Smithsferry Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Smithsferry Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	7% 10% 3 7% 4 4 4 3 4 6% 49%	135598
	Night	8% 10% 3 6% 4 3 4 3 5% 6% 48%	132090
Transparency	Day	36% 5% 9% 20% 19% 11%	130345
	Night	37% 5% 11% 21% 17% 8%	128342
Seeing	Day	15% 13% 29% 28% 13% 2	15717
	Night	16% 5% 20% 37% 20% 2	24985

By Month

Percentage Cloud Cover SW (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	6% 21% 10% 13% 5% 4 4 3 3 4 26%	3639
	11th to 20th	Day	9% 13% 7% 8% 3 3 4 2 4 6% 41%	3640
	21st to end	Day	12% 16% 3 6% 3 3 3 2 3 6% 43%	4004
February	1st to 10th	Day	12% 20% 6% 12% 6% 4 3 3 4 4 27%	1722
	11th to 20th	Day	10% 21% 6% 9% 4 5% 4 4 5% 5% 26%	1950
	21st to end	Day	10% 22% 8% 12% 5% 4 3 3 5% 5% 23%	1650
March	1st to 10th	Day	7% 15% 7% 12% 5% 5% 5% 4 6% 7% 28%	4518
	11th to 20th	Day	8% 16% 6% 9% 5% 4 3 3 3 4 39%	4819
	21st to end	Day	9% 20% 5% 12% 7% 6% 5% 4 5% 7% 20%	5722
April	1st to 10th	Day	12% 16% 4 10% 6% 5% 5% 3 5% 6% 29%	2400
	11th to 20th	Day	9% 13% 3 9% 6% 4 4 3 4 6% 38%	2433
	21st to end	Day	10% 12% 3 8% 5% 4 5% 4 5% 6% 37%	2600
May	1st to 10th	Day	6% 10% 3 7% 4 3 4 3 4 5% 51%	5880
	11th to 20th	Day	7% 8% 2 6% 5% 5% 5% 4 6% 6% 46%	6089
	21st to end	Day	6% 7% 2 5% 4 4 4 4 5% 6% 54%	6902
June	1st to 10th	Day	11% 7% 3 8% 6% 5% 6% 4 6% 7% 36%	2800
	11th to 20th	Day	10% 7% 3 6% 5% 4 4 4 5% 5% 46%	2800
	21st to end	Day	4 4 2 5% 3 3 4 3 4 6% 64%	2800
July	1st to 10th	Day	2 1 1 2 2 2 3 2 3 4 78%	6319
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 4 79%	6320
	21st to end	Day	2 3 1 3 2 3 3 3 5% 7% 69%	6880
August	1st to 10th	Day	2 3 1 5% 4 4 4 3 6% 8% 59%	6004
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 78%	5798
	21st to end	Day	5% 4 2 4 3 3 3 2 3 5% 65%	6076
September	1st to 10th	Day	4 3 1 3 2 3 2 3 4 5% 70%	2400
	11th to 20th	Day	7% 5% 1 5% 3 4 4 3 5% 6% 58%	2215
	21st to end	Day	7% 5% 1 4 3 3 4 3 4 5% 61%	2200
October	1st to 10th	Day	6% 4 2 5% 3 3 3 2 3 5% 64%	3600
	11th to 20th	Day	9% 6% 2 8% 4 4 4 3 5% 6% 51%	3109
	21st to end	Day	8% 12% 4 8% 5% 4 4 3 5% 6% 42%	3200
November	1st to 10th	Day	15% 12% 4 8% 5% 3 3 3 5% 6% 34%	1854
	11th to 20th	Day	15% 14% 4 8% 5% 5% 4 3 5% 6% 32%	1863
	21st to end	Day	11% 14% 4 9% 5% 5% 4 3 5% 6% 34%	1740
December	1st to 10th	Day	9% 11% 3 7% 4 3 3 2 4 5% 49%	2620
	11th to 20th	Day	12% 22% 7% 11% 4 5% 4 3 5% 5% 19%	3028
	21st to end	Day	12% 17% 6% 10% 4 4 4 3 5% 6% 29%	4004
January	1st to 10th	Night	10% 19% 7% 10% 5% 4 4 3 5% 5% 29%	6306
	11th to 20th	Night	10% 14% 5% 8% 4 3 3 3 4 5% 41%	6320
	21st to end	Night	11% 16% 3 6% 4 4 3 2 3 5% 43%	6952
February	1st to 10th	Night	13% 19% 4 10% 5% 4 4 3 4 5% 29%	2672
	11th to 20th	Night	11% 17% 6% 8% 5% 4 4 3 4 6% 33%	2338
	21st to end	Night	11% 18% 6% 10% 6% 4 4 3 4 5% 28%	1980
