Astronomers by Necessity
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astronomers by Necessity clear sky chart from 2005-11-06 to 2025-05-11.

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% 5% 2 4 3 3 3 3 5% 6% 57%	114322
	Night	8% 5% 2 4 3 3 3 3 4 6% 60%	114877
Transparency	Day	27% 4 9% 23% 26% 12%	118143
	Night	25% 3 8% 22% 27% 15%	119184
Seeing	Day	10% 8% 25% 30% 20% 6%	15706
	Night	10% 8% 33% 35% 13% 1	25744

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	10% 7% 4 6% 4 3 3 2 3 5% 55%	4077
	11th to 20th	Day	5% 4 1 3 2 2 2 3 4 6% 68%	4080
	21st to end	Day	18% 8% 2 4 2 2 2 2 3 4 53%	4696
February	1st to 10th	Day	12% 4 2 4 3 3 3 4 5% 5% 55%	2000
	11th to 20th	Day	11% 9% 3 5% 3 3 4 3 4 5% 52%	1949
	21st to end	Day	14% 6% 2 4 3 3 2 2 3 5% 57%	1650
March	1st to 10th	Day	11% 5% 2 4 3 3 3 2 5% 6% 56%	4736
	11th to 20th	Day	11% 4 2 4 2 3 3 3 5% 5% 59%	4980
	21st to end	Day	5% 4 1 5% 4 4 4 4 6% 7% 55%	5462
April	1st to 10th	Day	9% 5% 2 5% 3 4 4 4 4 7% 54%	2200
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 5% 55%	2320
	21st to end	Day	9% 5% 1 5% 4 3 4 3 5% 6% 56%	2565
May	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 5% 7% 48%	4212
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 4 5% 51%	4083
	21st to end	Day	9% 6% 2 6% 5% 5% 4 4 6% 8% 45%	4477
June	1st to 10th	Day	6% 3 1 4 3 3 4 4 5% 8% 57%	2470
	11th to 20th	Day	4 3 1 3 3 4 4 4 6% 9% 59%	2470
	21st to end	Day	5% 4 1 3 4 4 4 4 5% 7% 59%	2470
July	1st to 10th	Day	5% 4 2 5% 4 4 4 4 6% 8% 55%	4120
	11th to 20th	Day	3 2 1 3 3 3 4 3 5% 8% 65%	4120
	21st to end	Day	4 2 1 4 3 4 4 3 6% 9% 59%	4465
August	1st to 10th	Day	1 2 1 3 3 3 3 3 5% 7% 70%	4104
	11th to 20th	Day	3 2 1 2 3 3 3 3 6% 9% 63%	3773
	21st to end	Day	3 2 1 4 3 4 4 3 5% 8% 63%	4130
September	1st to 10th	Day	10% 4 1 5% 4 5% 4 3 5% 5% 54%	2090
	11th to 20th	Day	10% 5% 2 6% 4 3 4 3 5% 6% 51%	2090
	21st to end	Day	8% 3 2 4 3 3 4 3 4 6% 60%	2090
October	1st to 10th	Day	4 3 2 4 3 2 3 3 4 7% 64%	2090
	11th to 20th	Day	8% 4 1 4 2 2 2 3 4 4 67%	1971
	21st to end	Day	11% 5% 2 3 2 2 3 1 3 4 65%	2090
November	1st to 10th	Day	8% 4 2 4 3 3 3 3 4 6% 60%	1873
	11th to 20th	Day	4 3 1 3 3 3 3 4 5% 6% 64%	1800
	21st to end	Day	12% 5% 2 4 3 3 3 3 4 6% 56%	1800
December	1st to 10th	Day	13% 6% 2 5% 3 3 4 3 5% 6% 51%	2940
	11th to 20th	Day	9% 5% 1 5% 3 4 5% 3 4 5% 55%	3391
	21st to end	Day	10% 7% 2 5% 3 3 3 2 3 4 60%	4488
January	1st to 10th	Night	11% 5% 2 4 3 3 2 3 3 5% 58%	5888
	11th to 20th	Night	6% 3 1 3 2 2 2 2 3 5% 70%	5900
	21st to end	Night	14% 9% 3 5% 3 3 3 3 4 5% 50%	6324
February	1st to 10th	Night	10% 6% 2 5% 3 3 3 2 4 5% 57%	2400
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 3 6% 53%	2338
	21st to end	Night	11% 5% 2 4 3 3 3 3 4 5% 58%	1980
