F.Myers Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the F.Myers Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	12% 7% 2 6% 4 4 4 4 6% 8% 42%	127067
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 45%	125110
Transparency	Day	38% 6% 11% 23% 18% 4	130731
	Night	37% 6% 10% 22% 20% 5%	129521
Seeing	Day	15% 9% 22% 24% 20% 9%	16940
	Night	16% 8% 25% 30% 19% 3	26796

By Month

Percentage Cloud Cover SE (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% 10% 4 10% 3 3 3 2 4 6% 39%	3697
	11th to 20th	Day	15% 8% 2 7% 3 3 2 2 3 5% 51%	4006
	21st to end	Day	19% 8% 2 5% 3 3 4 3 4 6% 43%	4694
February	1st to 10th	Day	20% 11% 4 6% 4 3 3 3 4 6% 37%	2000
	11th to 20th	Day	20% 9% 3 8% 4 4 3 3 3 6% 38%	1948
	21st to end	Day	22% 10% 2 6% 5% 3 3 2 4 5% 36%	1650
March	1st to 10th	Day	16% 13% 4 8% 4 4 4 3 4 5% 36%	4480
	11th to 20th	Day	14% 8% 2 6% 4 3 3 3 4 5% 47%	4740
	21st to end	Day	12% 7% 3 7% 5% 4 5% 3 5% 6% 43%	5963
April	1st to 10th	Day	14% 7% 3 6% 4 4 4 4 5% 7% 44%	2400
	11th to 20th	Day	15% 7% 3 6% 4 3 3 3 4 6% 44%	2374
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 3 5% 8% 41%	2400
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 6% 5% 7% 9% 28%	5607
	11th to 20th	Day	14% 8% 3 9% 5% 5% 6% 5% 6% 8% 31%	5861
	21st to end	Day	16% 7% 3 6% 4 4 4 4 6% 8% 36%	6518
June	1st to 10th	Day	9% 7% 2 7% 5% 4 5% 4 6% 10% 41%	2800
	11th to 20th	Day	5% 4 2 6% 5% 5% 5% 5% 8% 10% 46%	2800
	21st to end	Day	5% 3 2 5% 5% 5% 5% 6% 8% 11% 45%	2800
July	1st to 10th	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 11% 40%	6360
	11th to 20th	Day	6% 5% 3 6% 5% 5% 6% 6% 9% 13% 37%	6360
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 5% 8% 11% 34%	6432
August	1st to 10th	Day	3 3 2 5% 3 4 5% 4 7% 10% 55%	3905
	11th to 20th	Day	8% 5% 2 6% 3 4 5% 5% 6% 9% 46%	3849
	21st to end	Day	5% 4 1 5% 4 4 4 3 5% 8% 56%	4312
September	1st to 10th	Day	9% 6% 2 5% 4 5% 5% 4 7% 9% 45%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 5% 7% 47%	2270
	21st to end	Day	12% 7% 2 5% 4 4 4 3 4 6% 50%	2200
October	1st to 10th	Day	8% 4 2 4 3 3 3 3 4 7% 58%	2191
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 7% 55%	2000
	21st to end	Day	18% 8% 2 4 3 2 3 3 3 5% 50%	2131
November	1st to 10th	Day	13% 5% 2 5% 3 3 2 3 4 6% 54%	1852
	11th to 20th	Day	14% 6% 2 6% 3 3 3 3 4 6% 51%	1890
	21st to end	Day	16% 8% 2 5% 3 3 4 3 5% 7% 44%	1890
December	1st to 10th	Day	16% 10% 2 6% 3 3 3 2 3 5% 46%	3020
	11th to 20th	Day	20% 9% 3 6% 4 4 3 3 4 5% 40%	3197
	21st to end	Day	15% 12% 3 7% 5% 5% 5% 4 5% 6% 32%	4070
January	1st to 10th	Night	17% 11% 3 9% 3 3 3 3 4 5% 39%	6488
	11th to 20th	Night	16% 9% 3 7% 4 3 3 2 3 4 46%	6189
	21st to end	Night	21% 8% 2 6% 3 3 3 3 4 5% 42%	6517
February	1st to 10th	Night	21% 10% 3 7% 4 3 4 3 4 5% 35%	2401
	11th to 20th	Night	21% 8% 3 7% 5% 4 4 2 4 5% 38%	2338
