Rockbottom Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rockbottom Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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% 7% 2 6% 4 4 4 4 5% 7% 39%	101225
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	97796
Transparency	Day	43% 7% 12% 22% 14% 2	104763
	Night	43% 7% 12% 22% 15% 2	102334
Seeing	Day	23% 16% 33% 19% 7% 2	15029
	Night	21% 14% 34% 24% 6%	23927

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	14% 8% 3 7% 4 4 4 3 4 5% 45%	2517
	11th to 20th	Day	16% 7% 4 7% 4 4 5% 3 4 5% 40%	2520
	21st to end	Day	21% 10% 4 5% 5% 4 4 3 4 5% 34%	2772
February	1st to 10th	Day	19% 8% 3 6% 5% 5% 4 4 6% 6% 36%	1770
	11th to 20th	Day	13% 8% 2 7% 5% 5% 6% 4 6% 6% 39%	1848
	21st to end	Day	18% 11% 3 6% 5% 4 4 3 4 6% 37%	1570
March	1st to 10th	Day	23% 14% 3 7% 4 3 4 2 4 5% 30%	3116
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 41%	3317
	21st to end	Day	22% 9% 2 6% 4 4 4 4 5% 6% 34%	4056
April	1st to 10th	Day	18% 10% 3 7% 4 4 4 3 5% 5% 38%	2280
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 4 6% 6% 39%	2259
	21st to end	Day	21% 6% 3 7% 5% 5% 4 4 5% 5% 36%	2470
May	1st to 10th	Day	24% 11% 3 7% 5% 5% 4 3 5% 6% 27%	4106
	11th to 20th	Day	23% 9% 3 7% 4 4 4 3 4 7% 32%	4173
	21st to end	Day	14% 5% 2 5% 4 4 4 3 5% 9% 44%	4837
June	1st to 10th	Day	19% 6% 2 6% 5% 4 5% 5% 6% 7% 36%	2660
	11th to 20th	Day	16% 5% 2 5% 5% 5% 5% 4 7% 8% 37%	2660
	21st to end	Day	18% 6% 2 7% 5% 4 5% 3 6% 9% 35%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 8% 36%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	4440
	21st to end	Day	16% 8% 3 7% 5% 4 5% 4 6% 8% 36%	4814
August	1st to 10th	Day	15% 6% 3 6% 6% 6% 6% 5% 6% 9% 33%	4174
	11th to 20th	Day	13% 8% 3 7% 4 4 5% 4 6% 7% 38%	3963
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 7% 9% 46%	4180
September	1st to 10th	Day	13% 6% 2 5% 3 4 4 4 5% 7% 46%	2280
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 4 6% 51%	2095
	21st to end	Day	17% 7% 2 6% 4 4 4 3 4 6% 44%	2090
October	1st to 10th	Day	22% 6% 2 5% 4 4 4 3 5% 5% 41%	2090
	11th to 20th	Day	14% 7% 2 5% 4 3 4 3 5% 7% 48%	1852
	21st to end	Day	20% 7% 2 6% 4 3 4 3 4 6% 41%	1881
November	1st to 10th	Day	16% 7% 3 5% 4 3 2 2 4 5% 49%	1698
	11th to 20th	Day	19% 6% 2 4 3 3 3 3 5% 5% 46%	1710
	21st to end	Day	17% 8% 2 5% 4 4 3 4 6% 5% 43%	1710
December	1st to 10th	Day	17% 9% 3 7% 4 4 5% 4 4 6% 38%	1539
	11th to 20th	Day	22% 7% 2 5% 3 2 3 3 4 4 45%	1906
	21st to end	Day	22% 11% 3 6% 4 3 4 3 5% 5% 34%	2772
January	1st to 10th	Night	18% 8% 2 4 3 3 3 2 4 5% 48%	4429
	11th to 20th	Night	15% 9% 4 9% 4 4 4 3 4 5% 39%	4440
	21st to end	Night	22% 13% 4 7% 4 4 3 3 5% 4 31%	4884
February	1st to 10th	Night	16% 9% 2 7% 5% 4 4 3 5% 5% 39%	2407
	11th to 20th	Night	14% 9% 3 6% 5% 5% 4 3 5% 6% 40%	2218