March	1st to 10th	Night	8% 8% 2 4 3 3 3 2 4 6% 56%	5126
	11th to 20th	Night	9% 4 1 4 2 2 3 3 5% 7% 59%	4980
	21st to end	Night	5% 5% 2 6% 4 3 3 3 4 6% 57%	5467
April	1st to 10th	Night	7% 4 2 4 4 3 3 4 6% 7% 57%	2200
	11th to 20th	Night	9% 5% 2 5% 4 4 3 3 5% 5% 55%	2026
	21st to end	Night	7% 4 1 3 3 3 4 3 4 7% 61%	1833
May	1st to 10th	Night	12% 7% 2 6% 3 3 3 3 5% 5% 49%	2913
	11th to 20th	Night	7% 6% 3 5% 4 3 4 3 4 6% 53%	2828
	21st to end	Night	7% 5% 2 4 3 3 4 3 6% 9% 54%	3090
June	1st to 10th	Night	3 2 1 3 3 3 4 3 8% 11% 59%	1710
	11th to 20th	Night	2 2 1 3 3 3 3 4 7% 9% 63%	1710
	21st to end	Night	3 3 1 4 2 3 3 3 4 7% 66%	1710
July	1st to 10th	Night	4 3 1 4 3 3 4 4 5% 9% 61%	2850
	11th to 20th	Night	2 1 2 2 2 3 2 5% 11% 71%	2850
	21st to end	Night	2 3 1 4 2 3 4 4 5% 9% 64%	3098
August	1st to 10th	Night	1 1 2 1 1 2 3 4 8% 76%	2836
	11th to 20th	Night	3 2 1 3 2 2 2 2 5% 9% 68%	3049
	21st to end	Night	3 3 1 3 2 3 3 3 6% 10% 63%	3535
September	1st to 10th	Night	8% 4 1 4 4 4 4 3 5% 7% 58%	2090
	11th to 20th	Night	9% 6% 2 6% 4 4 3 3 5% 7% 51%	2090
	21st to end	Night	7% 2 1 3 3 2 3 3 5% 7% 64%	2090
October	1st to 10th	Night	4 2 1 3 2 3 2 2 5% 8% 67%	2090
	11th to 20th	Night	8% 3 1 4 3 2 2 1 3 4 68%	2190
	21st to end	Night	10% 5% 1 3 2 3 2 2 4 4 64%	2508
November	1st to 10th	Night	8% 4 1 4 2 3 2 3 3 5% 64%	2422
	11th to 20th	Night	3 3 2 5% 3 3 3 3 4 5% 66%	2600
	21st to end	Night	13% 5% 2 4 3 3 3 3 4 7% 55%	2600
December	1st to 10th	Night	12% 6% 2 4 2 3 3 2 4 5% 56%	4250
	11th to 20th	Night	8% 5% 2 5% 4 3 4 3 4 6% 55%	4816
	21st to end	Night	9% 6% 2 5% 3 3 3 2 3 5% 60%	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	33% 2 6% 21% 27% 10%	4078
	11th to 20th	Day	20% 2 5% 22% 39% 12%	4080
	21st to end	Day	37% 3 4 15% 24% 18%	4696
February	1st to 10th	Day	32% 3 5% 17% 25% 17%	2000
	11th to 20th	Day	36% 2 7% 19% 22% 13%	1959
	21st to end	Day	33% 2 4 13% 26% 21%	1650
March	1st to 10th	Day	30% 3 5% 13% 24% 26%	4736
	11th to 20th	Day	28% 2 10% 12% 21% 26%	4980
	21st to end	Day	27% 4 10% 22% 25% 12%	5464
April	1st to 10th	Day	31% 4 9% 20% 21% 15%	2200
	11th to 20th	Day	32% 3 8% 19% 23% 16%	2320
	21st to end	Day	31% 4 8% 19% 30% 8%	2565
May	1st to 10th	Day	37% 4 9% 21% 17% 12%	4250
	11th to 20th	Day	36% 3 9% 20% 28% 3	4087
	21st to end	Day	37% 5% 13% 22% 13% 10%	4489
June	1st to 10th	Day	25% 4 14% 31% 21% 5%	2470
	11th to 20th	Day	21% 5% 13% 32% 23% 7%	2470
	21st to end	Day	25% 4 11% 26% 25% 8%	2470
July	1st to 10th	Day	27% 6% 12% 38% 15% 2	4056
	11th to 20th	Day	17% 4 12% 35% 27% 4	4051
	21st to end	Day	19% 5% 13% 38% 23% 2	4325
August	1st to 10th	Day	14% 4 10% 29% 37% 7%	4090
	11th to 20th	Day	19% 4 12% 34% 27% 4	3810
	21st to end	Day	19% 6% 17% 32% 24% 3	4129
September	1st to 10th	Day	31% 4 9% 20% 24% 11%	2090