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 6% 39%	1980
March	1st to 10th	Night	18% 10% 3 7% 4 4 4 3 5% 5% 37%	5373
	11th to 20th	Night	12% 8% 2 5% 4 3 3 3 5% 6% 50%	5261
	21st to end	Night	13% 7% 2 7% 4 4 4 4 5% 7% 43%	4964
April	1st to 10th	Night	12% 7% 2 6% 4 3 4 4 5% 7% 47%	2000
	11th to 20th	Night	15% 6% 2 5% 3 4 2 3 6% 7% 47%	1975
	21st to end	Night	13% 6% 2 6% 5% 4 4 4 6% 7% 44%	2000
May	1st to 10th	Night	18% 10% 3 8% 5% 4 4 3 5% 7% 34%	4273
	11th to 20th	Night	14% 10% 3 8% 5% 5% 4 4 4 6% 36%	4032
	21st to end	Night	14% 7% 2 7% 5% 4 4 4 6% 8% 36%	4334
June	1st to 10th	Night	8% 6% 2 8% 4 4 5% 4 7% 10% 41%	1600
	11th to 20th	Night	4 4 1 3 3 3 4 4 7% 11% 56%	1600
	21st to end	Night	3 3 1 4 4 4 4 6% 7% 13% 51%	1600
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 10% 41%	3618
	11th to 20th	Night	3 3 2 5% 4 4 6% 4 8% 14% 45%	3617
	21st to end	Night	7% 6% 3 7% 5% 5% 5% 5% 9% 12% 37%	4424
August	1st to 10th	Night	2 3 1 4 3 3 4 3 6% 9% 63%	3244
	11th to 20th	Night	4 3 1 5% 3 4 4 4 7% 11% 54%	3200
	21st to end	Night	4 3 1 5% 3 3 4 4 6% 8% 59%	3586
September	1st to 10th	Night	8% 5% 2 5% 3 3 3 4 6% 7% 53%	2000
	11th to 20th	Night	8% 6% 2 6% 4 3 3 3 5% 8% 51%	2108
	21st to end	Night	10% 5% 2 4 3 4 3 3 5% 7% 55%	2200
October	1st to 10th	Night	8% 4 1 3 3 2 3 3 4 6% 62%	2205
	11th to 20th	Night	11% 5% 1 3 2 3 3 2 4 5% 58%	2400
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 5% 50%	2706
November	1st to 10th	Night	12% 6% 2 4 3 3 3 2 5% 5% 55%	2666
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 7% 52%	2730
	21st to end	Night	16% 7% 3 4 4 3 3 3 4 5% 47%	2730
December	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 5% 45%	4380
	11th to 20th	Night	20% 8% 3 6% 3 3 3 3 3 5% 43%	5221
	21st to end	Night	18% 10% 4 9% 5% 4 4 3 4 5% 33%	7150

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	47% 3 11% 23% 15% 1	3698
	11th to 20th	Day	38% 3 6% 26% 25% 1	4006
	21st to end	Day	42% 4 7% 20% 22% 6%	4694
February	1st to 10th	Day	50% 4 9% 21% 15% 2	2000
	11th to 20th	Day	50% 3 8% 19% 16% 4	1958
	21st to end	Day	51% 3 8% 16% 15% 6%	1650
March	1st to 10th	Day	51% 4 5% 16% 15% 9%	4480
	11th to 20th	Day	39% 5% 7% 22% 19% 8%	4740
	21st to end	Day	41% 5% 9% 24% 20% 1	5965
April	1st to 10th	Day	38% 6% 9% 23% 21% 3	2400
	11th to 20th	Day	40% 5% 9% 21% 19% 5%	2374
	21st to end	Day	41% 7% 10% 25% 17% 1	2400
May	1st to 10th	Day	48% 10% 14% 18% 10% 1	5642
	11th to 20th	Day	49% 9% 14% 18% 9% 1	5865
	21st to end	Day	43% 8% 10% 23% 14% 2	6532
June	1st to 10th	Day	37% 8% 12% 22% 18% 2	2800
	11th to 20th	Day	29% 8% 15% 27% 18% 3	2800
	21st to end	Day	28% 9% 17% 24% 17% 3	2800
July	1st to 10th	Day	33% 10% 16% 27% 12% 1	6291
	11th to 20th	Day	32% 11% 21% 27% 7% 1	6285
	21st to end	Day	40% 10% 22% 22% 6%	6247
August	1st to 10th	Day	21% 8% 18% 29% 23% 2	3893
	11th to 20th	Day	32% 8% 16% 24% 17% 2	3885
