Old Town Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Town Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	127331
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	124388
Transparency	Day	39% 6% 11% 23% 17% 4	130924
	Night	39% 5% 11% 23% 18% 4	128865
Seeing	Day	20% 13% 28% 24% 12% 4	16504
	Night	18% 11% 30% 29% 11% 1	26101

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% 4 1 5% 4 3 4 3 5% 6% 51%	3789
	11th to 20th	Day	15% 8% 2 6% 4 3 3 3 4 5% 47%	4114
	21st to end	Day	15% 7% 2 7% 5% 4 4 3 4 6% 42%	4466
February	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 7% 39%	1800
	11th to 20th	Day	19% 7% 3 5% 3 3 3 4 6% 6% 40%	1823
	21st to end	Day	18% 8% 2 6% 5% 4 4 3 4 7% 40%	1650
March	1st to 10th	Day	18% 6% 1 5% 3 4 5% 3 5% 7% 42%	4836
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 4 6% 51%	4980
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 5% 7% 41%	5574
April	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 4 6% 7% 43%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 6% 48%	2374
	21st to end	Day	17% 6% 2 5% 3 4 5% 4 6% 7% 42%	2400
May	1st to 10th	Day	19% 8% 2 7% 5% 4 5% 4 7% 9% 31%	5718
	11th to 20th	Day	20% 7% 3 6% 4 4 5% 5% 6% 10% 31%	6170
	21st to end	Day	15% 5% 2 6% 5% 5% 5% 4 6% 9% 39%	6934
June	1st to 10th	Day	11% 4 2 6% 5% 4 5% 4 7% 10% 43%	2800
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 42%	2800
	21st to end	Day	8% 4 2 6% 4 4 5% 5% 8% 10% 43%	2800
July	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 5% 8% 11% 44%	6360
	11th to 20th	Day	9% 6% 3 7% 5% 5% 7% 6% 10% 12% 29%	6176
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	6424
August	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 4 8% 10% 33%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 9% 38%	3977
	21st to end	Day	9% 4 2 5% 4 4 5% 4 7% 9% 47%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 46%	2400
	11th to 20th	Day	11% 6% 3 4 4 3 4 3 6% 9% 47%	2400
	21st to end	Day	19% 9% 2 7% 3 4 4 4 6% 6% 36%	2110
October	1st to 10th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 48%	2000
	11th to 20th	Day	16% 4 2 4 3 3 3 3 5% 5% 51%	2000
	21st to end	Day	13% 6% 2 5% 3 3 3 3 5% 7% 49%	2200
November	1st to 10th	Day	11% 5% 1 4 4 4 4 3 5% 7% 52%	1942
	11th to 20th	Day	13% 6% 2 4 3 4 3 3 5% 8% 50%	1890
	21st to end	Day	14% 6% 2 5% 4 3 3 3 5% 6% 48%	1680
December	1st to 10th	Day	11% 5% 2 7% 4 4 4 3 6% 7% 48%	2680
	11th to 20th	Day	17% 8% 2 5% 3 3 3 3 5% 6% 45%	3070
	21st to end	Day	16% 8% 3 7% 4 4 4 3 6% 7% 39%	4048
January	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 4 5% 49%	6371
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 52%	5979
	21st to end	Night	14% 8% 3 6% 4 3 3 2 4 6% 47%	6490
February	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 6% 6% 41%	2600
	11th to 20th	Night	20% 6% 2 6% 3 3 3 3 4 6% 43%	2463
