Winfree Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winfree Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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% 6% 2 6% 4 4 5% 4 6% 8% 42%	148585
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	143450
Transparency	Day	38% 6% 13% 22% 17% 4	152190
	Night	38% 6% 11% 22% 18% 5%	147917
Seeing	Day	18% 11% 25% 25% 15% 6%	17726
	Night	17% 9% 26% 31% 15% 2	27718

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	16% 6% 2 4 3 4 4 3 4 7% 48%	5258
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 5% 56%	5280
	21st to end	Day	15% 6% 3 7% 3 4 4 3 4 5% 48%	5780
February	1st to 10th	Day	12% 8% 2 6% 5% 4 5% 4 6% 8% 42%	1890
	11th to 20th	Day	20% 6% 1 5% 4 4 4 3 5% 6% 42%	1843
	21st to end	Day	19% 8% 3 6% 4 4 4 3 5% 6% 38%	1839
March	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 7% 6% 39%	5045
	11th to 20th	Day	11% 5% 2 5% 3 3 4 3 6% 7% 51%	5090
	21st to end	Day	15% 6% 3 7% 5% 5% 4 4 5% 6% 41%	5586
April	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 7% 50%	2437
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 48%	2494
	21st to end	Day	15% 6% 2 6% 5% 4 4 3 5% 7% 42%	2587
May	1st to 10th	Day	12% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	7616
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 6% 8% 39%	7626
	21st to end	Day	19% 6% 2 6% 4 4 4 3 6% 8% 37%	8391
June	1st to 10th	Day	8% 4 2 5% 5% 5% 5% 5% 7% 10% 44%	2882
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 8% 11% 39%	2940
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 42%	2785
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 5% 8% 11% 42%	7680
	11th to 20th	Day	11% 5% 2 7% 5% 5% 6% 5% 9% 13% 32%	7680
	21st to end	Day	11% 6% 2 7% 6% 6% 6% 5% 8% 11% 34%	8340
August	1st to 10th	Day	14% 7% 3 8% 5% 5% 6% 5% 8% 12% 27%	6014
	11th to 20th	Day	8% 4 2 7% 5% 5% 6% 5% 8% 11% 37%	5890
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 11% 47%	6080
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 5% 7% 50%	2520
	11th to 20th	Day	13% 7% 2 6% 3 3 4 4 5% 8% 45%	2363
	21st to end	Day	22% 7% 2 7% 5% 4 4 3 5% 7% 35%	2274
October	1st to 10th	Day	15% 5% 2 5% 3 4 2 2 4 6% 52%	2100
	11th to 20th	Day	16% 4 1 4 3 3 3 3 4 5% 55%	2100
	21st to end	Day	13% 6% 2 5% 3 4 3 3 4 5% 54%	2280
November	1st to 10th	Day	12% 5% 2 4 3 4 3 3 4 6% 55%	2057
	11th to 20th	Day	14% 5% 1 4 4 3 4 3 4 7% 51%	1966
	21st to end	Day	14% 5% 1 5% 3 3 3 3 5% 7% 50%	1680
December	1st to 10th	Day	12% 7% 2 5% 3 3 4 4 5% 6% 47%	2680
	11th to 20th	Day	20% 6% 1 4 3 2 3 3 5% 6% 47%	3070
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 8% 37%	4442
January	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 6% 52%	7635
	11th to 20th	Night	11% 6% 2 5% 3 3 3 2 4 5% 56%	7680
	21st to end	Night	16% 6% 2 6% 3 3 3 2 4 6% 50%	8401
February	1st to 10th	Night	14% 6% 2 6% 4 3 4 4 6% 8% 44%	2730
