Asteria Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Asteria Observatory clear sky chart from 2005-11-06 to 2024-10-25.

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	15% 10% 3 7% 5% 4 4 4 6% 7% 34%	101108
	Night	15% 10% 4 7% 5% 4 4 3 5% 7% 36%	97658
Transparency	Day	48% 6% 11% 21% 13% 2	100921
	Night	48% 5% 11% 21% 13% 2	97515
Seeing	Day	22% 13% 27% 21% 12% 5%	14859
	Night	20% 11% 25% 27% 14% 2	24911

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	18% 20% 6% 9% 5% 5% 4 3 4 6% 21%	2836
	11th to 20th	Day	15% 20% 7% 12% 6% 6% 4 2 3 4 19%	2840
	21st to end	Day	28% 19% 8% 8% 5% 4 3 2 3 4 15%	3124
February	1st to 10th	Day	22% 13% 4 9% 5% 4 3 3 5% 5% 27%	1710
	11th to 20th	Day	18% 13% 5% 12% 5% 4 4 3 5% 5% 27%	1824
	21st to end	Day	20% 15% 5% 7% 4 3 3 3 4 5% 32%	1570
March	1st to 10th	Day	23% 15% 4 8% 4 4 4 3 5% 6% 25%	3301
	11th to 20th	Day	14% 8% 3 7% 5% 4 5% 3 5% 7% 38%	3480
	21st to end	Day	19% 9% 3 7% 5% 4 3 3 5% 7% 34%	3815
April	1st to 10th	Day	14% 13% 3 8% 5% 5% 5% 3 5% 6% 34%	2100
	11th to 20th	Day	17% 10% 3 6% 4 4 3 4 5% 6% 37%	2404
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 3 5% 7% 34%	2470
May	1st to 10th	Day	21% 11% 3 7% 5% 4 4 4 5% 7% 29%	4082
	11th to 20th	Day	21% 8% 3 7% 4 4 5% 4 6% 7% 32%	4081
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 5% 8% 10% 34%	4476
June	1st to 10th	Day	12% 6% 2 7% 5% 4 5% 4 7% 8% 38%	2470
	11th to 20th	Day	9% 7% 3 7% 6% 5% 5% 5% 6% 9% 39%	2470
	21st to end	Day	11% 6% 2 6% 6% 5% 7% 5% 8% 10% 35%	2470
July	1st to 10th	Day	11% 6% 3 7% 6% 5% 5% 5% 6% 8% 40%	4120
	11th to 20th	Day	8% 6% 2 5% 4 5% 5% 5% 8% 11% 41%	4120
	21st to end	Day	12% 8% 3 8% 6% 6% 6% 5% 7% 8% 32%	4466
August	1st to 10th	Day	8% 6% 2 7% 6% 6% 6% 5% 7% 10% 37%	4104
	11th to 20th	Day	8% 6% 2 7% 5% 5% 5% 5% 8% 11% 38%	4044
	21st to end	Day	5% 4 2 6% 5% 4 5% 4 6% 8% 50%	4447
September	1st to 10th	Day	15% 5% 2 5% 3 3 4 4 5% 8% 47%	2090
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 3 6% 7% 50%	2090
	21st to end	Day	18% 7% 3 6% 4 3 3 3 4 6% 42%	2090
October	1st to 10th	Day	11% 7% 2 6% 4 3 5% 3 7% 8% 44%	2027
	11th to 20th	Day	14% 8% 2 5% 4 4 4 4 5% 6% 44%	1900
	21st to end	Day	21% 10% 2 6% 4 3 4 2 4 6% 39%	2008
November	1st to 10th	Day	10% 7% 3 5% 4 4 3 3 4 7% 49%	1672
	11th to 20th	Day	16% 12% 4 7% 5% 3 3 3 5% 6% 35%	1710
	21st to end	Day	18% 13% 3 7% 4 4 5% 3 4 6% 33%	1710
December	1st to 10th	Day	22% 16% 6% 9% 6% 5% 3 2 4 5% 21%	1710
	11th to 20th	Day	17% 17% 6% 11% 4 4 4 3 4 6% 24%	2153
	21st to end	Day	20% 23% 5% 10% 5% 5% 4 3 5% 5% 15%	3124
January	1st to 10th	Night	15% 20% 7% 13% 6% 4 4 3 5% 4 19%	4109
	11th to 20th	Night	17% 17% 6% 15% 6% 4 4 3 4 5% 19%	4120
	21st to end	Night	26% 16% 7% 10% 7% 3 3 2 3 4 19%	4532
February	1st to 10th	Night	22% 13% 4 9% 5% 4 3 3 4 5% 29%	2470