	21st to end	Day	26% 7% 11% 29% 25% 3	4311
September	1st to 10th	Day	34% 8% 10% 26% 17% 5%	2400
	11th to 20th	Day	36% 7% 10% 20% 24% 4	2270
	21st to end	Day	35% 5% 10% 20% 24% 6%	2200
October	1st to 10th	Day	17% 3 7% 15% 43% 14%	3568
	11th to 20th	Day	20% 3 6% 24% 31% 16%	3240
	21st to end	Day	29% 3 5% 24% 33% 6%	3421
November	1st to 10th	Day	32% 4 7% 24% 26% 6%	1852
	11th to 20th	Day	35% 4 8% 22% 24% 7%	1890
	21st to end	Day	40% 4 8% 22% 21% 4	1890
December	1st to 10th	Day	42% 4 6% 27% 19% 2	3020
	11th to 20th	Day	48% 4 8% 22% 16% 3	3194
	21st to end	Day	51% 6% 10% 23% 10% 1	4070
January	1st to 10th	Night	48% 3 10% 20% 16% 2	6488
	11th to 20th	Night	43% 4 6% 19% 24% 3	6189
	21st to end	Night	46% 3 6% 21% 20% 4	6517
February	1st to 10th	Night	52% 4 9% 16% 17% 2	2401
	11th to 20th	Night	49% 5% 7% 18% 16% 5%	2339
	21st to end	Night	48% 4 8% 17% 18% 5%	1980
March	1st to 10th	Night	48% 4 7% 15% 18% 8%	5373
	11th to 20th	Night	35% 4 8% 21% 17% 15%	5261
	21st to end	Night	40% 5% 9% 23% 20% 3	4964
April	1st to 10th	Night	36% 6% 11% 23% 20% 5%	2000
	11th to 20th	Night	37% 4 8% 21% 22% 8%	1975
	21st to end	Night	38% 7% 10% 24% 18% 3	2000
May	1st to 10th	Night	48% 7% 13% 20% 11% 2	4303
	11th to 20th	Night	48% 8% 13% 19% 11% 1	4033
	21st to end	Night	42% 8% 12% 23% 14% 2	4342
June	1st to 10th	Night	37% 8% 11% 20% 23% 2	1600
	11th to 20th	Night	22% 7% 17% 31% 20% 4	1600
	21st to end	Night	22% 10% 18% 28% 17% 6%	1600
July	1st to 10th	Night	33% 10% 16% 25% 15% 1	3580
	11th to 20th	Night	25% 10% 22% 32% 8% 3	3572
	21st to end	Night	35% 11% 23% 23% 9%	4296
August	1st to 10th	Night	17% 7% 12% 39% 23% 1	3232
	11th to 20th	Night	21% 9% 19% 30% 18% 2	3230
	21st to end	Night	21% 6% 12% 30% 26% 4	3586
September	1st to 10th	Night	29% 7% 12% 25% 21% 7%	2000
	11th to 20th	Night	31% 6% 11% 19% 29% 4	2108
	21st to end	Night	29% 5% 10% 25% 25% 7%	2200
October	1st to 10th	Night	16% 4 6% 16% 43% 16%	3608
	11th to 20th	Night	17% 3 5% 18% 37% 19%	3899
	21st to end	Night	26% 4 6% 26% 25% 13%	4366
November	1st to 10th	Night	32% 4 7% 24% 26% 7%	2666
	11th to 20th	Night	32% 4 7% 23% 25% 10%	2730
	21st to end	Night	40% 4 7% 22% 22% 5%	2730
December	1st to 10th	Night	41% 4 8% 22% 23% 2	4380
	11th to 20th	Night	44% 3 6% 23% 20% 3	5223
	21st to end	Night	51% 6% 10% 22% 10% 3	7150

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	24% 20% 35% 16% 5%	330
	11th to 20th	Day	27% 19% 36% 13% 4 1	330
	21st to end	Day	23% 21% 41% 12% 3	363
February	1st to 10th	Day	27% 19% 31% 18% 4 1	300
	11th to 20th	Day	33% 22% 33% 9% 3	300
	21st to end	Day	31% 22% 34% 8% 4	249
March	1st to 10th	Day	26% 18% 36% 14% 5%	300
	11th to 20th	Day	27% 21% 33% 16% 3	300
	21st to end	Day	24% 15% 34% 19% 7% 1	546
April	1st to 10th	Day	20% 18% 28% 24% 8% 1	500
	11th to 20th	Day	20% 15% 30% 23% 9% 2	492
	21st to end	Day	16% 15% 30% 23% 12% 3	500