	11th to 20th	Day	34% 4 9% 21% 24% 8%	2090
	21st to end	Day	25% 4 8% 20% 29% 14%	2090
October	1st to 10th	Day	13% 2 6% 15% 36% 27%	3480
	11th to 20th	Day	17% 3 7% 22% 31% 20%	3250
	21st to end	Day	19% 2 3 26% 36% 14%	3428
November	1st to 10th	Day	26% 4 6% 24% 25% 15%	1873
	11th to 20th	Day	20% 3 8% 23% 30% 16%	1800
	21st to end	Day	32% 3 4 22% 26% 14%	1800
December	1st to 10th	Day	35% 3 5% 21% 27% 9%	2940
	11th to 20th	Day	32% 4 7% 14% 27% 16%	3389
	21st to end	Day	30% 2 3 24% 25% 15%	4488
January	1st to 10th	Night	30% 3 5% 23% 30% 9%	5888
	11th to 20th	Night	19% 1 4 19% 37% 20%	5900
	21st to end	Night	38% 3 5% 14% 23% 17%	6324
February	1st to 10th	Night	30% 3 6% 17% 25% 18%	2400
	11th to 20th	Night	35% 3 6% 15% 25% 16%	2338
	21st to end	Night	30% 3 6% 15% 22% 24%	1980
March	1st to 10th	Night	30% 3 4 14% 24% 25%	5126
	11th to 20th	Night	26% 2 8% 18% 15% 29%	4980
	21st to end	Night	28% 4 7% 20% 30% 12%	5467
April	1st to 10th	Night	27% 3 10% 21% 20% 19%	2200
	11th to 20th	Night	31% 4 10% 17% 23% 17%	2026
	21st to end	Night	25% 4 11% 23% 29% 8%	1833
May	1st to 10th	Night	39% 4 8% 20% 19% 10%	2940
	11th to 20th	Night	32% 5% 12% 21% 24% 6%	2829
	21st to end	Night	28% 5% 14% 27% 18% 8%	3099
June	1st to 10th	Night	18% 5% 18% 34% 20% 5%	1710
	11th to 20th	Night	17% 5% 13% 32% 24% 9%	1710
	21st to end	Night	20% 4 10% 29% 27% 10%	1710
July	1st to 10th	Night	20% 6% 11% 40% 18% 3	2805
	11th to 20th	Night	10% 3 10% 40% 31% 6%	2796
	21st to end	Night	17% 5% 12% 38% 26% 2	2999
August	1st to 10th	Night	8% 3 10% 29% 43% 8%	2826
	11th to 20th	Night	15% 2 14% 35% 27% 6%	3077
	21st to end	Night	16% 6% 21% 29% 24% 4	3535
September	1st to 10th	Night	27% 5% 8% 22% 26% 11%	2090
	11th to 20th	Night	31% 5% 11% 21% 22% 10%	2090
	21st to end	Night	21% 3 7% 23% 27% 19%	2090
October	1st to 10th	Night	11% 2 6% 16% 31% 35%	3480
	11th to 20th	Night	15% 2 8% 18% 33% 23%	3637
	21st to end	Night	20% 3 3 22% 38% 14%	4120
November	1st to 10th	Night	25% 3 7% 20% 25% 19%	2422
	11th to 20th	Night	22% 3 5% 23% 29% 19%	2600
	21st to end	Night	30% 3 6% 18% 28% 16%	2600
December	1st to 10th	Night	31% 3 5% 22% 27% 12%	4250
	11th to 20th	Night	31% 3 6% 15% 29% 16%	4817
	21st to end	Night	29% 3 5% 21% 26% 16%	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	13% 21% 36% 26% 4	300
	11th to 20th	Day	15% 18% 35% 25% 6% 1	300
	21st to end	Day	15% 16% 39% 23% 7%	330
February	1st to 10th	Day	17% 17% 39% 25% 2 1	300
	11th to 20th	Day	20% 18% 42% 16% 4	300
	21st to end	Day	21% 17% 41% 17% 3	249
March	1st to 10th	Day	19% 18% 37% 22% 4	300
	11th to 20th	Day	20% 16% 41% 18% 4 1	300
	21st to end	Day	13% 15% 46% 22% 4	330
April	1st to 10th	Day	15% 19% 42% 18% 6%	300
	11th to 20th	Day	15% 19% 45% 16% 5%	438
	21st to end	Day	12% 16% 43% 23% 7%	665
