Stars at Night Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stars at Night Observatory clear sky chart from 2005-11-06 to 2024-12-09.

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	12% 5% 2 5% 4 4 4 3 5% 7% 50%	101515
	Night	11% 5% 2 4 3 3 3 3 5% 7% 54%	98373
Transparency	Day	32% 6% 11% 24% 21% 6%	105232
	Night	29% 5% 10% 22% 24% 10%	102783
Seeing	Day	12% 6% 21% 28% 25% 9%	15269
	Night	13% 7% 27% 34% 18% 2	25157

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	13% 7% 2 6% 4 2 2 2 4 6% 52%	2847
	11th to 20th	Day	12% 6% 1 3 3 3 2 2 3 6% 59%	2850
	21st to end	Day	23% 9% 3 6% 2 2 2 2 2 4 45%	3135
February	1st to 10th	Day	19% 7% 3 6% 4 3 3 3 3 5% 43%	1767
	11th to 20th	Day	18% 8% 3 6% 4 3 3 2 3 6% 45%	1849
	21st to end	Day	22% 6% 3 4 2 3 1 2 4 4 49%	1638
March	1st to 10th	Day	15% 7% 2 5% 3 3 3 3 5% 7% 46%	3436
	11th to 20th	Day	19% 6% 2 5% 5% 4 5% 4 5% 6% 40%	3480
	21st to end	Day	15% 9% 3 5% 4 4 4 3 6% 7% 40%	3814
April	1st to 10th	Day	14% 7% 2 5% 4 4 3 3 6% 7% 46%	2090
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 5% 8% 44%	2206
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 7% 45%	2346
May	1st to 10th	Day	19% 9% 3 7% 5% 5% 4 3 5% 5% 37%	4082
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 6% 46%	4082
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 35%	4477
June	1st to 10th	Day	7% 4 1 5% 5% 4 5% 5% 7% 9% 49%	2470
	11th to 20th	Day	5% 4 1 6% 4 4 5% 4 7% 10% 51%	2470
	21st to end	Day	5% 4 2 4 4 5% 4 5% 6% 8% 55%	2470
July	1st to 10th	Day	7% 5% 2 5% 5% 4 4 4 6% 8% 50%	4120
	11th to 20th	Day	3 3 1 3 3 3 4 4 6% 9% 61%	4120
	21st to end	Day	7% 6% 2 7% 5% 4 5% 5% 7% 10% 41%	4465
August	1st to 10th	Day	1 1 1 2 3 3 3 3 5% 8% 71%	4104
	11th to 20th	Day	4 2 1 3 2 3 3 4 7% 10% 59%	4041
	21st to end	Day	3 3 1 4 4 4 5% 5% 8% 10% 54%	4133
September	1st to 10th	Day	8% 5% 2 5% 3 3 4 4 5% 8% 52%	2090
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 6% 6% 49%	2090
	21st to end	Day	7% 4 2 4 2 3 3 4 6% 7% 59%	2090
October	1st to 10th	Day	6% 3 1 5% 3 3 3 3 4 6% 63%	2000
	11th to 20th	Day	13% 4 1 3 3 2 3 2 4 5% 60%	1900
	21st to end	Day	15% 4 2 4 2 2 3 2 4 4 58%	2090
November	1st to 10th	Day	13% 5% 1 5% 3 3 3 3 4 5% 55%	1958
	11th to 20th	Day	8% 4 1 6% 3 3 3 3 4 5% 58%	1927
	21st to end	Day	16% 6% 1 4 3 3 3 2 3 6% 53%	1800
December	1st to 10th	Day	15% 5% 1 5% 4 4 3 3 5% 6% 48%	1782
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 5% 5% 45%	2161
	21st to end	Day	17% 8% 2 5% 3 3 3 3 4 5% 47%	3135
January	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 54%	4108
	11th to 20th	Night	13% 5% 1 3 2 2 2 2 4 5% 62%	4120
	21st to end	Night	22% 7% 2 5% 3 2 2 3 3 4 47%	4532
February	1st to 10th	Night	15% 7% 2 5% 3 3 3 3 4 6% 49%	2429
	11th to 20th	Night	17% 6% 2 5% 4 3 3 2 4 5% 48%	2218
	21st to end	Night	18% 5% 1 5% 3 3 3 3 3 6% 50%	1814
