Robert Ferguson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robert Ferguson Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	8% 4 1 4 3 2 3 3 4 6% 63%	140052
	Night	10% 5% 2 4 3 3 3 3 4 6% 58%	138907
Transparency	Day	24% 4 8% 20% 24% 20%	134949
	Night	28% 5% 9% 18% 21% 18%	135001
Seeing	Day	11% 10% 23% 30% 20% 6%	17049
	Night	11% 5% 16% 34% 29% 5%	26435

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	19% 7% 2 5% 4 3 3 3 6% 6% 42%	4999
	11th to 20th	Day	12% 5% 2 5% 3 3 2 2 3 6% 57%	5340
	21st to end	Day	15% 6% 3 7% 4 3 4 3 5% 8% 42%	5843
February	1st to 10th	Day	19% 5% 2 6% 4 3 5% 2 4 4 46%	1890
	11th to 20th	Day	16% 8% 2 6% 4 4 4 4 5% 7% 40%	1898
	21st to end	Day	16% 7% 2 6% 5% 3 3 3 4 6% 46%	1730
March	1st to 10th	Day	17% 11% 3 7% 5% 4 4 3 5% 6% 37%	5014
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 5% 6% 47%	5078
	21st to end	Day	16% 8% 2 7% 5% 4 5% 4 6% 8% 36%	5529
April	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 42%	2400
	11th to 20th	Day	10% 4 2 5% 4 4 4 3 5% 7% 52%	2369
	21st to end	Day	6% 5% 1 3 4 3 3 3 5% 9% 57%	2400
May	1st to 10th	Day	4 2 1 2 2 2 1 2 3 5% 77%	5748
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 6% 76%	6004
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 78%	6902
June	1st to 10th	Day	5% 5% 2 3 2 2 3 3 5% 8% 62%	2800
	11th to 20th	Day	1 1 1 3 2 2 2 2 4 6% 76%	2800
	21st to end	Day	2 1 1 2 1 2 2 2 3 5% 79%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 4 87%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 5% 83%	5953
	21st to end	Day	1 1 2 2 2 2 2 3 5% 80%	6402
August	1st to 10th	Day	1 1 1 1 2 3 3 4 6% 79%	5777
	11th to 20th	Day	1 1 1 1 2 1 2 4 86%	5605
	21st to end	Day	1 1 1 2 1 2 2 1 3 6% 80%	5984
September	1st to 10th	Day	1 1 1 2 2 2 1 1 3 6% 80%	2400
	11th to 20th	Day	3 2 1 4 2 3 3 3 4 7% 68%	2400
	21st to end	Day	4 3 1 2 2 1 2 2 4 5% 75%	2105
October	1st to 10th	Day	4 2 1 2 2 2 3 3 4 5% 73%	3280
	11th to 20th	Day	6% 3 1 4 3 2 2 2 3 6% 68%	3261
	21st to end	Day	6% 3 2 4 3 3 3 3 5% 7% 60%	3558
November	1st to 10th	Day	12% 3 1 4 3 3 3 3 4 7% 57%	2017
	11th to 20th	Day	12% 5% 3 7% 4 4 3 4 5% 6% 47%	1867
	21st to end	Day	12% 4 1 4 4 3 4 3 5% 5% 54%	1680
December	1st to 10th	Day	10% 5% 1 4 3 2 4 3 7% 9% 53%	2700
	11th to 20th	Day	28% 8% 2 5% 3 3 4 4 6% 8% 30%	3108
	21st to end	Day	28% 10% 3 6% 3 3 4 4 5% 7% 29%	4092
January	1st to 10th	Night	19% 7% 2 5% 3 3 4 3 5% 8% 43%	7905
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 6% 56%	7650
	21st to end	Night	15% 8% 2 7% 4 4 4 3 5% 6% 43%	8371
February	1st to 10th	Night	18% 6% 2 4 3 3 3 3 4 5% 49%	2730
	11th to 20th	Night	14% 8% 2 6% 4 3 4 3 5% 7% 43%	2607
	21st to end	Night	13% 6% 2 5% 3 3 4 2 4 6% 52%	2076
March	1st to 10th	Night	16% 11% 3 6% 4 3 4 3 4 6% 39%	5097