March	1st to 10th	Night	7% 16% 5% 8% 5% 3 3 2 4 7% 40%	5376
	11th to 20th	Night	10% 15% 4 8% 4 3 4 3 4 5% 40%	5181
	21st to end	Night	7% 13% 4 10% 6% 4 5% 4 7% 9% 31%	5236
April	1st to 10th	Night	11% 12% 3 7% 4 4 5% 3 5% 7% 41%	2000
	11th to 20th	Night	10% 10% 3 7% 4 4 4 4 4 5% 45%	1914
	21st to end	Night	8% 7% 3 6% 5% 4 5% 3 5% 6% 48%	1800
May	1st to 10th	Night	8% 8% 2 3 2 3 4 2 5% 7% 56%	4100
	11th to 20th	Night	6% 6% 2 6% 4 4 4 3 6% 7% 51%	3810
	21st to end	Night	5% 6% 2 6% 4 3 3 2 3 6% 60%	3972
June	1st to 10th	Night	11% 6% 1 5% 4 4 5% 4 6% 9% 45%	1600
	11th to 20th	Night	9% 6% 2 4 3 3 3 3 6% 6% 56%	1600
	21st to end	Night	3 4 1 4 3 4 4 3 4 6% 65%	1600
July	1st to 10th	Night	2 2 1 2 2 2 3 3 3 4 76%	3640
	11th to 20th	Night	1 1 1 1 2 2 2 4 5% 83%	3640
	21st to end	Night	2 3 1 3 2 3 3 4 6% 8% 65%	3970
August	1st to 10th	Night	3 3 2 4 3 4 4 4 6% 8% 60%	3796
	11th to 20th	Night	1 1 1 2 2 2 2 2 3 5% 79%	4051
	21st to end	Night	6% 4 1 4 2 3 3 2 4 5% 66%	4916
September	1st to 10th	Night	2 3 1 3 2 2 3 2 4 5% 72%	2000
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 5% 6% 60%	2185
	21st to end	Night	8% 4 1 3 3 2 2 2 4 5% 66%	2200
October	1st to 10th	Night	4 3 1 4 3 3 3 2 4 5% 67%	3600
	11th to 20th	Night	7% 6% 2 5% 3 3 4 2 4 6% 57%	4036
	21st to end	Night	9% 8% 2 6% 4 4 5% 4 7% 7% 44%	4600
November	1st to 10th	Night	15% 11% 3 7% 4 4 3 3 6% 6% 38%	2665
	11th to 20th	Night	17% 11% 3 7% 4 4 4 4 5% 6% 35%	2691
	21st to end	Night	12% 12% 3 8% 5% 3 4 3 4 6% 40%	2655
December	1st to 10th	Night	8% 10% 3 6% 3 3 3 3 4 6% 51%	4560
	11th to 20th	Night	11% 18% 7% 12% 6% 5% 5% 4 6% 6% 21%	5176
	21st to end	Night	12% 15% 5% 10% 5% 4 4 3 5% 6% 28%	6952

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	60% 7% 11% 16% 4	3639
	11th to 20th	Day	44% 6% 15% 25% 8% 2	3640
	21st to end	Day	44% 6% 12% 20% 16% 3	4004
February	1st to 10th	Day	60% 7% 12% 15% 4 1	1722
	11th to 20th	Day	59% 8% 11% 14% 7% 1	1960
	21st to end	Day	61% 8% 9% 15% 6% 1	1650
March	1st to 10th	Day	51% 6% 10% 18% 12% 3	4518
	11th to 20th	Day	48% 5% 8% 29% 10%	4819
	21st to end	Day	60% 9% 12% 16% 4	5722
April	1st to 10th	Day	54% 7% 14% 17% 8% 1	2400
	11th to 20th	Day	45% 6% 12% 21% 14% 2	2436
	21st to end	Day	46% 8% 12% 23% 12%	2600
May	1st to 10th	Day	34% 5% 7% 27% 24% 3	5880
	11th to 20th	Day	33% 5% 9% 28% 20% 5%	6092
	21st to end	Day	28% 6% 9% 30% 23% 4	6904
June	1st to 10th	Day	42% 8% 12% 22% 14% 1	2800
	11th to 20th	Day	36% 5% 9% 22% 21% 7%	2800
	21st to end	Day	21% 4 7% 20% 35% 13%	2800
July	1st to 10th	Day	11% 2 3 11% 26% 47%	6251
	11th to 20th	Day	11% 2 2 6% 26% 53%	6245
	21st to end	Day	16% 3 3 16% 34% 29%	6681
August	1st to 10th	Day	25% 4 5% 12% 27% 28%	4373
	11th to 20th	Day	12% 2 4 15% 43% 24%	4197
	21st to end	Day	17% 2 4 20% 33% 23%	4379
September	1st to 10th	Day	16% 4 5% 20% 32% 23%	2400
