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 2024-11-06.

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% 5% 4 5% 4 6% 7% 36%	101126
	Night	18% 8% 3 6% 4 4 4 4 5% 6% 36%	97920
Transparency	Day	45% 7% 12% 21% 13% 2	104652
	Night	46% 7% 12% 21% 13% 2	102484
Seeing	Day	24% 15% 30% 20% 9% 2	14914
	Night	22% 12% 30% 27% 8% 1	23797

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	15% 8% 2 6% 5% 4 4 4 7% 7% 39%	2517
	11th to 20th	Day	16% 8% 2 6% 6% 4 6% 4 5% 7% 35%	2520
	21st to end	Day	16% 11% 3 6% 4 5% 4 4 5% 6% 36%	2772
February	1st to 10th	Day	16% 10% 3 7% 4 5% 5% 4 5% 6% 35%	1658
	11th to 20th	Day	11% 11% 2 8% 5% 5% 5% 4 7% 5% 37%	1848
	21st to end	Day	16% 9% 3 7% 6% 5% 6% 4 5% 6% 34%	1570
March	1st to 10th	Day	23% 13% 3 7% 4 4 5% 3 5% 5% 27%	3116
	11th to 20th	Day	15% 9% 3 8% 5% 4 4 3 5% 6% 36%	3379
	21st to end	Day	22% 9% 3 6% 4 3 3 3 5% 7% 34%	3964
April	1st to 10th	Day	19% 12% 3 6% 5% 3 3 3 4 7% 35%	2280
	11th to 20th	Day	18% 6% 2 7% 5% 5% 4 4 5% 7% 37%	2333
	21st to end	Day	23% 9% 3 7% 5% 4 4 4 4 4 34%	2470
May	1st to 10th	Day	24% 11% 3 8% 4 5% 5% 4 5% 6% 26%	4106
	11th to 20th	Day	18% 8% 3 8% 6% 5% 5% 3 5% 7% 30%	4223
	21st to end	Day	15% 6% 2 5% 5% 5% 5% 5% 6% 9% 37%	4837
June	1st to 10th	Day	23% 7% 2 6% 5% 5% 5% 4 6% 7% 29%	2660
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 7% 38%	2660
	21st to end	Day	22% 7% 3 7% 5% 5% 5% 4 6% 7% 29%	2660
July	1st to 10th	Day	14% 6% 2 7% 5% 4 6% 5% 8% 10% 34%	4440
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 8% 11% 37%	4440
	21st to end	Day	19% 7% 2 6% 5% 4 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 5% 7% 9% 33%	4183
	11th to 20th	Day	17% 9% 3 7% 5% 5% 5% 5% 6% 7% 32%	4045
	21st to end	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 44%	4294
September	1st to 10th	Day	12% 4 2 5% 4 4 4 4 6% 8% 48%	2256
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 7% 52%	2090
	21st to end	Day	16% 5% 2 5% 4 3 4 3 6% 7% 44%	2090
October	1st to 10th	Day	19% 7% 3 6% 4 4 4 3 4 6% 39%	2090
	11th to 20th	Day	18% 6% 2 5% 4 4 4 3 4 7% 42%	1817
	21st to end	Day	24% 8% 2 5% 5% 4 4 3 4 6% 35%	1881
November	1st to 10th	Day	17% 7% 2 5% 3 3 3 3 5% 7% 44%	1717
	11th to 20th	Day	21% 5% 2 4 4 4 4 4 7% 7% 40%	1710
	21st to end	Day	21% 7% 2 5% 4 3 3 3 5% 7% 39%	1695
December	1st to 10th	Day	21% 12% 3 7% 4 4 4 3 5% 5% 33%	1395
	11th to 20th	Day	23% 9% 2 5% 4 3 3 3 5% 5% 37%	1824
	21st to end	Day	23% 12% 3 6% 4 4 4 4 5% 5% 29%	2772
January	1st to 10th	Night	15% 10% 3 6% 4 4 4 4 4 7% 39%	4429
	11th to 20th	Night	13% 9% 3 10% 6% 5% 6% 5% 7% 7% 28%	4440
	21st to end	Night	19% 11% 3 9% 5% 5% 5% 4 5% 6% 29%	4884
February	1st to 10th	Night	13% 10% 3 7% 5% 6% 5% 5% 5% 6% 34%	2518
	11th to 20th	Night	11% 11% 3 7% 5% 5% 5% 4 5% 5% 40%	2218
	21st to end	Night	13% 10% 3 7% 5% 5% 4 4 5% 6% 38%	1884
