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 2024-12-06.

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	9% 4 1 4 3 3 3 3 4 6% 62%	105346
	Night	10% 5% 2 4 3 3 3 3 4 6% 58%	103087
Transparency	Day	25% 4 8% 19% 23% 20%	105105
	Night	28% 5% 10% 18% 20% 19%	102919
Seeing	Day	10% 10% 23% 31% 20% 6%	15433
	Night	11% 5% 16% 33% 30% 6%	24731

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% 9% 3 7% 4 4 3 3 4 6% 39%	2676
	11th to 20th	Day	23% 9% 4 8% 5% 4 3 3 4 5% 33%	2860
	21st to end	Day	23% 8% 4 7% 4 3 4 3 4 7% 32%	3146
February	1st to 10th	Day	18% 5% 2 7% 5% 3 5% 2 4 4 46%	1710
	11th to 20th	Day	16% 8% 2 6% 3 4 4 4 4 7% 43%	1712
	21st to end	Day	16% 7% 2 5% 4 3 3 2 4 6% 48%	1570
March	1st to 10th	Day	22% 13% 3 8% 5% 4 3 3 4 5% 29%	3408
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 5% 6% 54%	3490
	21st to end	Day	19% 8% 2 6% 4 4 5% 3 5% 7% 36%	3855
April	1st to 10th	Day	15% 8% 2 6% 5% 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	10% 4 2 5% 4 4 4 3 5% 7% 52%	2249
	21st to end	Day	6% 5% 1 3 4 3 3 3 5% 9% 58%	2280
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 3 5% 72%	4017
	11th to 20th	Day	3 2 1 3 1 2 2 2 4 6% 73%	4182
	21st to end	Day	3 2 1 3 2 2 2 2 4 6% 73%	4813
June	1st to 10th	Day	6% 5% 2 3 2 2 3 3 5% 8% 61%	2660
	11th to 20th	Day	1 1 1 3 2 2 2 2 4 6% 75%	2660
	21st to end	Day	2 1 1 2 1 2 2 2 3 5% 78%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 87%	4420
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 6% 81%	4157
	21st to end	Day	1 2 1 2 2 1 3 5% 83%	4455
August	1st to 10th	Day	1 1 1 2 2 2 4 4 5% 7% 71%	4091
	11th to 20th	Day	1 1 1 1 2 1 3 5% 84%	3887
	21st to end	Day	1 1 1 3 2 2 2 1 2 5% 79%	4180
September	1st to 10th	Day	1 1 2 2 1 1 1 3 6% 82%	2280
	11th to 20th	Day	3 2 1 3 2 3 3 3 4 7% 68%	2280
	21st to end	Day	3 2 1 2 2 1 2 2 4 5% 76%	2000
October	1st to 10th	Day	3 2 1 2 2 2 2 3 4 5% 74%	3180
	11th to 20th	Day	5% 3 1 4 3 2 2 2 3 6% 68%	3161
	21st to end	Day	6% 3 2 4 3 3 3 3 5% 7% 60%	3448
November	1st to 10th	Day	11% 3 1 3 3 3 3 3 4 7% 57%	1919
	11th to 20th	Day	12% 5% 2 6% 4 4 3 4 5% 6% 49%	1778
	21st to end	Day	13% 4 1 4 4 3 5% 3 5% 5% 53%	1600
December	1st to 10th	Day	17% 8% 1 6% 3 3 4 3 4 5% 46%	1560
	11th to 20th	Day	33% 9% 2 5% 3 3 3 2 4 5% 31%	1928
	21st to end	Day	24% 9% 2 6% 3 3 3 4 4 7% 35%	2794
January	1st to 10th	Night	16% 9% 2 6% 3 3 4 3 4 7% 43%	4269
	11th to 20th	Night	22% 9% 3 7% 4 5% 4 3 5% 6% 32%	4110
	21st to end	Night	24% 10% 3 7% 4 4 3 3 4 6% 32%	4521
February	1st to 10th	Night	17% 6% 2 4 3 3 3 3 4 5% 49%	2470
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 5% 7% 45%	2353
	21st to end	Night	13% 6% 2 4 3 3 4 2 4 6% 53%	1884
March	1st to 10th	Night	22% 13% 3 6% 4 3 4 3 4 6% 34%	3463
