Furtman Farm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Furtman Farm Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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% 9% 3 8% 5% 4 4 4 5% 7% 37%	138753
	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 36%	134297
Transparency	Day	46% 5% 12% 25% 11% 1	142265
	Night	48% 4 12% 23% 12% 1	138861
Seeing	Day	17% 10% 30% 25% 13% 4	18429
	Night	17% 8% 31% 31% 11% 1	28296

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	10% 13% 7% 18% 9% 5% 4 3 4 5% 24%	4691
	11th to 20th	Day	6% 12% 7% 14% 8% 7% 7% 8% 7% 5% 19%	4710
	21st to end	Day	5% 13% 6% 13% 6% 5% 7% 5% 6% 8% 27%	5546
February	1st to 10th	Day	8% 13% 5% 11% 7% 5% 5% 4 7% 6% 31%	1890
	11th to 20th	Day	7% 12% 5% 10% 5% 5% 5% 4 5% 6% 36%	1844
	21st to end	Day	11% 11% 4 11% 6% 5% 4 3 5% 8% 33%	1662
March	1st to 10th	Day	12% 8% 3 7% 4 4 4 3 5% 8% 42%	4757
	11th to 20th	Day	9% 11% 4 9% 5% 4 5% 4 5% 8% 37%	5090
	21st to end	Day	20% 11% 4 9% 4 4 4 3 5% 6% 30%	5769
April	1st to 10th	Day	17% 12% 4 8% 5% 4 4 3 5% 6% 31%	2520
	11th to 20th	Day	20% 11% 4 9% 5% 4 3 3 4 7% 30%	2494
	21st to end	Day	21% 10% 3 7% 5% 5% 5% 3 5% 5% 31%	2696
May	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 6% 48%	8212
	11th to 20th	Day	16% 8% 3 7% 4 4 4 3 4 5% 43%	8222
	21st to end	Day	14% 8% 3 7% 5% 4 5% 4 5% 7% 40%	9047
June	1st to 10th	Day	12% 6% 2 6% 5% 4 4 4 7% 8% 43%	2940
	11th to 20th	Day	15% 7% 2 7% 6% 5% 5% 4 6% 7% 36%	2940
	21st to end	Day	12% 6% 2 5% 5% 4 5% 4 8% 11% 38%	2940
July	1st to 10th	Day	9% 5% 1 5% 4 4 4 4 7% 11% 45%	6500
	11th to 20th	Day	10% 4 2 4 3 4 4 3 6% 9% 51%	6431
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 10% 46%	6924
August	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 7% 9% 41%	4381
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 6% 8% 43%	4171
	21st to end	Day	13% 7% 2 7% 4 4 5% 3 5% 7% 41%	4413
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 45%	2400
	11th to 20th	Day	13% 6% 2 7% 4 5% 4 3 6% 8% 43%	2400
	21st to end	Day	16% 7% 2 6% 3 4 4 3 5% 7% 43%	2103
October	1st to 10th	Day	16% 8% 2 6% 3 3 4 4 6% 9% 38%	2000
	11th to 20th	Day	18% 12% 3 7% 4 5% 4 3 4 5% 34%	2000
	21st to end	Day	18% 13% 3 7% 4 4 3 3 5% 6% 33%	2090
November	1st to 10th	Day	13% 14% 4 9% 5% 5% 4 3 4 7% 33%	1762
	11th to 20th	Day	13% 15% 7% 11% 8% 4 4 3 5% 6% 25%	1680
	21st to end	Day	14% 13% 5% 9% 5% 4 5% 4 6% 6% 30%	1680
December	1st to 10th	Day	8% 15% 7% 17% 13% 5% 5% 3 4 4 20%	2690
	11th to 20th	Day	12% 16% 7% 10% 7% 4 4 4 4 5% 27%	3088
	21st to end	Day	19% 21% 7% 19% 7% 4 3 2 3 3 13%	4070
January	1st to 10th	Night	9% 14% 6% 16% 7% 5% 4 3 4 5% 27%	8232
	11th to 20th	Night	6% 14% 6% 14% 8% 7% 8% 7% 6% 5% 20%	8280
	21st to end	Night	5% 16% 6% 12% 5% 5% 5% 3 6% 6% 31%	8664
February	1st to 10th	Night	8% 11% 6% 11% 6% 5% 5% 4 6% 6% 33%	2730