	11th to 20th	Night	16% 8% 2 5% 3 3 3 2 4 5% 50%	5077
	21st to end	Night	18% 12% 3 5% 3 3 3 3 4 7% 40%	5433
April	1st to 10th	Night	11% 6% 2 6% 5% 3 4 3 5% 6% 51%	2000
	11th to 20th	Night	9% 5% 2 5% 4 3 3 3 4 6% 56%	1979
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 8% 61%	2000
May	1st to 10th	Night	4 3 1 3 2 2 2 1 3 5% 74%	4221
	11th to 20th	Night	2 2 2 3 3 3 4 2 4 6% 69%	3900
	21st to end	Night	2 1 2 2 2 2 2 2 4 81%	3972
June	1st to 10th	Night	4 4 1 4 2 3 3 2 5% 9% 64%	1600
	11th to 20th	Night	1 1 1 2 2 2 2 2 5% 6% 77%	1600
	21st to end	Night	2 2 1 3 2 1 2 2 4 5% 76%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 4 88%	3640
	11th to 20th	Night	1 1 1 2 3 2 2 2 4 7% 75%	4005
	21st to end	Night	1 2 1 4 4 3 4 4 6% 10% 62%	4472
August	1st to 10th	Night	2 1 1 2 2 2 2 2 4 6% 76%	4035
	11th to 20th	Night	3 2 2 2 2 3 5% 82%	4250
	21st to end	Night	2 2 1 2 2 1 2 2 3 5% 79%	5015
September	1st to 10th	Night	1 1 2 2 2 2 2 4 5% 79%	2000
	11th to 20th	Night	4 3 1 3 3 3 3 2 5% 7% 66%	2000
	21st to end	Night	3 2 1 3 2 2 2 2 4 5% 75%	2290
October	1st to 10th	Night	2 3 1 3 2 2 2 2 4 4 74%	3920
	11th to 20th	Night	4 3 1 4 3 3 4 2 4 5% 65%	3897
	21st to end	Night	7% 3 1 3 4 3 4 3 5% 6% 62%	4253
November	1st to 10th	Night	9% 5% 2 4 3 2 2 4 5% 8% 56%	2499
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 5% 8% 49%	2751
	21st to end	Night	11% 5% 2 5% 3 3 4 2 4 6% 55%	2940
December	1st to 10th	Night	11% 3 1 4 3 3 4 3 5% 7% 55%	4700
	11th to 20th	Night	25% 9% 2 5% 4 4 4 4 6% 7% 29%	5316
	21st to end	Night	31% 8% 2 6% 3 3 3 3 4 5% 32%	7106

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	40% 8% 13% 22% 10% 7%	5000
	11th to 20th	Day	31% 4 6% 12% 24% 23%	5340
	21st to end	Day	41% 5% 8% 19% 10% 18%	5843
February	1st to 10th	Day	41% 4 7% 18% 20% 9%	1890
	11th to 20th	Day	42% 6% 9% 18% 18% 6%	1907
	21st to end	Day	40% 5% 9% 23% 16% 7%	1730
March	1st to 10th	Day	46% 6% 9% 19% 18% 3	5014
	11th to 20th	Day	36% 6% 12% 27% 11% 8%	5078
	21st to end	Day	43% 9% 14% 22% 10% 2	5529
April	1st to 10th	Day	39% 7% 13% 20% 17% 4	2400
	11th to 20th	Day	31% 6% 13% 22% 20% 7%	2372
	21st to end	Day	24% 6% 14% 29% 23% 4	2400
May	1st to 10th	Day	12% 2 9% 26% 30% 20%	5748
	11th to 20th	Day	12% 3 7% 34% 33% 10%	6007
	21st to end	Day	10% 4 10% 33% 39% 4	6904
June	1st to 10th	Day	22% 5% 11% 28% 27% 7%	2800
	11th to 20th	Day	11% 4 10% 20% 37% 18%	2800
	21st to end	Day	11% 3 5% 15% 35% 30%	2800
July	1st to 10th	Day	6% 2 3 8% 23% 58%	6251
	11th to 20th	Day	7% 3 4 6% 16% 64%	5882
	21st to end	Day	9% 4 5% 17% 27% 39%	6217
August	1st to 10th	Day	12% 4 5% 12% 27% 41%	4206
	11th to 20th	Day	7% 2 4 10% 43% 34%	4052
	21st to end	Day	11% 3 5% 12% 37% 32%	4311
September	1st to 10th	Day	9% 3 5% 12% 30% 42%	2400
	11th to 20th	Day	18% 5% 8% 24% 34% 12%	2400
	21st to end	Day	14% 3 6% 20% 32% 25%	2105
