Blackstar II Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackstar II Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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	16% 8% 3 7% 6% 4 5% 5% 6% 7% 34%	74782
	Night	16% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	74969
Transparency	Day	47% 8% 12% 20% 12% 2	74487
	Night	46% 8% 12% 20% 12% 2	74798
Seeing	Day	24% 20% 29% 17% 8% 2	13780
	Night	22% 20% 27% 22% 8% 1	22923

By Month

Percentage Cloud Cover NE (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	11% 11% 5% 8% 7% 4 6% 6% 6% 8% 28%	1517
	11th to 20th	Day	16% 11% 4 9% 9% 4 6% 7% 4 6% 25%	1464
	21st to end	Day	11% 9% 5% 8% 8% 4 6% 6% 6% 6% 30%	1607
February	1st to 10th	Day	15% 11% 5% 8% 8% 4 6% 5% 5% 7% 24%	1641
	11th to 20th	Day	12% 8% 5% 11% 7% 4 5% 5% 7% 6% 30%	1748
	21st to end	Day	19% 11% 4 8% 8% 3 5% 4 4 4 29%	1480
March	1st to 10th	Day	13% 9% 4 9% 7% 3 5% 4 6% 6% 32%	1797
	11th to 20th	Day	15% 10% 5% 8% 6% 4 5% 5% 5% 6% 31%	1852
	21st to end	Day	18% 9% 4 7% 5% 4 4 4 4 5% 35%	2376
April	1st to 10th	Day	16% 10% 4 7% 6% 4 4 4 5% 6% 33%	2160
	11th to 20th	Day	16% 8% 3 6% 6% 3 5% 5% 5% 6% 36%	2134
	21st to end	Day	19% 9% 3 7% 6% 4 5% 5% 5% 5% 31%	2269
May	1st to 10th	Day	24% 9% 4 6% 5% 4 5% 5% 4 5% 29%	2340
	11th to 20th	Day	20% 6% 3 5% 4 3 4 5% 5% 8% 37%	2460
	21st to end	Day	16% 6% 2 6% 6% 4 4 5% 6% 8% 37%	2725
June	1st to 10th	Day	17% 7% 3 6% 7% 4 6% 5% 7% 8% 30%	2520
	11th to 20th	Day	19% 7% 3 5% 4 4 5% 5% 6% 9% 33%	2520
	21st to end	Day	17% 7% 3 7% 7% 4 5% 5% 6% 7% 31%	2520
July	1st to 10th	Day	15% 6% 2 5% 6% 4 6% 6% 6% 8% 36%	2520
	11th to 20th	Day	12% 6% 2 5% 6% 4 6% 6% 6% 10% 38%	2520
	21st to end	Day	13% 6% 3 6% 5% 4 6% 7% 7% 8% 34%	2744
August	1st to 10th	Day	12% 5% 2 6% 6% 4 5% 6% 8% 10% 35%	2459
	11th to 20th	Day	13% 7% 3 7% 6% 4 5% 6% 7% 8% 33%	2159
	21st to end	Day	13% 6% 3 6% 6% 3 5% 6% 6% 9% 38%	2376
September	1st to 10th	Day	13% 6% 3 7% 5% 4 5% 5% 5% 7% 41%	2160
	11th to 20th	Day	15% 6% 2 4 4 3 5% 4 4 7% 45%	2048
	21st to end	Day	16% 6% 3 5% 5% 3 4 5% 5% 7% 40%	1980
October	1st to 10th	Day	22% 10% 3 6% 6% 2 4 4 4 5% 34%	1926
	11th to 20th	Day	15% 8% 3 5% 5% 3 5% 5% 6% 7% 38%	1779
	21st to end	Day	22% 13% 3 6% 5% 3 4 4 5% 5% 29%	1782
November	1st to 10th	Day	12% 8% 4 6% 4 2 4 4 5% 6% 45%	1671
	11th to 20th	Day	17% 8% 4 6% 5% 3 4 6% 5% 7% 35%	1710
	21st to end	Day	16% 11% 3 5% 6% 3 5% 5% 5% 6% 34%	1620
December	1st to 10th	Day	17% 13% 5% 9% 7% 3 5% 4 5% 6% 26%	1520
	11th to 20th	Day	20% 11% 4 7% 6% 3 3 3 4 6% 33%	1906
	21st to end	Day	17% 13% 5% 11% 8% 4 4 4 5% 6% 23%	2772
January	1st to 10th	Night	13% 10% 5% 6% 7% 4 6% 5% 5% 7% 31%	2649
	11th to 20th	Night	16% 9% 4 8% 7% 5% 6% 5% 6% 7% 25%	2574
	21st to end	Night	10% 10% 4 8% 7% 4 7% 6% 6% 7% 30%	2745
February	1st to 10th	Night	15% 9% 4 8% 7% 4 7% 6% 6% 7% 26%	2318
	11th to 20th	Night	12% 9% 4 9% 7% 4 6% 6% 7% 7% 29%	2098