	11th to 20th	Night	7% 10% 4 9% 6% 6% 5% 5% 6% 5% 37%	2663
	21st to end	Night	11% 12% 3 9% 6% 5% 4 4 5% 6% 34%	2144
March	1st to 10th	Night	11% 7% 3 7% 4 4 4 3 5% 6% 47%	5358
	11th to 20th	Night	11% 9% 3 11% 5% 5% 5% 4 6% 6% 34%	5090
	21st to end	Night	21% 13% 4 10% 4 4 3 2 4 5% 29%	5401
April	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	2100
	11th to 20th	Night	21% 10% 4 6% 4 4 4 3 4 5% 35%	2075
	21st to end	Night	20% 11% 3 6% 4 4 4 4 5% 7% 33%	1918
May	1st to 10th	Night	12% 7% 3 7% 4 4 4 3 5% 7% 46%	4671
	11th to 20th	Night	17% 7% 2 6% 3 3 3 3 4 6% 46%	4670
	21st to end	Night	13% 7% 2 6% 5% 4 5% 4 6% 8% 40%	5139
June	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 6% 10% 43%	1680
	11th to 20th	Night	11% 6% 2 5% 4 5% 5% 4 7% 8% 43%	1680
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 7% 9% 43%	1680
July	1st to 10th	Night	8% 4 2 5% 3 4 5% 4 7% 9% 50%	3698
	11th to 20th	Night	8% 5% 2 5% 3 3 4 3 6% 9% 52%	3657
	21st to end	Night	10% 6% 2 6% 3 4 4 3 5% 7% 49%	3947
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 6% 7% 46%	2776
	11th to 20th	Night	14% 5% 3 6% 4 3 4 3 6% 7% 45%	2888
	21st to end	Night	14% 6% 3 6% 5% 4 4 3 6% 7% 43%	3487
September	1st to 10th	Night	10% 4 2 6% 3 3 4 4 4 7% 53%	2000
	11th to 20th	Night	9% 6% 2 5% 4 4 3 3 6% 7% 50%	2000
	21st to end	Night	16% 5% 2 6% 3 4 3 3 5% 7% 46%	2275
October	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 47%	2400
	11th to 20th	Night	17% 11% 3 6% 5% 4 4 3 5% 6% 37%	2400
	21st to end	Night	18% 11% 3 7% 5% 4 4 4 5% 6% 33%	2750
November	1st to 10th	Night	15% 11% 4 7% 4 4 4 3 5% 7% 37%	2755
	11th to 20th	Night	14% 12% 6% 11% 5% 4 3 3 4 5% 32%	2940
	21st to end	Night	14% 11% 4 11% 6% 4 4 4 5% 6% 33%	2940
December	1st to 10th	Night	8% 13% 5% 16% 9% 5% 5% 4 5% 6% 23%	4720
	11th to 20th	Night	12% 17% 6% 13% 6% 5% 5% 5% 5% 5% 21%	5339
	21st to end	Night	22% 19% 6% 14% 5% 4 3 2 3 4 18%	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	64% 4 8% 16% 8%	4692
	11th to 20th	Day	63% 4 8% 15% 10%	4710
	21st to end	Day	54% 5% 12% 21% 8%	5546
February	1st to 10th	Day	53% 4 12% 21% 10% 1	1890
	11th to 20th	Day	48% 4 12% 25% 9% 1	1853
	21st to end	Day	51% 4 12% 22% 10% 1	1662
March	1st to 10th	Day	41% 4 16% 23% 12% 4	4757
	11th to 20th	Day	45% 5% 14% 23% 11% 1	5090
	21st to end	Day	56% 4 10% 19% 10% 1	5771
April	1st to 10th	Day	53% 4 9% 18% 13% 2	2520
	11th to 20th	Day	56% 3 10% 19% 11% 1	2494
	21st to end	Day	56% 4 10% 20% 10%	2696
May	1st to 10th	Day	36% 5% 11% 28% 18% 2	8250
	11th to 20th	Day	43% 4 10% 23% 16% 4	8226
	21st to end	Day	43% 5% 9% 28% 14% 2	9061
June	1st to 10th	Day	37% 6% 16% 29% 12% 1	2940
	11th to 20th	Day	45% 6% 13% 22% 13% 1	2940
	21st to end	Day	37% 7% 18% 28% 9% 1	2940
July	1st to 10th	Day	31% 7% 18% 35% 9%	6431
	11th to 20th	Day	30% 5% 17% 35% 12% 1	6356
	21st to end	Day	33% 6% 16% 30% 15%	6728