May	1st to 10th	Day	20% 14% 26% 25% 13% 3	503
	11th to 20th	Day	17% 8% 22% 25% 20% 9%	527
	21st to end	Day	12% 5% 20% 30% 20% 12%	606
June	1st to 10th	Day	9% 6% 17% 28% 24% 15%	730
	11th to 20th	Day	6% 3 13% 31% 32% 15%	730
	21st to end	Day	6% 2 16% 32% 27% 16%	730
July	1st to 10th	Day	9% 2 11% 27% 29% 22%	721
	11th to 20th	Day	8% 3 9% 25% 35% 20%	718
	21st to end	Day	8% 2 10% 25% 34% 21%	720
August	1st to 10th	Day	7% 2 9% 28% 35% 18%	526
	11th to 20th	Day	11% 2 12% 29% 34% 12%	519
	21st to end	Day	10% 2 7% 23% 35% 22%	583
September	1st to 10th	Day	9% 2 14% 28% 35% 12%	530
	11th to 20th	Day	11% 2 10% 24% 36% 16%	530
	21st to end	Day	17% 5% 15% 30% 23% 11%	530
October	1st to 10th	Day	8% 4 18% 35% 27% 8%	521
	11th to 20th	Day	9% 11% 34% 30% 14% 3	330
	21st to end	Day	23% 12% 33% 21% 9% 2	363
November	1st to 10th	Day	17% 10% 31% 28% 12% 2	330
	11th to 20th	Day	12% 15% 36% 28% 8% 1	330
	21st to end	Day	24% 14% 31% 18% 11% 2	330
December	1st to 10th	Day	25% 19% 33% 18% 5% 1	330
	11th to 20th	Day	26% 16% 31% 20% 6% 1	330
	21st to end	Day	27% 16% 36% 17% 3	363
January	1st to 10th	Night	22% 15% 33% 25% 4	1050
	11th to 20th	Night	24% 16% 34% 22% 4	1050
	21st to end	Night	21% 12% 41% 24% 2	1104
February	1st to 10th	Night	25% 14% 34% 24% 3	801
	11th to 20th	Night	27% 14% 35% 23% 2	783
	21st to end	Night	24% 16% 36% 22% 2	660
March	1st to 10th	Night	22% 14% 32% 29% 5%	800
	11th to 20th	Night	22% 12% 34% 26% 5% 1	725
	21st to end	Night	17% 11% 34% 30% 8%	660
April	1st to 10th	Night	16% 10% 35% 28% 11% 1	600
	11th to 20th	Night	19% 10% 30% 29% 12%	593
	21st to end	Night	15% 7% 33% 32% 12% 2	600
May	1st to 10th	Night	16% 6% 29% 34% 14% 2	600
	11th to 20th	Night	13% 5% 22% 36% 22% 2	591
	21st to end	Night	7% 4 18% 38% 29% 5%	650
June	1st to 10th	Night	10% 3 14% 40% 29% 5%	600
	11th to 20th	Night	6% 2 10% 37% 43% 3	600
	21st to end	Night	4 2 11% 38% 40% 6%	600
July	1st to 10th	Night	6% 3 7% 32% 42% 10%	585
	11th to 20th	Night	8% 1 7% 28% 44% 11%	582
	21st to end	Night	6% 1 8% 29% 46% 10%	657
August	1st to 10th	Night	5% 1 7% 33% 45% 9%	597
	11th to 20th	Night	6% 1 7% 39% 41% 5%	594
	21st to end	Night	8% 2 8% 29% 45% 9%	660
September	1st to 10th	Night	9% 2 9% 34% 42% 6%	600
	11th to 20th	Night	9% 2 12% 31% 40% 6%	708
	21st to end	Night	11% 3 14% 35% 33% 4	800
October	1st to 10th	Night	9% 3 16% 36% 31% 4	800
	11th to 20th	Night	14% 5% 23% 37% 19% 2	800
	21st to end	Night	20% 7% 27% 30% 14% 2	880
November	1st to 10th	Night	15% 7% 25% 35% 16% 2	820
	11th to 20th	Night	13% 10% 32% 34% 11% 1	840
	21st to end	Night	20% 9% 30% 29% 10% 1	840
December	1st to 10th	Night	23% 11% 32% 27% 6%	840
	11th to 20th	Night	23% 11% 29% 31% 6%	971
	21st to end	Night	26% 14% 30% 26% 4	1155

Page generated at 2026-01-29 00:21:40. Version ac07

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