Blue Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Star Observatory clear sky chart from 2005-11-06 to 2024-01-22.

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	11% 5% 2 4 4 3 4 5% 5% 8% 48%	75026
	Night	11% 4 2 4 4 2 3 4 5% 7% 54%	74938
Transparency	Day	34% 6% 11% 22% 20% 7%	74751
	Night	31% 5% 11% 21% 22% 10%	74733
Seeing	Day	12% 6% 20% 27% 25% 10%	13957
	Night	13% 6% 26% 35% 18% 2	23967

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	15% 7% 2 4 3 2 3 3 4 5% 51%	1707
	11th to 20th	Day	17% 6% 3 5% 3 2 3 4 4 7% 47%	1710
	21st to end	Day	18% 6% 3 4 3 2 3 4 4 5% 49%	1791
February	1st to 10th	Day	20% 7% 4 6% 3 1 3 4 4 6% 41%	1701
	11th to 20th	Day	20% 8% 3 5% 5% 2 3 3 4 6% 42%	1749
	21st to end	Day	21% 6% 2 5% 4 1 3 3 4 5% 47%	1560
March	1st to 10th	Day	19% 7% 2 5% 5% 2 3 4 4 6% 42%	1980
	11th to 20th	Day	17% 7% 3 5% 4 3 5% 4 4 5% 42%	1980
	21st to end	Day	15% 6% 3 5% 4 3 3 4 4 5% 47%	2178
April	1st to 10th	Day	14% 7% 2 4 5% 3 4 4 5% 7% 44%	1980
	11th to 20th	Day	14% 6% 3 5% 5% 3 5% 4 6% 8% 42%	2077
	21st to end	Day	12% 5% 3 5% 5% 3 4 6% 4 7% 47%	2186
May	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 5% 4 7% 44%	2340
	11th to 20th	Day	11% 6% 2 6% 7% 3 5% 5% 6% 7% 43%	2302
	21st to end	Day	11% 5% 3 6% 6% 4 5% 6% 7% 10% 37%	2531
June	1st to 10th	Day	6% 3 1 4 4 3 5% 6% 6% 11% 51%	2340
	11th to 20th	Day	5% 3 2 4 5% 3 5% 5% 6% 12% 51%	2340
	21st to end	Day	5% 3 1 4 5% 3 5% 7% 7% 11% 49%	2340
July	1st to 10th	Day	6% 4 2 6% 6% 4 7% 8% 8% 10% 39%	2340
	11th to 20th	Day	3 3 1 3 4 3 4 5% 6% 11% 56%	2340
	21st to end	Day	3 3 2 4 5% 4 6% 5% 8% 11% 49%	2546
August	1st to 10th	Day	3 2 1 2 3 2 4 4 5% 9% 63%	2328
	11th to 20th	Day	5% 3 2 4 4 3 4 5% 7% 11% 51%	2301
	21st to end	Day	5% 3 1 4 6% 4 6% 6% 7% 10% 48%	2313
September	1st to 10th	Day	9% 4 3 4 4 3 5% 5% 6% 8% 49%	1980
	11th to 20th	Day	10% 5% 2 4 5% 3 5% 6% 6% 9% 44%	1980
	21st to end	Day	6% 4 2 4 4 3 5% 5% 5% 8% 55%	1980
October	1st to 10th	Day	6% 4 1 3 4 2 4 4 5% 7% 60%	1900
	11th to 20th	Day	12% 4 2 4 3 2 3 4 4 6% 57%	1800
	21st to end	Day	14% 5% 2 3 3 1 2 3 3 4 59%	1980
November	1st to 10th	Day	12% 5% 2 4 5% 2 3 3 4 6% 54%	1858
	11th to 20th	Day	10% 6% 2 4 3 2 3 4 4 6% 55%	1872
	21st to end	Day	18% 7% 3 4 4 3 3 4 4 5% 46%	1710
December	1st to 10th	Day	16% 6% 2 4 4 2 3 4 5% 5% 49%	1710
	11th to 20th	Day	16% 7% 3 4 5% 3 3 4 5% 6% 43%	2161
	21st to end	Day	15% 8% 3 6% 5% 2 3 3 4 5% 46%	3135
January	1st to 10th	Night	14% 6% 2 4 4 2 2 3 3 5% 54%	2458
	11th to 20th	Night	17% 6% 2 4 4 2 3 3 4 5% 51%	2470
	21st to end	Night	16% 6% 3 4 4 2 4 3 3 5% 50%	2600
February	1st to 10th	Night	17% 6% 3 4 4 2 3 4 4 5% 48%	2274
	11th to 20th	Night	20% 6% 3 4 4 2 3 3 4 6% 46%	2098
	21st to end	Night	20% 6% 2 5% 4 2 3 3 3 5% 47%	1696
March	1st to 10th	Night	17% 5% 3 5% 5% 2 4 4 5% 7% 44%	1980
