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 2025-03-12.

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% 10% 4 9% 5% 5% 4 4 5% 7% 35%	109082
	Night	12% 11% 4 9% 5% 4 4 4 5% 6% 35%	110163
Transparency	Day	47% 5% 13% 24% 11% 1	112639
	Night	49% 5% 11% 23% 12% 1	114746
Seeing	Day	17% 10% 30% 26% 13% 4	16459
	Night	17% 8% 31% 31% 11% 1	26552

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	9% 12% 7% 17% 8% 5% 4 2 4 4 28%	3617
	11th to 20th	Day	6% 11% 8% 12% 7% 8% 8% 8% 8% 5% 19%	3620
	21st to end	Day	6% 17% 7% 15% 6% 5% 4 3 5% 5% 26%	4277
February	1st to 10th	Day	8% 13% 5% 10% 7% 5% 5% 4 7% 6% 32%	1800
	11th to 20th	Day	7% 12% 5% 10% 5% 5% 5% 4 5% 6% 36%	1754
	21st to end	Day	11% 11% 4 10% 6% 5% 4 3 5% 8% 33%	1585
March	1st to 10th	Day	10% 10% 3 8% 5% 5% 4 4 5% 6% 40%	3342
	11th to 20th	Day	8% 10% 4 10% 5% 5% 4 4 5% 7% 38%	3491
	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% 12% 3 7% 4 5% 5% 3 4 5% 34%	1900
	21st to end	Day	19% 13% 3 7% 4 4 3 3 5% 6% 33%	1986
November	1st to 10th	Day	14% 15% 3 8% 5% 5% 4 3 4 7% 33%	1677
	11th to 20th	Day	13% 15% 7% 11% 8% 4 4 3 5% 6% 24%	1600
	21st to end	Day	14% 12% 5% 9% 5% 4 5% 4 6% 6% 31%	1600
December	1st to 10th	Day	8% 15% 7% 17% 13% 5% 5% 2 4 4 20%	2610
	11th to 20th	Day	12% 17% 7% 11% 7% 4 3 3 4 4 27%	3008
	21st to end	Day	19% 21% 6% 19% 7% 4 3 2 3 3 13%	3982
January	1st to 10th	Night	9% 11% 6% 16% 8% 5% 4 3 4 5% 30%	6348
	11th to 20th	Night	7% 13% 5% 12% 8% 7% 8% 6% 8% 5% 20%	6360
	21st to end	Night	7% 20% 8% 13% 5% 4 4 3 5% 5% 27%	6697
February	1st to 10th	Night	8% 11% 6% 11% 6% 5% 5% 4 6% 6% 34%	2600
	11th to 20th	Night	7% 10% 4 9% 6% 6% 5% 5% 6% 5% 37%	2533
	21st to end	Night	12% 13% 3 9% 5% 5% 4 4 5% 6% 34%	2045
March	1st to 10th	Night	9% 8% 3 7% 4 4 4 4 6% 7% 45%	3760
	11th to 20th	Night	8% 10% 4 12% 5% 4 4 4 5% 4 39%	3497
	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	16% 11% 3 6% 4 4 4 3 5% 6% 37%	2280
	21st to end	Night	18% 11% 3 7% 5% 4 4 4 5% 6% 33%	2612
November	1st to 10th	Night	16% 11% 4 7% 4 3 4 3 5% 7% 36%	2621
	11th to 20th	Night	15% 12% 6% 11% 6% 4 3 3 4 5% 31%	2800
	21st to end	Night	14% 10% 4 10% 6% 4 4 4 5% 6% 34%	2800
December	1st to 10th	Night	8% 13% 6% 16% 9% 5% 5% 4 5% 7% 23%	4580
	11th to 20th	Night	12% 18% 6% 13% 6% 5% 5% 5% 5% 5% 21%	5199
	21st to end	Night	22% 19% 6% 14% 5% 4 3 3 3 4 17%	6996

