Blackhole Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackhole Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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% 8% 3 6% 5% 3 5% 5% 5% 7% 37%	74963
	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 39%	74736
Transparency	Day	46% 6% 11% 22% 14% 2	74686
	Night	45% 6% 11% 21% 15% 3	74532
Seeing	Day	19% 12% 25% 22% 15% 7%	13734
	Night	19% 9% 25% 29% 16% 2	23782

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	17% 11% 6% 9% 7% 4 4 4 5% 5% 28%	1706
	11th to 20th	Day	21% 14% 5% 11% 6% 2 3 5% 4 4 25%	1644
	21st to end	Day	18% 13% 6% 8% 6% 3 5% 3 4 6% 29%	1782
February	1st to 10th	Day	20% 12% 4 9% 6% 3 4 4 5% 5% 28%	1696
	11th to 20th	Day	19% 11% 5% 8% 6% 3 5% 4 5% 5% 29%	1748
	21st to end	Day	19% 11% 5% 7% 6% 3 4 5% 5% 6% 31%	1480
March	1st to 10th	Day	19% 9% 3 6% 4 3 4 5% 5% 6% 36%	1843
	11th to 20th	Day	19% 9% 4 7% 6% 3 5% 5% 5% 5% 32%	1980
	21st to end	Day	19% 10% 4 7% 6% 3 5% 4 4 7% 32%	2178
April	1st to 10th	Day	14% 9% 3 7% 5% 2 4 4 5% 8% 38%	2069
	11th to 20th	Day	17% 7% 3 6% 5% 3 4 5% 5% 6% 39%	2201
	21st to end	Day	18% 7% 3 6% 5% 4 5% 5% 6% 6% 35%	2340
May	1st to 10th	Day	19% 7% 3 5% 6% 3 5% 5% 6% 7% 34%	2340
	11th to 20th	Day	18% 7% 4 7% 6% 4 5% 4 6% 7% 33%	2301
	21st to end	Day	9% 5% 2 7% 6% 4 6% 6% 7% 10% 38%	2531
June	1st to 10th	Day	11% 5% 2 5% 6% 4 5% 5% 6% 8% 43%	2340
	11th to 20th	Day	9% 4 3 6% 6% 4 5% 6% 7% 9% 41%	2340
	21st to end	Day	8% 4 3 7% 7% 4 6% 5% 7% 8% 40%	2413
July	1st to 10th	Day	12% 6% 3 5% 5% 3 5% 6% 7% 9% 37%	2349
	11th to 20th	Day	9% 5% 2 6% 6% 4 7% 6% 6% 10% 39%	2340
	21st to end	Day	12% 5% 2 5% 5% 4 5% 6% 7% 8% 38%	2547
August	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 6% 7% 11% 40%	2327
	11th to 20th	Day	9% 6% 3 6% 7% 4 6% 6% 8% 10% 35%	2303
	21st to end	Day	9% 5% 2 5% 5% 3 5% 6% 7% 10% 43%	2423
September	1st to 10th	Day	14% 6% 2 4 4 3 4 4 5% 8% 45%	2015
	11th to 20th	Day	10% 4 2 5% 5% 3 5% 4 5% 8% 48%	1980
	21st to end	Day	14% 6% 3 5% 4 2 4 4 4 7% 47%	1980
October	1st to 10th	Day	12% 5% 2 4 5% 2 4 4 5% 8% 50%	1980
	11th to 20th	Day	14% 6% 3 5% 4 2 4 4 5% 5% 47%	1807
	21st to end	Day	20% 9% 2 6% 4 2 4 3 4 6% 41%	1901
November	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 3 6% 54%	1672
	11th to 20th	Day	15% 9% 4 6% 5% 2 4 3 5% 6% 41%	1710
	21st to end	Day	18% 8% 4 6% 5% 2 3 4 4 5% 40%	1710
December	1st to 10th	Day	23% 12% 5% 6% 6% 3 5% 4 4 5% 28%	1710
	11th to 20th	Day	17% 12% 4 8% 6% 3 5% 4 4 5% 32%	2153
	21st to end	Day	18% 17% 5% 9% 6% 4 5% 5% 4 6% 20%	3124
January	1st to 10th	Night	17% 11% 5% 11% 6% 4 4 4 5% 5% 28%	2459
	11th to 20th	Night	21% 13% 5% 9% 6% 3 4 4 4 4 26%	2378
	21st to end	Night	18% 12% 5% 9% 6% 3 4 3 4 5% 31%	2574
February	1st to 10th	Night	23% 12% 5% 7% 6% 2 4 4 4 5% 27%	2259
	11th to 20th	Night	20% 11% 4 8% 7% 3 4 4 4 5% 30%	2098
