Starfront Observatories
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starfront Observatories clear sky chart from 2005-11-06 to 2024-08-15.

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	9% 5% 2 5% 4 3 4 3 5% 7% 55%	95376
	Night	9% 4 1 4 3 3 3 3 4 7% 59%	93423
Transparency	Day	28% 6% 10% 22% 23% 12%	95152
	Night	25% 5% 9% 22% 24% 14%	93253
Seeing	Day	9% 6% 22% 30% 24% 8%	14718
	Night	10% 5% 28% 40% 16% 1	24617

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% 2 5% 3 2 3 2 3 4 59%	2847
	11th to 20th	Day	8% 6% 2 3 2 2 2 2 5% 6% 62%	2850
	21st to end	Day	13% 6% 2 5% 3 3 3 2 4 5% 53%	3432
February	1st to 10th	Day	12% 5% 2 5% 3 2 3 3 4 7% 54%	1900
	11th to 20th	Day	12% 7% 2 4 3 3 3 3 4 5% 54%	1849
	21st to end	Day	13% 6% 1 4 3 3 3 2 4 5% 56%	1570
March	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 4 5% 51%	3244
	11th to 20th	Day	18% 7% 2 5% 4 3 3 3 4 7% 44%	3480
	21st to end	Day	6% 5% 2 6% 4 4 4 4 7% 8% 50%	3814
April	1st to 10th	Day	9% 5% 2 4 3 3 3 4 5% 7% 55%	2090
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 5% 7% 52%	2249
	21st to end	Day	9% 5% 2 4 4 3 3 2 4 6% 56%	2366
May	1st to 10th	Day	13% 5% 2 6% 5% 4 4 3 5% 5% 48%	4082
	11th to 20th	Day	13% 6% 2 5% 4 3 4 3 4 5% 51%	4082
	21st to end	Day	9% 5% 2 5% 4 4 4 5% 6% 8% 47%	4477
June	1st to 10th	Day	5% 2 1 4 5% 4 4 3 5% 7% 61%	2470
	11th to 20th	Day	3 2 1 4 4 4 4 4 6% 8% 61%	2470
	21st to end	Day	5% 4 1 4 4 4 4 4 6% 8% 56%	2470
July	1st to 10th	Day	4 3 2 5% 4 5% 5% 4 5% 8% 56%	4120
	11th to 20th	Day	2 2 1 3 3 4 3 4 6% 9% 63%	4120
	21st to end	Day	5% 4 1 4 3 3 5% 4 6% 9% 57%	4465
August	1st to 10th	Day	1 2 1 3 2 3 3 3 4 8% 70%	2375
	11th to 20th	Day	8% 3 1 4 5% 5% 5% 5% 6% 9% 49%	2172
	21st to end	Day	4 3 2 5% 4 4 5% 5% 7% 8% 54%	2376
September	1st to 10th	Day	8% 5% 2 6% 4 5% 4 3 4 7% 53%	1980
	11th to 20th	Day	11% 4 2 6% 5% 5% 4 4 5% 6% 49%	1980
	21st to end	Day	8% 3 1 4 3 4 3 3 5% 6% 59%	1980
October	1st to 10th	Day	5% 3 2 3 3 3 4 3 5% 6% 63%	1980
	11th to 20th	Day	9% 3 2 4 2 2 2 2 4 6% 64%	1921
	21st to end	Day	12% 4 2 4 2 2 2 1 3 4 63%	1980
November	1st to 10th	Day	11% 4 1 5% 3 2 2 2 3 6% 60%	1759
	11th to 20th	Day	7% 3 1 4 3 3 3 3 4 7% 62%	1710
	21st to end	Day	12% 6% 2 4 4 3 3 2 4 5% 55%	1710
December	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 4 5% 55%	1710
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 5% 6% 53%	2161
	21st to end	Day	12% 5% 1 4 2 2 2 2 3 5% 61%	3135
January	1st to 10th	Night	12% 5% 1 3 2 2 3 2 3 5% 60%	4108
	11th to 20th	Night	9% 5% 1 4 2 2 2 1 3 4 68%	4120
	21st to end	Night	14% 7% 2 4 2 2 3 2 4 5% 54%	4263
February	1st to 10th	Night	11% 5% 3 5% 3 2 2 3 4 6% 56%	2280
	11th to 20th	Night	13% 6% 2 4 2 2 2 3 4 5% 57%	2218
	21st to end	Night	12% 5% 1 4 3 3 3 3 5% 6% 57%	1884
