Roger Twitchell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Roger Twitchell Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	18% 8% 2 6% 4 4 5% 4 6% 7% 36%	126213
	Night	17% 8% 3 7% 4 4 4 4 5% 6% 36%	123781
Transparency	Day	46% 6% 11% 21% 14% 2	129721
	Night	46% 6% 11% 21% 13% 2	128361
Seeing	Day	24% 16% 30% 20% 9% 2	16178
	Night	22% 12% 30% 27% 8% 1	25040

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	13% 7% 3 7% 5% 5% 6% 5% 7% 7% 35%	3597
	11th to 20th	Day	12% 8% 3 6% 5% 4 5% 4 6% 7% 40%	3570
	21st to end	Day	11% 9% 3 6% 4 4 5% 4 5% 5% 44%	3960
February	1st to 10th	Day	15% 10% 3 7% 5% 5% 5% 4 6% 6% 35%	1747
	11th to 20th	Day	11% 10% 2 7% 5% 5% 5% 4 7% 6% 37%	1948
	21st to end	Day	15% 9% 3 7% 6% 4 6% 4 5% 6% 34%	1650
March	1st to 10th	Day	22% 12% 3 8% 5% 5% 5% 3 4 4 30%	4476
	11th to 20th	Day	18% 11% 3 7% 5% 4 4 3 4 5% 36%	4837
	21st to end	Day	26% 9% 3 6% 4 3 3 3 5% 7% 31%	5684
April	1st to 10th	Day	19% 12% 3 7% 5% 3 3 3 4 6% 35%	2400
	11th to 20th	Day	18% 6% 2 7% 5% 5% 4 4 5% 7% 37%	2457
	21st to end	Day	23% 9% 3 7% 5% 3 4 4 4 4 35%	2600
May	1st to 10th	Day	33% 10% 2 7% 4 4 4 4 4 5% 22%	5886
	11th to 20th	Day	19% 9% 3 9% 6% 5% 5% 3 5% 7% 29%	6059
	21st to end	Day	19% 7% 3 6% 4 5% 5% 5% 6% 8% 32%	6949
June	1st to 10th	Day	23% 8% 2 6% 5% 5% 5% 4 6% 7% 29%	2800
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 6% 7% 38%	2800
	21st to end	Day	22% 7% 2 7% 5% 5% 5% 4 6% 7% 30%	2800
July	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 5% 8% 10% 30%	6360
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 5% 7% 10% 37%	6360
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 8% 42%	6926
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 5% 7% 9% 34%	4316
	11th to 20th	Day	16% 9% 3 7% 5% 5% 5% 5% 5% 7% 33%	4175
	21st to end	Day	10% 5% 2 5% 4 4 5% 5% 7% 9% 44%	4430
September	1st to 10th	Day	12% 4 2 5% 3 4 4 4 6% 8% 48%	2375
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 7% 53%	2200
	21st to end	Day	16% 5% 2 5% 3 3 4 3 6% 7% 45%	2200
October	1st to 10th	Day	18% 7% 3 6% 4 4 4 3 4 6% 41%	2200
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 4 8% 41%	1912
	21st to end	Day	23% 8% 2 5% 5% 4 4 3 5% 6% 35%	1980
November	1st to 10th	Day	16% 7% 2 5% 3 3 3 4 6% 7% 44%	1779
	11th to 20th	Day	20% 4 2 4 4 3 4 4 7% 6% 42%	1800
	21st to end	Day	22% 7% 2 5% 3 4 3 3 6% 7% 39%	1784
December	1st to 10th	Day	17% 9% 2 7% 4 4 4 4 6% 8% 35%	2384
	11th to 20th	Day	19% 8% 2 5% 3 3 3 3 6% 7% 40%	2852
	21st to end	Day	19% 10% 3 6% 4 4 5% 4 5% 5% 34%	3960
January	1st to 10th	Night	12% 8% 3 8% 5% 5% 5% 4 5% 6% 38%	6349
	11th to 20th	Night	12% 10% 4 10% 6% 6% 6% 5% 6% 7% 27%	6315
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 4 6% 6% 38%	6996
February	1st to 10th	Night	13% 11% 3 7% 5% 6% 5% 5% 6% 6% 34%	2649
	11th to 20th	Night	11% 11% 2 7% 5% 5% 5% 4 5% 5% 40%	2338