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% 4 7% 19% 10%	3618
	11th to 20th	Day	62% 4 7% 17% 10%	3620
	21st to end	Day	59% 5% 12% 18% 6%	4277
February	1st to 10th	Day	52% 4 12% 21% 10% 1	1800
	11th to 20th	Day	48% 5% 13% 24% 9% 1	1763
	21st to end	Day	51% 4 12% 21% 11% 1	1585
March	1st to 10th	Day	45% 4 15% 22% 12% 2	3342
	11th to 20th	Day	45% 5% 13% 25% 12% 1	3491
	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	33% 3 11% 36% 16%	3160
	11th to 20th	Day	45% 4 8% 28% 14% 1	3140
	21st to end	Day	48% 4 14% 25% 8%	3250
November	1st to 10th	Day	51% 5% 11% 20% 12% 1	1677
	11th to 20th	Day	62% 4 13% 15% 6% 1	1600
	21st to end	Day	53% 6% 15% 20% 6% 1	1600
December	1st to 10th	Day	68% 4 9% 15% 3	2610
	11th to 20th	Day	61% 3 8% 16% 11% 1	3005
	21st to end	Day	77% 4 6% 7% 5%	3982
January	1st to 10th	Night	57% 4 10% 17% 12%	6348
	11th to 20th	Night	61% 4 10% 15% 10%	6360
	21st to end	Night	59% 4 11% 19% 6%	6697
February	1st to 10th	Night	50% 4 12% 21% 11% 1	2600
	11th to 20th	Night	47% 5% 13% 23% 10% 1	2534
	21st to end	Night	51% 4 12% 21% 10% 1	2045
March	1st to 10th	Night	38% 5% 16% 24% 16% 2	3760
	11th to 20th	Night	47% 5% 10% 24% 14%	3497
	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	27% 3 7% 36% 23% 4	3800
	11th to 20th	Night	42% 5% 11% 22% 19% 1	3779
	21st to end	Night	50% 4 11% 24% 10%	4298
November	1st to 10th	Night	47% 4 12% 24% 11% 1	2621
	11th to 20th	Night	56% 4 13% 17% 10%	2800
	21st to end	Night	51% 5% 15% 22% 6% 1	2800
December	1st to 10th	Night	61% 6% 14% 15% 3	4580
	11th to 20th	Night	65% 4 9% 15% 6%	5201
	21st to end	Night	72% 3 6% 13% 5%	6996

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	14% 13% 35% 25% 9% 3	300
	11th to 20th	Day	15% 10% 38% 29% 8%	300
	21st to end	Day	16% 10% 39% 20% 14% 1	330
February	1st to 10th	Day	15% 13% 44% 21% 5% 2	300
	11th to 20th	Day	12% 18% 44% 18% 7% 2	300
	21st to end	Day	16% 18% 45% 18% 3	249
March	1st to 10th	Day	19% 12% 41% 17% 10% 1	300
	11th to 20th	Day	17% 10% 41% 23% 8% 2	273
	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% 17% 32% 21% 6% 1	300
	21st to end	Day	27% 14% 39% 15% 5% 1	330
November	1st to 10th	Day	22% 12% 34% 23% 8% 1	300
	11th to 20th	Day	21% 17% 39% 16% 7% 1	300
	21st to end	Day	24% 12% 36% 22% 5%	300
December	1st to 10th	Day	16% 11% 40% 26% 8%	300
	11th to 20th	Day	20% 16% 38% 20% 6%	300
	21st to end	Day	29% 12% 38% 17% 3 1	330
January	1st to 10th	Night	14% 10% 35% 31% 10% 1	1000
	11th to 20th	Night	18% 8% 38% 28% 8% 1	1000
	21st to end	Night	18% 10% 35% 29% 7% 1	1100
February	1st to 10th	Night	13% 11% 39% 28% 8% 1	1000
	11th to 20th	Night	12% 11% 41% 28% 7% 1	979
	21st to end	Night	19% 9% 34% 29% 8%	725
March	1st to 10th	Night	16% 11% 37% 28% 7% 1	746
	11th to 20th	Night	17% 8% 41% 28% 6% 1	575
	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	20% 12% 33% 28% 7%	760
	21st to end	Night	23% 10% 31% 29% 7%	836
November	1st to 10th	Night	21% 9% 35% 26% 8%	865
	11th to 20th	Night	21% 13% 32% 27% 7% 1	1000
	21st to end	Night	20% 12% 36% 26% 6%	1000
December	1st to 10th	Night	16% 11% 35% 30% 9% 1	1000
	11th to 20th	Night	23% 9% 37% 23% 8% 1	1000
	21st to end	Night	26% 10% 31% 25% 7% 1	1100

Page generated at 2025-03-09 07:08:45. Version ac07

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