March	1st to 10th	Night	9% 4 1 3 3 3 3 3 4 6% 59%	3615
	11th to 20th	Night	15% 5% 2 4 3 3 4 4 6% 7% 47%	3480
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 5% 7% 53%	3817
April	1st to 10th	Night	6% 4 1 3 3 3 4 4 6% 8% 58%	2090
	11th to 20th	Night	11% 5% 1 5% 3 4 4 2 5% 8% 53%	1877
	21st to end	Night	9% 3 1 3 3 2 2 3 4 6% 64%	1810
May	1st to 10th	Night	15% 5% 2 4 3 3 3 3 4 7% 51%	2823
	11th to 20th	Night	9% 5% 2 5% 4 3 4 3 5% 6% 55%	2823
	21st to end	Night	8% 4 3 5% 3 3 3 3 5% 9% 54%	3090
June	1st to 10th	Night	3 3 1 2 4 3 4 3 6% 10% 63%	1710
	11th to 20th	Night	3 2 2 2 3 3 4 5% 10% 66%	1710
	21st to end	Night	5% 3 1 3 3 3 3 3 5% 9% 64%	1710
July	1st to 10th	Night	4 2 1 3 3 3 3 3 4 9% 65%	2850
	11th to 20th	Night	1 1 2 1 2 2 2 4 8% 75%	2850
	21st to end	Night	3 3 1 4 3 3 3 4 6% 8% 62%	3098
August	1st to 10th	Night	1 1 1 2 2 2 2 2 5% 9% 75%	1780
	11th to 20th	Night	6% 3 1 4 4 4 4 4 5% 8% 56%	1828
	21st to end	Night	3 2 1 4 3 4 4 3 6% 10% 59%	1980
September	1st to 10th	Night	6% 4 1 4 4 4 4 4 5% 8% 57%	1979
	11th to 20th	Night	9% 5% 1 5% 3 3 4 4 6% 7% 52%	1980
	21st to end	Night	6% 4 1 3 3 3 3 4 4 8% 58%	1980
October	1st to 10th	Night	5% 2 1 3 2 3 3 3 4 6% 67%	1980
	11th to 20th	Night	10% 4 1 3 3 2 3 3 4 4 63%	2020
	21st to end	Night	12% 4 1 3 2 2 2 2 3 4 64%	2376
November	1st to 10th	Night	9% 4 2 4 2 2 4 3 4 5% 62%	2316
	11th to 20th	Night	6% 3 1 3 3 3 3 3 4 6% 65%	2470
	21st to end	Night	15% 4 2 4 3 3 3 2 4 6% 54%	2470
December	1st to 10th	Night	12% 4 1 4 3 3 3 3 4 6% 59%	2470
	11th to 20th	Night	10% 6% 2 4 4 3 4 3 4 6% 53%	3036
	21st to end	Night	12% 5% 1 4 3 3 3 3 4 5% 58%	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	29% 3 4 16% 30% 18%	2848
	11th to 20th	Day	24% 4 7% 18% 31% 17%	2850
	21st to end	Day	33% 5% 6% 16% 28% 12%	3432
February	1st to 10th	Day	33% 4 5% 18% 21% 20%	1900
	11th to 20th	Day	34% 3 6% 20% 20% 17%	1859
	21st to end	Day	31% 4 5% 13% 25% 22%	1570
March	1st to 10th	Day	36% 5% 6% 9% 23% 21%	3244
	11th to 20th	Day	40% 6% 11% 13% 18% 11%	3480
	21st to end	Day	28% 9% 11% 15% 23% 14%	3814
April	1st to 10th	Day	29% 6% 10% 19% 19% 18%	2090
	11th to 20th	Day	32% 6% 8% 15% 23% 16%	2249
	21st to end	Day	30% 4 7% 19% 26% 13%	2366
May	1st to 10th	Day	36% 7% 10% 15% 20% 12%	4120
	11th to 20th	Day	35% 6% 7% 17% 25% 9%	4086
	21st to end	Day	32% 9% 13% 24% 14% 9%	4489
June	1st to 10th	Day	23% 6% 13% 28% 24% 5%	2470
	11th to 20th	Day	20% 6% 14% 28% 23% 9%	2470
	21st to end	Day	25% 7% 11% 25% 23% 9%	2470
July	1st to 10th	Day	25% 7% 15% 38% 15% 1	4056
	11th to 20th	Day	16% 5% 15% 34% 24% 5%	4051
	21st to end	Day	22% 5% 12% 35% 22% 2	4325
August	1st to 10th	Day	14% 4 14% 35% 29% 4	2361
	11th to 20th	Day	31% 7% 15% 28% 15% 4	2172
	21st to end	Day	26% 8% 15% 27% 19% 5%	2376