	21st to end	Night	13% 10% 3 7% 5% 5% 5% 4 6% 7% 37%	1980
March	1st to 10th	Night	23% 10% 3 7% 4 4 4 4 5% 5% 30%	5364
	11th to 20th	Night	15% 10% 4 8% 5% 5% 6% 4 6% 6% 33%	5122
	21st to end	Night	24% 9% 3 6% 4 4 4 3 6% 7% 30%	5245
April	1st to 10th	Night	20% 11% 3 6% 5% 4 4 2 4 6% 35%	2000
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	1890
	21st to end	Night	23% 8% 2 6% 4 4 5% 3 4 7% 35%	1800
May	1st to 10th	Night	32% 10% 3 7% 4 4 3 3 5% 5% 23%	4054
	11th to 20th	Night	17% 10% 4 9% 5% 5% 5% 4 6% 8% 28%	3863
	21st to end	Night	17% 8% 3 8% 4 4 5% 4 6% 7% 33%	3926
June	1st to 10th	Night	23% 8% 2 6% 5% 4 5% 3 6% 6% 34%	1600
	11th to 20th	Night	18% 6% 3 5% 4 4 4 4 6% 8% 40%	1600
	21st to end	Night	20% 7% 2 6% 4 3 5% 5% 6% 7% 35%	1600
July	1st to 10th	Night	12% 6% 3 7% 4 5% 5% 5% 7% 9% 38%	3600
	11th to 20th	Night	15% 8% 2 7% 5% 4 4 4 5% 8% 38%	3600
	21st to end	Night	11% 4 2 4 3 4 5% 4 7% 7% 49%	3926
August	1st to 10th	Night	21% 8% 3 7% 4 5% 5% 3 5% 6% 34%	2801
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 4 5% 9% 36%	2872
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	3452
September	1st to 10th	Night	13% 5% 2 4 3 4 3 3 5% 6% 50%	2025
	11th to 20th	Night	15% 5% 2 3 3 3 3 3 5% 6% 53%	2200
	21st to end	Night	16% 6% 2 5% 3 4 3 3 5% 8% 45%	2200
October	1st to 10th	Night	18% 7% 3 6% 4 5% 4 4 5% 6% 41%	2200
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 3 6% 7% 40%	2485
	21st to end	Night	24% 9% 2 4 4 4 4 3 5% 5% 36%	2860
November	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	2564
	11th to 20th	Night	17% 7% 2 5% 4 4 5% 3 5% 6% 43%	2600
	21st to end	Night	16% 8% 2 6% 5% 3 4 4 5% 7% 40%	2614
December	1st to 10th	Night	17% 10% 3 7% 4 4 4 4 6% 7% 33%	4778
	11th to 20th	Night	24% 8% 2 5% 4 4 4 3 4 4 37%	5317
	21st to end	Night	20% 11% 3 7% 4 4 4 3 4 6% 36%	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	44% 6% 10% 26% 13% 1	3598
	11th to 20th	Day	42% 5% 12% 26% 12% 3	3600
	21st to end	Day	41% 5% 9% 20% 18% 6%	3960
February	1st to 10th	Day	48% 5% 11% 20% 13% 3	1747
	11th to 20th	Day	46% 5% 12% 19% 14% 3	1957
	21st to end	Day	48% 7% 10% 18% 13% 5%	1650
March	1st to 10th	Day	56% 5% 7% 11% 13% 8%	4479
	11th to 20th	Day	49% 6% 13% 18% 12% 3	4837
	21st to end	Day	52% 4 5% 16% 18% 5%	5684
April	1st to 10th	Day	51% 4 9% 18% 15% 4	2400
	11th to 20th	Day	45% 7% 9% 21% 15% 3	2457
	21st to end	Day	53% 5% 8% 17% 14% 4	2600
May	1st to 10th	Day	63% 8% 8% 14% 6% 1	5897
	11th to 20th	Day	53% 8% 12% 17% 9% 2	6063
	21st to end	Day	46% 8% 12% 20% 13% 1	6949
June	1st to 10th	Day	52% 6% 12% 19% 10%	2800
	11th to 20th	Day	43% 8% 11% 20% 15% 2	2800
	21st to end	Day	52% 8% 10% 18% 11% 1	2800
July	1st to 10th	Day	47% 7% 14% 21% 11%	6290
	11th to 20th	Day	40% 7% 14% 24% 13% 1	6285
	21st to end	Day	38% 6% 12% 26% 17%	6728