	21st to end	Night	24% 13% 5% 6% 5% 3 3 4 5% 4 28%	1776
March	1st to 10th	Night	16% 9% 4 6% 5% 3 5% 4 5% 5% 40%	2116
	11th to 20th	Night	19% 9% 4 5% 6% 3 4 4 5% 6% 34%	1980
	21st to end	Night	15% 8% 4 8% 6% 3 4 4 5% 8% 34%	2178
April	1st to 10th	Night	13% 6% 2 5% 5% 4 5% 6% 6% 8% 39%	1906
	11th to 20th	Night	15% 6% 3 5% 5% 3 5% 4 5% 7% 42%	1706
	21st to end	Night	17% 6% 3 7% 4 3 5% 5% 5% 8% 38%	1620
May	1st to 10th	Night	17% 7% 3 6% 6% 3 5% 5% 6% 8% 34%	1620
	11th to 20th	Night	15% 7% 3 6% 5% 4 5% 5% 6% 8% 37%	1592
	21st to end	Night	8% 5% 2 5% 6% 3 6% 5% 8% 10% 42%	1744
June	1st to 10th	Night	7% 5% 2 4 5% 3 5% 5% 8% 9% 48%	1620
	11th to 20th	Night	5% 5% 3 5% 6% 3 6% 6% 6% 10% 47%	1620
	21st to end	Night	8% 4 2 4 4 3 6% 6% 8% 11% 46%	1518
July	1st to 10th	Night	9% 7% 3 5% 4 4 5% 6% 6% 10% 42%	1584
	11th to 20th	Night	5% 4 2 5% 5% 4 5% 5% 7% 10% 46%	1620
	21st to end	Night	9% 4 2 5% 4 3 6% 4 7% 8% 47%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 3 5% 5% 6% 11% 46%	1611
	11th to 20th	Night	8% 5% 2 5% 6% 3 4 6% 6% 10% 44%	1598
	21st to end	Night	7% 5% 2 4 4 3 4 4 6% 9% 53%	1929
September	1st to 10th	Night	11% 5% 2 4 4 3 4 3 6% 8% 50%	1926
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 5% 8% 52%	1980
	21st to end	Night	12% 5% 3 5% 5% 2 4 4 4 5% 51%	1980
October	1st to 10th	Night	10% 5% 2 4 4 3 3 4 6% 7% 53%	1980
	11th to 20th	Night	14% 6% 2 5% 4 3 4 4 4 8% 45%	2151
	21st to end	Night	18% 8% 3 5% 5% 3 4 4 4 5% 41%	2452
November	1st to 10th	Night	13% 6% 2 4 4 2 4 4 4 5% 51%	2406
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 4 4 6% 42%	2470
	21st to end	Night	18% 10% 3 5% 5% 3 4 4 4 6% 38%	2470
December	1st to 10th	Night	22% 11% 4 6% 5% 3 5% 4 4 6% 30%	2470
	11th to 20th	Night	17% 10% 5% 8% 6% 3 4 3 4 6% 33%	3042
	21st to end	Night	18% 16% 6% 10% 8% 4 5% 4 4 6% 19%	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	57% 4 9% 19% 10% 1	1707
	11th to 20th	Day	64% 4 10% 16% 7%	1644
	21st to end	Day	58% 3 11% 20% 7% 2	1782
February	1st to 10th	Day	59% 4 9% 18% 8% 2	1696
	11th to 20th	Day	57% 4 9% 19% 9% 2	1758
	21st to end	Day	55% 5% 8% 20% 10% 3	1480
March	1st to 10th	Day	49% 4 10% 23% 13% 2	1843
	11th to 20th	Day	53% 6% 9% 18% 11% 3	1980
	21st to end	Day	53% 5% 10% 21% 9% 2	2178
April	1st to 10th	Day	44% 6% 10% 25% 12% 3	2069
	11th to 20th	Day	45% 5% 9% 20% 16% 4	2201
	21st to end	Day	48% 6% 11% 23% 10% 2	2340
May	1st to 10th	Day	48% 7% 12% 22% 9% 2	2340
	11th to 20th	Day	50% 6% 12% 19% 11% 1	2305
	21st to end	Day	39% 9% 17% 22% 12% 1	2531
June	1st to 10th	Day	37% 8% 11% 26% 16% 1	2340
	11th to 20th	Day	37% 9% 14% 23% 16% 2	2340
	21st to end	Day	39% 8% 13% 22% 15% 3	2413
July	1st to 10th	Day	41% 9% 14% 18% 16% 2	2284
	11th to 20th	Day	39% 9% 13% 22% 15% 2	2271
	21st to end	Day	41% 7% 15% 23% 12% 2	2405
August	1st to 10th	Day	37% 9% 14% 26% 13% 1	2314