August	1st to 10th	Day	39% 5% 16% 27% 13% 1	4366
	11th to 20th	Day	37% 6% 14% 34% 8%	4210
	21st to end	Day	40% 6% 16% 26% 11% 1	4412
September	1st to 10th	Day	37% 5% 15% 32% 11%	2400
	11th to 20th	Day	39% 5% 21% 27% 7%	2400
	21st to end	Day	40% 5% 15% 31% 9%	2103
October	1st to 10th	Day	33% 3 10% 37% 16%	3260
	11th to 20th	Day	45% 4 7% 27% 15% 1	3240
	21st to end	Day	49% 4 14% 25% 8%	3354
November	1st to 10th	Day	52% 5% 11% 19% 12% 1	1762
	11th to 20th	Day	60% 4 13% 17% 6% 1	1680
	21st to end	Day	53% 6% 14% 20% 6% 1	1680
December	1st to 10th	Day	69% 4 9% 15% 3	2690
	11th to 20th	Day	61% 3 9% 16% 11% 1	3085
	21st to end	Day	77% 4 6% 8% 5%	4070
January	1st to 10th	Night	60% 4 12% 15% 10%	8233
	11th to 20th	Night	64% 3 9% 15% 9%	8280
	21st to end	Night	54% 4 10% 21% 10%	8664
February	1st to 10th	Night	51% 4 12% 21% 11% 1	2730
	11th to 20th	Night	47% 5% 13% 23% 10% 1	2664
	21st to end	Night	51% 4 13% 21% 10% 1	2144
March	1st to 10th	Night	38% 4 16% 22% 17% 4	5358
	11th to 20th	Night	49% 5% 12% 21% 12%	5090
	21st to end	Night	59% 3 9% 15% 14%	5401
April	1st to 10th	Night	52% 5% 6% 19% 17% 2	2100
	11th to 20th	Night	51% 4 9% 20% 13% 2	2075
	21st to end	Night	50% 5% 9% 21% 14% 1	1918
May	1st to 10th	Night	38% 4 10% 28% 18% 1	4695
	11th to 20th	Night	39% 4 9% 22% 19% 6%	4671
	21st to end	Night	41% 5% 10% 27% 15% 1	5149
June	1st to 10th	Night	36% 5% 16% 28% 13% 2	1680
	11th to 20th	Night	37% 6% 13% 23% 18% 2	1680
	21st to end	Night	35% 7% 15% 28% 12% 2	1680
July	1st to 10th	Night	30% 6% 19% 33% 11% 1	3660
	11th to 20th	Night	29% 5% 15% 37% 13% 1	3612
	21st to end	Night	34% 6% 12% 29% 17% 1	3832
August	1st to 10th	Night	36% 5% 14% 27% 16% 3	2767
	11th to 20th	Night	37% 6% 12% 33% 10% 1	2915
	21st to end	Night	39% 7% 14% 28% 11% 1	3487
September	1st to 10th	Night	31% 5% 15% 36% 13% 1	2000
	11th to 20th	Night	32% 5% 18% 34% 12%	2000
	21st to end	Night	37% 4 14% 31% 12% 1	2275
October	1st to 10th	Night	28% 3 8% 36% 23% 4	3920
	11th to 20th	Night	42% 5% 11% 22% 19% 1	3899
	21st to end	Night	50% 4 11% 24% 10%	4436
November	1st to 10th	Night	47% 4 12% 24% 11% 1	2755
	11th to 20th	Night	55% 4 13% 18% 10% 1	2940
	21st to end	Night	52% 5% 14% 22% 6% 1	2940
December	1st to 10th	Night	62% 5% 14% 15% 3	4720
	11th to 20th	Night	65% 4 9% 15% 7%	5341
	21st to end	Night	72% 3 7% 13% 5%	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	15% 12% 35% 27% 8% 3	330
	11th to 20th	Day	15% 10% 40% 28% 8%	330
	21st to end	Day	15% 10% 41% 21% 13% 1	363
February	1st to 10th	Day	15% 14% 44% 20% 5% 2	330
	11th to 20th	Day	12% 17% 44% 19% 7% 2	330
	21st to end	Day	16% 18% 44% 18% 4	273
March	1st to 10th	Day	18% 15% 40% 17% 9% 1	330
	11th to 20th	Day	17% 12% 42% 20% 7% 2	330
	21st to end	Day	22% 14% 33% 19% 9% 2	446
April	1st to 10th	Day	24% 16% 33% 22% 4 1	540
	11th to 20th	Day	25% 13% 31% 22% 9%	532
	21st to end	Day	27% 10% 29% 23% 8% 2	713