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 4 57%	3490
	21st to end	Night	17% 13% 3 6% 4 3 3 3 4 6% 38%	3785
April	1st to 10th	Night	11% 7% 2 6% 4 3 3 3 5% 6% 51%	1900
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 7% 57%	1879
	21st to end	Night	6% 3 1 3 3 3 2 3 5% 8% 62%	1900
May	1st to 10th	Night	5% 2 1 3 2 2 3 2 3 4 73%	2939
	11th to 20th	Night	2 2 2 3 3 3 4 2 5% 7% 68%	2716
	21st to end	Night	3 1 1 2 2 2 3 2 3 5% 77%	2762
June	1st to 10th	Night	4 4 1 4 2 3 3 2 5% 8% 63%	1520
	11th to 20th	Night	2 1 1 2 2 2 2 2 5% 5% 77%	1520
	21st to end	Night	2 2 1 3 2 1 2 2 3 5% 75%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	2540
	11th to 20th	Night	1 1 2 2 1 2 2 3 6% 80%	2793
	21st to end	Night	1 1 2 3 3 3 3 5% 9% 70%	3109
August	1st to 10th	Night	2 1 1 3 2 2 3 3 5% 9% 67%	2848
	11th to 20th	Night	1 2 2 2 2 2 3 5% 81%	2954
	21st to end	Night	2 2 1 2 2 2 2 1 3 4 79%	3498
September	1st to 10th	Night	1 2 2 1 3 2 4 5% 80%	1900
	11th to 20th	Night	4 3 1 3 3 3 3 2 5% 7% 66%	1900
	21st to end	Night	2 2 1 3 2 2 2 2 4 5% 76%	2175
October	1st to 10th	Night	2 3 1 3 2 2 2 2 4 4 75%	3800
	11th to 20th	Night	3 3 1 4 3 3 4 2 4 5% 65%	3777
	21st to end	Night	7% 3 1 3 4 3 4 2 5% 6% 63%	4121
November	1st to 10th	Night	9% 5% 2 4 3 2 2 3 4 8% 56%	2377
	11th to 20th	Night	11% 6% 2 4 3 3 4 3 5% 8% 51%	2620
	21st to end	Night	11% 5% 2 4 3 3 3 2 4 6% 56%	2800
December	1st to 10th	Night	19% 6% 2 5% 3 3 3 2 5% 7% 45%	2736
	11th to 20th	Night	32% 8% 2 5% 3 2 3 3 5% 6% 31%	3276
	21st to end	Night	23% 8% 2 6% 4 4 4 3 4 5% 36%	4862

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	45% 6% 12% 26% 7% 3	2676
	11th to 20th	Day	54% 6% 10% 13% 10% 7%	2860
	21st to end	Day	52% 6% 9% 17% 11% 5%	3146
February	1st to 10th	Day	42% 5% 7% 19% 19% 8%	1710
	11th to 20th	Day	41% 6% 8% 19% 19% 7%	1721
	21st to end	Day	39% 4 9% 25% 16% 7%	1570
March	1st to 10th	Day	56% 6% 10% 14% 13% 2	3408
	11th to 20th	Day	30% 6% 10% 30% 14% 11%	3490
	21st to end	Day	44% 9% 14% 20% 11% 2	3855
April	1st to 10th	Day	40% 7% 13% 20% 17% 4	2280
	11th to 20th	Day	32% 6% 14% 22% 20% 6%	2252
	21st to end	Day	23% 6% 14% 29% 24% 4	2280
May	1st to 10th	Day	17% 3 12% 29% 24% 16%	4017
	11th to 20th	Day	15% 3 7% 30% 31% 13%	4185
	21st to end	Day	14% 5% 13% 37% 27% 4	4814
June	1st to 10th	Day	23% 5% 11% 28% 25% 7%	2660
	11th to 20th	Day	12% 4 11% 20% 37% 17%	2660
	21st to end	Day	12% 3 5% 15% 35% 30%	2660
July	1st to 10th	Day	7% 2 3 8% 15% 66%	4351
	11th to 20th	Day	7% 3 5% 8% 19% 58%	4086
	21st to end	Day	8% 3 4 7% 26% 53%	4315
August	1st to 10th	Day	13% 4 5% 12% 26% 41%	4076
	11th to 20th	Day	7% 2 4 10% 44% 33%	3926
	21st to end	Day	11% 3 5% 12% 37% 32%	4179
September	1st to 10th	Day	8% 3 5% 11% 30% 44%	2280
	11th to 20th	Day	18% 5% 8% 24% 34% 12%	2280
	21st to end	Day	13% 3 6% 20% 33% 26%	2000