	11th to 20th	Day	41% 8% 14% 24% 13% 1	2303
	21st to end	Day	34% 8% 14% 24% 16% 4	2423
September	1st to 10th	Day	39% 6% 12% 20% 18% 6%	2015
	11th to 20th	Day	34% 6% 11% 24% 23% 2	1980
	21st to end	Day	37% 4 9% 26% 21% 2	1980
October	1st to 10th	Day	33% 6% 11% 24% 24% 3	1980
	11th to 20th	Day	38% 4 9% 26% 20% 3	1807
	21st to end	Day	46% 3 9% 24% 16% 2	1901
November	1st to 10th	Day	34% 4 8% 28% 22% 4	1672
	11th to 20th	Day	45% 4 7% 22% 18% 4	1710
	21st to end	Day	47% 3 10% 21% 17% 2	1710
December	1st to 10th	Day	58% 5% 11% 17% 9% 1	1710
	11th to 20th	Day	54% 5% 8% 18% 14% 1	2150
	21st to end	Day	64% 5% 10% 14% 6% 1	3124
January	1st to 10th	Night	58% 5% 8% 17% 11% 1	2459
	11th to 20th	Night	62% 4 9% 16% 8% 1	2378
	21st to end	Night	56% 4 9% 20% 8% 2	2574
February	1st to 10th	Night	59% 5% 10% 19% 6% 1	2259
	11th to 20th	Night	56% 4 8% 20% 11% 1	2099
	21st to end	Night	59% 4 10% 15% 9% 3	1776
March	1st to 10th	Night	47% 5% 7% 21% 17% 2	2116
	11th to 20th	Night	50% 4 12% 21% 12% 2	1980
	21st to end	Night	49% 5% 12% 21% 11% 2	2178
April	1st to 10th	Night	40% 8% 10% 24% 16% 2	1906
	11th to 20th	Night	40% 5% 11% 19% 18% 7%	1706
	21st to end	Night	45% 6% 12% 21% 13% 3	1620
May	1st to 10th	Night	47% 8% 12% 20% 11% 3	1620
	11th to 20th	Night	44% 8% 14% 20% 12% 2	1593
	21st to end	Night	35% 9% 16% 20% 17% 2	1745
June	1st to 10th	Night	30% 8% 16% 26% 16% 4	1620
	11th to 20th	Night	30% 9% 14% 23% 20% 4	1620
	21st to end	Night	30% 10% 16% 25% 16% 4	1518
July	1st to 10th	Night	37% 9% 14% 19% 17% 4	1539
	11th to 20th	Night	31% 7% 15% 24% 20% 4	1566
	21st to end	Night	33% 8% 15% 27% 15% 2	1672
August	1st to 10th	Night	32% 6% 15% 28% 17% 2	1600
	11th to 20th	Night	33% 9% 15% 25% 17% 1	1598
	21st to end	Night	27% 8% 13% 27% 19% 5%	1929
September	1st to 10th	Night	33% 6% 14% 23% 17% 7%	1926
	11th to 20th	Night	31% 6% 11% 24% 25% 3	1980
	21st to end	Night	37% 5% 7% 22% 26% 3	1980
October	1st to 10th	Night	30% 4 12% 27% 22% 5%	1980
	11th to 20th	Night	38% 5% 9% 24% 21% 3	2151
	21st to end	Night	45% 4 8% 22% 18% 3	2452
November	1st to 10th	Night	36% 4 9% 24% 23% 4	2406
	11th to 20th	Night	44% 4 8% 19% 20% 5%	2470
	21st to end	Night	48% 4 9% 23% 14% 2	2470
December	1st to 10th	Night	55% 5% 11% 18% 10% 1	2470
	11th to 20th	Night	53% 3 9% 22% 12% 1	3044
	21st to end	Night	66% 4 9% 14% 7%	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	28% 20% 34% 13% 4	270
	11th to 20th	Day	33% 24% 31% 10% 1	248
	21st to end	Day	28% 25% 33% 11% 2	264
February	1st to 10th	Day	33% 22% 33% 10% 3	240
	11th to 20th	Day	35% 28% 29% 8%	240
	21st to end	Day	36% 20% 34% 9% 2	198
March	1st to 10th	Day	28% 25% 33% 11% 2	283
	11th to 20th	Day	25% 23% 31% 16% 5%	420
	21st to end	Day	30% 18% 29% 16% 7%	462
April	1st to 10th	Day	22% 17% 29% 21% 9% 1	420
	11th to 20th	Day	23% 19% 25% 21% 9% 4	412
	21st to end	Day	25% 15% 33% 17% 9% 2	420