	21st to end	Night	17% 8% 2 5% 3 4 3 3 5% 7% 43%	1980
March	1st to 10th	Night	16% 6% 2 6% 4 3 3 3 4 5% 47%	5012
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 5% 8% 50%	4980
	21st to end	Night	13% 7% 3 6% 4 4 4 3 5% 6% 44%	5358
April	1st to 10th	Night	15% 7% 2 6% 5% 5% 4 4 5% 6% 41%	2000
	11th to 20th	Night	14% 5% 2 6% 3 3 4 4 5% 7% 46%	1975
	21st to end	Night	16% 5% 2 5% 3 4 3 4 7% 7% 43%	2000
May	1st to 10th	Night	18% 10% 3 7% 5% 5% 5% 4 6% 8% 29%	4218
	11th to 20th	Night	22% 8% 3 6% 4 4 4 4 6% 9% 31%	3721
	21st to end	Night	17% 6% 3 7% 4 4 5% 4 7% 9% 35%	3920
June	1st to 10th	Night	10% 5% 2 6% 4 5% 6% 4 8% 8% 44%	1600
	11th to 20th	Night	13% 5% 1 6% 4 5% 5% 5% 6% 10% 40%	1600
	21st to end	Night	9% 5% 2 6% 5% 5% 4 5% 8% 10% 42%	1600
July	1st to 10th	Night	8% 6% 3 7% 5% 5% 6% 5% 7% 10% 40%	3598
	11th to 20th	Night	9% 8% 3 9% 6% 5% 7% 5% 8% 11% 29%	3776
	21st to end	Night	9% 6% 2 7% 5% 5% 5% 4 8% 10% 39%	4427
August	1st to 10th	Night	16% 6% 2 8% 5% 5% 5% 4 7% 9% 33%	2915
	11th to 20th	Night	11% 7% 3 8% 6% 5% 5% 5% 7% 8% 35%	3038
	21st to end	Night	9% 5% 2 5% 4 4 3 3 6% 8% 51%	3586
September	1st to 10th	Night	13% 5% 3 7% 4 5% 4 3 5% 7% 45%	2000
	11th to 20th	Night	9% 7% 2 5% 4 3 4 3 5% 8% 48%	2000
	21st to end	Night	18% 8% 2 8% 4 4 4 3 6% 6% 36%	2286
October	1st to 10th	Night	16% 6% 3 5% 3 3 3 3 4 6% 47%	2400
	11th to 20th	Night	15% 5% 2 5% 3 4 3 2 4 6% 51%	2400
	21st to end	Night	14% 5% 2 4 3 3 3 4 5% 6% 50%	2640
November	1st to 10th	Night	12% 5% 2 4 4 3 4 3 5% 5% 54%	2572
	11th to 20th	Night	14% 7% 2 4 4 3 3 3 4 7% 49%	2730
	21st to end	Night	13% 5% 1 4 5% 3 4 4 6% 7% 48%	2938
December	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 5% 7% 52%	4720
	11th to 20th	Night	20% 6% 2 5% 3 4 4 3 5% 7% 40%	5345
	21st to end	Night	16% 7% 3 6% 4 4 5% 4 5% 7% 39%	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	35% 4 6% 26% 22% 7%	3790
	11th to 20th	Day	40% 4 6% 23% 19% 8%	4114
	21st to end	Day	43% 5% 8% 28% 14% 3	4466
February	1st to 10th	Day	44% 5% 12% 25% 13% 2	1800
	11th to 20th	Day	44% 4 8% 24% 18% 3	1832
	21st to end	Day	45% 5% 11% 20% 16% 4	1650
March	1st to 10th	Day	41% 5% 8% 22% 16% 8%	4836
	11th to 20th	Day	33% 5% 8% 26% 21% 6%	4980
	21st to end	Day	43% 5% 10% 19% 17% 7%	5574
April	1st to 10th	Day	40% 5% 10% 20% 19% 6%	2400
	11th to 20th	Day	37% 6% 9% 22% 20% 7%	2374
	21st to end	Day	40% 6% 10% 22% 17% 4	2400
May	1st to 10th	Day	48% 7% 14% 19% 11% 1	5751
	11th to 20th	Day	47% 8% 11% 24% 9% 1	6174
	21st to end	Day	41% 8% 12% 22% 16% 2	6949
June	1st to 10th	Day	35% 7% 14% 26% 16% 2	2800
	11th to 20th	Day	35% 10% 13% 24% 16% 2	2800
	21st to end	Day	32% 9% 15% 24% 18% 2	2800
July	1st to 10th	Day	33% 9% 20% 20% 14% 4	6290
	11th to 20th	Day	43% 10% 17% 21% 7% 1	6101
	21st to end	Day	37% 7% 11% 22% 22% 1	6240
August	1st to 10th	Day	44% 7% 16% 25% 8%	4221