	21st to end	Night	17% 10% 4 8% 6% 3 5% 5% 6% 6% 31%	1776
March	1st to 10th	Night	15% 9% 4 7% 5% 4 6% 5% 7% 6% 32%	2150
	11th to 20th	Night	14% 10% 3 7% 7% 4 6% 6% 6% 7% 31%	2105
	21st to end	Night	16% 8% 4 8% 5% 3 4 4 5% 5% 37%	1980
April	1st to 10th	Night	16% 11% 4 6% 6% 3 4 6% 6% 7% 32%	1800
	11th to 20th	Night	17% 9% 3 6% 5% 2 4 5% 5% 7% 36%	1775
	21st to end	Night	20% 9% 4 6% 4 2 4 4 5% 6% 35%	1689
May	1st to 10th	Night	23% 11% 3 6% 5% 3 4 5% 5% 5% 30%	1620
	11th to 20th	Night	16% 8% 5% 6% 5% 3 4 5% 6% 7% 35%	1447
	21st to end	Night	15% 8% 3 5% 6% 3 4 4 6% 8% 39%	1550
June	1st to 10th	Night	18% 8% 3 6% 6% 3 3 4 5% 7% 38%	1440
	11th to 20th	Night	18% 7% 2 5% 5% 3 5% 5% 5% 8% 37%	1440
	21st to end	Night	15% 5% 2 5% 5% 3 5% 5% 5% 8% 42%	1440
July	1st to 10th	Night	13% 6% 3 5% 5% 3 4 5% 6% 8% 44%	1440
	11th to 20th	Night	14% 5% 2 5% 6% 2 4 7% 6% 10% 39%	1440
	21st to end	Night	10% 6% 2 5% 5% 3 4 5% 6% 8% 46%	1574
August	1st to 10th	Night	11% 4 2 5% 5% 2 5% 5% 6% 8% 47%	1460
	11th to 20th	Night	12% 6% 2 6% 7% 3 6% 5% 6% 8% 39%	1739
	21st to end	Night	15% 6% 2 5% 4 3 3 4 5% 8% 45%	1980
September	1st to 10th	Night	13% 7% 2 6% 5% 3 5% 5% 5% 7% 43%	1800
	11th to 20th	Night	14% 5% 3 5% 4 3 4 4 4 6% 50%	1912
	21st to end	Night	16% 8% 3 5% 4 2 4 5% 4 6% 44%	1980
October	1st to 10th	Night	21% 8% 3 5% 5% 3 4 3 4 5% 38%	2029
	11th to 20th	Night	15% 8% 3 6% 5% 3 4 5% 5% 5% 41%	2179
	21st to end	Night	21% 10% 4 6% 5% 3 3 4 4 7% 32%	2574
November	1st to 10th	Night	14% 8% 3 5% 5% 2 4 5% 5% 6% 44%	2407
	11th to 20th	Night	17% 7% 4 6% 6% 4 5% 5% 5% 6% 36%	2470
	21st to end	Night	13% 9% 4 6% 5% 3 5% 5% 6% 6% 35%	2557
December	1st to 10th	Night	18% 11% 4 7% 6% 3 5% 5% 7% 7% 26%	2660
	11th to 20th	Night	19% 10% 5% 8% 6% 3 5% 5% 5% 6% 28%	3288
	21st to end	Night	19% 12% 6% 11% 6% 3 5% 5% 5% 5% 23%	4884

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	49% 8% 10% 23% 9% 1	1518
	11th to 20th	Day	54% 10% 12% 14% 8% 2	1464
	21st to end	Day	50% 8% 12% 17% 11% 2	1607
February	1st to 10th	Day	56% 8% 12% 13% 10% 1	1641
	11th to 20th	Day	49% 9% 10% 19% 12% 2	1757
	21st to end	Day	55% 7% 8% 17% 11% 1	1480
March	1st to 10th	Day	48% 8% 11% 19% 11% 2	1800
	11th to 20th	Day	50% 7% 11% 17% 13% 3	1852
	21st to end	Day	50% 6% 7% 16% 16% 4	2376
April	1st to 10th	Day	51% 6% 8% 21% 11% 3	2160
	11th to 20th	Day	46% 7% 9% 21% 15% 2	2134
	21st to end	Day	51% 8% 8% 18% 12% 3	2269
May	1st to 10th	Day	56% 6% 6% 18% 11% 2	2340
	11th to 20th	Day	44% 6% 10% 23% 15% 2	2464
	21st to end	Day	42% 9% 15% 21% 11% 2	2725
June	1st to 10th	Day	48% 8% 13% 21% 9% 1	2520
	11th to 20th	Day	45% 8% 14% 21% 11% 1	2520
	21st to end	Day	49% 7% 14% 19% 10% 1	2520
July	1st to 10th	Day	43% 6% 13% 23% 13% 1	2450
	11th to 20th	Day	40% 9% 15% 25% 10%	2445
	21st to end	Day	43% 8% 15% 19% 14% 1	2591
August	1st to 10th	Day	40% 9% 15% 24% 11% 1	2445