	21st to end	Night	18% 10% 2 6% 4 3 3 2 5% 6% 40%	1884
March	1st to 10th	Night	28% 11% 2 5% 3 3 4 3 4 5% 32%	3724
	11th to 20th	Night	11% 10% 3 7% 5% 5% 4 3 5% 5% 41%	3641
	21st to end	Night	23% 9% 2 5% 3 3 4 4 6% 5% 34%	3574
April	1st to 10th	Night	17% 8% 3 7% 5% 4 3 3 5% 6% 37%	1900
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 5% 5% 40%	1870
	21st to end	Night	20% 7% 2 7% 4 4 3 3 5% 5% 39%	1710
May	1st to 10th	Night	25% 10% 3 8% 5% 4 4 3 5% 6% 28%	2834
	11th to 20th	Night	21% 10% 3 8% 5% 4 5% 5% 5% 7% 28%	2735
	21st to end	Night	16% 6% 2 6% 5% 4 4 3 5% 8% 42%	2738
June	1st to 10th	Night	17% 8% 3 5% 6% 4 4 3 6% 7% 38%	1520
	11th to 20th	Night	20% 6% 3 5% 4 4 4 4 6% 8% 37%	1520
	21st to end	Night	14% 6% 2 5% 4 4 5% 3 7% 9% 39%	1520
July	1st to 10th	Night	13% 5% 2 7% 5% 5% 6% 5% 7% 10% 35%	2520
	11th to 20th	Night	12% 7% 3 8% 4 4 5% 3 7% 9% 39%	2520
	21st to end	Night	15% 8% 3 7% 5% 4 4 3 5% 7% 37%	2739
August	1st to 10th	Night	19% 6% 3 7% 5% 5% 5% 4 6% 7% 33%	2724
	11th to 20th	Night	11% 7% 2 8% 6% 5% 6% 4 7% 8% 36%	2861
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 7% 56%	3476
September	1st to 10th	Night	12% 6% 2 5% 5% 3 3 3 6% 6% 48%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 6% 53%	2085
	21st to end	Night	17% 7% 2 6% 4 3 4 3 5% 6% 44%	2090
October	1st to 10th	Night	20% 7% 2 5% 4 3 3 2 5% 6% 42%	2090
	11th to 20th	Night	16% 4 2 4 3 3 4 4 5% 6% 48%	2323
	21st to end	Night	22% 8% 2 6% 4 3 3 3 4 6% 40%	2717
November	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 4 49%	2453
	11th to 20th	Night	19% 7% 2 5% 4 3 3 3 4 5% 47%	2470
	21st to end	Night	16% 7% 2 5% 4 4 3 3 5% 7% 45%	2470
December	1st to 10th	Night	18% 9% 3 7% 5% 4 4 3 4 6% 39%	2638
	11th to 20th	Night	19% 6% 2 5% 3 3 4 3 5% 6% 42%	3288
	21st to end	Night	20% 10% 3 7% 4 4 4 3 5% 6% 35%	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	40% 5% 10% 32% 11% 2	2518
	11th to 20th	Day	46% 6% 10% 25% 12% 2	2520
	21st to end	Day	50% 7% 11% 21% 10% 2	2772
February	1st to 10th	Day	46% 6% 14% 19% 12% 2	1770
	11th to 20th	Day	44% 7% 11% 20% 15% 3	1857
	21st to end	Day	49% 5% 10% 20% 13% 3	1570
March	1st to 10th	Day	56% 5% 11% 14% 11% 3	3119
	11th to 20th	Day	34% 9% 18% 24% 13% 3	3317
	21st to end	Day	48% 5% 8% 16% 17% 6%	4056
April	1st to 10th	Day	47% 6% 10% 18% 16% 3	2280
	11th to 20th	Day	41% 7% 12% 19% 17% 3	2259
	21st to end	Day	47% 7% 9% 17% 16% 4	2470
May	1st to 10th	Day	54% 9% 12% 14% 9% 1	4120
	11th to 20th	Day	50% 8% 15% 16% 10% 2	4177
	21st to end	Day	35% 9% 13% 24% 18% 2	4837
June	1st to 10th	Day	44% 8% 14% 23% 10% 1	2660
	11th to 20th	Day	41% 9% 14% 19% 15% 2	2660
	21st to end	Day	45% 9% 13% 20% 13% 1	2660
July	1st to 10th	Day	43% 8% 12% 21% 15% 1	4370
	11th to 20th	Day	38% 8% 16% 24% 12% 1	4365
	21st to end	Day	46% 7% 12% 20% 13% 2	4661
August	1st to 10th	Day	46% 9% 16% 21% 8% 1	4160
