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 2005-11-05 to 2024-10-14.

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	13% 9% 3 8% 5% 5% 4 4 5% 7% 36%	101435
	Night	13% 10% 4 9% 5% 4 4 3 5% 6% 36%	97234
Transparency	Day	47% 5% 13% 24% 10% 1	101228
	Night	48% 5% 12% 23% 11% 1	97117
Seeing	Day	17% 10% 30% 26% 14% 4	16012
	Night	17% 8% 31% 31% 11% 1	25851

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	13% 16% 8% 15% 6% 4 4 3 5% 5% 22%	2527
	11th to 20th	Day	6% 10% 6% 14% 9% 11% 9% 8% 6% 5% 16%	2530
	21st to end	Day	9% 18% 7% 16% 6% 5% 5% 3 5% 5% 20%	2980
February	1st to 10th	Day	8% 13% 5% 10% 6% 4 5% 4 7% 6% 33%	1710
	11th to 20th	Day	7% 12% 5% 10% 5% 5% 5% 4 5% 6% 35%	1664
	21st to end	Day	12% 11% 4 10% 6% 5% 4 3 5% 7% 33%	1508
March	1st to 10th	Day	10% 10% 3 8% 5% 5% 4 4 5% 6% 40%	3238
	11th to 20th	Day	8% 10% 4 10% 5% 5% 5% 4 5% 7% 38%	3480
	21st to end	Day	19% 13% 4 10% 4 5% 4 3 5% 6% 26%	3940
April	1st to 10th	Day	16% 12% 4 8% 5% 4 4 3 5% 6% 32%	2280
	11th to 20th	Day	21% 11% 4 9% 5% 4 3 2 4 6% 31%	2254
	21st to end	Day	21% 10% 3 8% 4 5% 5% 3 5% 5% 31%	2440
May	1st to 10th	Day	17% 8% 2 7% 4 4 4 4 5% 6% 40%	4399
	11th to 20th	Day	15% 7% 2 6% 4 5% 5% 3 5% 7% 41%	4399
	21st to end	Day	18% 8% 3 7% 5% 4 5% 4 5% 7% 34%	4824
June	1st to 10th	Day	12% 5% 1 5% 5% 4 4 3 7% 8% 44%	2660
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 4 6% 7% 37%	2660
	21st to end	Day	11% 5% 2 5% 5% 4 5% 4 8% 11% 39%	2660
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 5% 7% 11% 40%	4440
	11th to 20th	Day	7% 4 1 4 4 4 4 4 6% 10% 52%	4440
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 9% 49%	4813
August	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 7% 9% 41%	4246
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 6% 8% 43%	4041
	21st to end	Day	13% 7% 3 8% 5% 5% 4 3 5% 7% 41%	4278
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 46%	2280
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 3 5% 8% 44%	2280
	21st to end	Day	17% 7% 2 6% 3 4 4 3 5% 7% 43%	1999
October	1st to 10th	Day	17% 8% 2 6% 4 3 4 4 6% 9% 37%	1900
	11th to 20th	Day	18% 13% 3 8% 4 5% 5% 4 4 5% 32%	1830
	21st to end	Day	19% 13% 3 7% 5% 4 3 2 5% 6% 33%	1882
November	1st to 10th	Day	13% 15% 3 8% 5% 5% 4 3 4 7% 34%	1592
	11th to 20th	Day	14% 15% 8% 11% 8% 4 4 3 5% 6% 23%	1520
	21st to end	Day	14% 12% 5% 8% 5% 4 5% 4 6% 6% 32%	1520
December	1st to 10th	Day	10% 16% 7% 14% 8% 5% 5% 3 5% 5% 21%	1520
	11th to 20th	Day	13% 15% 7% 11% 8% 4 5% 4 5% 5% 22%	1918
	21st to end	Day	23% 19% 6% 16% 6% 3 3 2 3 3 17%	2783
January	1st to 10th	Night	12% 13% 7% 14% 5% 4 4 3 5% 5% 26%	4428
	11th to 20th	Night	7% 12% 5% 14% 9% 8% 8% 5% 5% 5% 21%	4440
	21st to end	Night	9% 23% 8% 14% 4 4 4 3 4 4 22%	4683
February	1st to 10th	Night	7% 11% 6% 11% 6% 5% 5% 4 6% 6% 35%	2470