March	1st to 10th	Night	26% 11% 2 5% 4 4 4 4 4 5% 31%	3724
	11th to 20th	Night	13% 9% 4 8% 6% 6% 7% 5% 6% 5% 31%	3580
	21st to end	Night	24% 10% 3 6% 4 4 4 3 6% 7% 30%	3665
April	1st to 10th	Night	20% 11% 3 5% 5% 4 4 2 4 6% 36%	1900
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 4 7% 37%	1795
	21st to end	Night	23% 8% 2 6% 4 4 5% 3 4 7% 35%	1710
May	1st to 10th	Night	22% 10% 3 7% 5% 5% 4 4 6% 6% 28%	2834
	11th to 20th	Night	18% 10% 4 9% 5% 5% 5% 5% 6% 8% 26%	2699
	21st to end	Night	15% 7% 3 6% 4 4 4 4 6% 7% 40%	2738
June	1st to 10th	Night	23% 7% 2 7% 4 3 5% 3 6% 6% 33%	1520
	11th to 20th	Night	18% 6% 3 5% 4 4 3 4 6% 8% 39%	1520
	21st to end	Night	21% 7% 2 6% 4 3 5% 4 6% 7% 34%	1520
July	1st to 10th	Night	11% 5% 3 7% 5% 5% 6% 5% 8% 9% 37%	2520
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 4 5% 8% 41%	2520
	21st to end	Night	15% 6% 2 5% 4 4 5% 4 6% 6% 42%	2739
August	1st to 10th	Night	21% 8% 3 7% 4 5% 5% 3 5% 6% 33%	2715
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 4 5% 9% 35%	2782
	21st to end	Night	11% 6% 2 6% 5% 5% 5% 3 6% 8% 44%	3346
September	1st to 10th	Night	13% 5% 2 4 4 4 3 3 6% 6% 50%	1924
	11th to 20th	Night	16% 5% 2 4 3 3 3 3 5% 5% 52%	2090
	21st to end	Night	16% 6% 2 5% 3 4 3 3 5% 8% 45%	2090
October	1st to 10th	Night	19% 7% 3 6% 4 5% 4 4 5% 6% 38%	2090
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 4 6% 7% 40%	2360
	21st to end	Night	24% 9% 2 4 4 4 4 3 5% 5% 36%	2717
November	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	2483
	11th to 20th	Night	18% 7% 2 5% 4 4 5% 3 5% 6% 42%	2470
	21st to end	Night	16% 8% 2 6% 5% 3 4 3 5% 7% 41%	2483
December	1st to 10th	Night	20% 10% 3 7% 5% 4 4 3 5% 6% 32%	2774
	11th to 20th	Night	21% 9% 2 6% 4 3 4 3 5% 5% 37%	3355
	21st to end	Night	23% 10% 3 7% 4 4 3 3 4 6% 33%	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	41% 7% 12% 26% 11% 1	2518
	11th to 20th	Day	47% 6% 13% 23% 10% 2	2520
	21st to end	Day	47% 7% 12% 21% 12% 1	2772
February	1st to 10th	Day	48% 5% 12% 20% 12% 3	1658
	11th to 20th	Day	47% 5% 12% 19% 14% 3	1857
	21st to end	Day	48% 7% 10% 18% 12% 5%	1570
March	1st to 10th	Day	59% 6% 8% 11% 14% 3	3119
	11th to 20th	Day	46% 7% 11% 18% 14% 4	3379
	21st to end	Day	50% 4 7% 14% 20% 5%	3964
April	1st to 10th	Day	51% 4 9% 18% 14% 4	2280
	11th to 20th	Day	45% 8% 10% 21% 14% 3	2333
	21st to end	Day	53% 5% 8% 17% 13% 4	2470
May	1st to 10th	Day	56% 9% 9% 16% 8% 1	4120
	11th to 20th	Day	51% 9% 13% 16% 10% 2	4227
	21st to end	Day	39% 9% 14% 21% 15% 1	4837
June	1st to 10th	Day	52% 7% 13% 19% 9%	2660
	11th to 20th	Day	43% 8% 11% 19% 15% 3	2660
	21st to end	Day	53% 8% 11% 17% 10% 1	2660
July	1st to 10th	Day	42% 7% 14% 24% 12%	4370
	11th to 20th	Day	39% 8% 16% 27% 10%	4365
	21st to end	Day	45% 7% 12% 21% 15%	4661
August	1st to 10th	Day	44% 9% 14% 24% 8%	4169
	11th to 20th	Day	49% 7% 14% 23% 7%	4084