	11th to 20th	Day	43% 7% 15% 26% 9% 1	4002
	21st to end	Day	32% 7% 13% 30% 17% 1	4180
September	1st to 10th	Day	36% 7% 12% 24% 19% 2	2280
	11th to 20th	Day	35% 5% 9% 26% 22% 3	2095
	21st to end	Day	41% 5% 10% 20% 21% 2	2090
October	1st to 10th	Day	41% 6% 9% 22% 20% 3	3480
	11th to 20th	Day	31% 4 8% 27% 26% 4	3042
	21st to end	Day	32% 5% 12% 30% 18% 3	3081
November	1st to 10th	Day	40% 4 12% 25% 17% 2	1698
	11th to 20th	Day	39% 5% 11% 23% 20% 2	1710
	21st to end	Day	41% 6% 14% 24% 12% 2	1710
December	1st to 10th	Day	46% 6% 15% 16% 17% 1	1539
	11th to 20th	Day	42% 5% 14% 18% 17% 4	1906
	21st to end	Day	50% 8% 12% 19% 11% 1	2772
January	1st to 10th	Night	39% 4 13% 28% 12% 3	4429
	11th to 20th	Night	45% 6% 13% 22% 12% 3	4440
	21st to end	Night	55% 6% 10% 16% 11% 2	4884
February	1st to 10th	Night	45% 6% 15% 19% 13% 2	2407
	11th to 20th	Night	45% 5% 10% 20% 16% 4	2220
	21st to end	Night	45% 5% 8% 24% 14% 4	1884
March	1st to 10th	Night	53% 5% 10% 20% 10% 3	3734
	11th to 20th	Night	40% 9% 15% 18% 15% 2	3642
	21st to end	Night	49% 5% 7% 13% 18% 7%	3574
April	1st to 10th	Night	47% 6% 11% 19% 15% 3	1900
	11th to 20th	Night	45% 6% 9% 19% 18% 2	1870
	21st to end	Night	45% 7% 10% 19% 15% 4	1710
May	1st to 10th	Night	54% 9% 12% 15% 8% 2	2840
	11th to 20th	Night	52% 11% 14% 13% 8% 2	2736
	21st to end	Night	39% 9% 15% 22% 14% 1	2739
June	1st to 10th	Night	45% 8% 12% 22% 12% 1	1520
	11th to 20th	Night	45% 8% 13% 21% 13% 1	1520
	21st to end	Night	37% 11% 16% 22% 13% 1	1520
July	1st to 10th	Night	42% 8% 14% 21% 14% 1	2480
	11th to 20th	Night	40% 8% 17% 22% 14% 1	2472
	21st to end	Night	46% 8% 12% 18% 15% 1	2651
August	1st to 10th	Night	50% 9% 12% 18% 10% 1	2714
	11th to 20th	Night	42% 10% 15% 23% 9%	2888
	21st to end	Night	26% 7% 16% 30% 21% 1	3476
September	1st to 10th	Night	37% 6% 13% 24% 19% 1	1900
	11th to 20th	Night	33% 6% 11% 22% 26% 2	2085
	21st to end	Night	40% 7% 9% 23% 19% 2	2090
October	1st to 10th	Night	38% 6% 16% 23% 15% 2	3480
	11th to 20th	Night	26% 5% 10% 27% 28% 4	3859
	21st to end	Night	34% 4 12% 31% 17% 2	4467
November	1st to 10th	Night	38% 7% 10% 23% 19% 4	2453
	11th to 20th	Night	40% 4 12% 22% 19% 3	2470
	21st to end	Night	40% 5% 14% 26% 13% 2	2470
December	1st to 10th	Night	46% 5% 13% 22% 13% 1	2638
	11th to 20th	Night	42% 7% 15% 19% 16% 2	3288
	21st to end	Night	48% 8% 10% 24% 10% 1	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% 26% 27% 16% 4	180
	11th to 20th	Day	32% 24% 32% 9% 3	180
	21st to end	Day	24% 25% 36% 12% 4	198
February	1st to 10th	Day	31% 20% 33% 14% 1	362
	11th to 20th	Day	26% 30% 32% 11% 1	456
	21st to end	Day	32% 18% 31% 15% 3 1	382
March	1st to 10th	Day	30% 20% 35% 12% 3	460
	11th to 20th	Day	28% 25% 31% 13% 3 1	460
	21st to end	Day	31% 19% 32% 11% 5% 1	506
April	1st to 10th	Day	31% 23% 27% 13% 5% 1	460
	11th to 20th	Day	29% 23% 31% 11% 5% 1	452