March	1st to 10th	Night	17% 8% 2 7% 4 4 4 3 4 6% 41%	3419
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 5% 6% 45%	3480
	21st to end	Night	14% 5% 3 6% 4 3 4 3 5% 6% 47%	3817
April	1st to 10th	Night	10% 5% 1 5% 3 3 4 4 6% 8% 52%	2090
	11th to 20th	Night	13% 6% 2 5% 3 4 4 3 5% 6% 49%	1922
	21st to end	Night	11% 5% 1 5% 3 3 2 3 6% 7% 53%	1829
May	1st to 10th	Night	19% 8% 3 6% 4 3 3 3 5% 8% 37%	2823
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 5% 7% 49%	2823
	21st to end	Night	10% 6% 2 6% 4 4 4 4 7% 8% 45%	3090
June	1st to 10th	Night	5% 3 1 4 4 4 3 4 6% 10% 57%	1710
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 8% 63%	1710
	21st to end	Night	3 3 1 4 3 3 4 3 7% 11% 58%	1710
July	1st to 10th	Night	5% 3 1 4 4 3 4 4 6% 11% 55%	2850
	11th to 20th	Night	3 2 1 3 2 2 2 3 5% 9% 67%	2850
	21st to end	Night	4 4 2 5% 4 4 5% 4 8% 9% 51%	3098
August	1st to 10th	Night	1 1 2 2 1 2 2 4 9% 75%	2836
	11th to 20th	Night	4 2 1 3 2 2 3 3 5% 9% 66%	2798
	21st to end	Night	3 2 1 3 3 4 4 3 7% 9% 62%	3520
September	1st to 10th	Night	7% 5% 2 4 3 2 3 4 6% 8% 56%	2090
	11th to 20th	Night	11% 4 1 5% 3 3 4 4 5% 6% 55%	2090
	21st to end	Night	6% 3 1 4 3 3 3 3 5% 6% 64%	2090
October	1st to 10th	Night	6% 3 1 3 3 3 3 2 5% 6% 67%	2161
	11th to 20th	Night	12% 4 1 3 2 2 2 2 4 5% 62%	2280
	21st to end	Night	15% 4 1 4 2 2 1 2 3 4 61%	2508
November	1st to 10th	Night	12% 5% 2 4 2 3 3 2 3 4 60%	2340
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 4 6% 59%	2469
	21st to end	Night	15% 6% 2 4 3 3 3 3 3 5% 54%	2600
December	1st to 10th	Night	14% 6% 2 4 3 2 3 2 4 6% 53%	2581
	11th to 20th	Night	14% 6% 3 6% 3 4 3 3 4 5% 51%	3036
	21st to end	Night	16% 6% 2 5% 3 2 2 2 3 5% 53%	4532

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	34% 5% 7% 21% 30% 4	2848
	11th to 20th	Day	29% 4 10% 24% 30% 4	2850
	21st to end	Day	47% 3 4 17% 22% 8%	3135
February	1st to 10th	Day	46% 3 7% 17% 19% 9%	1767
	11th to 20th	Day	44% 3 6% 20% 19% 8%	1859
	21st to end	Day	41% 3 5% 15% 25% 11%	1638
March	1st to 10th	Day	35% 7% 8% 17% 24% 8%	3436
	11th to 20th	Day	43% 8% 9% 21% 16% 4	3480
	21st to end	Day	41% 8% 11% 18% 18% 4	3814
April	1st to 10th	Day	37% 6% 10% 20% 18% 9%	2090
	11th to 20th	Day	39% 7% 9% 17% 21% 8%	2206
	21st to end	Day	36% 7% 10% 20% 22% 4	2346
May	1st to 10th	Day	46% 9% 8% 22% 12% 3	4120
	11th to 20th	Day	37% 7% 10% 24% 21% 1	4086
	21st to end	Day	44% 11% 13% 18% 11% 4	4489
June	1st to 10th	Day	28% 9% 16% 28% 16% 2	2470
	11th to 20th	Day	27% 8% 16% 26% 18% 5%	2470
	21st to end	Day	25% 7% 14% 28% 22% 3	2470
July	1st to 10th	Day	29% 8% 20% 32% 10%	4056
	11th to 20th	Day	18% 6% 13% 40% 22% 2	4051
	21st to end	Day	35% 9% 14% 31% 11%	4325
August	1st to 10th	Day	12% 5% 13% 32% 34% 5%	4090
	11th to 20th	Day	19% 5% 17% 40% 17% 2	4080