May	1st to 10th	Day	23% 17% 29% 18% 8% 5%	423
	11th to 20th	Day	18% 10% 30% 24% 13% 6%	447
	21st to end	Day	15% 6% 23% 25% 20% 10%	485
June	1st to 10th	Day	12% 6% 20% 29% 23% 9%	450
	11th to 20th	Day	11% 6% 22% 30% 21% 10%	450
	21st to end	Day	11% 6% 26% 28% 19% 11%	523
July	1st to 10th	Day	17% 4 15% 18% 23% 23%	454
	11th to 20th	Day	11% 5% 17% 25% 26% 15%	444
	21st to end	Day	14% 3 16% 28% 25% 15%	488
August	1st to 10th	Day	13% 4 16% 25% 27% 15%	446
	11th to 20th	Day	12% 5% 17% 32% 22% 12%	439
	21st to end	Day	9% 4 15% 30% 25% 16%	495
September	1st to 10th	Day	12% 4 15% 26% 28% 14%	450
	11th to 20th	Day	11% 3 15% 28% 29% 15%	450
	21st to end	Day	18% 8% 20% 24% 24% 6%	450
October	1st to 10th	Day	14% 7% 23% 29% 20% 7%	450
	11th to 20th	Day	16% 13% 31% 24% 13% 3	450
	21st to end	Day	24% 13% 33% 22% 8% 1	416
November	1st to 10th	Day	16% 14% 33% 26% 7% 3	270
	11th to 20th	Day	24% 19% 28% 20% 7% 1	270
	21st to end	Day	25% 14% 38% 17% 4 1	270
December	1st to 10th	Day	31% 18% 30% 13% 6%	270
	11th to 20th	Day	24% 17% 39% 15% 5%	270
	21st to end	Day	31% 20% 37% 11% 1	297
January	1st to 10th	Night	23% 19% 32% 22% 5%	760
	11th to 20th	Night	31% 15% 31% 20% 3	732
	21st to end	Night	25% 15% 34% 24% 2	792
February	1st to 10th	Night	31% 12% 32% 22% 3	720
	11th to 20th	Night	28% 16% 34% 21% 1	703
	21st to end	Night	33% 15% 33% 18% 1	592
March	1st to 10th	Night	24% 15% 32% 23% 6%	720
	11th to 20th	Night	25% 12% 32% 24% 7%	720
	21st to end	Night	22% 11% 32% 28% 7%	792
April	1st to 10th	Night	17% 14% 29% 31% 8%	646
	11th to 20th	Night	16% 12% 28% 29% 14% 1	533
	21st to end	Night	20% 6% 34% 30% 9% 1	540
May	1st to 10th	Night	19% 8% 30% 29% 11% 2	540
	11th to 20th	Night	18% 5% 23% 35% 17% 2	531
	21st to end	Night	10% 2 21% 33% 28% 5%	584
June	1st to 10th	Night	9% 2 16% 43% 26% 4	540
	11th to 20th	Night	7% 2 16% 40% 30% 4	540
	21st to end	Night	9% 4 19% 39% 27% 3	540
July	1st to 10th	Night	11% 2 12% 30% 34% 10%	525
	11th to 20th	Night	5% 2 13% 37% 39% 4	522
	21st to end	Night	10% 3 10% 35% 36% 5%	591
August	1st to 10th	Night	9% 2 7% 37% 38% 7%	537
	11th to 20th	Night	9% 3 9% 35% 36% 7%	534
	21st to end	Night	8% 1 12% 33% 38% 8%	594
September	1st to 10th	Night	14% 1 12% 32% 35% 6%	666
	11th to 20th	Night	11% 3 13% 32% 34% 7%	720
	21st to end	Night	14% 4 19% 34% 26% 3	720
October	1st to 10th	Night	13% 5% 20% 33% 25% 4	720
	11th to 20th	Night	17% 7% 26% 33% 16% 1	720
	21st to end	Night	22% 11% 28% 27% 11% 1	792
November	1st to 10th	Night	16% 8% 25% 31% 17% 2	740
	11th to 20th	Night	21% 12% 29% 28% 10% 1	760
	21st to end	Night	24% 9% 28% 28% 9% 1	760
December	1st to 10th	Night	29% 13% 27% 26% 5%	760
	11th to 20th	Night	24% 11% 33% 26% 6% 1	760
	21st to end	Night	28% 14% 31% 23% 4	836

Page generated at 2024-01-10 19:35:20. Version ac07

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