	11th to 20th	Night	18% 14% 4 9% 5% 4 4 3 4 5% 29%	2237
	21st to end	Night	20% 16% 6% 7% 4 4 3 3 3 5% 29%	1884
March	1st to 10th	Night	21% 12% 3 7% 3 3 4 3 4 6% 33%	3551
	11th to 20th	Night	14% 9% 3 7% 4 4 4 3 6% 7% 39%	3480
	21st to end	Night	19% 8% 3 6% 4 4 3 3 4 5% 40%	3817
April	1st to 10th	Night	15% 11% 3 6% 4 3 4 4 6% 7% 37%	2090
	11th to 20th	Night	13% 9% 2 5% 5% 4 4 4 7% 8% 38%	1730
	21st to end	Night	14% 8% 3 7% 5% 5% 4 3 6% 7% 37%	1710
May	1st to 10th	Night	27% 9% 3 6% 4 4 4 3 5% 6% 27%	2813
	11th to 20th	Night	19% 9% 3 6% 3 3 3 3 4 7% 40%	2812
	21st to end	Night	10% 6% 2 6% 4 4 4 4 7% 10% 42%	3079
June	1st to 10th	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 44%	1710
	11th to 20th	Night	8% 5% 2 5% 4 5% 5% 4 8% 10% 45%	1710
	21st to end	Night	9% 5% 2 6% 4 3 4 5% 8% 9% 44%	1710
July	1st to 10th	Night	9% 7% 3 6% 4 4 4 3 6% 9% 46%	2840
	11th to 20th	Night	8% 5% 2 5% 5% 5% 5% 4 6% 10% 45%	2840
	21st to end	Night	9% 7% 3 9% 5% 5% 6% 4 7% 8% 37%	3087
August	1st to 10th	Night	7% 7% 3 8% 5% 5% 5% 4 7% 10% 39%	2825
	11th to 20th	Night	9% 8% 3 7% 4 4 4 4 5% 7% 43%	2785
	21st to end	Night	5% 3 2 5% 5% 4 5% 5% 7% 10% 51%	3178
September	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 6% 51%	2085
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 5% 7% 54%	2090
	21st to end	Night	15% 6% 3 6% 3 4 3 3 4 5% 47%	2090
October	1st to 10th	Night	9% 6% 2 6% 3 4 4 2 5% 7% 52%	2152
	11th to 20th	Night	15% 7% 2 6% 3 4 3 3 5% 7% 45%	2280
	21st to end	Night	19% 8% 3 5% 4 4 4 4 4 6% 40%	2452
November	1st to 10th	Night	14% 7% 2 5% 4 3 2 3 4 6% 49%	2406
	11th to 20th	Night	16% 11% 3 8% 4 3 4 3 5% 6% 37%	2470
	21st to end	Night	19% 12% 3 6% 5% 4 4 3 4 6% 33%	2470
December	1st to 10th	Night	22% 14% 5% 8% 5% 4 4 3 4 5% 26%	2470
	11th to 20th	Night	17% 14% 7% 9% 5% 3 4 3 4 6% 28%	3042
	21st to end	Night	20% 21% 5% 10% 7% 5% 4 3 4 5% 17%	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	65% 4 13% 12% 6%	2837
	11th to 20th	Day	70% 3 12% 12% 4	2840
	21st to end	Day	73% 4 7% 12% 3 1	3124
February	1st to 10th	Day	59% 5% 12% 15% 8% 1	1710
	11th to 20th	Day	59% 4 10% 17% 10%	1834
	21st to end	Day	57% 4 10% 20% 9% 1	1570
March	1st to 10th	Day	59% 5% 8% 20% 6% 1	3301
	11th to 20th	Day	45% 5% 11% 25% 14% 1	3480
	21st to end	Day	49% 5% 10% 23% 13%	3815
April	1st to 10th	Day	51% 5% 10% 21% 11% 3	2100
	11th to 20th	Day	46% 6% 10% 20% 15% 4	2404
	21st to end	Day	50% 5% 11% 20% 12% 2	2470
May	1st to 10th	Day	53% 7% 9% 21% 9% 1	4120
	11th to 20th	Day	50% 7% 11% 18% 13% 1	4085
	21st to end	Day	39% 11% 14% 17% 15% 4	4489
June	1st to 10th	Day	40% 8% 12% 23% 16% 2	2470
	11th to 20th	Day	40% 8% 13% 21% 16% 1	2470
	21st to end	Day	40% 9% 13% 21% 14% 2	2470
July	1st to 10th	Day	42% 6% 11% 23% 16% 2	4055
	11th to 20th	Day	33% 8% 11% 32% 16% 1	4051
	21st to end	Day	46% 7% 11% 17% 16% 3	4324