September	1st to 10th	Day	32% 6% 9% 22% 22% 10%	1980
	11th to 20th	Day	35% 7% 10% 20% 22% 7%	1980
	21st to end	Day	25% 5% 10% 22% 24% 14%	1980
October	1st to 10th	Day	20% 5% 10% 25% 26% 13%	1980
	11th to 20th	Day	24% 4 7% 18% 27% 20%	1921
	21st to end	Day	28% 3 5% 18% 26% 20%	1980
November	1st to 10th	Day	28% 4 6% 23% 25% 13%	1759
	11th to 20th	Day	23% 4 8% 21% 26% 18%	1710
	21st to end	Day	33% 4 6% 18% 25% 15%	1710
December	1st to 10th	Day	33% 3 7% 18% 26% 12%	1710
	11th to 20th	Day	31% 5% 9% 14% 25% 16%	2159
	21st to end	Day	28% 3 4 19% 29% 17%	3135
January	1st to 10th	Night	27% 4 4 16% 30% 19%	4108
	11th to 20th	Night	22% 3 4 19% 28% 23%	4120
	21st to end	Night	33% 5% 6% 15% 27% 15%	4263
February	1st to 10th	Night	30% 4 7% 16% 23% 20%	2280
	11th to 20th	Night	31% 3 7% 18% 22% 19%	2218
	21st to end	Night	29% 4 7% 14% 23% 23%	1884
March	1st to 10th	Night	25% 6% 7% 14% 28% 19%	3615
	11th to 20th	Night	36% 7% 11% 17% 16% 13%	3480
	21st to end	Night	27% 7% 12% 19% 24% 11%	3817
April	1st to 10th	Night	23% 6% 11% 20% 19% 21%	2090
	11th to 20th	Night	31% 6% 9% 17% 20% 17%	1877
	21st to end	Night	21% 5% 12% 23% 27% 11%	1810
May	1st to 10th	Night	34% 6% 8% 21% 19% 11%	2850
	11th to 20th	Night	28% 7% 12% 21% 20% 11%	2824
	21st to end	Night	26% 8% 13% 27% 17% 8%	3099
June	1st to 10th	Night	17% 7% 16% 33% 23% 4	1710
	11th to 20th	Night	14% 5% 14% 34% 24% 9%	1710
	21st to end	Night	18% 5% 12% 29% 27% 9%	1710
July	1st to 10th	Night	18% 6% 16% 39% 19% 2	2805
	11th to 20th	Night	10% 3 12% 40% 30% 5%	2796
	21st to end	Night	19% 5% 14% 38% 23% 1	2999
August	1st to 10th	Night	10% 3 14% 37% 31% 5%	1770
	11th to 20th	Night	25% 6% 13% 32% 19% 6%	1828
	21st to end	Night	19% 7% 18% 30% 20% 6%	1980
September	1st to 10th	Night	24% 8% 11% 25% 22% 11%	1979
	11th to 20th	Night	30% 5% 15% 23% 19% 8%	1980
	21st to end	Night	24% 4 10% 24% 23% 16%	1980
October	1st to 10th	Night	17% 5% 12% 27% 22% 16%	1980
	11th to 20th	Night	25% 4 8% 16% 24% 23%	2020
	21st to end	Night	25% 3 6% 17% 27% 21%	2376
November	1st to 10th	Night	25% 4 8% 19% 24% 19%	2316
	11th to 20th	Night	21% 4 7% 23% 25% 20%	2470
	21st to end	Night	34% 3 7% 16% 25% 16%	2470
December	1st to 10th	Night	28% 3 8% 20% 26% 15%	2470
	11th to 20th	Night	32% 4 6% 16% 24% 18%	3037
	21st to end	Night	29% 4 5% 15% 28% 19%	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	12% 11% 38% 31% 7% 1	270
	11th to 20th	Day	15% 13% 26% 29% 14% 2	270
	21st to end	Day	11% 13% 38% 28% 9% 1	297
February	1st to 10th	Day	15% 16% 34% 24% 10% 1	270
	11th to 20th	Day	18% 11% 39% 26% 4 1	270
	21st to end	Day	19% 12% 41% 22% 4 1	225
March	1st to 10th	Day	14% 16% 37% 25% 7%	270
	11th to 20th	Day	16% 14% 33% 27% 9% 1	270
	21st to end	Day	10% 15% 45% 23% 7%	297
April	1st to 10th	Day	13% 13% 45% 23% 6% 1	270
