The Rock Bottom Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Rock Bottom Observatory clear sky chart from 2005-11-06 to 2024-05-14.

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	4 6% 2 5% 4 4 4 4 6% 8% 52%	83656
	Night	4 6% 2 5% 3 3 3 3 5% 7% 59%	85978
Transparency	Day	27% 5% 15% 25% 21% 7%	83363
	Night	25% 5% 13% 27% 23% 8%	85793
Seeing	Day	8% 16% 38% 24% 11% 2	14888
	Night	7% 15% 39% 25% 12% 2	25053

By Month

Percentage Cloud Cover SW (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	5% 3 7% 4 4 3 3 6% 8% 58%	2538
	11th to 20th	Day	4 3 7% 4 4 4 4 6% 10% 54%	2613
	21st to end	Day	1 5% 3 5% 4 4 4 4 6% 7% 57%	3146
February	1st to 10th	Day	1 5% 3 8% 3 4 3 4 5% 6% 56%	1710
	11th to 20th	Day	5% 3 4 4 3 4 4 5% 9% 60%	1796
	21st to end	Day	1 6% 2 6% 5% 3 3 3 5% 6% 59%	1570
March	1st to 10th	Day	2 8% 3 6% 3 3 4 4 5% 7% 54%	3130
	11th to 20th	Day	3 11% 3 8% 4 4 4 4 5% 8% 46%	3480
	21st to end	Day	3 8% 2 7% 4 4 5% 5% 6% 9% 47%	4036
April	1st to 10th	Day	4 7% 2 4 2 3 4 4 6% 10% 54%	2280
	11th to 20th	Day	7% 9% 4 6% 4 4 4 3 7% 9% 43%	2251
	21st to end	Day	7% 9% 2 6% 5% 5% 5% 4 6% 9% 43%	2280
May	1st to 10th	Day	10% 8% 3 4 5% 5% 5% 5% 6% 9% 40%	2428
	11th to 20th	Day	14% 8% 2 6% 4 5% 4 5% 7% 9% 36%	2522
	21st to end	Day	7% 6% 2 5% 5% 4 6% 6% 8% 10% 41%	2724
June	1st to 10th	Day	4 3 1 5% 3 4 6% 5% 9% 11% 48%	2520
	11th to 20th	Day	6% 4 2 4 3 4 5% 6% 7% 9% 52%	2520
	21st to end	Day	4 4 1 4 4 5% 5% 6% 8% 9% 50%	2520
July	1st to 10th	Day	6% 4 2 4 3 5% 6% 6% 9% 9% 45%	2520
	11th to 20th	Day	3 3 1 4 4 5% 5% 5% 8% 9% 51%	2520
	21st to end	Day	5% 3 2 6% 5% 6% 6% 6% 9% 10% 42%	2693
August	1st to 10th	Day	5% 4 1 5% 5% 6% 6% 6% 8% 11% 42%	2328
	11th to 20th	Day	5% 3 1 4 4 5% 6% 4 8% 10% 52%	2153
	21st to end	Day	5% 3 2 5% 4 4 5% 5% 7% 9% 50%	2376
September	1st to 10th	Day	5% 4 1 4 3 4 3 4 6% 8% 58%	2160
	11th to 20th	Day	6% 3 2 3 3 3 3 4 6% 9% 59%	2154
	21st to end	Day	7% 4 1 4 3 2 3 3 4 6% 63%	1971
October	1st to 10th	Day	6% 7% 2 4 3 4 4 3 5% 6% 57%	1800
	11th to 20th	Day	3 4 1 3 3 3 2 3 4 6% 67%	1800
	21st to end	Day	4 6% 2 4 3 1 3 3 5% 6% 62%	1921
November	1st to 10th	Day	2 4 1 3 3 3 3 3 5% 8% 67%	1673
	11th to 20th	Day	2 5% 2 4 2 2 3 3 7% 7% 64%	1710