May	1st to 10th	Day	11% 13% 40% 29% 6%	703
	11th to 20th	Day	14% 9% 32% 30% 13% 3	676
	21st to end	Day	9% 7% 35% 32% 14% 3	734
June	1st to 10th	Day	6% 3 18% 43% 24% 5%	680
	11th to 20th	Day	4 1 21% 49% 21% 4	680
	21st to end	Day	4 2 13% 38% 33% 10%	680
July	1st to 10th	Day	4 1 8% 37% 37% 13%	671
	11th to 20th	Day	2 1 6% 38% 40% 13%	668
	21st to end	Day	2 1 5% 36% 40% 15%	739
August	1st to 10th	Day	3 5% 33% 45% 13%	676
	11th to 20th	Day	6% 2 6% 31% 42% 13%	512
	21st to end	Day	4 1 7% 31% 37% 19%	515
September	1st to 10th	Day	13% 1 8% 16% 43% 19%	300
	11th to 20th	Day	4 2 7% 29% 43% 16%	300
	21st to end	Day	10% 2 7% 36% 32% 13%	300
October	1st to 10th	Day	4 2 14% 38% 31% 12%	300
	11th to 20th	Day	9% 5% 33% 34% 17% 2	300
	21st to end	Day	18% 8% 28% 28% 15% 2	330
November	1st to 10th	Day	15% 6% 34% 29% 12% 3	300
	11th to 20th	Day	8% 13% 35% 29% 12% 4	300
	21st to end	Day	15% 19% 37% 19% 9% 2	300
December	1st to 10th	Day	16% 14% 32% 28% 8% 1	300
	11th to 20th	Day	12% 14% 44% 23% 7%	300
	21st to end	Day	18% 14% 37% 25% 6%	330
January	1st to 10th	Night	12% 10% 39% 31% 7%	1000
	11th to 20th	Night	14% 14% 33% 32% 6%	1000
	21st to end	Night	13% 13% 39% 28% 7%	1025
February	1st to 10th	Night	14% 11% 43% 28% 5% 1	800
	11th to 20th	Night	16% 16% 40% 25% 3	784
	21st to end	Night	14% 17% 47% 20% 2	660
March	1st to 10th	Night	15% 15% 42% 24% 3	695
	11th to 20th	Night	15% 12% 43% 23% 6%	600
	21st to end	Night	7% 15% 48% 26% 4	660
April	1st to 10th	Night	9% 17% 47% 24% 3	600
	11th to 20th	Night	12% 12% 50% 23% 2	593
	21st to end	Night	10% 13% 47% 23% 6% 1	600
May	1st to 10th	Night	13% 11% 45% 27% 5%	600
	11th to 20th	Night	10% 6% 42% 30% 12% 1	563
	21st to end	Night	8% 4 44% 32% 12%	617
June	1st to 10th	Night	2 3 31% 44% 18% 2	570
	11th to 20th	Night	3 2 33% 53% 9%	570
	21st to end	Night	5% 1 19% 51% 22% 1	570
July	1st to 10th	Night	4 2 13% 50% 30% 1	555
	11th to 20th	Night	2 11% 57% 28% 1	552
	21st to end	Night	2 1 8% 58% 31% 1	624
August	1st to 10th	Night	1 8% 58% 31% 1	567
	11th to 20th	Night	7% 1 7% 50% 31% 3	564
	21st to end	Night	4 2 5% 48% 36% 5%	627
September	1st to 10th	Night	8% 2 10% 45% 32% 4	570
	11th to 20th	Night	11% 1 12% 44% 29% 2	570
	21st to end	Night	8% 1 17% 45% 27% 2	570
October	1st to 10th	Night	6% 2 25% 43% 22% 2	570
	11th to 20th	Night	9% 5% 33% 40% 12% 1	670
	21st to end	Night	13% 9% 31% 33% 13% 1	836
November	1st to 10th	Night	11% 5% 33% 38% 12% 1	862
	11th to 20th	Night	6% 10% 37% 36% 11% 1	1000
	21st to end	Night	15% 11% 38% 26% 9% 1	1000
December	1st to 10th	Night	11% 11% 36% 34% 7%	1000
	11th to 20th	Night	13% 11% 38% 33% 6%	1000
	21st to end	Night	13% 12% 39% 28% 7%	1100

Page generated at 2025-05-08 08:33:57. Version ac07

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