	11th to 20th	Day	13% 16% 45% 22% 4	447
	21st to end	Day	11% 14% 40% 26% 8% 1	546
May	1st to 10th	Day	10% 11% 35% 36% 8%	653
	11th to 20th	Day	13% 6% 30% 33% 14% 3	675
	21st to end	Day	11% 4 29% 38% 16% 2	734
June	1st to 10th	Day	6% 2 15% 42% 29% 5%	680
	11th to 20th	Day	3 1 16% 49% 27% 5%	680
	21st to end	Day	5% 1 10% 36% 38% 11%	680
July	1st to 10th	Day	6% 1 5% 33% 41% 14%	671
	11th to 20th	Day	3 1 3 28% 48% 17%	668
	21st to end	Day	2 4 26% 47% 21%	739
August	1st to 10th	Day	2 3 26% 45% 24%	593
	11th to 20th	Day	7% 1 3 27% 43% 20%	454
	21st to end	Day	4 6% 26% 41% 23%	495
September	1st to 10th	Day	7% 4 18% 41% 29%	270
	11th to 20th	Day	6% 2 4 19% 41% 27%	270
	21st to end	Day	12% 2 4 30% 32% 19%	270
October	1st to 10th	Day	2 2 12% 39% 35% 10%	270
	11th to 20th	Day	9% 3 27% 33% 23% 5%	270
	21st to end	Day	19% 7% 29% 21% 21% 5%	297
November	1st to 10th	Day	16% 5% 24% 33% 16% 7%	270
	11th to 20th	Day	13% 10% 23% 33% 17% 6%	270
	21st to end	Day	17% 16% 28% 24% 13% 2	270
December	1st to 10th	Day	17% 9% 26% 31% 14% 3	270
	11th to 20th	Day	10% 11% 38% 29% 12%	270
	21st to end	Day	20% 10% 36% 20% 14%	297
January	1st to 10th	Night	13% 5% 35% 38% 9%	950
	11th to 20th	Night	14% 8% 31% 38% 9%	950
	21st to end	Night	14% 8% 34% 37% 7%	880
February	1st to 10th	Night	14% 8% 39% 34% 6% 1	760
	11th to 20th	Night	15% 8% 41% 32% 4	744
	21st to end	Night	15% 12% 42% 28% 3	628
March	1st to 10th	Night	14% 8% 40% 33% 4	693
	11th to 20th	Night	15% 8% 40% 32% 5%	570
	21st to end	Night	9% 10% 47% 31% 3	627
April	1st to 10th	Night	8% 8% 51% 29% 4	570
	11th to 20th	Night	12% 10% 49% 26% 3	563
	21st to end	Night	9% 8% 51% 26% 7%	570
May	1st to 10th	Night	9% 7% 45% 35% 4	570
	11th to 20th	Night	10% 4 39% 37% 10% 1	561
	21st to end	Night	9% 3 36% 40% 11% 1	617
June	1st to 10th	Night	3 2 25% 53% 16% 1	570
	11th to 20th	Night	4 24% 60% 12%	570
	21st to end	Night	5% 1 18% 49% 26% 1	570
July	1st to 10th	Night	5% 1 8% 48% 35% 2	555
	11th to 20th	Night	2 7% 55% 35% 2	552
	21st to end	Night	2 4 53% 38% 2	624
August	1st to 10th	Night	1 5% 53% 40% 1	567
	11th to 20th	Night	6% 5% 42% 43% 3	549
	21st to end	Night	4 1 5% 41% 45% 4	594
September	1st to 10th	Night	7% 7% 45% 37% 3	540
	11th to 20th	Night	11% 1 7% 43% 37% 1	540
	21st to end	Night	8% 1 11% 48% 30% 1	540
October	1st to 10th	Night	5% 2 15% 50% 26% 1	540
	11th to 20th	Night	10% 4 22% 44% 18% 1	580
	21st to end	Night	14% 7% 24% 37% 17% 1	792
November	1st to 10th	Night	11% 3 24% 44% 17%	836
	11th to 20th	Night	6% 6% 30% 43% 13% 1	950
	21st to end	Night	17% 6% 28% 37% 11% 1	950
December	1st to 10th	Night	13% 5% 29% 44% 9%	950
	11th to 20th	Night	14% 7% 33% 39% 7%	950
	21st to end	Night	14% 7% 35% 36% 9%	1045

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