	11th to 20th	Day	44% 9% 14% 23% 9% 1	2159
	21st to end	Day	40% 7% 13% 25% 13% 1	2376
September	1st to 10th	Day	43% 6% 12% 25% 13% 1	2160
	11th to 20th	Day	38% 7% 9% 23% 21% 2	2048
	21st to end	Day	41% 7% 11% 23% 15% 3	1980
October	1st to 10th	Day	53% 5% 8% 19% 12% 3	1926
	11th to 20th	Day	42% 7% 14% 24% 11% 1	1779
	21st to end	Day	55% 7% 10% 15% 11% 3	1782
November	1st to 10th	Day	39% 6% 14% 25% 13% 2	1671
	11th to 20th	Day	44% 10% 10% 20% 15% 2	1710
	21st to end	Day	47% 7% 14% 21% 9% 2	1620
December	1st to 10th	Day	56% 7% 13% 13% 11% 1	1520
	11th to 20th	Day	49% 8% 12% 18% 11% 1	1906
	21st to end	Day	58% 9% 10% 12% 11%	2772
January	1st to 10th	Night	47% 9% 12% 18% 13% 2	2649
	11th to 20th	Night	53% 11% 11% 15% 8% 2	2574
	21st to end	Night	47% 8% 10% 18% 14% 2	2745
February	1st to 10th	Night	52% 9% 12% 17% 9% 2	2318
	11th to 20th	Night	48% 9% 11% 18% 12% 2	2100
	21st to end	Night	49% 8% 10% 21% 9% 3	1776
March	1st to 10th	Night	46% 9% 11% 20% 11% 2	2160
	11th to 20th	Night	47% 9% 11% 20% 10% 3	2106
	21st to end	Night	46% 7% 8% 18% 17% 4	1980
April	1st to 10th	Night	49% 7% 10% 20% 12% 2	1800
	11th to 20th	Night	47% 6% 10% 21% 15% 1	1775
	21st to end	Night	48% 7% 10% 18% 14% 4	1689
May	1st to 10th	Night	54% 6% 9% 16% 13% 2	1620
	11th to 20th	Night	45% 7% 11% 19% 14% 2	1448
	21st to end	Night	42% 9% 15% 21% 12% 1	1551
June	1st to 10th	Night	44% 7% 13% 25% 11%	1440
	11th to 20th	Night	45% 7% 16% 21% 11%	1440
	21st to end	Night	38% 9% 15% 24% 12% 1	1440
July	1st to 10th	Night	36% 7% 16% 26% 14% 1	1400
	11th to 20th	Night	38% 8% 15% 29% 9%	1392
	21st to end	Night	34% 8% 18% 24% 14% 2	1486
August	1st to 10th	Night	33% 8% 18% 29% 11% 1	1450
	11th to 20th	Night	40% 8% 18% 24% 9%	1739
	21st to end	Night	36% 7% 12% 28% 15% 1	1980
September	1st to 10th	Night	39% 7% 13% 25% 16% 1	1800
	11th to 20th	Night	35% 6% 11% 26% 21% 1	1912
	21st to end	Night	40% 6% 8% 25% 18% 2	1980
October	1st to 10th	Night	48% 6% 11% 22% 12% 2	2029
	11th to 20th	Night	42% 7% 14% 24% 12% 1	2179
	21st to end	Night	52% 6% 11% 19% 10% 1	2574
November	1st to 10th	Night	40% 7% 11% 21% 19% 3	2407
	11th to 20th	Night	45% 9% 11% 19% 15% 2	2470
	21st to end	Night	44% 9% 14% 22% 10%	2557
December	1st to 10th	Night	51% 11% 12% 17% 9%	2660
	11th to 20th	Night	48% 14% 13% 14% 11% 1	3288
	21st to end	Night	56% 11% 8% 16% 9%	4884

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	28% 30% 30% 8% 3 1	180
	11th to 20th	Day	31% 34% 25% 8% 2	166
	21st to end	Day	22% 33% 30% 12% 2 1	187
February	1st to 10th	Day	37% 36% 22% 3 2	261
	11th to 20th	Day	29% 33% 27% 9% 1	416
	21st to end	Day	32% 28% 27% 12% 1	346
March	1st to 10th	Day	28% 27% 31% 11% 2	420
	11th to 20th	Day	31% 29% 27% 9% 3 1	420
	21st to end	Day	33% 27% 23% 10% 5% 3	462
April	1st to 10th	Day	33% 24% 25% 11% 6% 1	420
	11th to 20th	Day	28% 26% 28% 12% 5% 2	412