	21st to end	Day	34% 7% 14% 25% 18% 2	4294
September	1st to 10th	Day	33% 8% 14% 27% 19% 1	2256
	11th to 20th	Day	32% 5% 10% 30% 20% 3	2090
	21st to end	Day	38% 5% 12% 27% 17% 1	2090
October	1st to 10th	Day	45% 7% 11% 22% 14% 1	3480
	11th to 20th	Day	36% 4 12% 27% 20% 1	2994
	21st to end	Day	38% 4 12% 23% 20% 4	3081
November	1st to 10th	Day	39% 4 15% 24% 15% 3	1717
	11th to 20th	Day	41% 7% 12% 20% 17% 3	1710
	21st to end	Day	45% 5% 17% 21% 11% 1	1695
December	1st to 10th	Day	51% 6% 12% 19% 10% 2	1395
	11th to 20th	Day	48% 5% 9% 19% 17% 2	1825
	21st to end	Day	56% 6% 11% 17% 9% 2	2772
January	1st to 10th	Night	43% 7% 13% 25% 9% 2	4429
	11th to 20th	Night	47% 11% 14% 16% 10% 2	4440
	21st to end	Night	54% 7% 8% 19% 11% 2	4884
February	1st to 10th	Night	48% 7% 13% 17% 13% 3	2518
	11th to 20th	Night	44% 5% 11% 21% 16% 3	2220
	21st to end	Night	44% 6% 11% 21% 13% 4	1884
March	1st to 10th	Night	54% 4 11% 17% 10% 3	3734
	11th to 20th	Night	48% 10% 11% 17% 12% 3	3581
	21st to end	Night	53% 4 7% 14% 18% 4	3665
April	1st to 10th	Night	50% 4 8% 19% 16% 3	1900
	11th to 20th	Night	47% 5% 9% 22% 16% 1	1795
	21st to end	Night	50% 6% 11% 16% 13% 4	1710
May	1st to 10th	Night	54% 9% 11% 17% 9% 1	2840
	11th to 20th	Night	51% 11% 13% 17% 7% 2	2700
	21st to end	Night	40% 7% 19% 20% 13% 1	2739
June	1st to 10th	Night	51% 7% 12% 20% 11% 1	1520
	11th to 20th	Night	42% 7% 15% 21% 13% 2	1520
	21st to end	Night	48% 7% 14% 19% 10% 1	1520
July	1st to 10th	Night	41% 8% 18% 21% 12% 1	2480
	11th to 20th	Night	41% 7% 14% 27% 11%	2472
	21st to end	Night	40% 8% 13% 26% 12%	2651
August	1st to 10th	Night	52% 7% 13% 21% 8%	2705
	11th to 20th	Night	45% 8% 15% 20% 11%	2809
	21st to end	Night	36% 7% 15% 25% 16% 1	3346
September	1st to 10th	Night	34% 6% 13% 31% 15%	1924
	11th to 20th	Night	34% 4 11% 30% 18% 2	2090
	21st to end	Night	37% 6% 12% 28% 15% 2	2090
October	1st to 10th	Night	45% 7% 12% 22% 13% 1	3480
	11th to 20th	Night	35% 5% 9% 28% 21% 2	3922
	21st to end	Night	40% 4 11% 27% 17% 2	4467
November	1st to 10th	Night	42% 6% 11% 26% 14% 2	2483
	11th to 20th	Night	42% 6% 11% 23% 16% 2	2470
	21st to end	Night	42% 7% 14% 23% 13% 1	2483
December	1st to 10th	Night	52% 6% 10% 21% 10% 1	2774
	11th to 20th	Night	48% 5% 10% 19% 17% 1	3355
	21st to end	Night	53% 5% 11% 21% 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	33% 16% 27% 20% 5%	180
	11th to 20th	Day	28% 18% 33% 17% 2 2	180
	21st to end	Day	19% 24% 32% 19% 6%	198
February	1st to 10th	Day	29% 18% 31% 18% 3	308
	11th to 20th	Day	27% 25% 32% 13% 3	456
	21st to end	Day	25% 21% 30% 20% 4	382
March	1st to 10th	Day	27% 19% 34% 15% 5%	460
	11th to 20th	Day	28% 20% 32% 15% 4 1	460
	21st to end	Day	29% 22% 27% 13% 6% 2	506
April	1st to 10th	Day	34% 20% 24% 15% 5% 2	460
	11th to 20th	Day	29% 21% 28% 14% 6% 2	452
	21st to end	Day	32% 20% 29% 11% 6% 2	460
