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 2025-08-05.

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%	125470
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	122586
Transparency	Day	43% 6% 12% 21% 15% 3	128990
	Night	43% 6% 12% 21% 16% 2	127140
Seeing	Day	23% 17% 32% 19% 7% 2	15969
	Night	21% 15% 34% 23% 6%	24838

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% 7% 3 9% 5% 5% 5% 4 5% 6% 41%	3597
	11th to 20th	Day	13% 7% 3 6% 3 4 5% 3 4 5% 47%	3570
	21st to end	Day	17% 8% 3 5% 4 3 4 3 4 6% 44%	3960
February	1st to 10th	Day	18% 8% 3 6% 5% 5% 3 5% 6% 7% 36%	1864
	11th to 20th	Day	13% 8% 2 7% 5% 5% 6% 4 6% 6% 39%	1948
	21st to end	Day	18% 11% 3 6% 5% 4 4 3 4 6% 38%	1650
March	1st to 10th	Day	20% 11% 3 7% 4 3 4 3 4 5% 36%	4476
	11th to 20th	Day	16% 8% 2 6% 4 4 5% 4 6% 7% 39%	4747
	21st to end	Day	25% 9% 2 6% 3 4 4 4 5% 6% 31%	5804
April	1st to 10th	Day	19% 10% 3 7% 4 4 4 3 5% 5% 38%	2400
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 4 6% 6% 39%	2379
	21st to end	Day	21% 5% 3 7% 5% 5% 4 4 5% 5% 37%	2600
May	1st to 10th	Day	30% 10% 3 6% 4 4 4 3 4 6% 25%	5886
	11th to 20th	Day	21% 8% 3 8% 5% 4 4 4 4 7% 32%	5995
	21st to end	Day	19% 6% 2 7% 4 4 4 4 5% 8% 35%	6949
June	1st to 10th	Day	19% 6% 2 6% 5% 4 5% 5% 6% 7% 35%	2800
	11th to 20th	Day	17% 5% 2 5% 4 5% 5% 4 7% 8% 37%	2800
	21st to end	Day	17% 6% 2 7% 5% 4 5% 3 6% 8% 36%	2800
July	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 5% 7% 9% 33%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 10% 38%	6360
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 8% 44%	6926
August	1st to 10th	Day	14% 6% 3 6% 6% 6% 6% 4 6% 9% 34%	4228
	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	15% 6% 3 5% 4 3 3 2 4 5% 50%	1761
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 5% 6% 48%	1800
	21st to end	Day	18% 8% 2 5% 3 4 3 3 6% 5% 43%	1800
December	1st to 10th	Day	15% 8% 2 5% 3 3 4 3 4 6% 47%	2633
	11th to 20th	Day	19% 7% 2 5% 3 2 3 2 4 5% 47%	2986
	21st to end	Day	20% 11% 3 6% 3 3 3 3 4 5% 39%	3960
January	1st to 10th	Night	14% 7% 2 5% 3 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	13% 8% 3 9% 4 4 4 3 5% 5% 40%	6315
	21st to end	Night	16% 10% 3 7% 5% 4 3 3 5% 5% 39%	6996
February	1st to 10th	Night	16% 9% 2 7% 5% 4 4 3 5% 5% 39%	2533
	11th to 20th	Night	14% 9% 3 6% 5% 5% 4 3 6% 6% 40%	2338
	21st to end	Night	17% 10% 2 7% 5% 3 3 2 5% 6% 40%	1980
March	1st to 10th	Night	23% 9% 2 5% 3 3 4 3 4 6% 38%	5364
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 4 5% 39%	5211
	21st to end	Night	24% 10% 2 6% 3 3 4 4 5% 5% 34%	5112
April	1st to 10th	Night	18% 8% 3 7% 5% 4 3 3 5% 6% 36%	2000
	11th to 20th	Night	19% 7% 2 6% 5% 4 4 3 5% 5% 40%	1970
	21st to end	Night	20% 7% 2 7% 4 4 3 3 5% 6% 40%	1800
May	1st to 10th	Night	32% 10% 2 6% 4 3 3 3 4 6% 26%	4054
	11th to 20th	Night	17% 10% 3 9% 5% 5% 6% 4 6% 7% 29%	3927
	21st to end	Night	20% 8% 3 7% 6% 4 4 4 5% 7% 33%	3926
June	1st to 10th	Night	17% 8% 3 6% 6% 4 4 3 6% 7% 37%	1600
	11th to 20th	Night	20% 6% 3 5% 4 4 4 4 6% 8% 36%	1600
	21st to end	Night	14% 6% 2 6% 4 4 5% 4 7% 9% 40%	1600
July	1st to 10th	Night	13% 6% 2 7% 4 5% 5% 4 6% 10% 37%	3600
	11th to 20th	Night	12% 7% 3 7% 4 4 5% 3 7% 9% 39%	3600
	21st to end	Night	12% 7% 3 6% 4 4 4 3 5% 6% 46%	3926
August	1st to 10th	Night	19% 6% 3 7% 5% 5% 5% 4 6% 6% 33%	2758
	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% 5% 2 5% 4 4 3 3 5% 4 49%	2537
	11th to 20th	Night	18% 7% 2 4 4 3 3 3 4 5% 48%	2600
	21st to end	Night	17% 7% 2 5% 4 4 3 3 5% 6% 45%	2600
December	1st to 10th	Night	17% 9% 2 6% 4 4 3 2 4 5% 43%	4544
	11th to 20th	Night	22% 7% 2 6% 3 3 3 2 4 5% 42%	5208
	21st to end	Night	17% 10% 3 6% 4 4 3 3 4 6% 40%	6996