	11th to 20th	Night	20% 8% 3 4 3 3 3 3 5% 5% 42%	2663
	21st to end	Night	15% 6% 3 6% 4 4 4 3 5% 6% 43%	1963
March	1st to 10th	Night	16% 5% 2 6% 4 4 4 3 5% 6% 44%	5028
	11th to 20th	Night	10% 3 1 4 3 3 4 4 5% 8% 54%	5090
	21st to end	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	5588
April	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 7% 46%	2181
	11th to 20th	Night	11% 5% 2 5% 4 3 4 4 6% 8% 49%	2075
	21st to end	Night	13% 5% 2 6% 4 3 4 4 5% 8% 45%	2050
May	1st to 10th	Night	13% 7% 2 8% 5% 5% 5% 4 6% 8% 37%	5237
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 6% 7% 39%	5234
	21st to end	Night	21% 6% 2 6% 4 4 4 4 5% 8% 36%	5761
June	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 7% 11% 43%	1727
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 7% 8% 44%	1680
	21st to end	Night	7% 4 2 4 4 5% 5% 5% 7% 11% 44%	1824
July	1st to 10th	Night	10% 6% 3 7% 5% 5% 6% 5% 7% 10% 37%	5278
	11th to 20th	Night	13% 7% 2 8% 5% 6% 6% 5% 8% 10% 30%	5277
	21st to end	Night	12% 7% 3 9% 6% 7% 6% 4 7% 8% 31%	5736
August	1st to 10th	Night	18% 8% 3 7% 6% 5% 5% 5% 7% 11% 26%	4135
	11th to 20th	Night	10% 6% 3 8% 6% 6% 6% 6% 7% 10% 34%	4095
	21st to end	Night	10% 6% 2 6% 5% 4 5% 3 6% 9% 44%	5045
September	1st to 10th	Night	11% 5% 2 5% 3 5% 4 4 5% 8% 48%	2100
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 4 5% 7% 43%	2253
	21st to end	Night	24% 8% 3 6% 4 4 3 3 4 5% 36%	2344
October	1st to 10th	Night	16% 6% 2 6% 3 3 3 2 4 6% 49%	2520
	11th to 20th	Night	14% 5% 2 5% 3 4 4 3 4 5% 51%	2520
	21st to end	Night	14% 4 1 4 3 3 3 3 4 6% 56%	2737
November	1st to 10th	Night	13% 6% 3 5% 3 3 3 3 3 4 53%	2461
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 4 6% 51%	2647
	21st to end	Night	12% 4 2 5% 3 5% 4 4 5% 7% 50%	2940
December	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	4720
	11th to 20th	Night	19% 7% 2 6% 4 4 3 3 4 6% 40%	5345
	21st to end	Night	17% 5% 2 5% 4 4 4 4 5% 7% 42%	6750

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	37% 3 10% 22% 23% 5%	5259
	11th to 20th	Day	32% 3 5% 19% 28% 14%	5280
	21st to end	Day	41% 3 6% 26% 19% 4	5780
February	1st to 10th	Day	40% 5% 12% 23% 17% 3	1890
	11th to 20th	Day	42% 4 9% 21% 18% 4	1852
	21st to end	Day	46% 5% 11% 17% 16% 5%	1839
March	1st to 10th	Day	42% 5% 9% 18% 17% 9%	5045
	11th to 20th	Day	32% 5% 8% 22% 22% 10%	5090
	21st to end	Day	43% 5% 10% 18% 18% 6%	5586
April	1st to 10th	Day	34% 5% 12% 24% 19% 7%	2437
	11th to 20th	Day	34% 5% 12% 24% 20% 5%	2494
	21st to end	Day	41% 6% 11% 24% 15% 4	2587
May	1st to 10th	Day	39% 8% 13% 25% 14% 1	7651
	11th to 20th	Day	42% 6% 12% 24% 15% 1	7630
	21st to end	Day	43% 7% 12% 18% 16% 4	8405
June	1st to 10th	Day	33% 9% 16% 25% 16% 1	2882
	11th to 20th	Day	34% 10% 17% 22% 16% 1	2940
	21st to end	Day	33% 10% 18% 23% 14% 2	2785
July	1st to 10th	Day	34% 10% 20% 23% 11% 3	7615
	11th to 20th	Day	40% 10% 19% 22% 8%	7611
	21st to end	Day	41% 9% 14% 21% 14% 1	8156