October	1st to 10th	Day	13% 3 5% 11% 26% 42%	3180
	11th to 20th	Day	19% 4 6% 21% 28% 21%	3161
	21st to end	Day	23% 4 10% 22% 29% 13%	3448
November	1st to 10th	Day	27% 5% 9% 27% 20% 12%	1919
	11th to 20th	Day	36% 5% 12% 22% 17% 8%	1778
	21st to end	Day	32% 5% 11% 18% 20% 14%	1600
December	1st to 10th	Day	40% 6% 8% 19% 19% 8%	1560
	11th to 20th	Day	57% 4 7% 12% 16% 5%	1928
	21st to end	Day	48% 6% 14% 19% 9% 4	2794
January	1st to 10th	Night	40% 7% 19% 23% 8% 3	4269
	11th to 20th	Night	51% 8% 12% 12% 10% 8%	4110
	21st to end	Night	53% 6% 10% 12% 13% 6%	4521
February	1st to 10th	Night	37% 5% 8% 19% 19% 11%	2470
	11th to 20th	Night	39% 5% 11% 18% 19% 8%	2355
	21st to end	Night	33% 4 11% 27% 20% 6%	1884
March	1st to 10th	Night	49% 9% 11% 13% 14% 3	3463
	11th to 20th	Night	31% 4 10% 22% 22% 10%	3490
	21st to end	Night	43% 8% 13% 22% 10% 3	3785
April	1st to 10th	Night	35% 6% 14% 20% 21% 5%	1900
	11th to 20th	Night	29% 5% 12% 27% 17% 10%	1879
	21st to end	Night	21% 5% 13% 30% 25% 6%	1900
May	1st to 10th	Night	17% 4 7% 27% 28% 17%	2939
	11th to 20th	Night	19% 4 12% 18% 26% 21%	2717
	21st to end	Night	13% 5% 10% 38% 29% 7%	2762
June	1st to 10th	Night	20% 7% 12% 23% 30% 9%	1520
	11th to 20th	Night	11% 4 9% 22% 35% 19%	1520
	21st to end	Night	14% 3 7% 14% 31% 31%	1520
July	1st to 10th	Night	6% 3 3 8% 15% 66%	2500
	11th to 20th	Night	9% 3 6% 9% 15% 58%	2742
	21st to end	Night	13% 7% 8% 7% 17% 49%	3011
August	1st to 10th	Night	16% 4 8% 10% 21% 40%	2839
	11th to 20th	Night	9% 3 7% 8% 34% 39%	2981
	21st to end	Night	13% 3 5% 11% 31% 36%	3498
September	1st to 10th	Night	9% 2 6% 11% 28% 44%	1900
	11th to 20th	Night	21% 3 7% 20% 33% 15%	1900
	21st to end	Night	14% 3 6% 20% 27% 30%	2175
October	1st to 10th	Night	14% 3 5% 10% 19% 48%	3800
	11th to 20th	Night	21% 6% 8% 21% 23% 21%	3777
	21st to end	Night	23% 4 7% 20% 26% 19%	4121
November	1st to 10th	Night	27% 5% 11% 26% 18% 13%	2377
	11th to 20th	Night	31% 5% 13% 24% 18% 9%	2620
	21st to end	Night	31% 4 9% 20% 22% 14%	2800
December	1st to 10th	Night	39% 5% 9% 17% 18% 11%	2736
	11th to 20th	Night	54% 5% 10% 12% 15% 4	3276
	21st to end	Night	48% 7% 12% 17% 12% 4	4862

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	31% 13% 25% 17% 11% 3	265
	11th to 20th	Day	23% 11% 24% 25% 14% 3	370
	21st to end	Day	17% 10% 25% 29% 15% 4	407
February	1st to 10th	Day	22% 11% 21% 25% 18% 4	370
	11th to 20th	Day	21% 18% 25% 22% 13% 1	387
	21st to end	Day	17% 15% 26% 28% 13% 1	382
March	1st to 10th	Day	25% 17% 28% 20% 9% 2	460
	11th to 20th	Day	22% 14% 27% 20% 13% 4	460
	21st to end	Day	18% 18% 30% 24% 9% 1	506
April	1st to 10th	Day	17% 14% 29% 25% 11% 4	460
	11th to 20th	Day	15% 16% 28% 23% 13% 5%	452
	21st to end	Day	7% 16% 28% 28% 13% 7%	460
