Black Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Star Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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	17% 9% 4 7% 6% 3 5% 5% 5% 7% 33%	74804
	Night	17% 9% 4 7% 6% 3 5% 5% 5% 6% 33%	74994
Transparency	Day	50% 4 11% 21% 12% 2	74509
	Night	50% 4 12% 21% 12% 1	74823
Seeing	Day	26% 19% 28% 18% 8% 2	13770
	Night	24% 18% 27% 22% 7% 1	22925

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	12% 13% 5% 9% 7% 4 6% 5% 6% 7% 26%	1517
	11th to 20th	Day	16% 13% 5% 10% 8% 4 5% 6% 4 6% 24%	1486
	21st to end	Day	12% 11% 6% 9% 8% 5% 5% 5% 5% 6% 28%	1617
February	1st to 10th	Day	17% 12% 5% 9% 8% 4 7% 5% 6% 5% 22%	1630
	11th to 20th	Day	13% 10% 7% 11% 8% 3 5% 5% 4 6% 28%	1748
	21st to end	Day	19% 13% 5% 10% 6% 4 4 3 4 4 28%	1480
March	1st to 10th	Day	15% 10% 5% 10% 6% 3 5% 4 5% 7% 30%	1797
	11th to 20th	Day	16% 11% 6% 9% 6% 3 4 5% 5% 6% 30%	1854
	21st to end	Day	19% 9% 5% 7% 6% 3 5% 4 3 5% 35%	2376
April	1st to 10th	Day	17% 12% 4 8% 6% 3 4 4 5% 5% 32%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 6% 5% 5% 6% 35%	2134
	21st to end	Day	20% 9% 4 6% 7% 4 5% 5% 5% 5% 31%	2261
May	1st to 10th	Day	24% 10% 3 6% 6% 3 5% 4 5% 5% 30%	2340
	11th to 20th	Day	20% 6% 3 5% 4 3 4 4 6% 7% 38%	2474
	21st to end	Day	16% 6% 2 6% 5% 3 5% 6% 7% 8% 37%	2725
June	1st to 10th	Day	18% 6% 3 6% 7% 4 5% 6% 6% 9% 30%	2520
	11th to 20th	Day	20% 6% 2 6% 5% 4 4 5% 6% 10% 33%	2520
	21st to end	Day	17% 7% 3 7% 6% 4 6% 5% 5% 8% 31%	2520
July	1st to 10th	Day	15% 5% 3 5% 6% 4 6% 5% 7% 8% 36%	2520
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 5% 6% 10% 38%	2520
	21st to end	Day	14% 6% 3 6% 6% 4 5% 6% 7% 8% 35%	2744
August	1st to 10th	Day	12% 7% 3 6% 5% 3 6% 6% 7% 9% 35%	2470
	11th to 20th	Day	14% 8% 3 6% 7% 4 6% 5% 7% 8% 33%	2148
	21st to end	Day	14% 5% 3 6% 6% 3 5% 5% 6% 9% 38%	2376
September	1st to 10th	Day	15% 6% 3 6% 5% 3 4 5% 6% 7% 40%	2160
	11th to 20th	Day	16% 6% 2 5% 4 2 3 4 5% 7% 45%	2056
	21st to end	Day	17% 6% 2 5% 4 2 4 5% 6% 7% 41%	1980
October	1st to 10th	Day	23% 9% 3 7% 5% 3 4 3 4 4 35%	1917
	11th to 20th	Day	18% 8% 3 5% 6% 2 4 5% 5% 7% 37%	1787
	21st to end	Day	24% 12% 4 7% 5% 3 4 4 4 5% 28%	1782
November	1st to 10th	Day	13% 8% 4 7% 5% 2 3 3 4 7% 44%	1671
	11th to 20th	Day	18% 10% 4 6% 6% 2 4 4 6% 6% 33%	1710
	21st to end	Day	17% 11% 5% 7% 6% 3 5% 4 5% 6% 32%	1606
December	1st to 10th	Day	19% 13% 6% 9% 7% 4 5% 4 5% 5% 24%	1520
	11th to 20th	Day	20% 13% 6% 6% 6% 2 4 3 3 4 32%	1906
	21st to end	Day	19% 14% 6% 11% 6% 4 4 4 5% 5% 22%	2772
January	1st to 10th	Night	14% 13% 5% 9% 8% 3 6% 4 4 6% 29%	2649
	11th to 20th	Night	18% 11% 4 10% 8% 5% 6% 5% 5% 5% 23%	2602
	21st to end	Night	11% 10% 6% 9% 7% 4 6% 5% 6% 6% 28%	2736
February	1st to 10th	Night	16% 10% 5% 8% 8% 4 7% 6% 6% 6% 24%	2328
	11th to 20th	Night	13% 9% 6% 9% 7% 4 6% 6% 6% 6% 26%	2098