October	1st to 10th	Day	14% 3 5% 11% 26% 41%	3280
	11th to 20th	Day	19% 4 6% 22% 28% 21%	3261
	21st to end	Day	23% 4 10% 22% 29% 12%	3558
November	1st to 10th	Day	27% 5% 9% 26% 21% 11%	2017
	11th to 20th	Day	38% 5% 12% 21% 16% 7%	1867
	21st to end	Day	31% 4 11% 19% 21% 13%	1680
December	1st to 10th	Day	27% 4 7% 27% 26% 8%	2700
	11th to 20th	Day	52% 5% 9% 17% 14% 4	3108
	21st to end	Day	54% 6% 12% 17% 8% 3	4092
January	1st to 10th	Night	39% 8% 18% 19% 13% 4	7905
	11th to 20th	Night	29% 5% 7% 14% 24% 21%	7650
	21st to end	Night	41% 5% 8% 13% 18% 15%	8371
February	1st to 10th	Night	38% 5% 8% 18% 19% 12%	2730
	11th to 20th	Night	40% 5% 11% 19% 18% 7%	2609
	21st to end	Night	34% 4 11% 25% 19% 7%	2076
March	1st to 10th	Night	42% 8% 10% 18% 18% 4	5097
	11th to 20th	Night	37% 5% 10% 21% 19% 8%	5077
	21st to end	Night	42% 7% 12% 24% 12% 2	5433
April	1st to 10th	Night	34% 6% 14% 21% 21% 5%	2000
	11th to 20th	Night	29% 5% 12% 26% 17% 11%	1979
	21st to end	Night	22% 5% 13% 30% 25% 6%	2000
May	1st to 10th	Night	15% 3 5% 25% 31% 21%	4218
	11th to 20th	Night	18% 4 10% 25% 27% 15%	3901
	21st to end	Night	10% 4 8% 29% 39% 10%	3972
June	1st to 10th	Night	19% 7% 11% 23% 31% 9%	1600
	11th to 20th	Night	10% 4 9% 21% 36% 21%	1600
	21st to end	Night	14% 3 7% 14% 32% 31%	1600
July	1st to 10th	Night	5% 2 3 7% 23% 60%	3600
	11th to 20th	Night	11% 5% 5% 8% 12% 60%	3954
	21st to end	Night	17% 8% 11% 10% 19% 36%	4345
August	1st to 10th	Night	16% 4 8% 11% 22% 40%	2929
	11th to 20th	Night	9% 3 7% 9% 34% 39%	3075
	21st to end	Night	13% 3 5% 11% 31% 36%	3608
September	1st to 10th	Night	10% 2 6% 11% 28% 43%	2000
	11th to 20th	Night	21% 4 7% 21% 32% 15%	2000
	21st to end	Night	15% 3 6% 20% 28% 29%	2290
October	1st to 10th	Night	15% 3 5% 11% 19% 47%	3920
	11th to 20th	Night	21% 6% 8% 21% 24% 21%	3897
	21st to end	Night	23% 4 7% 20% 26% 19%	4253
November	1st to 10th	Night	27% 5% 10% 26% 19% 13%	2499
	11th to 20th	Night	33% 5% 12% 24% 18% 8%	2751
	21st to end	Night	31% 4 9% 21% 22% 14%	2940
December	1st to 10th	Night	28% 5% 7% 22% 26% 13%	4700
	11th to 20th	Night	52% 5% 10% 14% 15% 5%	5316
	21st to end	Night	55% 5% 10% 17% 10% 3	7106

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	30% 13% 25% 19% 11% 3	315
	11th to 20th	Day	20% 10% 23% 26% 18% 3	430
	21st to end	Day	16% 10% 24% 30% 16% 4	473
February	1st to 10th	Day	22% 11% 21% 27% 16% 3	430
	11th to 20th	Day	21% 20% 25% 21% 12% 1	453
	21st to end	Day	17% 14% 27% 29% 12% 1	446
March	1st to 10th	Day	23% 17% 29% 20% 9% 2	540
	11th to 20th	Day	22% 13% 27% 20% 14% 4	529
	21st to end	Day	19% 19% 29% 23% 9% 1	550
April	1st to 10th	Day	16% 15% 28% 26% 11% 4	500
	11th to 20th	Day	14% 16% 28% 23% 13% 4	492
	21st to end	Day	7% 16% 28% 28% 14% 7%	500
May	1st to 10th	Day	10% 14% 31% 26% 14% 4	503