	11th to 20th	Day	39% 8% 13% 26% 14% 1	4016
	21st to end	Day	32% 6% 14% 28% 18% 2	4312
September	1st to 10th	Day	36% 7% 13% 19% 21% 4	2400
	11th to 20th	Day	34% 6% 11% 26% 20% 4	2400
	21st to end	Day	47% 6% 10% 18% 16% 2	2110
October	1st to 10th	Day	37% 4 7% 17% 29% 5%	3260
	11th to 20th	Day	25% 3 6% 29% 26% 10%	3240
	21st to end	Day	23% 2 8% 30% 27% 10%	3538
November	1st to 10th	Day	32% 4 11% 27% 21% 5%	1942
	11th to 20th	Day	34% 4 11% 25% 22% 5%	1890
	21st to end	Day	38% 4 9% 28% 18% 3	1680
December	1st to 10th	Day	36% 4 12% 27% 17% 4	2680
	11th to 20th	Day	41% 4 13% 29% 11% 3	3066
	21st to end	Day	43% 6% 12% 23% 16% 1	4048
January	1st to 10th	Night	37% 3 6% 25% 24% 5%	6371
	11th to 20th	Night	35% 3 7% 25% 23% 7%	5979
	21st to end	Night	40% 4 7% 28% 19% 3	6490
February	1st to 10th	Night	42% 5% 12% 24% 15% 2	2600
	11th to 20th	Night	44% 3 8% 22% 18% 5%	2465
	21st to end	Night	41% 4 10% 22% 17% 6%	1980
March	1st to 10th	Night	39% 4 7% 24% 17% 9%	5012
	11th to 20th	Night	32% 5% 10% 25% 21% 7%	4980
	21st to end	Night	40% 5% 8% 22% 20% 6%	5358
April	1st to 10th	Night	42% 5% 9% 19% 21% 4	2000
	11th to 20th	Night	37% 6% 11% 21% 20% 6%	1975
	21st to end	Night	39% 6% 10% 24% 16% 5%	2000
May	1st to 10th	Night	52% 7% 12% 17% 9% 3	4248
	11th to 20th	Night	48% 8% 11% 21% 11% 1	3722
	21st to end	Night	43% 8% 13% 22% 11% 2	3927
June	1st to 10th	Night	35% 9% 11% 26% 17% 2	1600
	11th to 20th	Night	37% 9% 13% 25% 13% 2	1600
	21st to end	Night	34% 7% 15% 21% 19% 3	1600
July	1st to 10th	Night	39% 10% 17% 19% 14% 2	3560
	11th to 20th	Night	46% 11% 19% 17% 7% 1	3731
	21st to end	Night	37% 9% 14% 22% 18%	4297
August	1st to 10th	Night	45% 9% 16% 21% 8% 1	2904
	11th to 20th	Night	44% 7% 12% 24% 12% 1	3065
	21st to end	Night	32% 5% 11% 30% 20% 2	3586
September	1st to 10th	Night	38% 7% 10% 22% 18% 6%	2000
	11th to 20th	Night	33% 6% 9% 27% 21% 3	2000
	21st to end	Night	47% 6% 10% 18% 16% 3	2286
October	1st to 10th	Night	40% 4 9% 21% 19% 7%	3920
	11th to 20th	Night	24% 3 10% 22% 32% 8%	3899
	21st to end	Night	23% 4 9% 31% 26% 8%	4252
November	1st to 10th	Night	33% 5% 9% 26% 22% 6%	2572
	11th to 20th	Night	37% 4 9% 22% 22% 7%	2730
	21st to end	Night	34% 5% 10% 26% 22% 3	2938
December	1st to 10th	Night	33% 4 12% 27% 21% 3	4720
	11th to 20th	Night	45% 4 11% 20% 16% 5%	5348
	21st to end	Night	43% 6% 14% 22% 15% 1	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	29% 20% 33% 14% 4	245
	11th to 20th	Day	29% 25% 29% 13% 4	318
	21st to end	Day	22% 26% 34% 13% 5%	330
February	1st to 10th	Day	26% 24% 34% 12% 4	300
	11th to 20th	Day	36% 19% 36% 7% 1	370
	21st to end	Day	30% 18% 32% 18% 2	414
March	1st to 10th	Day	25% 22% 31% 18% 3 1	500
	11th to 20th	Day	28% 18% 32% 15% 5% 1	500
	21st to end	Day	26% 18% 33% 17% 5% 1	550
April	1st to 10th	Day	23% 22% 32% 16% 5% 1	500
	11th to 20th	Day	20% 20% 33% 18% 6% 3	492