	11th to 20th	Day	26% 5% 9% 21% 27% 12%	2215
	21st to end	Day	25% 4 7% 27% 28% 9%	2200
October	1st to 10th	Day	25% 4 6% 19% 38% 9%	3600
	11th to 20th	Day	34% 3 13% 23% 24% 3	3109
	21st to end	Day	43% 5% 9% 25% 18% 1	3200
November	1st to 10th	Day	49% 5% 13% 21% 11% 1	1854
	11th to 20th	Day	53% 6% 15% 19% 7%	1863
	21st to end	Day	49% 6% 14% 18% 11% 2	1740
December	1st to 10th	Day	37% 6% 16% 34% 6% 1	2620
	11th to 20th	Day	64% 7% 14% 11% 3	3028
	21st to end	Day	56% 5% 13% 18% 7%	4004
January	1st to 10th	Night	56% 7% 14% 18% 4 1	6306
	11th to 20th	Night	46% 6% 13% 22% 12% 1	6320
	21st to end	Night	44% 7% 11% 22% 12% 5%	6952
February	1st to 10th	Night	57% 8% 14% 15% 5% 1	2672
	11th to 20th	Night	51% 8% 14% 16% 10% 1	2339
	21st to end	Night	56% 6% 10% 17% 9% 2	1980
March	1st to 10th	Night	43% 6% 9% 24% 13% 5%	5376
	11th to 20th	Night	45% 5% 7% 27% 14% 1	5181
	21st to end	Night	46% 11% 17% 21% 6%	5236
April	1st to 10th	Night	42% 7% 13% 25% 12% 1	2000
	11th to 20th	Night	40% 6% 11% 25% 16% 3	1914
	21st to end	Night	36% 6% 15% 28% 15% 1	1800
May	1st to 10th	Night	27% 5% 8% 30% 27% 2	4097
	11th to 20th	Night	29% 5% 10% 31% 23% 2	3810
	21st to end	Night	26% 5% 8% 36% 22% 3	3972
June	1st to 10th	Night	34% 8% 14% 29% 14% 1	1600
	11th to 20th	Night	29% 5% 10% 26% 23% 8%	1600
	21st to end	Night	19% 4 8% 21% 34% 14%	1600
July	1st to 10th	Night	12% 2 4 10% 26% 46%	3600
	11th to 20th	Night	7% 2 2 10% 27% 53%	3592
	21st to end	Night	17% 2 4 18% 32% 28%	3857
August	1st to 10th	Night	24% 3 5% 12% 33% 24%	2755
	11th to 20th	Night	11% 2 3 15% 40% 29%	2928
	21st to end	Night	17% 2 4 19% 32% 26%	3535
September	1st to 10th	Night	16% 3 6% 19% 33% 24%	2000
	11th to 20th	Night	24% 4 9% 19% 28% 16%	2185
	21st to end	Night	22% 3 6% 27% 29% 13%	2200
October	1st to 10th	Night	21% 3 7% 20% 35% 14%	3600
	11th to 20th	Night	29% 3 13% 23% 28% 4	4036
	21st to end	Night	36% 5% 12% 24% 20% 1	4600
November	1st to 10th	Night	44% 7% 13% 22% 13% 2	2665
	11th to 20th	Night	48% 7% 13% 22% 9%	2691
	21st to end	Night	46% 5% 13% 22% 12% 3	2655
December	1st to 10th	Night	35% 6% 22% 28% 8% 1	4560
	11th to 20th	Night	61% 7% 15% 14% 3	5176
	21st to end	Night	55% 6% 15% 16% 7% 1	6952

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	21% 16% 28% 22% 12% 2	200
	11th to 20th	Day	23% 13% 23% 23% 12% 8%	200
	21st to end	Day	21% 10% 28% 25% 11% 5%	220
February	1st to 10th	Day	28% 17% 26% 19% 8% 2	277
	11th to 20th	Day	21% 17% 34% 21% 6%	495
	21st to end	Day	24% 19% 27% 21% 7% 1	414
March	1st to 10th	Day	24% 17% 31% 23% 4 1	500
	11th to 20th	Day	23% 15% 28% 25% 9% 1	500
	21st to end	Day	26% 17% 30% 20% 6% 1	550
April	1st to 10th	Day	22% 18% 27% 21% 9% 2	500
	11th to 20th	Day	21% 20% 29% 21% 8% 1	492
	21st to end	Day	17% 20% 31% 22% 9% 1	500
May	1st to 10th	Day	13% 18% 32% 25% 11% 1	503