	11th to 20th	Day	10% 12% 29% 26% 18% 5%	528
	21st to end	Day	6% 10% 28% 32% 18% 6%	573
June	1st to 10th	Day	7% 10% 26% 33% 17% 7%	530
	11th to 20th	Day	4 10% 30% 37% 15% 3	530
	21st to end	Day	1 5% 27% 36% 25% 7%	530
July	1st to 10th	Day	2 5% 21% 44% 24% 6%	526
	11th to 20th	Day	1 1 15% 41% 32% 10%	524
	21st to end	Day	1 1 13% 45% 30% 9%	575
August	1st to 10th	Day	1 3 20% 40% 27% 10%	527
	11th to 20th	Day	2 4 22% 36% 27% 9%	517
	21st to end	Day	2 2 18% 38% 31% 9%	583
September	1st to 10th	Day	1 4 20% 38% 28% 9%	530
	11th to 20th	Day	5% 6% 23% 35% 23% 9%	530
	21st to end	Day	4 8% 23% 32% 25% 8%	530
October	1st to 10th	Day	7% 8% 23% 27% 25% 10%	530
	11th to 20th	Day	8% 10% 19% 26% 27% 10%	530
	21st to end	Day	12% 9% 20% 30% 24% 6%	583
November	1st to 10th	Day	14% 8% 18% 24% 29% 7%	493
	11th to 20th	Day	12% 12% 18% 25% 24% 9%	317
	21st to end	Day	14% 6% 21% 30% 23% 6%	220
December	1st to 10th	Day	17% 11% 19% 23% 23% 7%	220
	11th to 20th	Day	25% 12% 19% 20% 18% 7%	220
	21st to end	Day	27% 12% 24% 21% 14% 2	242
January	1st to 10th	Night	22% 6% 16% 31% 20% 5%	840
	11th to 20th	Night	19% 5% 15% 38% 19% 4	840
	21st to end	Night	18% 5% 18% 34% 22% 4	924
February	1st to 10th	Night	20% 6% 17% 35% 20% 3	840
	11th to 20th	Night	17% 8% 21% 31% 19% 4	824
	21st to end	Night	16% 8% 20% 36% 17% 2	692
March	1st to 10th	Night	19% 9% 23% 29% 18% 3	840
	11th to 20th	Night	19% 6% 18% 31% 23% 3	828
	21st to end	Night	18% 7% 25% 31% 17% 2	880
April	1st to 10th	Night	13% 7% 25% 34% 18% 3	800
	11th to 20th	Night	11% 10% 22% 31% 22% 4	791
	21st to end	Night	7% 8% 25% 32% 24% 4	800
May	1st to 10th	Night	6% 7% 22% 33% 27% 5%	656
	11th to 20th	Night	5% 3 19% 34% 35% 5%	522
	21st to end	Night	4 6% 21% 33% 31% 5%	433
June	1st to 10th	Night	6% 4 19% 38% 33% 2	400
	11th to 20th	Night	2 5% 16% 46% 31% 2	400
	21st to end	Night	3 2 13% 42% 38% 3	400
July	1st to 10th	Night	1 1 8% 38% 48% 4	390
	11th to 20th	Night	1 1 5% 32% 51% 11%	542
	21st to end	Night	2 5% 33% 52% 8%	657
August	1st to 10th	Night	3 1 8% 31% 48% 9%	597
	11th to 20th	Night	1 1 10% 36% 42% 9%	675
	21st to end	Night	1 1 7% 36% 47% 8%	880
September	1st to 10th	Night	1 1 10% 36% 45% 7%	800
	11th to 20th	Night	5% 2 12% 35% 38% 8%	800
	21st to end	Night	4 3 13% 36% 36% 10%	800
October	1st to 10th	Night	5% 4 15% 33% 34% 8%	800
	11th to 20th	Night	7% 5% 14% 32% 35% 7%	800
	21st to end	Night	10% 4 13% 32% 31% 10%	880
November	1st to 10th	Night	10% 6% 15% 29% 33% 8%	820
	11th to 20th	Night	13% 5% 16% 33% 27% 5%	840
	21st to end	Night	13% 5% 16% 37% 24% 6%	840
December	1st to 10th	Night	18% 6% 14% 34% 22% 6%	840
	11th to 20th	Night	25% 6% 15% 31% 20% 3	840
	21st to end	Night	25% 6% 20% 32% 16% 2	924

Page generated at 2026-03-14 23:23:50. Version ac07

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