August	1st to 10th	Day	39% 8% 13% 29% 11% 1	4091
	11th to 20th	Day	38% 7% 10% 30% 15% 1	4083
	21st to end	Day	30% 7% 14% 30% 17% 3	4447
September	1st to 10th	Day	35% 7% 10% 21% 22% 5%	2090
	11th to 20th	Day	33% 7% 11% 25% 23% 3	2090
	21st to end	Day	44% 4 9% 23% 19% 1	2090
October	1st to 10th	Day	36% 6% 12% 23% 22% 1	2027
	11th to 20th	Day	40% 5% 10% 26% 17% 2	1900
	21st to end	Day	48% 4 11% 21% 14% 2	2008
November	1st to 10th	Day	35% 6% 9% 29% 18% 3	1672
	11th to 20th	Day	50% 4 8% 20% 15% 3	1710
	21st to end	Day	53% 4 10% 21% 12% 1	1710
December	1st to 10th	Day	66% 5% 9% 16% 4	1710
	11th to 20th	Day	62% 3 9% 17% 8%	2150
	21st to end	Day	71% 5% 10% 9% 4	3124
January	1st to 10th	Night	67% 4 9% 14% 5%	4109
	11th to 20th	Night	68% 4 11% 13% 4 1	4120
	21st to end	Night	71% 3 7% 14% 3 1	4532
February	1st to 10th	Night	58% 4 11% 18% 8% 1	2470
	11th to 20th	Night	57% 4 9% 17% 12% 1	2238
	21st to end	Night	60% 4 9% 16% 9% 2	1884
March	1st to 10th	Night	52% 4 8% 23% 11% 1	3551
	11th to 20th	Night	42% 5% 12% 27% 12% 1	3480
	21st to end	Night	46% 4 9% 27% 13% 1	3817
April	1st to 10th	Night	45% 6% 11% 21% 15% 2	2090
	11th to 20th	Night	41% 6% 11% 18% 18% 5%	1730
	21st to end	Night	45% 6% 10% 22% 14% 2	1710
May	1st to 10th	Night	55% 7% 9% 18% 9% 2	2840
	11th to 20th	Night	44% 7% 11% 25% 12% 2	2813
	21st to end	Night	33% 9% 15% 25% 13% 4	3087
June	1st to 10th	Night	34% 8% 12% 24% 20% 3	1710
	11th to 20th	Night	31% 9% 16% 23% 19% 2	1710
	21st to end	Night	32% 9% 14% 23% 18% 3	1710
July	1st to 10th	Night	36% 5% 11% 24% 18% 5%	2795
	11th to 20th	Night	34% 7% 12% 30% 16% 1	2786
	21st to end	Night	43% 7% 12% 20% 13% 6%	2988
August	1st to 10th	Night	38% 8% 15% 27% 11% 1	2814
	11th to 20th	Night	39% 7% 9% 23% 21% 1	2812
	21st to end	Night	28% 6% 14% 29% 20% 4	3178
September	1st to 10th	Night	32% 7% 12% 23% 21% 6%	2085
	11th to 20th	Night	30% 6% 10% 25% 26% 3	2090
	21st to end	Night	39% 5% 9% 21% 25% 1	2090
October	1st to 10th	Night	32% 7% 10% 25% 22% 3	2152
	11th to 20th	Night	39% 6% 9% 25% 18% 3	2280
	21st to end	Night	45% 5% 10% 21% 17% 3	2452
November	1st to 10th	Night	37% 3 11% 24% 20% 4	2406
	11th to 20th	Night	48% 4 8% 20% 16% 3	2470
	21st to end	Night	53% 4 11% 20% 11% 1	2470
December	1st to 10th	Night	61% 5% 9% 18% 6%	2470
	11th to 20th	Night	58% 4 11% 20% 7%	3044
	21st to end	Night	70% 4 9% 14% 4	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	29% 23% 33% 10% 4	270
	11th to 20th	Day	32% 26% 33% 8% 1	270
	21st to end	Day	36% 23% 30% 10% 2	297
February	1st to 10th	Day	36% 24% 24% 13% 3	270
	11th to 20th	Day	36% 27% 31% 3 2	270
	21st to end	Day	37% 26% 30% 6% 1	225
March	1st to 10th	Day	32% 24% 27% 13% 4	373
	11th to 20th	Day	31% 20% 31% 12% 5%	460
	21st to end	Day	35% 17% 32% 13% 4	506
April	1st to 10th	Day	27% 19% 30% 15% 8% 1	460
	11th to 20th	Day	27% 18% 27% 18% 7% 3	452