	21st to end	Night	17% 12% 4 9% 7% 3 5% 4 6% 5% 29%	1776
March	1st to 10th	Night	16% 10% 5% 9% 6% 4 5% 5% 5% 5% 31%	2150
	11th to 20th	Night	15% 11% 4 8% 9% 4 6% 5% 5% 5% 30%	2099
	21st to end	Night	17% 11% 4 6% 5% 3 4 4 4 5% 36%	1980
April	1st to 10th	Night	18% 11% 4 6% 6% 3 5% 5% 6% 6% 31%	1800
	11th to 20th	Night	17% 10% 3 6% 6% 3 4 5% 4 7% 35%	1775
	21st to end	Night	21% 10% 3 5% 5% 3 4 5% 4 6% 35%	1697
May	1st to 10th	Night	23% 11% 4 5% 6% 3 4 4 5% 6% 30%	1620
	11th to 20th	Night	18% 8% 3 6% 5% 4 5% 4 5% 7% 35%	1433
	21st to end	Night	17% 7% 2 6% 5% 3 4 4 6% 8% 39%	1550
June	1st to 10th	Night	20% 6% 3 6% 6% 2 4 3 5% 7% 38%	1440
	11th to 20th	Night	18% 6% 3 5% 5% 3 6% 4 5% 8% 37%	1440
	21st to end	Night	14% 5% 2 4 5% 4 5% 6% 6% 8% 41%	1440
July	1st to 10th	Night	13% 5% 3 5% 5% 3 4 5% 6% 8% 43%	1440
	11th to 20th	Night	15% 5% 2 5% 4 3 5% 7% 7% 9% 38%	1440
	21st to end	Night	12% 6% 2 4 4 3 6% 6% 4 8% 45%	1574
August	1st to 10th	Night	11% 5% 2 5% 4 3 5% 5% 6% 8% 46%	1450
	11th to 20th	Night	12% 6% 2 6% 7% 4 6% 5% 6% 8% 38%	1749
	21st to end	Night	14% 6% 2 4 5% 2 5% 4 5% 8% 44%	1980
September	1st to 10th	Night	14% 7% 2 6% 5% 3 5% 5% 5% 6% 44%	1800
	11th to 20th	Night	15% 5% 2 4 3 2 3 3 3 7% 51%	1903
	21st to end	Night	17% 6% 3 6% 5% 2 3 4 4 6% 44%	1980
October	1st to 10th	Night	22% 8% 2 5% 5% 3 4 3 5% 5% 37%	2042
	11th to 20th	Night	16% 8% 3 5% 6% 3 4 5% 5% 6% 40%	2171
	21st to end	Night	22% 12% 3 7% 6% 3 4 4 4 6% 30%	2574
November	1st to 10th	Night	14% 9% 3 6% 4 2 4 4 5% 6% 42%	2407
	11th to 20th	Night	18% 9% 4 7% 5% 3 5% 4 4 6% 34%	2470
	21st to end	Night	13% 11% 4 8% 6% 4 5% 5% 5% 6% 33%	2569
December	1st to 10th	Night	19% 13% 5% 10% 7% 3 4 5% 5% 6% 23%	2660
	11th to 20th	Night	20% 12% 5% 10% 6% 3 4 5% 4 5% 25%	3288
	21st to end	Night	20% 14% 7% 12% 6% 3 4 4 4 4 22%	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	54% 3 10% 22% 10% 1	1518
	11th to 20th	Day	60% 4 12% 15% 7% 2	1486
	21st to end	Day	54% 4 12% 17% 11% 2	1617
February	1st to 10th	Day	60% 5% 11% 14% 9% 1	1630
	11th to 20th	Day	54% 5% 8% 19% 12% 2	1757
	21st to end	Day	59% 3 8% 17% 11% 1	1480
March	1st to 10th	Day	52% 3 12% 20% 11% 2	1800
	11th to 20th	Day	53% 3 10% 18% 13% 2	1854
	21st to end	Day	53% 2 7% 17% 16% 4	2376
April	1st to 10th	Day	54% 3 8% 21% 11% 2	2160
	11th to 20th	Day	49% 4 10% 22% 15% 1	2134
	21st to end	Day	54% 3 9% 19% 12% 3	2261
May	1st to 10th	Day	58% 3 6% 19% 11% 2	2340
	11th to 20th	Day	45% 4 9% 26% 15% 2	2478
	21st to end	Day	44% 6% 16% 22% 11% 2	2725
June	1st to 10th	Day	50% 5% 13% 22% 9%	2520
	11th to 20th	Day	48% 4 14% 22% 11%	2520
	21st to end	Day	50% 5% 13% 21% 10% 1	2520
July	1st to 10th	Day	44% 5% 12% 24% 14% 1	2450
	11th to 20th	Day	42% 5% 15% 27% 11%	2445
	21st to end	Day	45% 5% 15% 21% 14% 1	2591
August	1st to 10th	Day	42% 6% 16% 25% 11% 1	2456