May	1st to 10th	Day	19% 12% 28% 27% 10% 3	753
	11th to 20th	Day	19% 12% 28% 26% 12% 3	775
	21st to end	Day	19% 9% 25% 26% 17% 4	844
June	1st to 10th	Day	12% 7% 29% 30% 17% 4	780
	11th to 20th	Day	16% 9% 34% 28% 11% 2	780
	21st to end	Day	15% 8% 29% 27% 15% 6%	780
July	1st to 10th	Day	8% 4 21% 30% 25% 11%	771
	11th to 20th	Day	10% 8% 24% 28% 21% 9%	744
	21st to end	Day	9% 5% 25% 32% 20% 9%	794
August	1st to 10th	Day	12% 5% 23% 31% 22% 7%	726
	11th to 20th	Day	15% 5% 19% 31% 23% 6%	715
	21st to end	Day	13% 7% 23% 30% 23% 4	645
September	1st to 10th	Day	14% 8% 25% 29% 18% 5%	530
	11th to 20th	Day	11% 8% 29% 31% 18% 3	530
	21st to end	Day	21% 8% 25% 23% 16% 7%	379
October	1st to 10th	Day	18% 11% 29% 23% 13% 6%	330
	11th to 20th	Day	24% 18% 31% 21% 6% 1	330
	21st to end	Day	26% 13% 39% 15% 5% 1	363
November	1st to 10th	Day	22% 14% 33% 23% 8% 1	330
	11th to 20th	Day	20% 17% 38% 16% 8% 1	330
	21st to end	Day	24% 12% 36% 23% 5%	330
December	1st to 10th	Day	15% 10% 42% 25% 8%	330
	11th to 20th	Day	20% 17% 37% 21% 5%	330
	21st to end	Day	29% 12% 38% 17% 3 1	363
January	1st to 10th	Night	14% 10% 35% 31% 9%	1050
	11th to 20th	Night	18% 9% 38% 27% 7% 1	1050
	21st to end	Night	17% 10% 36% 30% 7% 1	1155
February	1st to 10th	Night	13% 11% 39% 28% 8% 1	1050
	11th to 20th	Night	12% 11% 40% 29% 7% 1	1029
	21st to end	Night	18% 9% 35% 30% 7%	760
March	1st to 10th	Night	16% 11% 37% 28% 7% 1	783
	11th to 20th	Night	18% 8% 41% 26% 6% 1	630
	21st to end	Night	21% 10% 29% 28% 10% 2	693
April	1st to 10th	Night	25% 10% 32% 27% 7%	630
	11th to 20th	Night	24% 10% 30% 29% 7% 1	623
	21st to end	Night	26% 6% 28% 30% 8% 1	630
May	1st to 10th	Night	18% 9% 35% 31% 7% 1	630
	11th to 20th	Night	18% 8% 30% 33% 9% 2	621
	21st to end	Night	14% 5% 29% 37% 14% 1	683
June	1st to 10th	Night	12% 6% 29% 39% 13% 1	630
	11th to 20th	Night	14% 6% 29% 43% 9%	630
	21st to end	Night	12% 5% 22% 43% 16% 1	630
July	1st to 10th	Night	6% 3 23% 41% 25% 3	615
	11th to 20th	Night	10% 4 24% 40% 20% 2	597
	21st to end	Night	7% 3 21% 44% 21% 3	657
August	1st to 10th	Night	10% 2 16% 48% 23% 1	597
	11th to 20th	Night	15% 4 18% 35% 26% 3	594
	21st to end	Night	13% 3 21% 38% 23% 2	660
September	1st to 10th	Night	11% 5% 22% 40% 19% 3	600
	11th to 20th	Night	11% 5% 28% 35% 19% 2	600
	21st to end	Night	16% 7% 30% 30% 16% 1	725
October	1st to 10th	Night	19% 8% 26% 33% 12% 1	800
	11th to 20th	Night	21% 11% 33% 27% 7%	800
	21st to end	Night	23% 10% 31% 29% 7%	880
November	1st to 10th	Night	21% 10% 36% 26% 8%	909
	11th to 20th	Night	20% 13% 32% 27% 7% 1	1050
	21st to end	Night	20% 12% 36% 25% 6%	1050
December	1st to 10th	Night	16% 11% 35% 29% 9% 1	1050
	11th to 20th	Night	22% 10% 36% 22% 8% 1	1050
	21st to end	Night	26% 10% 32% 25% 7% 1	1155

Page generated at 2026-07-13 02:14:48. Version ac07

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