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 2024-07-05.

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	10% 5% 2 4 3 3 3 3 4 6% 59%	89724
	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	89652
Transparency	Day	28% 5% 9% 21% 22% 15%	89443
	Night	31% 5% 10% 19% 20% 14%	89456
Seeing	Day	11% 10% 23% 30% 20% 6%	14862
	Night	11% 5% 16% 33% 30% 6%	24164

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 2 1 2 2 2 2 2 3 4 79%	2579
	11th to 20th	Day	1 1 1 2 1 2 2 2 4 7% 78%	2374
	21st to end	Day	1 1 1 2 2 2 2 2 4 6% 79%	2546
August	1st to 10th	Day	1 2 1 3 2 2 4 2 3 6% 74%	2328
	11th to 20th	Day	1 1 2 2 2 2 2 3 6% 78%	2224
	21st to end	Day	1 2 1 3 2 2 2 2 3 6% 76%	2376
September	1st to 10th	Day	1 1 2 2 2 1 1 3 6% 81%	2160
	11th to 20th	Day	4 2 1 3 2 3 3 3 4 7% 69%	2160
	21st to end	Day	3 3 1 2 2 2 2 2 4 5% 75%	1896
October	1st to 10th	Day	6% 3 1 3 2 3 3 2 4 5% 70%	1800
	11th to 20th	Day	7% 3 1 3 2 2 3 2 3 6% 66%	1800
	21st to end	Day	10% 3 2 4 2 2 2 2 3 6% 63%	1980
November	1st to 10th	Day	12% 3 1 4 4 3 3 3 5% 7% 56%	1821
	11th to 20th	Day	12% 5% 2 6% 4 4 3 3 5% 6% 50%	1690
	21st to end	Day	12% 4 1 4 4 4 5% 4 5% 5% 53%	1520
December	1st to 10th	Day	17% 8% 1 6% 4 3 4 3 4 6% 45%	1520
	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 2 1 1 2 1 3 5% 82%	1475
	11th to 20th	Night	1 1 2 2 2 3 2 4 7% 77%	1581
	21st to end	Night	2 1 1 3 3 3 3 4 5% 9% 68%	1773
August	1st to 10th	Night	2 1 1 3 3 3 3 3 4 8% 67%	1611
	11th to 20th	Night	1 1 3 2 4 3 3 4 7% 72%	1674
	21st to end	Night	1 2 1 3 3 2 3 2 5% 7% 72%	1980
September	1st to 10th	Night	1 2 2 2 3 2 4 6% 79%	1800
	11th to 20th	Night	4 3 1 3 3 3 3 3 4 7% 66%	1800
	21st to end	Night	3 2 1 3 2 2 2 2 4 5% 75%	2059
October	1st to 10th	Night	4 4 1 3 2 2 3 2 4 5% 69%	2160
	11th to 20th	Night	5% 3 1 4 2 3 4 2 5% 6% 64%	2160
	21st to end	Night	9% 3 1 3 3 2 3 2 4 6% 62%	2376
November	1st to 10th	Night	9% 5% 2 4 3 2 2 3 4 8% 56%	2255
	11th to 20th	Night	11% 6% 2 4 3 3 4 3 5% 8% 51%	2489
	21st to end	Night	10% 5% 2 4 3 3 3 2 4 6% 57%	2660
December	1st to 10th	Night	20% 6% 2 5% 3 3 3 2 5% 7% 44%	2660
	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	11% 4 4 13% 25% 43%	2510
	11th to 20th	Day	10% 5% 7% 13% 28% 37%	2303
	21st to end	Day	10% 3 5% 10% 27% 45%	2405
August	1st to 10th	Day	15% 4 6% 16% 31% 28%	2312
	11th to 20th	Day	11% 3 5% 12% 37% 31%	2224
	21st to end	Day	13% 3 6% 14% 34% 30%	2376
September	1st to 10th	Day	9% 3 5% 11% 30% 42%	2160
	11th to 20th	Day	17% 5% 8% 24% 34% 13%	2160
	21st to end	Day	13% 3 6% 21% 32% 25%	1896
October	1st to 10th	Day	18% 4 6% 15% 32% 25%	1800
	11th to 20th	Day	21% 5% 8% 19% 25% 23%	1800
