% 3 22% 46% 23% 2	646
	11th to 20th	Night	8% 2 16% 41% 29% 3	591
	21st to end	Night	7% 1 10% 37% 39% 6%	650
June	1st to 10th	Night	12% 2 9% 34% 37% 6%	600
	11th to 20th	Night	7% 2 8% 24% 40% 19%	600
	21st to end	Night	7% 2 5% 24% 45% 18%	600
July	1st to 10th	Night	9% 1 5% 19% 43% 24%	585
	11th to 20th	Night	6% 2 2 16% 48% 26%	582
	21st to end	Night	6% 1 5% 19% 48% 21%	657
August	1st to 10th	Night	8% 1 3 20% 48% 21%	597
	11th to 20th	Night	6% 1 4 14% 48% 27%	619
	21st to end	Night	13% 1 8% 22% 37% 19%	880
September	1st to 10th	Night	11% 3 8% 25% 38% 17%	800
	11th to 20th	Night	13% 2 10% 30% 35% 11%	793
	21st to end	Night	13% 2 8% 29% 37% 11%	760
October	1st to 10th	Night	13% 4 17% 36% 26% 4	760
	11th to 20th	Night	6% 4 18% 37% 32% 4	760
	21st to end	Night	11% 5% 17% 38% 26% 3	836
November	1st to 10th	Night	12% 3 23% 36% 23% 2	780
	11th to 20th	Night	12% 7% 23% 37% 19% 2	800
	21st to end	Night	10% 6% 30% 36% 16% 1	800
December	1st to 10th	Night	9% 7% 27% 35% 21% 1	800
	11th to 20th	Night	12% 7% 28% 36% 17% 1	800
	21st to end	Night	14% 8% 27% 35% 16%	880

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