August	1st to 10th	Day	44% 9% 14% 25% 8%	4302
	11th to 20th	Day	48% 7% 14% 23% 8%	4214
	21st to end	Day	34% 7% 14% 25% 18% 2	4430
September	1st to 10th	Day	33% 7% 13% 27% 19% 1	2375
	11th to 20th	Day	32% 4 10% 30% 21% 2	2200
	21st to end	Day	37% 5% 11% 27% 18% 1	2200
October	1st to 10th	Day	44% 7% 10% 23% 15% 1	3590
	11th to 20th	Day	36% 4 12% 27% 20% 1	3089
	21st to end	Day	38% 4 11% 23% 20% 4	3180
November	1st to 10th	Day	38% 5% 15% 25% 15% 3	1779
	11th to 20th	Day	39% 7% 11% 20% 19% 4	1800
	21st to end	Day	45% 5% 17% 22% 11% 1	1784
December	1st to 10th	Day	45% 6% 15% 23% 9% 1	2384
	11th to 20th	Day	42% 5% 10% 19% 18% 6%	2853
	21st to end	Day	52% 5% 9% 17% 12% 6%	3960
January	1st to 10th	Night	44% 6% 11% 27% 10% 1	6349
	11th to 20th	Night	51% 9% 12% 17% 9% 2	6360
	21st to end	Night	45% 6% 8% 19% 17% 5%	6996
February	1st to 10th	Night	48% 7% 13% 17% 13% 3	2649
	11th to 20th	Night	44% 5% 11% 20% 16% 3	2340
	21st to end	Night	44% 6% 11% 21% 14% 4	1980
March	1st to 10th	Night	55% 4 9% 13% 13% 6%	5374
	11th to 20th	Night	48% 8% 16% 16% 10% 2	5123
	21st to end	Night	52% 4 6% 15% 20% 4	5245
April	1st to 10th	Night	51% 4 8% 19% 16% 3	2000
	11th to 20th	Night	47% 5% 9% 21% 16% 1	1890
	21st to end	Night	50% 5% 11% 16% 14% 4	1800
May	1st to 10th	Night	62% 7% 9% 14% 7% 1	4060
	11th to 20th	Night	52% 9% 10% 20% 7% 1	3864
	21st to end	Night	48% 7% 16% 20% 10% 1	3927
June	1st to 10th	Night	51% 7% 11% 20% 11% 1	1600
	11th to 20th	Night	42% 7% 14% 22% 13% 2	1600
	21st to end	Night	47% 7% 14% 20% 10% 1	1600
July	1st to 10th	Night	41% 6% 18% 23% 11%	3560
	11th to 20th	Night	44% 7% 13% 25% 12%	3552
	21st to end	Night	33% 7% 15% 33% 11%	3809
August	1st to 10th	Night	52% 7% 13% 21% 8%	2791
	11th to 20th	Night	45% 8% 15% 21% 11%	2899
	21st to end	Night	36% 7% 15% 26% 16% 1	3452
September	1st to 10th	Night	34% 6% 13% 31% 16%	2025
	11th to 20th	Night	32% 5% 11% 31% 19% 2	2200
	21st to end	Night	37% 6% 12% 28% 15% 2	2200
October	1st to 10th	Night	44% 6% 12% 22% 14% 1	3590
	11th to 20th	Night	35% 5% 10% 28% 20% 2	4047
	21st to end	Night	40% 4 10% 27% 16% 2	4610
November	1st to 10th	Night	41% 5% 11% 27% 15% 1	2564
	11th to 20th	Night	41% 6% 11% 23% 16% 3	2600
	21st to end	Night	43% 7% 14% 22% 13% 1	2614
December	1st to 10th	Night	49% 4 11% 28% 8%	4778
	11th to 20th	Night	49% 4 9% 16% 19% 3	5317
	21st to end	Night	50% 5% 10% 19% 15% 2	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	36% 16% 26% 18% 5%	200
	11th to 20th	Day	27% 20% 33% 18% 2 2	200
	21st to end	Day	20% 25% 33% 18% 5%	220
February	1st to 10th	Day	28% 20% 31% 17% 3	337
	11th to 20th	Day	28% 25% 32% 12% 2	496
	21st to end	Day	24% 22% 31% 19% 4	414
March	1st to 10th	Day	28% 20% 34% 15% 4	500
	11th to 20th	Day	29% 21% 32% 14% 4 1	500
	21st to end	Day	30% 21% 28% 13% 6% 2	550
April	1st to 10th	Day	35% 21% 24% 14% 5% 1	500