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	41% 6% 8% 27% 16% 2	3598
	11th to 20th	Day	39% 5% 10% 22% 18% 5%	3600
	21st to end	Day	41% 5% 10% 21% 19% 3	3960
February	1st to 10th	Day	46% 6% 14% 19% 13% 2	1864
	11th to 20th	Day	44% 7% 10% 20% 15% 3	1957
	21st to end	Day	48% 5% 9% 20% 14% 3	1650
March	1st to 10th	Day	50% 5% 11% 14% 11% 9%	4479
	11th to 20th	Day	39% 7% 15% 26% 10% 2	4747
	21st to end	Day	52% 4 7% 15% 16% 6%	5804
April	1st to 10th	Day	48% 6% 10% 18% 16% 4	2400
	11th to 20th	Day	41% 7% 12% 19% 18% 3	2379
	21st to end	Day	47% 6% 9% 17% 17% 4	2600
May	1st to 10th	Day	58% 8% 13% 12% 8% 1	5897
	11th to 20th	Day	51% 7% 14% 16% 10% 1	5999
	21st to end	Day	45% 8% 12% 21% 13% 1	6949
June	1st to 10th	Day	44% 8% 14% 22% 11% 1	2800
	11th to 20th	Day	42% 9% 14% 19% 15% 2	2800
	21st to end	Day	44% 8% 13% 20% 14% 1	2800
July	1st to 10th	Day	46% 8% 11% 20% 13% 1	6290
	11th to 20th	Day	39% 8% 16% 24% 12% 1	6285
	21st to end	Day	39% 5% 12% 27% 15% 1	6728
August	1st to 10th	Day	45% 9% 16% 21% 9% 1	4214
	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	39% 4 12% 25% 17% 2	1761
	11th to 20th	Day	38% 5% 10% 22% 23% 3	1800
	21st to end	Day	42% 6% 14% 25% 12% 2	1800
December	1st to 10th	Day	39% 4 16% 27% 13% 1	2633
	11th to 20th	Day	40% 4 15% 18% 19% 5%	2986
	21st to end	Day	48% 6% 9% 19% 15% 5%	3960
January	1st to 10th	Night	36% 5% 11% 32% 15% 2	6349
	11th to 20th	Night	44% 6% 13% 21% 13% 4	6360
	21st to end	Night	47% 5% 10% 16% 18% 5%	6996
February	1st to 10th	Night	46% 5% 14% 20% 13% 2	2533
	11th to 20th	Night	45% 5% 10% 20% 16% 4	2340
	21st to end	Night	46% 4 8% 24% 15% 4	1980
March	1st to 10th	Night	47% 5% 10% 17% 15% 6%	5374
	11th to 20th	Night	44% 7% 17% 18% 12% 2	5212
	21st to end	Night	51% 5% 6% 12% 21% 6%	5112
April	1st to 10th	Night	48% 6% 10% 19% 15% 3	2000
	11th to 20th	Night	45% 6% 9% 19% 18% 2	1970
	21st to end	Night	45% 7% 10% 18% 16% 4	1800
May	1st to 10th	Night	59% 7% 13% 12% 8% 1	4060
	11th to 20th	Night	51% 10% 13% 15% 9% 2	3928
	21st to end	Night	49% 8% 13% 18% 11% 1	3927
June	1st to 10th	Night	45% 8% 12% 22% 12% 1	1600
	11th to 20th	Night	45% 8% 13% 21% 12% 1	1600
	21st to end	Night	37% 10% 15% 23% 14% 1	1600
July	1st to 10th	Night	42% 7% 13% 22% 15%	3560
	11th to 20th	Night	40% 7% 19% 19% 13% 1	3552
	21st to end	Night	38% 6% 12% 28% 15% 1	3809
August	1st to 10th	Night	49% 9% 12% 18% 11% 1	2748
	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	37% 7% 10% 23% 20% 4	2537
	11th to 20th	Night	39% 4 11% 21% 20% 4	2600
	21st to end	Night	40% 5% 14% 26% 13% 2	2600
December	1st to 10th	Night	43% 4 12% 29% 12% 1	4544
	11th to 20th	Night	45% 5% 14% 15% 19% 2	5208
	21st to end	Night	44% 6% 9% 22% 15% 3	6996