	11th to 20th	Night	7% 11% 4 9% 6% 6% 5% 4 6% 5% 37%	2403
	21st to end	Night	12% 13% 3 9% 5% 5% 4 4 5% 6% 35%	1946
March	1st to 10th	Night	8% 8% 3 7% 5% 4 4 4 6% 7% 44%	3643
	11th to 20th	Night	9% 10% 4 12% 5% 4 4 4 5% 4 39%	3480
	21st to end	Night	23% 13% 4 10% 4 3 3 2 4 5% 28%	3690
April	1st to 10th	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 34%	1900
	11th to 20th	Night	22% 10% 4 6% 4 4 4 4 4 5% 34%	1875
	21st to end	Night	20% 11% 2 6% 4 4 4 4 5% 7% 34%	1735
May	1st to 10th	Night	15% 8% 3 8% 5% 4 4 3 4 7% 38%	2506
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 48%	2506
	21st to end	Night	13% 8% 3 7% 5% 5% 4 4 6% 7% 37%	2743
June	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 7% 10% 43%	1520
	11th to 20th	Night	11% 5% 2 5% 4 5% 5% 4 7% 8% 44%	1520
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 7% 9% 44%	1520
July	1st to 10th	Night	11% 4 2 5% 3 3 5% 3 7% 9% 48%	2530
	11th to 20th	Night	5% 4 1 5% 4 3 4 3 6% 8% 56%	2530
	21st to end	Night	7% 4 2 5% 3 4 4 3 5% 8% 56%	2750
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 6% 8% 46%	2691
	11th to 20th	Night	14% 5% 3 5% 3 3 4 3 6% 7% 46%	2798
	21st to end	Night	14% 6% 3 6% 5% 4 4 3 6% 7% 42%	3380
September	1st to 10th	Night	9% 4 2 6% 4 3 4 4 4 7% 53%	1900
	11th to 20th	Night	9% 6% 2 5% 4 4 3 3 6% 7% 51%	1900
	21st to end	Night	16% 6% 2 6% 3 4 3 3 5% 7% 45%	2161
October	1st to 10th	Night	15% 8% 2 4 3 3 4 3 5% 6% 47%	2280
	11th to 20th	Night	17% 11% 3 7% 4 4 4 3 5% 6% 36%	2203
	21st to end	Night	18% 11% 3 7% 4 4 4 4 5% 6% 33%	2474
November	1st to 10th	Night	16% 10% 4 7% 4 4 4 3 5% 7% 37%	2486
	11th to 20th	Night	14% 12% 6% 11% 6% 4 4 3 4 5% 31%	2660
	21st to end	Night	14% 10% 4 9% 5% 4 4 4 5% 6% 35%	2660
December	1st to 10th	Night	10% 15% 5% 13% 8% 4 4 3 5% 6% 26%	2660
	11th to 20th	Night	13% 16% 5% 11% 7% 6% 6% 5% 5% 5% 23%	3279
	21st to end	Night	24% 20% 4 10% 5% 4 3 3 3 5% 19%	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	64% 5% 9% 17% 4 1	2528
	11th to 20th	Day	67% 5% 9% 14% 5%	2530
	21st to end	Day	64% 5% 11% 15% 5%	2980
February	1st to 10th	Day	51% 4 12% 22% 10% 1	1710
	11th to 20th	Day	49% 5% 13% 24% 9% 1	1673
	21st to end	Day	52% 4 11% 21% 11% 1	1508
March	1st to 10th	Day	45% 4 15% 22% 12% 1	3238
	11th to 20th	Day	45% 5% 13% 24% 12% 1	3480
	21st to end	Day	59% 4 10% 21% 6% 1	3940
April	1st to 10th	Day	53% 4 10% 19% 13% 2	2280
	11th to 20th	Day	55% 4 9% 19% 12% 1	2254
	21st to end	Day	56% 4 11% 19% 9%	2440
May	1st to 10th	Day	45% 5% 11% 25% 13% 2	4440
	11th to 20th	Day	42% 5% 14% 26% 12% 1	4403
	21st to end	Day	49% 5% 12% 22% 11% 1	4837
June	1st to 10th	Day	36% 6% 17% 29% 11% 1	2660
	11th to 20th	Day	44% 6% 13% 22% 14% 1	2660
	21st to end	Day	36% 7% 18% 28% 9% 1	2660
July	1st to 10th	Day	36% 8% 18% 26% 12%	4371
	11th to 20th	Day	26% 6% 17% 35% 15% 1	4365
	21st to end	Day	31% 6% 16% 31% 16%	4662