	21st to end	Day	30% 22% 30% 13% 4 2	420
May	1st to 10th	Day	36% 21% 24% 11% 4 3	423
	11th to 20th	Day	21% 24% 34% 15% 4 1	447
	21st to end	Day	20% 14% 31% 21% 11% 2	485
June	1st to 10th	Day	18% 18% 32% 20% 10% 2	450
	11th to 20th	Day	21% 16% 31% 21% 10% 1	450
	21st to end	Day	19% 17% 34% 19% 9% 2	450
July	1st to 10th	Day	17% 10% 32% 27% 12% 2	445
	11th to 20th	Day	14% 7% 36% 31% 10% 2	444
	21st to end	Day	13% 11% 39% 25% 10% 2	488
August	1st to 10th	Day	13% 11% 28% 31% 12% 5%	446
	11th to 20th	Day	16% 10% 29% 27% 12% 7%	439
	21st to end	Day	14% 14% 26% 26% 14% 6%	495
September	1st to 10th	Day	18% 12% 26% 23% 16% 6%	450
	11th to 20th	Day	18% 14% 25% 20% 13% 9%	450
	21st to end	Day	19% 16% 28% 20% 12% 6%	450
October	1st to 10th	Day	28% 15% 26% 15% 12% 4	450
	11th to 20th	Day	21% 19% 30% 21% 8% 1	439
	21st to end	Day	33% 19% 26% 17% 5% 1	396
November	1st to 10th	Day	23% 23% 24% 21% 8% 1	365
	11th to 20th	Day	29% 28% 24% 12% 5% 1	370
	21st to end	Day	30% 24% 26% 17% 2 1	280
December	1st to 10th	Day	32% 31% 24% 10% 3	180
	11th to 20th	Day	38% 24% 30% 6% 2	180
	21st to end	Day	29% 31% 27% 10% 2 1	198
January	1st to 10th	Night	23% 29% 31% 14% 2	760
	11th to 20th	Night	26% 26% 30% 15% 2	736
	21st to end	Night	19% 31% 29% 18% 3	792
February	1st to 10th	Night	25% 27% 33% 14% 2	720
	11th to 20th	Night	23% 31% 31% 14% 1	703
	21st to end	Night	29% 30% 26% 14% 2	592
March	1st to 10th	Night	26% 27% 27% 17% 2	720
	11th to 20th	Night	23% 28% 29% 17% 3 1	720
	21st to end	Night	24% 25% 26% 18% 5% 1	792
April	1st to 10th	Night	25% 23% 31% 17% 4	720
	11th to 20th	Night	23% 22% 29% 19% 6%	711
	21st to end	Night	28% 22% 27% 20% 4	609
May	1st to 10th	Night	28% 16% 27% 21% 8%	540
	11th to 20th	Night	20% 17% 32% 26% 5%	386
	21st to end	Night	20% 8% 28% 30% 14% 1	389
June	1st to 10th	Night	21% 10% 30% 28% 11% 1	360
	11th to 20th	Night	19% 9% 26% 35% 10% 1	360
	21st to end	Night	17% 7% 27% 36% 13% 1	360
July	1st to 10th	Night	15% 6% 24% 39% 15% 1	350
	11th to 20th	Night	16% 3 23% 41% 16% 1	348
	21st to end	Night	10% 7% 25% 40% 17%	394
August	1st to 10th	Night	12% 4 24% 42% 18% 1	386
	11th to 20th	Night	16% 7% 22% 33% 19% 3	675
	21st to end	Night	16% 7% 22% 33% 20% 2	792
September	1st to 10th	Night	15% 9% 28% 31% 15% 2	720
	11th to 20th	Night	15% 11% 24% 27% 18% 5%	720
	21st to end	Night	20% 12% 24% 27% 14% 3	720
October	1st to 10th	Night	27% 12% 25% 23% 11% 2	720
	11th to 20th	Night	21% 18% 26% 25% 9% 1	720
	21st to end	Night	27% 20% 27% 20% 5% 1	792
November	1st to 10th	Night	19% 21% 29% 22% 8%	740
	11th to 20th	Night	24% 26% 24% 17% 7% 1	760
	21st to end	Night	22% 25% 29% 19% 4 1	760
December	1st to 10th	Night	25% 29% 27% 14% 4	760
	11th to 20th	Night	25% 28% 28% 16% 3	760
	21st to end	Night	27% 32% 26% 14% 2	836

Page generated at 2024-01-11 13:32:03. Version ac07

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