	11th to 20th	Day	31% 21% 28% 13% 5% 2	492
	21st to end	Day	32% 19% 30% 11% 6% 2	500
May	1st to 10th	Day	36% 17% 24% 17% 3 2	503
	11th to 20th	Day	22% 19% 34% 15% 7% 2	527
	21st to end	Day	22% 15% 33% 21% 7% 1	573
June	1st to 10th	Day	23% 14% 35% 18% 8% 2	530
	11th to 20th	Day	22% 15% 31% 21% 10% 2	530
	21st to end	Day	24% 12% 34% 20% 9% 2	530
July	1st to 10th	Day	18% 10% 33% 27% 10% 2	525
	11th to 20th	Day	17% 7% 34% 28% 10% 3	524
	21st to end	Day	16% 9% 35% 26% 13% 2	576
August	1st to 10th	Day	13% 8% 31% 31% 14% 4	526
	11th to 20th	Day	16% 11% 27% 24% 15% 6%	519
	21st to end	Day	13% 10% 31% 28% 13% 5%	583
September	1st to 10th	Day	17% 10% 26% 26% 16% 6%	530
	11th to 20th	Day	17% 11% 24% 22% 16% 10%	530
	21st to end	Day	18% 13% 29% 24% 12% 6%	530
October	1st to 10th	Day	19% 17% 23% 21% 16% 4	530
	11th to 20th	Day	26% 14% 27% 19% 13% 1	436
	21st to end	Day	32% 15% 26% 20% 6%	462
November	1st to 10th	Day	26% 14% 29% 21% 9% 1	401
	11th to 20th	Day	30% 15% 30% 18% 7% 1	400
	21st to end	Day	34% 17% 24% 19% 6% 1	384
December	1st to 10th	Day	27% 20% 29% 17% 8%	200
	11th to 20th	Day	36% 16% 32% 13% 4	200
	21st to end	Day	32% 18% 26% 17% 6% 1	220
January	1st to 10th	Night	22% 17% 35% 23% 3	800
	11th to 20th	Night	25% 18% 29% 24% 4	800
	21st to end	Night	18% 18% 37% 23% 3	880
February	1st to 10th	Night	21% 18% 32% 25% 3	800
	11th to 20th	Night	20% 19% 39% 19% 3	783
	21st to end	Night	22% 16% 34% 24% 4	660
March	1st to 10th	Night	27% 15% 34% 21% 2	800
	11th to 20th	Night	23% 16% 33% 22% 5% 1	800
	21st to end	Night	26% 16% 30% 21% 5% 1	880
April	1st to 10th	Night	29% 15% 30% 23% 3	800
	11th to 20th	Night	23% 14% 35% 23% 5%	706
	21st to end	Night	27% 15% 32% 22% 4	600
May	1st to 10th	Night	27% 10% 30% 27% 5% 1	600
	11th to 20th	Night	23% 12% 31% 27% 7%	526
	21st to end	Night	21% 7% 27% 36% 8%	433
June	1st to 10th	Night	25% 6% 30% 31% 8%	400
	11th to 20th	Night	19% 6% 28% 35% 14%	400
	21st to end	Night	22% 5% 26% 37% 10%	400
July	1st to 10th	Night	16% 3 25% 42% 14%	390
	11th to 20th	Night	17% 3 23% 38% 18% 1	388
	21st to end	Night	16% 3 19% 44% 18% 1	438
August	1st to 10th	Night	15% 4 22% 40% 19% 1	522
	11th to 20th	Night	16% 4 28% 34% 16% 2	592
	21st to end	Night	14% 7% 22% 36% 19% 2	798
September	1st to 10th	Night	15% 6% 29% 33% 17% 1	800
	11th to 20th	Night	17% 7% 24% 27% 21% 4	800
	21st to end	Night	17% 10% 27% 29% 17% 1	800
October	1st to 10th	Night	21% 10% 21% 33% 14% 1	800
	11th to 20th	Night	20% 12% 30% 25% 12% 1	800
	21st to end	Night	29% 11% 29% 26% 5%	880
November	1st to 10th	Night	21% 13% 30% 28% 7% 1	789
	11th to 20th	Night	23% 13% 30% 25% 7% 1	800
	21st to end	Night	24% 15% 29% 26% 5%	800
December	1st to 10th	Night	27% 13% 34% 20% 6%	860
	11th to 20th	Night	26% 16% 32% 22% 4	835
	21st to end	Night	27% 16% 30% 23% 3	880

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