	21st to end	Day	23% 9% 21% 34% 12% 2	4132
September	1st to 10th	Day	30% 6% 11% 23% 19% 11%	2090
	11th to 20th	Day	34% 8% 12% 18% 22% 7%	2090
	21st to end	Day	24% 6% 10% 22% 27% 11%	2090
October	1st to 10th	Day	14% 3 7% 13% 32% 32%	3325
	11th to 20th	Day	18% 2 4 22% 31% 22%	3140
	21st to end	Day	23% 4 5% 30% 28% 9%	3428
November	1st to 10th	Day	33% 4 8% 22% 24% 9%	1958
	11th to 20th	Day	28% 4 9% 18% 28% 12%	1927
	21st to end	Day	35% 3 6% 21% 25% 9%	1800
December	1st to 10th	Day	36% 5% 9% 20% 23% 7%	1782
	11th to 20th	Day	40% 5% 7% 15% 24% 9%	2159
	21st to end	Day	38% 6% 6% 22% 21% 7%	3135
January	1st to 10th	Night	32% 6% 6% 18% 30% 8%	4108
	11th to 20th	Night	27% 5% 9% 19% 28% 12%	4120
	21st to end	Night	42% 5% 6% 15% 25% 8%	4532
February	1st to 10th	Night	37% 4 8% 18% 21% 11%	2429
	11th to 20th	Night	39% 4 7% 16% 24% 10%	2218
	21st to end	Night	38% 4 8% 14% 21% 15%	1814
March	1st to 10th	Night	42% 6% 8% 14% 19% 11%	3419
	11th to 20th	Night	39% 7% 10% 18% 20% 6%	3480
	21st to end	Night	37% 6% 9% 19% 25% 4	3817
April	1st to 10th	Night	30% 6% 12% 21% 18% 14%	2090
	11th to 20th	Night	35% 6% 9% 20% 18% 13%	1922
	21st to end	Night	30% 6% 11% 21% 27% 5%	1829
May	1st to 10th	Night	42% 10% 10% 19% 13% 5%	2850
	11th to 20th	Night	34% 8% 14% 19% 24% 2	2824
	21st to end	Night	32% 11% 15% 23% 13% 5%	3099
June	1st to 10th	Night	22% 8% 17% 29% 20% 5%	1710
	11th to 20th	Night	20% 5% 13% 34% 20% 7%	1710
	21st to end	Night	19% 8% 13% 30% 25% 5%	1710
July	1st to 10th	Night	23% 5% 20% 38% 13% 1	2805
	11th to 20th	Night	15% 5% 12% 42% 23% 3	2796
	21st to end	Night	26% 8% 17% 35% 13% 1	2999
August	1st to 10th	Night	8% 3 11% 31% 39% 7%	2826
	11th to 20th	Night	15% 4 14% 42% 20% 5%	2825
	21st to end	Night	18% 7% 20% 32% 21% 2	3520
September	1st to 10th	Night	25% 7% 11% 23% 22% 11%	2090
	11th to 20th	Night	29% 5% 12% 22% 21% 10%	2090
	21st to end	Night	21% 5% 9% 21% 29% 16%	2090
October	1st to 10th	Night	11% 3 6% 15% 27% 39%	3603
	11th to 20th	Night	17% 2 5% 17% 32% 27%	3779
	21st to end	Night	23% 2 4 26% 28% 16%	4120
November	1st to 10th	Night	29% 4 7% 20% 26% 14%	2340
	11th to 20th	Night	28% 4 7% 18% 28% 16%	2469
	21st to end	Night	34% 3 8% 16% 27% 11%	2600
December	1st to 10th	Night	34% 5% 8% 20% 23% 10%	2581
	11th to 20th	Night	38% 4 6% 16% 25% 12%	3037
	21st to end	Night	35% 4 6% 18% 27% 10%	4532

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	18% 12% 38% 24% 8%	270
	11th to 20th	Day	16% 17% 40% 19% 7% 1	270
	21st to end	Day	24% 11% 33% 23% 7% 1	297
February	1st to 10th	Day	21% 14% 39% 19% 7%	270
	11th to 20th	Day	25% 14% 41% 15% 4	270
	21st to end	Day	30% 15% 40% 13% 2	225
March	1st to 10th	Day	22% 12% 34% 29% 3	270
	11th to 20th	Day	21% 18% 36% 21% 4 1	270
	21st to end	Day	20% 13% 39% 22% 6%	297
April	1st to 10th	Day	19% 15% 34% 25% 6% 1	270