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	30% 27% 26% 14% 4	200
	11th to 20th	Day	33% 23% 33% 9% 3	200
	21st to end	Day	26% 25% 35% 10% 3	220
February	1st to 10th	Day	31% 22% 32% 13% 1	396
	11th to 20th	Day	28% 30% 31% 10% 1	496
	21st to end	Day	31% 20% 32% 14% 3	414
March	1st to 10th	Day	31% 22% 33% 11% 3	500
	11th to 20th	Day	28% 25% 30% 13% 3 1	500
	21st to end	Day	32% 19% 32% 11% 4 1	550
April	1st to 10th	Day	33% 23% 26% 12% 5% 1	500
	11th to 20th	Day	32% 23% 30% 10% 5% 1	492
	21st to end	Day	27% 21% 32% 15% 3 2	500
May	1st to 10th	Day	32% 20% 29% 14% 4 1	503
	11th to 20th	Day	24% 15% 40% 14% 5% 1	527
	21st to end	Day	19% 14% 36% 21% 8% 1	573
June	1st to 10th	Day	18% 13% 39% 20% 8% 2	530
	11th to 20th	Day	18% 14% 38% 22% 6% 2	530
	21st to end	Day	17% 14% 36% 22% 7% 3	530
July	1st to 10th	Day	17% 11% 31% 25% 14% 2	525
	11th to 20th	Day	13% 8% 34% 30% 12% 3	524
	21st to end	Day	13% 9% 37% 31% 9% 2	576
August	1st to 10th	Day	10% 9% 34% 32% 13% 2	502
	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	22% 17% 32% 20% 7% 2	395
	11th to 20th	Day	29% 20% 29% 18% 4 1	400
	21st to end	Day	29% 19% 30% 16% 6%	400
December	1st to 10th	Day	22% 26% 32% 17% 3	220
	11th to 20th	Day	34% 25% 31% 9% 3 1	200
	21st to end	Day	25% 23% 33% 15% 3 2	220
January	1st to 10th	Night	23% 20% 39% 17% 2	800
	11th to 20th	Night	27% 19% 35% 17% 2	800
	21st to end	Night	19% 22% 38% 19% 3	880
February	1st to 10th	Night	24% 18% 39% 18% 1	800
	11th to 20th	Night	23% 20% 41% 15% 1	783
	21st to end	Night	25% 21% 34% 17% 2	660
March	1st to 10th	Night	25% 20% 36% 17% 3	800
	11th to 20th	Night	22% 21% 38% 16% 3	800
	21st to end	Night	26% 18% 34% 18% 3	880
April	1st to 10th	Night	25% 19% 34% 19% 3	800
	11th to 20th	Night	23% 21% 33% 18% 5%	786
	21st to end	Night	22% 21% 36% 19% 3	600
May	1st to 10th	Night	30% 14% 32% 20% 3	600
	11th to 20th	Night	19% 16% 39% 21% 5%	551
	21st to end	Night	18% 9% 39% 29% 5%	433
June	1st to 10th	Night	19% 7% 38% 30% 7%	400
	11th to 20th	Night	19% 9% 37% 30% 5% 1	400
	21st to end	Night	15% 5% 39% 35% 7%	400
July	1st to 10th	Night	15% 6% 31% 40% 9% 1	390
	11th to 20th	Night	13% 5% 31% 40% 11%	388
	21st to end	Night	12% 5% 32% 43% 8%	438
August	1st to 10th	Night	12% 4 32% 41% 10% 1	515
	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% 14% 34% 25% 8% 1	780
	11th to 20th	Night	23% 19% 33% 19% 6% 1	800
	21st to end	Night	23% 19% 34% 18% 5% 1	800
December	1st to 10th	Night	26% 19% 34% 17% 5%	800
	11th to 20th	Night	25% 18% 33% 21% 3	800
	21st to end	Night	25% 17% 37% 18% 3	880

Page generated at 2025-08-02 01:32:28. Version ac07

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