	11th to 20th	Day	46% 5% 15% 24% 9% 1	2148
	21st to end	Day	41% 5% 11% 27% 14% 1	2376
September	1st to 10th	Day	44% 4 12% 25% 14% 1	2160
	11th to 20th	Day	40% 4 8% 24% 21% 2	2056
	21st to end	Day	43% 5% 10% 25% 15% 2	1980
October	1st to 10th	Day	54% 3 7% 21% 12% 3	1917
	11th to 20th	Day	45% 3 14% 25% 11% 1	1787
	21st to end	Day	57% 4 12% 14% 10% 3	1782
November	1st to 10th	Day	41% 4 15% 24% 14% 2	1671
	11th to 20th	Day	50% 4 9% 21% 15% 2	1710
	21st to end	Day	51% 3 13% 22% 9% 1	1606
December	1st to 10th	Day	59% 4 12% 14% 10% 1	1520
	11th to 20th	Day	54% 3 12% 19% 10% 1	1906
	21st to end	Day	64% 3 10% 13% 10%	2772
January	1st to 10th	Night	54% 3 11% 18% 13% 2	2649
	11th to 20th	Night	59% 4 12% 15% 8% 2	2602
	21st to end	Night	52% 3 10% 20% 13% 2	2736
February	1st to 10th	Night	57% 4 12% 17% 8% 2	2328
	11th to 20th	Night	54% 5% 10% 17% 12% 1	2100
	21st to end	Night	54% 3 11% 19% 10% 2	1776
March	1st to 10th	Night	52% 4 10% 21% 11% 2	2160
	11th to 20th	Night	53% 4 10% 21% 10% 2	2100
	21st to end	Night	50% 3 8% 18% 17% 4	1980
April	1st to 10th	Night	52% 3 10% 21% 11% 2	1800
	11th to 20th	Night	49% 4 9% 22% 15% 1	1775
	21st to end	Night	50% 4 10% 20% 14% 3	1697
May	1st to 10th	Night	56% 3 9% 17% 14% 2	1620
	11th to 20th	Night	48% 3 11% 21% 14% 2	1434
	21st to end	Night	45% 5% 15% 22% 11% 1	1551
June	1st to 10th	Night	46% 5% 11% 26% 11%	1440
	11th to 20th	Night	47% 4 16% 22% 11%	1440
	21st to end	Night	39% 7% 15% 26% 12% 1	1440
July	1st to 10th	Night	37% 5% 15% 28% 14% 1	1400
	11th to 20th	Night	41% 5% 15% 30% 10%	1392
	21st to end	Night	36% 6% 17% 25% 14% 2	1486
August	1st to 10th	Night	36% 4 18% 31% 11% 1	1440
	11th to 20th	Night	42% 5% 17% 27% 8%	1749
	21st to end	Night	38% 4 12% 29% 15% 1	1980
September	1st to 10th	Night	40% 5% 12% 27% 15% 1	1800
	11th to 20th	Night	36% 5% 10% 26% 22% 1	1903
	21st to end	Night	41% 4 10% 26% 18% 2	1980
October	1st to 10th	Night	49% 4 11% 22% 12% 2	2042
	11th to 20th	Night	44% 4 14% 25% 11% 1	2171
	21st to end	Night	54% 3 11% 20% 10% 1	2574
November	1st to 10th	Night	42% 4 11% 22% 18% 3	2407
	11th to 20th	Night	50% 4 10% 20% 14% 3	2470
	21st to end	Night	50% 3 15% 23% 9% 1	2569
December	1st to 10th	Night	58% 4 12% 17% 9%	2660
	11th to 20th	Night	58% 5% 12% 14% 10% 1	3288
	21st to end	Night	64% 4 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	32% 27% 31% 6% 3 1	180
	11th to 20th	Day	37% 26% 28% 8% 1 1	172
	21st to end	Day	25% 32% 29% 10% 3 1	191
February	1st to 10th	Day	42% 30% 24% 2 2	250
	11th to 20th	Day	35% 29% 24% 10% 2	416
	21st to end	Day	33% 27% 29% 9% 1	346
March	1st to 10th	Day	30% 26% 31% 12% 1	420
	11th to 20th	Day	33% 26% 26% 11% 4	420
	21st to end	Day	34% 26% 22% 11% 4 3	462
April	1st to 10th	Day	37% 23% 22% 13% 5% 1	420
	11th to 20th	Day	30% 25% 25% 13% 5% 1	412
	21st to end	Day	31% 21% 30% 12% 3 2	420