	21st to end	Day	28% 17% 31% 16% 6% 2	460
May	1st to 10th	Day	28% 18% 29% 16% 8% 2	463
	11th to 20th	Day	22% 14% 29% 21% 9% 5%	487
	21st to end	Day	15% 9% 26% 26% 18% 6%	529
June	1st to 10th	Day	15% 9% 26% 29% 15% 6%	490
	11th to 20th	Day	14% 8% 26% 32% 14% 5%	490
	21st to end	Day	11% 11% 29% 29% 12% 7%	490
July	1st to 10th	Day	13% 4 21% 27% 18% 17%	485
	11th to 20th	Day	10% 7% 23% 28% 21% 11%	484
	21st to end	Day	12% 3 26% 28% 20% 11%	532
August	1st to 10th	Day	12% 3 21% 30% 23% 10%	486
	11th to 20th	Day	13% 5% 23% 32% 16% 11%	479
	21st to end	Day	10% 4 20% 31% 23% 12%	539
September	1st to 10th	Day	14% 4 20% 24% 26% 11%	490
	11th to 20th	Day	11% 3 15% 29% 30% 12%	490
	21st to end	Day	24% 8% 21% 25% 18% 4	490
October	1st to 10th	Day	12% 7% 29% 27% 19% 5%	490
	11th to 20th	Day	21% 12% 31% 21% 12% 3	490
	21st to end	Day	26% 15% 31% 20% 8%	495
November	1st to 10th	Day	17% 17% 35% 22% 8% 1	270
	11th to 20th	Day	31% 18% 27% 17% 6% 1	270
	21st to end	Day	28% 19% 34% 14% 4	270
December	1st to 10th	Day	30% 21% 34% 12% 3 1	270
	11th to 20th	Day	24% 27% 34% 8% 6%	270
	21st to end	Day	33% 23% 29% 12% 3	297
January	1st to 10th	Night	30% 19% 30% 17% 4	760
	11th to 20th	Night	31% 20% 29% 18% 3	760
	21st to end	Night	27% 19% 31% 20% 2	836
February	1st to 10th	Night	30% 16% 30% 23% 1	760
	11th to 20th	Night	30% 22% 29% 18% 1	743
	21st to end	Night	31% 18% 31% 19% 2	628
March	1st to 10th	Night	25% 18% 29% 23% 4	760
	11th to 20th	Night	26% 18% 28% 21% 7% 1	760
	21st to end	Night	26% 15% 31% 21% 7%	836
April	1st to 10th	Night	22% 16% 33% 22% 6% 1	760
	11th to 20th	Night	17% 13% 35% 24% 10% 1	568
	21st to end	Night	18% 11% 38% 26% 6% 1	570
May	1st to 10th	Night	20% 11% 30% 26% 11% 2	570
	11th to 20th	Night	19% 7% 29% 28% 16% 1	561
	21st to end	Night	10% 5% 24% 32% 26% 2	617
June	1st to 10th	Night	12% 4 23% 39% 21% 1	570
	11th to 20th	Night	10% 3 22% 38% 24% 2	570
	21st to end	Night	11% 4 23% 36% 24% 2	570
July	1st to 10th	Night	12% 2 17% 33% 28% 7%	555
	11th to 20th	Night	6% 2 18% 37% 31% 5%	552
	21st to end	Night	11% 3 17% 32% 33% 4	624
August	1st to 10th	Night	7% 2 15% 40% 34% 3	567
	11th to 20th	Night	8% 3 16% 38% 30% 5%	564
	21st to end	Night	9% 2 16% 32% 34% 7%	627
September	1st to 10th	Night	14% 3 16% 33% 28% 5%	755
	11th to 20th	Night	10% 4 14% 36% 32% 5%	760
	21st to end	Night	18% 7% 22% 31% 18% 3	760
October	1st to 10th	Night	14% 6% 23% 33% 21% 4	760
	11th to 20th	Night	19% 10% 26% 30% 13% 1	760
	21st to end	Night	23% 13% 26% 27% 10% 1	812
November	1st to 10th	Night	19% 10% 24% 28% 17% 1	740
	11th to 20th	Night	23% 17% 26% 24% 9% 1	760
	21st to end	Night	27% 13% 25% 27% 7% 1	760
December	1st to 10th	Night	31% 17% 26% 21% 4 1	760
	11th to 20th	Night	26% 16% 29% 22% 6%	760
	21st to end	Night	30% 18% 27% 24% 3	836

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