August	1st to 10th	Day	39% 5% 15% 27% 13% 1	4231
	11th to 20th	Day	37% 6% 14% 35% 8%	4080
	21st to end	Day	41% 6% 16% 25% 11% 1	4277
September	1st to 10th	Day	36% 5% 15% 32% 12%	2280
	11th to 20th	Day	38% 5% 21% 28% 8%	2280
	21st to end	Day	41% 5% 15% 31% 8% 1	1999
October	1st to 10th	Day	44% 5% 13% 29% 9% 1	1900
	11th to 20th	Day	54% 5% 10% 21% 9% 1	1830
	21st to end	Day	54% 4 12% 21% 8%	1882
November	1st to 10th	Day	51% 5% 11% 21% 12% 1	1592
	11th to 20th	Day	63% 4 13% 14% 6% 1	1520
	21st to end	Day	51% 6% 15% 20% 6% 1	1520
December	1st to 10th	Day	65% 5% 12% 14% 4	1520
	11th to 20th	Day	63% 5% 13% 14% 4 1	1915
	21st to end	Day	73% 5% 7% 9% 7%	2783
January	1st to 10th	Night	59% 5% 11% 18% 6% 1	4428
	11th to 20th	Night	63% 5% 12% 14% 6%	4440
	21st to end	Night	65% 5% 8% 16% 5% 1	4683
February	1st to 10th	Night	50% 4 12% 22% 11% 1	2470
	11th to 20th	Night	47% 5% 13% 23% 9% 1	2404
	21st to end	Night	51% 4 12% 21% 10% 1	1946
March	1st to 10th	Night	38% 5% 16% 24% 15% 2	3643
	11th to 20th	Night	47% 5% 10% 24% 14%	3480
	21st to end	Night	61% 3 7% 17% 11%	3690
April	1st to 10th	Night	51% 5% 6% 19% 17% 2	1900
	11th to 20th	Night	52% 4 7% 20% 14% 2	1875
	21st to end	Night	50% 5% 9% 21% 13% 1	1735
May	1st to 10th	Night	45% 5% 12% 20% 16% 1	2530
	11th to 20th	Night	35% 5% 9% 29% 19% 3	2507
	21st to end	Night	44% 7% 13% 23% 13% 1	2751
June	1st to 10th	Night	35% 5% 17% 28% 13% 2	1520
	11th to 20th	Night	36% 6% 14% 23% 19% 2	1520
	21st to end	Night	34% 7% 14% 29% 13% 2	1520
July	1st to 10th	Night	32% 6% 21% 26% 14% 1	2490
	11th to 20th	Night	25% 5% 12% 40% 16% 2	2482
	21st to end	Night	26% 6% 13% 32% 21% 2	2662
August	1st to 10th	Night	35% 5% 14% 28% 15% 3	2682
	11th to 20th	Night	36% 6% 12% 33% 11% 1	2825
	21st to end	Night	40% 7% 14% 27% 11% 1	3380
September	1st to 10th	Night	30% 5% 15% 37% 14% 1	1900
	11th to 20th	Night	31% 5% 17% 35% 12%	1900
	21st to end	Night	38% 4 14% 31% 12% 1	2161
October	1st to 10th	Night	39% 4 12% 32% 12% 2	2280
	11th to 20th	Night	48% 6% 12% 24% 10% 1	2208
	21st to end	Night	50% 6% 12% 21% 10% 1	2474
November	1st to 10th	Night	47% 4 12% 24% 12% 1	2486
	11th to 20th	Night	56% 4 13% 16% 10%	2660
	21st to end	Night	50% 6% 15% 22% 6% 1	2660
December	1st to 10th	Night	59% 4 14% 17% 5%	2660
	11th to 20th	Night	61% 6% 12% 16% 5% 1	3281
	21st to end	Night	69% 4 8% 14% 5%	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	16% 12% 34% 26% 9% 3	270
	11th to 20th	Day	15% 10% 37% 30% 7%	270
	21st to end	Day	16% 8% 39% 21% 15% 1	297
February	1st to 10th	Day	15% 12% 43% 22% 6% 2	270
	11th to 20th	Day	13% 19% 43% 17% 6% 2	270
	21st to end	Day	18% 16% 45% 19% 3	225
March	1st to 10th	Day	19% 13% 39% 18% 10% 1	270
	11th to 20th	Day	17% 10% 40% 23% 8% 2	270
	21st to end	Day	22% 13% 31% 21% 10% 3	372
April	1st to 10th	Day	23% 15% 33% 22% 5% 1	460
	11th to 20th	Day	25% 12% 29% 22% 10%	452