May	1st to 10th	Day	34% 18% 25% 18% 3 2	463
	11th to 20th	Day	22% 19% 35% 16% 7% 2	487
	21st to end	Day	20% 16% 33% 22% 8% 1	529
June	1st to 10th	Day	22% 15% 34% 19% 8% 2	490
	11th to 20th	Day	21% 14% 31% 21% 11% 2	490
	21st to end	Day	24% 12% 32% 20% 9% 1	490
July	1st to 10th	Day	18% 9% 33% 27% 10% 2	485
	11th to 20th	Day	15% 7% 35% 29% 11% 4	484
	21st to end	Day	15% 9% 35% 25% 13% 3	532
August	1st to 10th	Day	14% 9% 29% 31% 14% 3	486
	11th to 20th	Day	17% 11% 27% 25% 15% 5%	479
	21st to end	Day	13% 11% 29% 28% 13% 6%	539
September	1st to 10th	Day	17% 10% 27% 25% 17% 6%	490
	11th to 20th	Day	18% 11% 23% 22% 15% 10%	490
	21st to end	Day	17% 12% 29% 23% 11% 6%	490
October	1st to 10th	Day	20% 17% 24% 21% 16% 3	490
	11th to 20th	Day	26% 14% 27% 20% 13% 1	405
	21st to end	Day	33% 15% 25% 20% 7%	429
November	1st to 10th	Day	26% 14% 28% 21% 9% 2	381
	11th to 20th	Day	30% 14% 30% 19% 7% 1	370
	21st to end	Day	33% 16% 24% 19% 7% 1	355
December	1st to 10th	Day	27% 20% 29% 16% 8%	180
	11th to 20th	Day	34% 15% 33% 13% 4	180
	21st to end	Day	34% 18% 27% 15% 5% 1	198
January	1st to 10th	Night	22% 16% 35% 23% 3	760
	11th to 20th	Night	25% 18% 30% 23% 4	760
	21st to end	Night	19% 17% 37% 23% 3	836
February	1st to 10th	Night	21% 18% 33% 26% 3	760
	11th to 20th	Night	19% 19% 39% 20% 3	743
	21st to end	Night	23% 16% 33% 24% 4	628
March	1st to 10th	Night	26% 15% 35% 21% 2	760
	11th to 20th	Night	22% 17% 33% 22% 5% 1	760
	21st to end	Night	26% 16% 31% 21% 5% 1	836
April	1st to 10th	Night	28% 15% 30% 24% 3	760
	11th to 20th	Night	23% 13% 36% 23% 5%	671
	21st to end	Night	27% 15% 31% 22% 5%	570
May	1st to 10th	Night	26% 10% 30% 28% 5% 1	570
	11th to 20th	Night	23% 11% 31% 28% 7%	500
	21st to end	Night	21% 8% 27% 36% 8%	411
June	1st to 10th	Night	25% 6% 29% 31% 8%	380
	11th to 20th	Night	18% 6% 27% 34% 14%	380
	21st to end	Night	23% 4 25% 37% 10%	380
July	1st to 10th	Night	16% 3 25% 41% 15%	370
	11th to 20th	Night	17% 3 23% 39% 18% 1	368
	21st to end	Night	16% 3 19% 43% 18% 1	416
August	1st to 10th	Night	15% 4 22% 41% 18% 1	496
	11th to 20th	Night	16% 4 28% 35% 16% 1	562
	21st to end	Night	14% 7% 22% 35% 19% 2	758
September	1st to 10th	Night	15% 5% 29% 32% 18% 1	760
	11th to 20th	Night	18% 7% 24% 26% 21% 4	760
	21st to end	Night	17% 10% 26% 29% 17% 1	760
October	1st to 10th	Night	22% 10% 21% 33% 13% 1	760
	11th to 20th	Night	20% 12% 29% 25% 12% 1	760
	21st to end	Night	30% 12% 29% 25% 5%	836
November	1st to 10th	Night	21% 12% 30% 28% 7% 1	764
	11th to 20th	Night	23% 13% 30% 26% 7% 1	760
	21st to end	Night	23% 16% 29% 26% 6%	760
December	1st to 10th	Night	27% 13% 35% 20% 5%	814
	11th to 20th	Night	25% 17% 31% 23% 4	792
	21st to end	Night	28% 16% 31% 23% 3	836

Page generated at 2024-11-03 13:19:04. Version ac07

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