	11th to 20th	Night	14% 5% 3 5% 4 3 4 4 5% 6% 49%	1980
	21st to end	Night	13% 5% 2 4 3 2 4 4 4 7% 53%	2178
April	1st to 10th	Night	11% 4 2 3 3 2 4 4 6% 7% 53%	1980
	11th to 20th	Night	13% 4 2 4 4 3 3 5% 5% 6% 50%	1829
	21st to end	Night	10% 4 2 5% 4 3 3 3 5% 8% 54%	1769
May	1st to 10th	Night	9% 5% 3 6% 5% 3 3 3 5% 8% 51%	1620
	11th to 20th	Night	10% 4 3 4 4 2 4 5% 6% 9% 50%	1593
	21st to end	Night	7% 4 2 4 4 3 5% 4 6% 10% 52%	1745
June	1st to 10th	Night	5% 4 2 3 3 2 3 5% 6% 9% 60%	1620
	11th to 20th	Night	3 1 1 3 4 2 3 3 5% 10% 65%	1620
	21st to end	Night	4 3 2 3 3 2 4 5% 6% 9% 58%	1620
July	1st to 10th	Night	4 3 2 5% 5% 3 6% 5% 7% 10% 50%	1620
	11th to 20th	Night	1 2 1 2 3 2 3 4 5% 10% 69%	1620
	21st to end	Night	2 2 1 3 4 3 4 6% 8% 11% 56%	1772
August	1st to 10th	Night	2 1 1 2 2 1 3 4 6% 10% 69%	1611
	11th to 20th	Night	6% 3 2 4 4 3 3 5% 6% 12% 52%	1600
	21st to end	Night	3 3 1 5% 4 3 4 5% 8% 10% 54%	2036
September	1st to 10th	Night	7% 4 2 4 5% 2 3 4 5% 9% 55%	1980
	11th to 20th	Night	10% 4 2 4 5% 3 3 4 4 8% 54%	1980
	21st to end	Night	7% 3 1 3 3 2 4 4 4 7% 61%	1980
October	1st to 10th	Night	5% 3 1 2 3 2 3 5% 5% 7% 65%	2042
	11th to 20th	Night	12% 4 1 3 3 2 3 3 3 5% 61%	2160
	21st to end	Night	14% 4 2 3 3 2 2 2 3 4 60%	2376
November	1st to 10th	Night	11% 4 2 3 4 2 3 3 4 5% 60%	2220
	11th to 20th	Night	8% 5% 2 4 5% 3 4 3 4 6% 57%	2303
	21st to end	Night	16% 6% 2 4 4 2 4 4 4 5% 50%	2470
December	1st to 10th	Night	15% 5% 2 4 4 2 4 3 4 7% 52%	2470
	11th to 20th	Night	15% 7% 3 6% 5% 2 3 3 4 6% 47%	3036
	21st to end	Night	16% 7% 2 4 4 2 3 3 4 4 52%	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	36% 3 8% 19% 26% 8%	1708
	11th to 20th	Day	38% 4 8% 20% 22% 7%	1710
	21st to end	Day	38% 4 6% 15% 27% 10%	1791
February	1st to 10th	Day	46% 4 5% 14% 21% 10%	1701
	11th to 20th	Day	46% 4 7% 16% 19% 8%	1759
	21st to end	Day	42% 4 7% 13% 20% 14%	1560
March	1st to 10th	Day	43% 5% 7% 15% 21% 9%	1980
	11th to 20th	Day	44% 6% 7% 14% 22% 7%	1980
	21st to end	Day	40% 5% 9% 18% 22% 7%	2178
April	1st to 10th	Day	40% 6% 9% 19% 17% 8%	1980
	11th to 20th	Day	40% 7% 10% 15% 19% 8%	2077
	21st to end	Day	36% 5% 10% 21% 22% 6%	2186
May	1st to 10th	Day	40% 6% 9% 21% 18% 6%	2340
	11th to 20th	Day	40% 7% 12% 21% 18% 1	2306
	21st to end	Day	40% 10% 13% 20% 16% 1	2531
June	1st to 10th	Day	27% 8% 17% 31% 14% 3	2340
	11th to 20th	Day	26% 9% 15% 30% 15% 4	2340
	21st to end	Day	27% 10% 14% 26% 20% 3	2340
July	1st to 10th	Day	35% 11% 19% 24% 12%	2276
	11th to 20th	Day	21% 8% 19% 32% 18% 3	2271
	21st to end	Day	24% 11% 19% 33% 13% 1	2405
August	1st to 10th	Day	17% 7% 17% 37% 20% 2	2314
	11th to 20th	Day	26% 10% 17% 27% 17% 3	2301
	21st to end	Day	30% 10% 13% 29% 16% 2	2313
September	1st to 10th	Day	31% 8% 12% 22% 19% 8%	1980