	21st to end	Day	25% 4 8% 20% 26% 17%	1980
November	1st to 10th	Day	28% 5% 9% 27% 19% 12%	1821
	11th to 20th	Day	36% 5% 12% 22% 18% 8%	1690
	21st to end	Day	32% 5% 12% 18% 20% 13%	1520
December	1st to 10th	Day	41% 6% 8% 18% 19% 9%	1520
	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	10% 4 5% 13% 26% 42%	1435
	11th to 20th	Night	11% 4 9% 14% 22% 39%	1530
	21st to end	Night	16% 7% 8% 10% 18% 40%	1674
August	1st to 10th	Night	19% 5% 8% 16% 24% 28%	1602
	11th to 20th	Night	14% 4 9% 12% 26% 35%	1674
	21st to end	Night	15% 5% 8% 15% 25% 32%	1980
September	1st to 10th	Night	9% 2 7% 11% 29% 41%	1800
	11th to 20th	Night	21% 3 7% 19% 33% 16%	1800
	21st to end	Night	15% 3 6% 21% 26% 30%	2059
October	1st to 10th	Night	19% 4 8% 17% 27% 25%	2160
	11th to 20th	Night	21% 5% 9% 19% 22% 24%	2160
	21st to end	Night	24% 5% 8% 19% 24% 21%	2376
November	1st to 10th	Night	28% 5% 11% 26% 17% 13%	2255
	11th to 20th	Night	31% 5% 12% 24% 18% 9%	2489
	21st to end	Night	30% 4 9% 21% 23% 14%	2660
December	1st to 10th	Night	40% 5% 9% 17% 18% 11%	2660
	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 20% 45% 23% 6%	463
	11th to 20th	Day	1 1 15% 40% 32% 11%	444
	21st to end	Day	1 1 12% 47% 30% 9%	487
August	1st to 10th	Day	1 4 18% 41% 28% 9%	447
	11th to 20th	Day	2 3 21% 38% 27% 10%	437
	21st to end	Day	2 2 18% 39% 31% 8%	495
September	1st to 10th	Day	1 4 17% 39% 30% 10%	450
	11th to 20th	Day	6% 4 23% 36% 23% 9%	450
	21st to end	Day	4 7% 23% 34% 25% 8%	450
October	1st to 10th	Day	6% 8% 25% 27% 24% 10%	450
	11th to 20th	Day	7% 10% 18% 28% 26% 11%	450
	21st to end	Day	12% 8% 19% 30% 24% 6%	495
November	1st to 10th	Day	14% 7% 18% 24% 31% 7%	417
	11th to 20th	Day	9% 8% 17% 26% 29% 11%	260
	21st to end	Day	13% 6% 23% 29% 23% 6%	180
December	1st to 10th	Day	19% 13% 19% 23% 18% 8%	180
	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 8% 36% 50% 4	359
	11th to 20th	Night	1 1 5% 30% 52% 12%	486
	21st to end	Night	2 5% 33% 52% 8%	591
August	1st to 10th	Night	2 1 9% 30% 49% 9%	537
	11th to 20th	Night	1 1 8% 34% 45% 10%	606
	21st to end	Night	1 1 6% 34% 48% 9%	792
September	1st to 10th	Night	1 1 8% 35% 47% 8%	720
	11th to 20th	Night	5% 2 10% 35% 40% 8%	720
	21st to end	Night	3 3 12% 35% 36% 10%	720
October	1st to 10th	Night	5% 4 15% 33% 34% 9%	720
	11th to 20th	Night	7% 4 12% 33% 36% 7%	720
	21st to end	Night	10% 3 12% 30% 33% 11%	792
November	1st to 10th	Night	10% 5% 15% 30% 32% 8%	740
	11th to 20th	Night	12% 3 15% 34% 30% 6%	760
	21st to end	Night	12% 5% 15% 36% 26% 6%	760
December	1st to 10th	Night	20% 7% 14% 33% 21% 6%	760
	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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