	21st to end	Day	2 5% 2 5% 3 3 3 3 6% 8% 60%	1573
December	1st to 10th	Day	1 5% 2 5% 3 4 3 3 5% 9% 60%	1520
	11th to 20th	Day	2 4 2 3 3 3 4 3 6% 10% 59%	1926
	21st to end	Day	2 9% 3 8% 4 3 3 3 5% 6% 54%	2794
January	1st to 10th	Night	1 5% 4 7% 4 4 4 3 5% 8% 56%	4417
	11th to 20th	Night	1 2 2 4 3 3 5% 5% 6% 9% 62%	4384
	21st to end	Night	2 7% 3 6% 4 4 3 3 4 6% 58%	4521
February	1st to 10th	Night	1 7% 3 6% 4 4 3 3 6% 7% 57%	2470
	11th to 20th	Night	1 5% 3 6% 5% 3 4 3 5% 6% 59%	2270
	21st to end	Night	1 8% 2 5% 3 3 3 3 4 7% 60%	1884
March	1st to 10th	Night	1 7% 3 5% 3 3 3 3 5% 8% 58%	3734
	11th to 20th	Night	4 14% 3 4 3 2 2 2 4 6% 55%	3495
	21st to end	Night	3 8% 3 6% 4 4 3 3 4 8% 54%	3610
April	1st to 10th	Night	3 7% 2 5% 3 4 4 3 7% 6% 58%	1900
	11th to 20th	Night	7% 7% 2 6% 3 3 3 3 6% 6% 55%	1877
	21st to end	Night	11% 8% 2 4 4 3 3 4 5% 8% 49%	1900
May	1st to 10th	Night	11% 7% 2 5% 4 4 4 4 6% 7% 46%	1748
	11th to 20th	Night	15% 8% 2 4 2 3 3 4 6% 6% 47%	1445
	21st to end	Night	7% 6% 1 4 2 3 4 4 5% 8% 56%	1552
June	1st to 10th	Night	5% 3 1 3 3 3 4 3 5% 10% 59%	1440
	11th to 20th	Night	5% 3 1 3 2 3 2 3 6% 7% 63%	1440
	21st to end	Night	4 2 1 2 2 3 3 3 6% 8% 64%	1440
July	1st to 10th	Night	5% 4 1 4 3 4 4 4 8% 10% 54%	1440
	11th to 20th	Night	2 2 1 2 2 4 3 3 7% 10% 63%	1440
	21st to end	Night	5% 4 1 4 4 4 5% 5% 7% 10% 52%	1620
August	1st to 10th	Night	4 3 1 4 4 4 5% 4 7% 10% 54%	1611
	11th to 20th	Night	4 2 1 3 3 4 4 3 5% 8% 62%	1748
	21st to end	Night	4 3 1 4 4 4 5% 3 5% 7% 61%	1980
September	1st to 10th	Night	6% 3 1 4 4 3 3 3 4 6% 63%	1800
	11th to 20th	Night	6% 3 1 3 2 2 2 2 4 5% 68%	1804
	21st to end	Night	6% 3 1 3 3 3 3 3 5% 6% 65%	1986
October	1st to 10th	Night	6% 6% 2 3 2 3 3 3 5% 6% 62%	2160
	11th to 20th	Night	4 4 2 3 2 2 3 2 4 6% 69%	2160
	21st to end	Night	5% 7% 2 5% 3 3 3 2 4 6% 59%	2416
November	1st to 10th	Night	2 3 1 3 3 2 3 3 4 8% 67%	2405
	11th to 20th	Night	1 5% 1 5% 2 3 3 3 4 7% 66%	2470
	21st to end	Night	2 5% 2 4 3 3 4 3 5% 7% 62%	2607
December	1st to 10th	Night	1 4 2 5% 3 3 3 3 5% 8% 63%	2660
	11th to 20th	Night	1 6% 2 5% 3 3 4 3 7% 10% 57%	3271
	21st to end	Night	3 10% 2 6% 3 3 3 3 5% 7% 54%	4873