August	1st to 10th	Day	47% 10% 19% 18% 4	6001
	11th to 20th	Day	37% 9% 20% 25% 8%	5929
	21st to end	Day	30% 7% 20% 25% 16% 1	6080
September	1st to 10th	Day	33% 8% 13% 21% 21% 4	2520
	11th to 20th	Day	37% 7% 10% 25% 18% 3	2363
	21st to end	Day	49% 6% 10% 16% 16% 3	2274
October	1st to 10th	Day	34% 3 7% 20% 29% 6%	3360
	11th to 20th	Day	25% 3 7% 23% 27% 14%	3340
	21st to end	Day	23% 3 11% 26% 25% 12%	3618
November	1st to 10th	Day	31% 4 10% 25% 25% 5%	2057
	11th to 20th	Day	34% 4 10% 24% 21% 8%	1966
	21st to end	Day	34% 4 11% 24% 24% 4	1680
December	1st to 10th	Day	36% 5% 10% 23% 20% 7%	2680
	11th to 20th	Day	39% 3 13% 21% 19% 5%	3066
	21st to end	Day	44% 5% 12% 23% 14% 1	4442
January	1st to 10th	Night	33% 4 9% 24% 22% 9%	7636
	11th to 20th	Night	32% 3 5% 19% 30% 10%	7680
	21st to end	Night	37% 3 6% 27% 20% 6%	8401
February	1st to 10th	Night	38% 5% 12% 23% 19% 3	2730
	11th to 20th	Night	44% 4 9% 19% 19% 6%	2665
	21st to end	Night	40% 5% 10% 22% 16% 7%	1963
March	1st to 10th	Night	39% 5% 8% 20% 20% 9%	5028
	11th to 20th	Night	27% 5% 9% 20% 29% 10%	5090
	21st to end	Night	38% 5% 10% 22% 20% 6%	5588
April	1st to 10th	Night	37% 6% 12% 18% 22% 5%	2181
	11th to 20th	Night	34% 5% 12% 22% 21% 5%	2075
	21st to end	Night	37% 7% 12% 24% 17% 3	2050
May	1st to 10th	Night	44% 7% 13% 20% 12% 4	5264
	11th to 20th	Night	43% 6% 12% 24% 14% 1	5235
	21st to end	Night	46% 7% 13% 20% 12% 3	5771
June	1st to 10th	Night	33% 8% 15% 26% 16% 1	1727
	11th to 20th	Night	36% 8% 18% 20% 16% 2	1680
	21st to end	Night	30% 11% 17% 22% 17% 4	1824
July	1st to 10th	Night	41% 11% 16% 20% 11% 2	5235
	11th to 20th	Night	47% 11% 16% 18% 6% 1	5226
	21st to end	Night	49% 8% 12% 17% 13%	5606
August	1st to 10th	Night	51% 11% 16% 17% 5%	4124
	11th to 20th	Night	43% 9% 18% 22% 8%	4122
	21st to end	Night	37% 7% 15% 30% 11% 1	5045
September	1st to 10th	Night	34% 7% 14% 21% 18% 6%	2100
	11th to 20th	Night	38% 7% 11% 23% 19% 2	2253
	21st to end	Night	51% 5% 9% 17% 15% 3	2344
October	1st to 10th	Night	40% 3 9% 18% 24% 6%	4040
	11th to 20th	Night	27% 3 7% 20% 29% 13%	4019
	21st to end	Night	20% 3 7% 29% 28% 12%	4349
November	1st to 10th	Night	35% 4 8% 20% 25% 7%	2461
	11th to 20th	Night	33% 5% 9% 21% 24% 8%	2647
	21st to end	Night	34% 5% 9% 25% 22% 6%	2940
December	1st to 10th	Night	33% 4 7% 29% 24% 4	4720
	11th to 20th	Night	45% 4 10% 19% 15% 7%	5348
	21st to end	Night	39% 6% 11% 25% 17% 2	6750

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	27% 27% 26% 16% 4 1	330
	11th to 20th	Day	28% 24% 34% 13% 1	330
	21st to end	Day	27% 26% 32% 12% 3	363
February	1st to 10th	Day	22% 23% 37% 13% 5%	330
	11th to 20th	Day	41% 22% 28% 7% 3	330
	21st to end	Day	30% 20% 29% 18% 3	408
March	1st to 10th	Day	25% 17% 30% 22% 5% 1	540
	11th to 20th	Day	21% 22% 32% 19% 5%	540
	21st to end	Day	22% 17% 36% 20% 5% 1	594
April	1st to 10th	Day	22% 19% 30% 20% 7% 1	540