	11th to 20th	Day	34% 9% 14% 18% 17% 8%	1980
	21st to end	Day	26% 7% 10% 22% 23% 13%	1980
October	1st to 10th	Day	24% 4 13% 22% 20% 16%	1900
	11th to 20th	Day	30% 5% 7% 16% 26% 17%	1800
	21st to end	Day	32% 3 6% 17% 27% 16%	1980
November	1st to 10th	Day	33% 5% 8% 21% 25% 8%	1858
	11th to 20th	Day	32% 5% 8% 17% 26% 12%	1872
	21st to end	Day	43% 4 5% 18% 21% 10%	1710
December	1st to 10th	Day	38% 5% 10% 19% 21% 7%	1710
	11th to 20th	Day	42% 5% 7% 16% 22% 9%	2159
	21st to end	Day	42% 3 5% 20% 23% 7%	3135
January	1st to 10th	Night	34% 3 6% 19% 26% 12%	2458
	11th to 20th	Night	37% 3 7% 18% 24% 10%	2470
	21st to end	Night	38% 3 5% 14% 25% 14%	2600
February	1st to 10th	Night	39% 4 7% 14% 20% 15%	2274
	11th to 20th	Night	41% 3 7% 15% 23% 11%	2098
	21st to end	Night	41% 4 8% 13% 18% 16%	1696
March	1st to 10th	Night	40% 6% 8% 14% 20% 12%	1980
	11th to 20th	Night	37% 6% 9% 14% 26% 8%	1980
	21st to end	Night	32% 5% 11% 19% 24% 9%	2178
April	1st to 10th	Night	30% 5% 11% 22% 19% 13%	1980
	11th to 20th	Night	34% 6% 11% 17% 18% 14%	1829
	21st to end	Night	31% 5% 10% 21% 27% 7%	1769
May	1st to 10th	Night	32% 6% 13% 21% 19% 9%	1620
	11th to 20th	Night	31% 7% 14% 27% 17% 3	1594
	21st to end	Night	28% 8% 18% 27% 17% 3	1746
June	1st to 10th	Night	21% 8% 14% 34% 20% 3	1620
	11th to 20th	Night	16% 6% 18% 34% 21% 5%	1620
	21st to end	Night	22% 9% 13% 27% 25% 5%	1620
July	1st to 10th	Night	27% 9% 18% 32% 13% 2	1575
	11th to 20th	Night	12% 6% 21% 37% 21% 3	1566
	21st to end	Night	18% 9% 24% 33% 14% 1	1673
August	1st to 10th	Night	12% 6% 18% 37% 24% 4	1601
	11th to 20th	Night	25% 8% 16% 28% 18% 5%	1600
	21st to end	Night	23% 8% 18% 31% 17% 4	2036
September	1st to 10th	Night	27% 6% 14% 22% 20% 10%	1980
	11th to 20th	Night	30% 6% 15% 22% 19% 9%	1980
	21st to end	Night	22% 4 11% 21% 27% 15%	1980
October	1st to 10th	Night	19% 5% 10% 26% 21% 19%	2042
	11th to 20th	Night	27% 3 9% 16% 25% 20%	2160
	21st to end	Night	30% 3 6% 16% 25% 21%	2376
November	1st to 10th	Night	28% 5% 8% 20% 26% 13%	2220
	11th to 20th	Night	30% 4 7% 19% 26% 14%	2303
	21st to end	Night	37% 4 7% 16% 24% 11%	2470
December	1st to 10th	Night	34% 5% 9% 20% 22% 10%	2470
	11th to 20th	Night	40% 3 7% 14% 24% 12%	3037
	21st to end	Night	37% 3 6% 15% 27% 12%	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	19% 14% 34% 24% 9% 1	270
	11th to 20th	Day	20% 16% 34% 20% 9% 2	270
	21st to end	Day	23% 11% 42% 18% 7%	267
February	1st to 10th	Day	24% 14% 34% 21% 7%	240
	11th to 20th	Day	31% 18% 29% 18% 5%	240
	21st to end	Day	29% 19% 36% 13% 4	198
March	1st to 10th	Day	25% 15% 32% 22% 6%	240
	11th to 20th	Day	21% 13% 38% 21% 6% 1	240
	21st to end	Day	19% 12% 43% 18% 6% 2	264
April	1st to 10th	Day	21% 13% 38% 23% 6%	240
	11th to 20th	Day	19% 13% 31% 26% 10% 1	355
	21st to end	Day	14% 14% 34% 24% 12% 1	446