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	24% 3 17% 29% 21% 4	2539
	11th to 20th	Day	26% 3 24% 26% 17% 4	2613
	21st to end	Day	25% 3 18% 29% 22% 4	3146
February	1st to 10th	Day	28% 5% 16% 28% 17% 5%	1710
	11th to 20th	Day	20% 6% 14% 31% 25% 5%	1806
	21st to end	Day	26% 3 12% 27% 24% 8%	1570
March	1st to 10th	Day	27% 3 13% 24% 29% 5%	3130
	11th to 20th	Day	32% 6% 15% 31% 13% 2	3480
	21st to end	Day	31% 6% 18% 30% 13% 2	4036
April	1st to 10th	Day	24% 6% 13% 28% 20% 8%	2280
	11th to 20th	Day	35% 5% 14% 23% 18% 6%	2254
	21st to end	Day	35% 5% 14% 28% 16% 2	2280
May	1st to 10th	Day	38% 6% 15% 23% 14% 3	2428
	11th to 20th	Day	42% 7% 17% 21% 10% 2	2526
	21st to end	Day	34% 7% 19% 21% 14% 5%	2724
June	1st to 10th	Day	26% 7% 21% 22% 17% 7%	2520
	11th to 20th	Day	26% 7% 13% 18% 22% 14%	2520
	21st to end	Day	25% 6% 12% 23% 23% 11%	2520
July	1st to 10th	Day	28% 8% 14% 20% 20% 10%	2451
	11th to 20th	Day	24% 7% 12% 26% 20% 12%	2445
	21st to end	Day	31% 7% 18% 22% 14% 6%	2543
August	1st to 10th	Day	31% 8% 18% 21% 15% 7%	2313
	11th to 20th	Day	24% 6% 14% 23% 22% 11%	2153
	21st to end	Day	26% 6% 13% 21% 25% 9%	2376
September	1st to 10th	Day	22% 4 9% 23% 24% 18%	2160
	11th to 20th	Day	22% 4 12% 22% 27% 14%	2154
	21st to end	Day	23% 2 7% 22% 36% 9%	1971
October	1st to 10th	Day	25% 5% 12% 21% 28% 9%	1800
	11th to 20th	Day	18% 4 9% 26% 32% 11%	1800
	21st to end	Day	22% 3 12% 28% 24% 10%	1921
November	1st to 10th	Day	16% 4 13% 27% 29% 11%	1673
	11th to 20th	Day	18% 4 10% 33% 28% 7%	1710
	21st to end	Day	23% 4 10% 27% 26% 11%	1573
December	1st to 10th	Day	22% 4 18% 29% 21% 6%	1520
	11th to 20th	Day	20% 5% 12% 32% 28% 3	1924
	21st to end	Day	28% 6% 12% 23% 25% 5%	2794
January	1st to 10th	Night	27% 4 14% 30% 23% 3	4417
	11th to 20th	Night	19% 5% 20% 28% 21% 7%	4384
	21st to end	Night	28% 3 17% 30% 16% 6%	4521
February	1st to 10th	Night	26% 6% 14% 29% 20% 5%	2470
	11th to 20th	Night	26% 6% 10% 29% 23% 5%	2270
	21st to end	Night	25% 4 8% 28% 25% 10%	1884
March	1st to 10th	Night	25% 5% 9% 25% 31% 5%	3734
	11th to 20th	Night	31% 4 14% 26% 21% 3	3495
	21st to end	Night	28% 7% 16% 31% 17% 3	3610
April	1st to 10th	Night	25% 7% 12% 25% 23% 8%	1900
	11th to 20th	Night	30% 6% 12% 27% 19% 6%	1877
	21st to end	Night	35% 5% 9% 29% 19% 3	1900
May	1st to 10th	Night	35% 8% 14% 23% 18% 3	1748
	11th to 20th	Night	37% 6% 18% 23% 14% 2	1446
	21st to end	Night	26% 6% 20% 27% 14% 7%	1552
June	1st to 10th	Night	22% 5% 19% 30% 15% 9%	1440
	11th to 20th	Night	20% 5% 12% 20% 22% 21%	1440
	21st to end	Night	18% 6% 10% 25% 30% 12%	1440
July	1st to 10th	Night	24% 7% 16% 24% 21% 9%	1400
	11th to 20th	Night	17% 5% 14% 28% 22% 14%	1392
	21st to end	Night	24% 6% 20% 27% 16% 7%	1530
August	1st to 10th	Night	24% 6% 23% 26% 15% 7%	1602
	11th to 20th	Night	21% 3 13% 25% 26% 12%	1748
	21st to end	Night	21% 4 12% 25% 26% 11%	1980
September	1st to 10th	Night	22% 4 11% 21% 22% 20%	1800
	11th to 20th	Night	18% 4 12% 25% 24% 17%	1804
	21st to end	Night	20% 4 8% 24% 32% 13%	1986
October	1st to 10th	Night	23% 5% 11% 21% 30% 10%	2160
	11th to 20th	Night	18% 5% 9% 25% 28% 15%	2160
	21st to end	Night	27% 3 8% 26% 26% 9%	2416
November	1st to 10th	Night	17% 5% 10% 28% 28% 13%	2405
	11th to 20th	Night	20% 3 9% 33% 28% 7%	2470
	21st to end	Night	22% 4 11% 25% 30% 9%	2607
December	1st to 10th	Night	21% 4 16% 27% 23% 8%	2660
	11th to 20th	Night	21% 6% 10% 33% 26% 4	3272
	21st to end	Night	29% 6% 12% 25% 23% 5%	4873