	11th to 20th	Day	21% 11% 34% 24% 10%	404
	21st to end	Day	16% 13% 34% 26% 9% 2	526
May	1st to 10th	Day	14% 9% 31% 34% 10% 1	653
	11th to 20th	Day	16% 5% 26% 34% 16% 3	675
	21st to end	Day	14% 6% 25% 32% 20% 4	734
June	1st to 10th	Day	6% 2 13% 37% 32% 10%	680
	11th to 20th	Day	6% 2 15% 40% 30% 8%	680
	21st to end	Day	4 2 12% 32% 37% 14%	680
July	1st to 10th	Day	6% 1 9% 29% 39% 15%	671
	11th to 20th	Day	2 1 5% 30% 47% 15%	668
	21st to end	Day	4 1 4 26% 48% 17%	739
August	1st to 10th	Day	2 3 26% 50% 18%	676
	11th to 20th	Day	8% 1 5% 28% 41% 17%	665
	21st to end	Day	6% 1 4 21% 39% 28%	518
September	1st to 10th	Day	8% 1 3 17% 40% 30%	300
	11th to 20th	Day	7% 1 6% 24% 35% 28%	300
	21st to end	Day	9% 4 7% 22% 37% 21%	300
October	1st to 10th	Day	3 2 11% 30% 41% 13%	300
	11th to 20th	Day	11% 4 24% 33% 22% 6%	300
	21st to end	Day	23% 9% 20% 25% 19% 4	330
November	1st to 10th	Day	17% 6% 31% 25% 16% 5%	300
	11th to 20th	Day	12% 8% 30% 30% 17% 3	300
	21st to end	Day	18% 9% 33% 28% 10% 2	300
December	1st to 10th	Day	19% 7% 32% 29% 12% 2	294
	11th to 20th	Day	19% 13% 36% 26% 6%	270
	21st to end	Day	20% 13% 36% 23% 8% 1	297
January	1st to 10th	Night	17% 9% 36% 30% 7%	950
	11th to 20th	Night	18% 15% 33% 28% 5% 1	950
	21st to end	Night	17% 9% 37% 30% 6%	1045
February	1st to 10th	Night	18% 12% 37% 28% 5%	909
	11th to 20th	Night	22% 11% 39% 26% 2	744
	21st to end	Night	23% 15% 40% 18% 3	558
March	1st to 10th	Night	20% 11% 35% 28% 6%	570
	11th to 20th	Night	17% 11% 37% 31% 5%	570
	21st to end	Night	14% 11% 38% 31% 5%	627
April	1st to 10th	Night	11% 11% 46% 27% 5%	570
	11th to 20th	Night	16% 12% 38% 26% 8%	563
	21st to end	Night	14% 7% 41% 28% 10% 1	570
May	1st to 10th	Night	11% 8% 38% 34% 8% 1	570
	11th to 20th	Night	10% 3 32% 36% 17% 1	561
	21st to end	Night	7% 4 30% 38% 17% 3	617
June	1st to 10th	Night	6% 2 17% 41% 31% 3	570
	11th to 20th	Night	4 2 18% 51% 23% 2	570
	21st to end	Night	4 1 13% 39% 36% 7%	570
July	1st to 10th	Night	4 1 11% 39% 39% 7%	555
	11th to 20th	Night	3 5% 42% 45% 5%	552
	21st to end	Night	2 4 43% 45% 7%	624
August	1st to 10th	Night	2 3 41% 48% 5%	567
	11th to 20th	Night	8% 5% 35% 47% 5%	564
	21st to end	Night	4 1 4 35% 48% 8%	627
September	1st to 10th	Night	10% 1 8% 33% 43% 5%	570
	11th to 20th	Night	13% 2 5% 33% 44% 3	570
	21st to end	Night	7% 1 11% 38% 38% 5%	570
October	1st to 10th	Night	8% 1 17% 42% 29% 4	641
	11th to 20th	Night	13% 4 24% 39% 18% 2	760
	21st to end	Night	17% 7% 27% 31% 17% 1	836
November	1st to 10th	Night	12% 5% 29% 39% 15% 1	780
	11th to 20th	Night	9% 11% 29% 39% 11% 1	869
	21st to end	Night	18% 9% 32% 30% 10% 1	1000
December	1st to 10th	Night	17% 10% 33% 33% 7%	993
	11th to 20th	Night	20% 9% 33% 33% 5%	950
	21st to end	Night	20% 10% 35% 30% 6%	1045

Page generated at 2024-12-06 02:54:46. Version ac07

Copyright © 2024 Attilla Danko.