May	1st to 10th	Day	10% 15% 31% 27% 13% 3	463
	11th to 20th	Day	9% 13% 28% 27% 18% 5%	488
	21st to end	Day	6% 10% 25% 33% 18% 7%	529
June	1st to 10th	Day	8% 11% 25% 33% 17% 7%	490
	11th to 20th	Day	4 10% 28% 39% 16% 3	490
	21st to end	Day	1 4 28% 36% 26% 6%	490
July	1st to 10th	Day	2 4 19% 45% 24% 6%	486
	11th to 20th	Day	1 1 15% 42% 30% 11%	484
	21st to end	Day	1 1 13% 47% 30% 9%	531
August	1st to 10th	Day	1 3 18% 41% 27% 9%	487
	11th to 20th	Day	2 4 22% 36% 26% 9%	477
	21st to end	Day	2 2 18% 38% 31% 9%	539
September	1st to 10th	Day	1 3 19% 39% 29% 10%	490
	11th to 20th	Day	5% 6% 23% 35% 23% 9%	490
	21st to end	Day	3 7% 23% 32% 27% 8%	490
October	1st to 10th	Day	6% 7% 24% 28% 24% 11%	490
	11th to 20th	Day	7% 10% 19% 27% 26% 10%	490
	21st to end	Day	12% 9% 20% 30% 23% 6%	539
November	1st to 10th	Day	13% 8% 19% 23% 30% 7%	455
	11th to 20th	Day	11% 10% 18% 25% 26% 10%	288
	21st to end	Day	16% 7% 22% 30% 21% 6%	200
December	1st to 10th	Day	19% 12% 18% 24% 18% 9%	190
	11th to 20th	Day	24% 14% 21% 18% 18% 6%	180
	21st to end	Day	22% 12% 24% 23% 17% 3	198
January	1st to 10th	Night	21% 6% 16% 31% 22% 5%	760
	11th to 20th	Night	20% 4 15% 36% 19% 5%	760
	21st to end	Night	19% 5% 17% 32% 23% 4	836
February	1st to 10th	Night	19% 5% 16% 35% 21% 3	760
	11th to 20th	Night	15% 8% 20% 32% 21% 4	744
	21st to end	Night	17% 8% 20% 34% 19% 3	628
March	1st to 10th	Night	20% 8% 22% 29% 19% 3	760
	11th to 20th	Night	19% 6% 18% 31% 24% 3	760
	21st to end	Night	17% 7% 25% 32% 17% 2	836
April	1st to 10th	Night	13% 6% 25% 34% 18% 3	760
	11th to 20th	Night	11% 10% 22% 31% 22% 4	751
	21st to end	Night	7% 8% 25% 32% 24% 5%	760
May	1st to 10th	Night	6% 7% 23% 33% 26% 5%	623
	11th to 20th	Night	5% 2 18% 34% 36% 5%	496
	21st to end	Night	5% 6% 20% 33% 31% 5%	411
June	1st to 10th	Night	6% 4 19% 36% 34% 2	380
	11th to 20th	Night	2 5% 15% 45% 31% 2	380
	21st to end	Night	3 1 13% 42% 38% 3	380
July	1st to 10th	Night	1 1 7% 38% 49% 4	370
	11th to 20th	Night	1 1 5% 31% 51% 11%	514
	21st to end	Night	2 5% 33% 52% 8%	624
August	1st to 10th	Night	3 1 8% 31% 48% 8%	567
	11th to 20th	Night	1 1 9% 35% 43% 10%	641
	21st to end	Night	1 1 7% 35% 47% 8%	836
September	1st to 10th	Night	1 1 9% 36% 46% 8%	760
	11th to 20th	Night	5% 2 11% 35% 39% 8%	760
	21st to end	Night	3 3 13% 35% 37% 10%	760
October	1st to 10th	Night	5% 4 15% 33% 34% 8%	760
	11th to 20th	Night	7% 5% 13% 32% 36% 7%	760
	21st to end	Night	10% 4 13% 32% 31% 11%	836
November	1st to 10th	Night	9% 6% 15% 30% 32% 8%	780
	11th to 20th	Night	13% 4 16% 34% 28% 6%	800
	21st to end	Night	14% 5% 16% 35% 24% 6%	800
December	1st to 10th	Night	19% 7% 14% 33% 22% 6%	782
	11th to 20th	Night	25% 6% 15% 30% 21% 3	760
	21st to end	Night	22% 6% 20% 33% 17% 2	836

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