May	1st to 10th	Day	14% 9% 30% 35% 10% 1	603
	11th to 20th	Day	14% 6% 25% 32% 20% 4	625
	21st to end	Day	12% 4 23% 34% 22% 5%	679
June	1st to 10th	Day	6% 1 13% 37% 32% 12%	630
	11th to 20th	Day	5% 2 14% 40% 31% 10%	630
	21st to end	Day	3 1 12% 32% 34% 17%	630
July	1st to 10th	Day	5% 1 9% 33% 37% 15%	621
	11th to 20th	Day	2 4 30% 44% 19%	618
	21st to end	Day	2 1 3 25% 49% 19%	684
August	1st to 10th	Day	3 4 25% 48% 20%	626
	11th to 20th	Day	7% 1 3 23% 51% 15%	615
	21st to end	Day	8% 1 3 15% 41% 33%	432
September	1st to 10th	Day	8% 3 19% 37% 33%	270
	11th to 20th	Day	6% 1 5% 24% 38% 26%	270
	21st to end	Day	9% 4 5% 27% 31% 25%	270
October	1st to 10th	Day	4 1 15% 30% 38% 12%	270
	11th to 20th	Day	11% 5% 25% 27% 23% 8%	270
	21st to end	Day	22% 11% 21% 24% 17% 6%	297
November	1st to 10th	Day	16% 6% 26% 33% 13% 6%	270
	11th to 20th	Day	16% 9% 25% 30% 17% 3	270
	21st to end	Day	20% 16% 27% 24% 13% 1	270
December	1st to 10th	Day	19% 9% 31% 23% 15% 3	270
	11th to 20th	Day	26% 12% 29% 24% 8% 1	270
	21st to end	Day	22% 11% 38% 21% 7% 1	297
January	1st to 10th	Night	18% 10% 35% 30% 7%	950
	11th to 20th	Night	22% 11% 32% 30% 6%	950
	21st to end	Night	18% 10% 36% 30% 6%	1000
February	1st to 10th	Night	22% 11% 32% 30% 5%	834
	11th to 20th	Night	24% 11% 36% 25% 3	704
	21st to end	Night	24% 14% 37% 23% 2	512
March	1st to 10th	Night	18% 12% 36% 28% 6%	540
	11th to 20th	Night	16% 9% 39% 30% 6%	540
	21st to end	Night	13% 12% 40% 29% 6% 1	594
April	1st to 10th	Night	11% 9% 45% 29% 6%	540
	11th to 20th	Night	15% 11% 38% 27% 8% 1	533
	21st to end	Night	12% 7% 43% 28% 8% 1	540
May	1st to 10th	Night	10% 6% 40% 34% 10% 1	540
	11th to 20th	Night	12% 4 33% 34% 16% 2	531
	21st to end	Night	8% 3 28% 41% 19% 2	584
June	1st to 10th	Night	5% 2 14% 43% 33% 4	540
	11th to 20th	Night	3 1 17% 53% 24% 2	540
	21st to end	Night	5% 1 13% 37% 37% 6%	540
July	1st to 10th	Night	3 1 9% 47% 33% 8%	525
	11th to 20th	Night	2 5% 45% 41% 7%	522
	21st to end	Night	2 1 5% 44% 45% 4	591
August	1st to 10th	Night	2 1 4 45% 43% 5%	537
	11th to 20th	Night	7% 1 5% 41% 39% 7%	534
	21st to end	Night	3 1 4 36% 49% 7%	594
September	1st to 10th	Night	8% 1 8% 34% 42% 7%	540
	11th to 20th	Night	12% 1 7% 33% 43% 4	540
	21st to end	Night	8% 1 11% 36% 39% 4	540
October	1st to 10th	Night	6% 2 16% 47% 25% 5%	602
	11th to 20th	Night	14% 3 22% 39% 19% 3	720
	21st to end	Night	16% 8% 27% 32% 16% 2	792
November	1st to 10th	Night	12% 3 27% 39% 18% 2	740
	11th to 20th	Night	9% 8% 30% 40% 12% 1	783
	21st to end	Night	18% 9% 30% 29% 12% 1	950
December	1st to 10th	Night	18% 8% 32% 33% 9%	950
	11th to 20th	Night	23% 7% 31% 33% 5%	950
	21st to end	Night	21% 9% 32% 32% 6%	1045

Page generated at 2024-01-19 08:28:34. Version ac07

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