May	1st to 10th	Day	35% 21% 26% 11% 7% 1	423
	11th to 20th	Day	24% 19% 35% 17% 4 1	447
	21st to end	Day	21% 14% 28% 25% 10% 3	485
June	1st to 10th	Day	18% 18% 33% 19% 10% 2	450
	11th to 20th	Day	23% 14% 29% 20% 12% 1	450
	21st to end	Day	20% 16% 33% 19% 10% 2	450
July	1st to 10th	Day	16% 9% 32% 28% 12% 2	445
	11th to 20th	Day	15% 7% 32% 32% 12% 2	444
	21st to end	Day	14% 11% 38% 26% 9% 2	488
August	1st to 10th	Day	13% 12% 27% 31% 12% 5%	446
	11th to 20th	Day	15% 9% 31% 24% 14% 6%	439
	21st to end	Day	15% 13% 26% 27% 14% 6%	495
September	1st to 10th	Day	19% 12% 24% 24% 15% 6%	450
	11th to 20th	Day	20% 13% 24% 19% 14% 10%	450
	21st to end	Day	21% 14% 28% 21% 10% 6%	450
October	1st to 10th	Day	29% 13% 25% 18% 10% 4	450
	11th to 20th	Day	25% 18% 30% 19% 8% 2	444
	21st to end	Day	35% 19% 22% 19% 5% 1	396
November	1st to 10th	Day	24% 22% 25% 20% 9%	365
	11th to 20th	Day	31% 26% 24% 14% 4 1	370
	21st to end	Day	33% 23% 25% 16% 2 1	266
December	1st to 10th	Day	33% 28% 26% 9% 4	180
	11th to 20th	Day	39% 26% 27% 7% 1	180
	21st to end	Day	37% 24% 27% 10% 2 1	198
January	1st to 10th	Night	25% 28% 32% 13% 2	760
	11th to 20th	Night	31% 23% 28% 15% 3	744
	21st to end	Night	23% 26% 32% 16% 2	792
February	1st to 10th	Night	28% 26% 31% 13% 2	720
	11th to 20th	Night	26% 28% 31% 13% 1	703
	21st to end	Night	31% 27% 25% 14% 2	592
March	1st to 10th	Night	28% 25% 30% 15% 2	720
	11th to 20th	Night	26% 26% 29% 16% 3	720
	21st to end	Night	29% 21% 25% 19% 6% 1	792
April	1st to 10th	Night	28% 21% 30% 17% 4	720
	11th to 20th	Night	26% 22% 26% 19% 6%	711
	21st to end	Night	29% 19% 28% 19% 5%	617
May	1st to 10th	Night	29% 15% 29% 20% 7%	540
	11th to 20th	Night	23% 13% 34% 26% 4	372
	21st to end	Night	18% 8% 29% 31% 13% 1	389
June	1st to 10th	Night	20% 10% 29% 28% 12%	360
	11th to 20th	Night	19% 8% 28% 34% 11% 1	360
	21st to end	Night	14% 9% 26% 39% 12% 1	360
July	1st to 10th	Night	16% 4 24% 39% 15% 1	350
	11th to 20th	Night	15% 6% 22% 42% 15% 1	348
	21st to end	Night	13% 5% 24% 42% 16%	394
August	1st to 10th	Night	13% 3 25% 41% 17% 1	376
	11th to 20th	Night	16% 5% 21% 35% 20% 3	685
	21st to end	Night	16% 6% 23% 34% 19% 2	792
September	1st to 10th	Night	16% 9% 28% 29% 16% 2	720
	11th to 20th	Night	17% 10% 26% 24% 18% 5%	720
	21st to end	Night	21% 11% 25% 28% 13% 3	720
October	1st to 10th	Night	28% 10% 24% 25% 10% 3	720
	11th to 20th	Night	23% 15% 30% 23% 8% 1	720
	21st to end	Night	28% 20% 28% 19% 5% 1	792
November	1st to 10th	Night	21% 21% 28% 22% 8%	740
	11th to 20th	Night	25% 24% 27% 16% 6% 1	760
	21st to end	Night	25% 23% 28% 19% 4 1	760
December	1st to 10th	Night	29% 27% 24% 15% 4	760
	11th to 20th	Night	32% 23% 27% 16% 3	760
	21st to end	Night	31% 28% 26% 14% 2	836

Page generated at 2024-01-13 19:28:31. Version ac07

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