	11th to 20th	Day	20% 16% 30% 21% 11% 3	532
	21st to end	Day	21% 11% 31% 24% 12% 1	540
May	1st to 10th	Day	20% 15% 33% 21% 9% 2	543
	11th to 20th	Day	20% 10% 25% 26% 15% 4	567
	21st to end	Day	20% 6% 22% 28% 17% 6%	617
June	1st to 10th	Day	10% 5% 28% 32% 19% 6%	570
	11th to 20th	Day	11% 6% 23% 32% 18% 9%	570
	21st to end	Day	9% 4 23% 34% 23% 8%	570
July	1st to 10th	Day	9% 4 16% 28% 30% 13%	565
	11th to 20th	Day	7% 1 14% 34% 30% 13%	564
	21st to end	Day	12% 3 18% 31% 25% 12%	620
August	1st to 10th	Day	16% 5% 17% 28% 24% 11%	566
	11th to 20th	Day	11% 3 16% 32% 26% 12%	559
	21st to end	Day	11% 3 14% 29% 26% 17%	627
September	1st to 10th	Day	12% 5% 15% 27% 25% 16%	570
	11th to 20th	Day	16% 3 14% 28% 27% 12%	570
	21st to end	Day	27% 8% 20% 24% 16% 4	570
October	1st to 10th	Day	21% 6% 17% 31% 19% 7%	570
	11th to 20th	Day	17% 11% 22% 28% 19% 3	570
	21st to end	Day	16% 11% 28% 28% 12% 4	615
November	1st to 10th	Day	17% 9% 31% 29% 11% 3	535
	11th to 20th	Day	19% 15% 30% 25% 10% 2	418
	21st to end	Day	20% 22% 25% 21% 9% 2	220
December	1st to 10th	Day	25% 15% 35% 18% 5%	220
	11th to 20th	Day	25% 15% 30% 25% 5% 1	220
	21st to end	Day	27% 14% 33% 19% 6% 2	333
January	1st to 10th	Night	19% 17% 37% 24% 3	840
	11th to 20th	Night	18% 18% 34% 28% 2	840
	21st to end	Night	18% 15% 42% 23% 3	924
February	1st to 10th	Night	16% 15% 40% 26% 3	840
	11th to 20th	Night	26% 16% 37% 20% 1	823
	21st to end	Night	20% 15% 39% 24% 3	692
March	1st to 10th	Night	16% 15% 37% 27% 4	840
	11th to 20th	Night	22% 13% 34% 25% 6% 1	840
	21st to end	Night	17% 13% 37% 27% 5%	924
April	1st to 10th	Night	18% 16% 35% 25% 6%	840
	11th to 20th	Night	14% 13% 33% 30% 10%	831
	21st to end	Night	16% 7% 32% 37% 8%	790
May	1st to 10th	Night	16% 9% 32% 30% 13% 1	630
	11th to 20th	Night	20% 6% 23% 35% 16% 1	621
	21st to end	Night	19% 2 15% 33% 28% 3	683
June	1st to 10th	Night	9% 1 18% 42% 28% 2	467
	11th to 20th	Night	12% 2 17% 41% 25% 3	420
	21st to end	Night	8% 2 12% 45% 30% 3	564
July	1st to 10th	Night	11% 2 10% 36% 36% 5%	615
	11th to 20th	Night	7% 1 9% 33% 44% 5%	612
	21st to end	Night	9% 2 8% 43% 33% 5%	690
August	1st to 10th	Night	14% 3 9% 39% 29% 5%	627
	11th to 20th	Night	14% 2 6% 41% 33% 4	624
	21st to end	Night	12% 3 11% 37% 33% 5%	924
September	1st to 10th	Night	13% 2 9% 34% 35% 6%	840
	11th to 20th	Night	17% 3 10% 34% 31% 5%	840
	21st to end	Night	27% 5% 17% 30% 21% 1	840
October	1st to 10th	Night	19% 4 18% 29% 26% 5%	840
	11th to 20th	Night	16% 8% 23% 35% 17% 1	840
	21st to end	Night	15% 7% 26% 35% 15% 2	913
November	1st to 10th	Night	16% 7% 28% 35% 12% 2	820
	11th to 20th	Night	17% 11% 31% 30% 11% 1	840
	21st to end	Night	14% 11% 33% 33% 9% 1	840
December	1st to 10th	Night	21% 10% 36% 26% 5%	840
	11th to 20th	Night	18% 10% 35% 32% 5%	840
	21st to end	Night	21% 12% 33% 31% 4	924

Page generated at 2026-10-26 04:37:46. Version ac07

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