	21st to end	Day	27% 10% 28% 23% 9% 3	617
May	1st to 10th	Day	20% 13% 30% 28% 8% 2	653
	11th to 20th	Day	18% 11% 28% 27% 12% 4	675
	21st to end	Day	19% 10% 25% 26% 16% 4	734
June	1st to 10th	Day	12% 7% 29% 29% 17% 5%	680
	11th to 20th	Day	14% 9% 34% 29% 11% 3	680
	21st to end	Day	14% 8% 29% 27% 16% 7%	680
July	1st to 10th	Day	8% 5% 23% 30% 24% 11%	671
	11th to 20th	Day	10% 8% 24% 27% 21% 10%	668
	21st to end	Day	9% 4 24% 33% 20% 10%	739
August	1st to 10th	Day	12% 5% 22% 33% 22% 7%	676
	11th to 20th	Day	15% 5% 19% 31% 24% 7%	665
	21st to end	Day	14% 7% 24% 30% 22% 4	598
September	1st to 10th	Day	14% 7% 26% 30% 18% 5%	490
	11th to 20th	Day	12% 7% 29% 30% 19% 3	490
	21st to end	Day	23% 9% 26% 24% 12% 6%	347
October	1st to 10th	Day	19% 11% 28% 22% 13% 6%	300
	11th to 20th	Day	23% 18% 30% 22% 6% 1	279
	21st to end	Day	28% 13% 40% 14% 5% 1	297
November	1st to 10th	Day	21% 12% 33% 24% 9% 1	270
	11th to 20th	Day	20% 16% 40% 16% 7% 1	270
	21st to end	Day	24% 11% 38% 21% 5%	270
December	1st to 10th	Day	15% 11% 39% 25% 9%	270
	11th to 20th	Day	21% 15% 37% 21% 6%	270
	21st to end	Day	28% 12% 39% 18% 2 1	297
January	1st to 10th	Night	15% 10% 35% 30% 10% 1	950
	11th to 20th	Night	18% 7% 38% 27% 8% 1	950
	21st to end	Night	19% 9% 35% 30% 7% 1	1045
February	1st to 10th	Night	13% 11% 39% 29% 8% 1	950
	11th to 20th	Night	12% 11% 40% 28% 8% 1	929
	21st to end	Night	19% 9% 34% 29% 8%	690
March	1st to 10th	Night	16% 11% 37% 28% 7% 1	709
	11th to 20th	Night	17% 8% 41% 28% 6% 1	570
	21st to end	Night	22% 11% 28% 27% 11% 2	627
April	1st to 10th	Night	24% 9% 33% 27% 7%	570
	11th to 20th	Night	25% 9% 29% 29% 7% 1	563
	21st to end	Night	26% 6% 28% 30% 9% 2	570
May	1st to 10th	Night	19% 9% 35% 31% 6% 1	570
	11th to 20th	Night	17% 7% 31% 33% 9% 2	561
	21st to end	Night	14% 6% 29% 37% 14% 1	617
June	1st to 10th	Night	12% 6% 30% 39% 12% 1	570
	11th to 20th	Night	13% 5% 29% 44% 9%	570
	21st to end	Night	11% 5% 23% 43% 17% 2	570
July	1st to 10th	Night	6% 3 23% 40% 26% 3	555
	11th to 20th	Night	10% 5% 24% 39% 20% 2	552
	21st to end	Night	7% 3 21% 44% 22% 3	624
August	1st to 10th	Night	10% 2 16% 49% 22% 1	567
	11th to 20th	Night	15% 4 18% 35% 26% 3	564
	21st to end	Night	13% 3 21% 37% 24% 2	627
September	1st to 10th	Night	10% 5% 23% 40% 19% 3	570
	11th to 20th	Night	11% 5% 28% 35% 18% 2	570
	21st to end	Night	17% 7% 31% 28% 15% 1	689
October	1st to 10th	Night	20% 8% 26% 33% 12% 1	760
	11th to 20th	Night	21% 12% 31% 28% 7%	735
	21st to end	Night	23% 10% 30% 29% 7%	792
November	1st to 10th	Night	21% 9% 35% 26% 9%	820
	11th to 20th	Night	20% 13% 32% 27% 8% 1	950
	21st to end	Night	20% 12% 36% 26% 6%	950
December	1st to 10th	Night	16% 11% 34% 30% 9% 1	950
	11th to 20th	Night	23% 9% 37% 22% 8% 1	950
	21st to end	Night	27% 10% 32% 24% 6% 1	1045

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