	21st to end	Day	25% 15% 32% 20% 7% 2	500
May	1st to 10th	Day	24% 17% 29% 20% 8% 2	503
	11th to 20th	Day	22% 14% 28% 24% 9% 2	527
	21st to end	Day	18% 9% 25% 28% 17% 3	573
June	1st to 10th	Day	13% 9% 27% 29% 15% 7%	530
	11th to 20th	Day	17% 7% 26% 29% 16% 4	530
	21st to end	Day	11% 5% 28% 32% 18% 5%	530
July	1st to 10th	Day	14% 2 17% 38% 20% 11%	525
	11th to 20th	Day	6% 3 21% 36% 27% 7%	524
	21st to end	Day	12% 3 21% 35% 22% 7%	576
August	1st to 10th	Day	14% 4 21% 31% 22% 8%	526
	11th to 20th	Day	14% 4 17% 34% 23% 9%	519
	21st to end	Day	11% 3 22% 32% 20% 11%	583
September	1st to 10th	Day	16% 6% 23% 26% 16% 12%	530
	11th to 20th	Day	13% 5% 22% 35% 17% 8%	530
	21st to end	Day	27% 10% 23% 26% 9% 5%	530
October	1st to 10th	Day	20% 7% 24% 26% 16% 7%	530
	11th to 20th	Day	17% 17% 28% 22% 14% 3	530
	21st to end	Day	17% 14% 37% 21% 9% 2	583
November	1st to 10th	Day	15% 16% 34% 23% 9% 3	474
	11th to 20th	Day	20% 16% 34% 22% 7% 2	430
	21st to end	Day	25% 22% 28% 16% 8%	220
December	1st to 10th	Day	21% 22% 38% 14% 4 1	220
	11th to 20th	Day	25% 20% 31% 16% 8%	220
	21st to end	Day	28% 17% 31% 19% 5%	242
January	1st to 10th	Night	19% 19% 39% 20% 3	840
	11th to 20th	Night	22% 15% 33% 25% 4	825
	21st to end	Night	19% 17% 37% 25% 2	880
February	1st to 10th	Night	20% 16% 34% 27% 3	800
	11th to 20th	Night	27% 20% 33% 19% 1	783
	21st to end	Night	25% 17% 37% 19% 3	660
March	1st to 10th	Night	20% 16% 38% 23% 2	800
	11th to 20th	Night	22% 17% 37% 19% 5% 1	800
	21st to end	Night	19% 17% 39% 21% 4 1	880
April	1st to 10th	Night	20% 20% 36% 21% 4	800
	11th to 20th	Night	18% 15% 33% 28% 6%	791
	21st to end	Night	21% 11% 34% 26% 8%	800
May	1st to 10th	Night	21% 10% 31% 27% 11%	684
	11th to 20th	Night	19% 8% 27% 36% 9%	462
	21st to end	Night	15% 6% 24% 36% 16% 1	433
June	1st to 10th	Night	11% 3 32% 36% 18% 1	400
	11th to 20th	Night	15% 5% 27% 39% 15% 1	400
	21st to end	Night	11% 3 26% 43% 17% 1	400
July	1st to 10th	Night	8% 3 21% 42% 23% 2	390
	11th to 20th	Night	6% 3 16% 43% 29% 3	463
	21st to end	Night	9% 2 20% 42% 25% 3	657
August	1st to 10th	Night	11% 2 13% 43% 29% 2	597
	11th to 20th	Night	14% 4 15% 40% 25% 2	692
	21st to end	Night	11% 3 19% 36% 27% 4	880
September	1st to 10th	Night	15% 3 22% 30% 26% 4	800
	11th to 20th	Night	13% 4 16% 39% 25% 4	800
	21st to end	Night	23% 5% 23% 32% 17% 1	800
October	1st to 10th	Night	19% 6% 22% 28% 21% 5%	800
	11th to 20th	Night	17% 11% 26% 32% 14% 1	800
	21st to end	Night	18% 10% 33% 26% 12% 1	880
November	1st to 10th	Night	14% 10% 34% 29% 12% 1	820
	11th to 20th	Night	19% 14% 32% 28% 8% 1	840
	21st to end	Night	18% 14% 33% 26% 9% 1	840
December	1st to 10th	Night	22% 13% 33% 24% 7% 1	840
	11th to 20th	Night	20% 12% 37% 27% 5%	840
	21st to end	Night	21% 13% 32% 30% 4	924

Page generated at 2026-01-14 01:14:29. Version ac07

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