	11th to 20th	Day	24% 14% 28% 23% 10% 1	528
	21st to end	Day	13% 13% 24% 31% 15% 3	573
June	1st to 10th	Day	13% 15% 28% 26% 15% 3	530
	11th to 20th	Day	16% 17% 33% 24% 9% 1	530
	21st to end	Day	5% 11% 35% 32% 14% 2	530
July	1st to 10th	Day	4 6% 41% 35% 12% 2	526
	11th to 20th	Day	1 11% 37% 36% 13% 2	524
	21st to end	Day	3 6% 33% 38% 18% 2	575
August	1st to 10th	Day	6% 9% 34% 34% 16% 2	527
	11th to 20th	Day	2 7% 29% 39% 20% 3	517
	21st to end	Day	7% 9% 28% 36% 19% 2	583
September	1st to 10th	Day	6% 6% 24% 41% 19% 4	530
	11th to 20th	Day	10% 8% 27% 33% 20% 1	530
	21st to end	Day	9% 10% 29% 32% 17% 2	530
October	1st to 10th	Day	13% 11% 25% 30% 18% 2	530
	11th to 20th	Day	17% 11% 24% 30% 15% 4	399
	21st to end	Day	18% 16% 22% 29% 13% 2	363
November	1st to 10th	Day	25% 11% 24% 27% 13% 2	330
	11th to 20th	Day	20% 13% 28% 26% 10% 3	321
	21st to end	Day	19% 7% 28% 26% 16% 3	270
December	1st to 10th	Day	29% 10% 17% 19% 16% 9%	200
	11th to 20th	Day	29% 14% 25% 23% 8% 2	200
	21st to end	Day	23% 16% 24% 23% 11% 2	220
January	1st to 10th	Night	25% 7% 22% 28% 16% 3	800
	11th to 20th	Night	24% 7% 21% 26% 18% 3	800
	21st to end	Night	21% 5% 21% 31% 19% 4	880
February	1st to 10th	Night	23% 8% 25% 31% 12% 2	800
	11th to 20th	Night	22% 5% 24% 33% 14% 1	784
	21st to end	Night	21% 7% 24% 33% 13% 2	660
March	1st to 10th	Night	20% 11% 23% 31% 13% 2	800
	11th to 20th	Night	18% 8% 22% 34% 16% 1	800
	21st to end	Night	21% 8% 22% 37% 11% 1	880
April	1st to 10th	Night	18% 8% 22% 34% 17% 2	800
	11th to 20th	Night	16% 9% 28% 32% 13% 2	726
	21st to end	Night	11% 6% 27% 39% 16% 2	600
May	1st to 10th	Night	11% 6% 24% 39% 19% 1	600
	11th to 20th	Night	11% 6% 18% 35% 26% 3	484
	21st to end	Night	13% 5% 18% 36% 26% 2	433
June	1st to 10th	Night	12% 6% 16% 40% 23% 4	400
	11th to 20th	Night	11% 4 22% 41% 21% 2	400
	21st to end	Night	5% 1 16% 51% 27% 1	400
July	1st to 10th	Night	2 2 15% 49% 29% 2	390
	11th to 20th	Night	1 2 18% 56% 22% 1	388
	21st to end	Night	3 1 11% 51% 31% 2	438
August	1st to 10th	Night	7% 3 11% 45% 31% 2	487
	11th to 20th	Night	1 2 12% 46% 36% 3	594
	21st to end	Night	6% 2 13% 44% 30% 4	833
September	1st to 10th	Night	5% 1 11% 49% 32% 3	800
	11th to 20th	Night	7% 3 13% 41% 31% 5%	800
	21st to end	Night	10% 2 18% 38% 26% 6%	800
October	1st to 10th	Night	11% 3 19% 41% 24% 3	800
	11th to 20th	Night	12% 6% 17% 39% 21% 4	800
	21st to end	Night	18% 4 22% 36% 18% 3	880
November	1st to 10th	Night	24% 5% 19% 35% 16% 2	820
	11th to 20th	Night	25% 5% 21% 32% 15% 1	828
	21st to end	Night	19% 7% 22% 33% 16% 4	800
December	1st to 10th	Night	20% 6% 22% 34% 16% 2	800
	11th to 20th	Night	28% 6% 20% 31% 14% 2	800
	21st to end	Night	30% 8% 25% 27% 10% 2	880

Page generated at 2025-11-14 22:55:59. Version ac07

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