	21st to end	Day	28% 20% 33% 15% 3 2	460
May	1st to 10th	Day	30% 22% 30% 14% 4 1	463
	11th to 20th	Day	24% 15% 40% 15% 5% 1	487
	21st to end	Day	18% 14% 37% 22% 9% 1	529
June	1st to 10th	Day	18% 13% 40% 20% 8% 2	490
	11th to 20th	Day	16% 14% 39% 22% 7% 2	490
	21st to end	Day	17% 14% 37% 20% 7% 3	490
July	1st to 10th	Day	16% 11% 31% 25% 15% 2	485
	11th to 20th	Day	14% 8% 34% 30% 11% 3	484
	21st to end	Day	13% 9% 36% 32% 9% 1	532
August	1st to 10th	Day	10% 10% 34% 32% 12% 2	486
	11th to 20th	Day	15% 9% 32% 27% 14% 3	479
	21st to end	Day	11% 9% 34% 27% 14% 5%	539
September	1st to 10th	Day	19% 9% 30% 23% 13% 5%	490
	11th to 20th	Day	20% 9% 29% 19% 13% 9%	490
	21st to end	Day	20% 13% 31% 23% 9% 5%	490
October	1st to 10th	Day	26% 13% 27% 20% 10% 4	490
	11th to 20th	Day	19% 16% 32% 21% 10% 3	439
	21st to end	Day	27% 15% 32% 18% 7% 1	429
November	1st to 10th	Day	21% 17% 32% 20% 7% 2	374
	11th to 20th	Day	29% 20% 28% 18% 4 1	370
	21st to end	Day	27% 19% 31% 17% 7%	370
December	1st to 10th	Day	21% 28% 32% 16% 3	199
	11th to 20th	Day	33% 23% 32% 8% 3	180
	21st to end	Day	26% 24% 33% 12% 3 2	198
January	1st to 10th	Night	22% 18% 40% 17% 2	760
	11th to 20th	Night	27% 20% 35% 16% 2	760
	21st to end	Night	19% 22% 37% 19% 3	836
February	1st to 10th	Night	24% 17% 39% 19% 1	760
	11th to 20th	Night	22% 21% 42% 15% 1	743
	21st to end	Night	26% 20% 33% 17% 3	628
March	1st to 10th	Night	24% 19% 36% 17% 3	760
	11th to 20th	Night	22% 21% 39% 16% 3	760
	21st to end	Night	26% 18% 34% 18% 4	836
April	1st to 10th	Night	23% 20% 34% 20% 3	760
	11th to 20th	Night	22% 21% 33% 19% 5%	746
	21st to end	Night	22% 20% 36% 19% 3	570
May	1st to 10th	Night	29% 15% 32% 21% 4	570
	11th to 20th	Night	20% 16% 39% 20% 4	523
	21st to end	Night	18% 9% 38% 29% 5%	411
June	1st to 10th	Night	19% 7% 38% 29% 8%	380
	11th to 20th	Night	19% 9% 37% 29% 5% 1	380
	21st to end	Night	15% 5% 40% 34% 6%	380
July	1st to 10th	Night	15% 5% 31% 39% 9% 1	370
	11th to 20th	Night	13% 5% 31% 41% 11%	368
	21st to end	Night	13% 5% 31% 43% 8%	416
August	1st to 10th	Night	12% 4 33% 40% 11% 1	505
	11th to 20th	Night	12% 5% 32% 37% 14%	602
	21st to end	Night	14% 6% 24% 39% 15% 2	836
September	1st to 10th	Night	16% 5% 33% 33% 12%	760
	11th to 20th	Night	16% 8% 27% 28% 19% 2	760
	21st to end	Night	21% 8% 28% 29% 12% 2	760
October	1st to 10th	Night	24% 11% 27% 26% 10% 2	760
	11th to 20th	Night	19% 12% 32% 24% 12% 1	760
	21st to end	Night	28% 13% 33% 20% 6%	836
November	1st to 10th	Night	19% 13% 34% 25% 8% 1	755
	11th to 20th	Night	24% 18% 33% 18% 6% 1	760
	21st to end	Night	23% 19% 35% 18% 5% 1	760
December	1st to 10th	Night	26% 19% 34% 16% 4	760
	11th to 20th	Night	24% 19% 33% 21% 3	760
	21st to end	Night	26% 17% 37% 17% 2	836

Page generated at 2024-11-01 22:47:37. Version ac07

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