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	7% 18% 46% 19% 7% 3	270
	11th to 20th	Day	5% 29% 47% 16% 3 1	270
	21st to end	Day	2 21% 53% 18% 5% 1	297
February	1st to 10th	Day	6% 35% 49% 9% 1 1	270
	11th to 20th	Day	6% 29% 51% 12% 2	270
	21st to end	Day	8% 33% 45% 13%	225
March	1st to 10th	Day	6% 31% 43% 16% 4	270
	11th to 20th	Day	11% 23% 43% 19% 4	450
	21st to end	Day	13% 28% 42% 15% 2	506
April	1st to 10th	Day	10% 32% 41% 15% 2	460
	11th to 20th	Day	13% 31% 40% 13% 3	452
	21st to end	Day	13% 22% 42% 19% 4 1	460
May	1st to 10th	Day	14% 21% 38% 21% 6% 1	577
	11th to 20th	Day	19% 9% 35% 25% 10% 2	641
	21st to end	Day	12% 13% 32% 26% 15% 2	679
June	1st to 10th	Day	5% 7% 33% 30% 21% 5%	630
	11th to 20th	Day	7% 10% 41% 26% 15% 2	630
	21st to end	Day	6% 8% 36% 29% 17% 4	630
July	1st to 10th	Day	7% 3 24% 37% 25% 5%	621
	11th to 20th	Day	6% 3 25% 37% 24% 6%	618
	21st to end	Day	6% 4 23% 32% 29% 6%	635
August	1st to 10th	Day	8% 5% 31% 38% 16% 2	447
	11th to 20th	Day	6% 4 31% 37% 18% 4	437
	21st to end	Day	6% 6% 31% 37% 16% 3	495
September	1st to 10th	Day	6% 9% 35% 29% 17% 3	450
	11th to 20th	Day	5% 7% 40% 33% 13% 2	444
	21st to end	Day	7% 10% 32% 29% 18% 4	270
October	1st to 10th	Day	9% 14% 33% 23% 17% 5%	270
	11th to 20th	Day	3 21% 44% 24% 5% 3	270
	21st to end	Day	8% 23% 44% 20% 4 1	297
November	1st to 10th	Day	3 21% 49% 17% 9% 1	270
	11th to 20th	Day	7% 26% 45% 16% 5% 1	270
	21st to end	Day	6% 21% 49% 21% 2 1	270
December	1st to 10th	Day	6% 26% 49% 15% 4 1	270
	11th to 20th	Day	5% 22% 60% 12% 1	270
	21st to end	Day	7% 27% 47% 14% 4	297
January	1st to 10th	Night	7% 22% 46% 17% 6% 1	950
	11th to 20th	Night	6% 32% 42% 15% 5% 1	917
	21st to end	Night	5% 23% 53% 15% 3 1	836
February	1st to 10th	Night	6% 27% 53% 11% 4	760
	11th to 20th	Night	5% 27% 48% 17% 3	744
	21st to end	Night	7% 26% 53% 13% 2	628
March	1st to 10th	Night	5% 19% 51% 19% 6%	760
	11th to 20th	Night	7% 18% 42% 26% 7%	760
	21st to end	Night	8% 19% 49% 20% 3 1	708
April	1st to 10th	Night	9% 16% 49% 23% 3	570
	11th to 20th	Night	11% 18% 43% 20% 8%	563
	21st to end	Night	14% 13% 40% 23% 9% 1	570
May	1st to 10th	Night	13% 10% 36% 28% 12% 1	570
	11th to 20th	Night	19% 6% 26% 32% 15% 2	542
	21st to end	Night	8% 9% 31% 33% 17% 2	584
June	1st to 10th	Night	6% 2 27% 37% 23% 4	540
	11th to 20th	Night	7% 4 34% 39% 15% 1	540
	21st to end	Night	5% 3 21% 42% 26% 3	540
July	1st to 10th	Night	6% 2 12% 39% 35% 5%	525
	11th to 20th	Night	2 1 14% 40% 38% 6%	522
	21st to end	Night	7% 2 8% 37% 41% 6%	591
August	1st to 10th	Night	6% 2 12% 45% 31% 3	537
	11th to 20th	Night	6% 2 12% 44% 31% 5%	534
	21st to end	Night	4 2 15% 38% 34% 6%	594
September	1st to 10th	Night	6% 3 19% 40% 26% 6%	540
	11th to 20th	Night	8% 4 24% 40% 18% 6%	540
	21st to end	Night	8% 5% 30% 37% 18% 3	546
October	1st to 10th	Night	9% 9% 35% 29% 14% 3	720
	11th to 20th	Night	6% 13% 46% 25% 9% 1	720
	21st to end	Night	8% 15% 50% 18% 7% 2	832
November	1st to 10th	Night	5% 17% 50% 21% 6% 2	925
	11th to 20th	Night	5% 26% 49% 16% 4 1	950
	21st to end	Night	5% 18% 51% 22% 3	950
December	1st to 10th	Night	8% 24% 50% 16% 3	950
	11th to 20th	Night	6% 22% 51% 17% 4 1	950
	21st to end	Night	7% 26% 48% 14% 5%	1045

Page generated at 2024-05-11 03:13:32. Version ac07

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