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 2025-06-10.

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	14% 6% 2 6% 4 4 5% 4 6% 8% 42%	119214
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	118983
Transparency	Day	38% 6% 12% 22% 17% 5%	122861
	Night	37% 6% 11% 22% 19% 5%	123473
Seeing	Day	19% 11% 25% 25% 15% 6%	15836
	Night	17% 9% 26% 31% 15% 2	25916

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	14% 5% 2 4 3 3 4 4 4 6% 51%	4056
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 4 5% 53%	4060
	21st to end	Day	17% 6% 2 6% 3 4 4 3 4 5% 47%	4466
February	1st to 10th	Day	12% 8% 2 6% 5% 4 5% 4 6% 7% 41%	1800
	11th to 20th	Day	20% 6% 1 5% 4 4 4 3 5% 6% 41%	1753
	21st to end	Day	18% 8% 3 6% 4 4 4 3 5% 6% 39%	1754
March	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 7% 6% 39%	4935
	11th to 20th	Day	11% 5% 2 5% 3 3 4 3 6% 7% 52%	4980
	21st to end	Day	15% 6% 3 7% 5% 5% 4 4 5% 6% 41%	5465
April	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 6% 49%	2321
	11th to 20th	Day	13% 6% 2 5% 4 4 4 4 5% 8% 47%	2374
	21st to end	Day	15% 5% 2 6% 5% 4 4 3 5% 7% 42%	2464
May	1st to 10th	Day	12% 5% 2 7% 5% 5% 7% 5% 8% 10% 34%	5862
	11th to 20th	Day	20% 7% 3 7% 5% 5% 5% 4 6% 8% 31%	5861
	21st to end	Day	14% 5% 2 5% 4 4 5% 4 7% 10% 42%	6433
June	1st to 10th	Day	9% 4 2 5% 5% 5% 5% 5% 6% 10% 44%	2731
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 8% 11% 39%	2660
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 10% 42%	2520
July	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 5% 9% 12% 40%	4120
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 12% 37%	4120
	21st to end	Day	12% 7% 3 8% 7% 6% 6% 5% 7% 9% 29%	4466
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 5% 7% 12% 31%	4104
	11th to 20th	Day	9% 4 2 7% 5% 5% 5% 5% 7% 10% 41%	3997
	21st to end	Day	8% 5% 2 5% 4 3 4 4 6% 11% 48%	4180
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 7% 48%	2280
	11th to 20th	Day	14% 7% 2 6% 3 3 4 4 5% 7% 44%	2138
	21st to end	Day	22% 7% 2 7% 4 3 4 3 4 6% 36%	2058
October	1st to 10th	Day	15% 6% 2 5% 3 3 3 2 4 6% 51%	1900
	11th to 20th	Day	16% 4 1 4 3 3 3 3 4 6% 54%	1900
	21st to end	Day	13% 6% 2 5% 3 4 3 3 4 5% 53%	2090
November	1st to 10th	Day	12% 5% 2 4 3 4 3 3 4 5% 55%	1957
	11th to 20th	Day	14% 5% 1 4 4 3 4 3 4 7% 50%	1873
	21st to end	Day	14% 5% 1 5% 3 3 3 3 5% 8% 50%	1600
December	1st to 10th	Day	12% 7% 2 5% 3 3 4 4 5% 6% 48%	2600
	11th to 20th	Day	21% 6% 1 4 3 2 3 3 5% 6% 46%	2990
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 8% 37%	4346
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 55%	5889
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 5% 56%	5900
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 6% 8% 43%	2600
	11th to 20th	Night	20% 8% 2 5% 3 3 3 3 5% 5% 42%	2533
	21st to end	Night	15% 7% 3 5% 4 4 4 3 5% 6% 44%	1872
March	1st to 10th	Night	16% 5% 2 6% 4 4 4 3 5% 6% 44%	4918
	11th to 20th	Night	10% 3 1 4 3 3 4 4 5% 8% 54%	4980
	21st to end	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	5467
April	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 7% 45%	2077
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 6% 7% 48%	1975
	21st to end	Night	14% 5% 2 6% 4 3 5% 4 5% 8% 44%	1952
May	1st to 10th	Night	15% 7% 2 8% 5% 5% 5% 4 6% 9% 34%	4033
	11th to 20th	Night	22% 8% 3 7% 4 4 4 3 6% 7% 31%	4032
	21st to end	Night	15% 5% 2 6% 4 5% 4 4 6% 9% 40%	4421
June	1st to 10th	Night	8% 4 2 7% 4 4 5% 4 7% 12% 43%	1643
	11th to 20th	Night	10% 5% 2 7% 5% 4 4 4 7% 8% 44%	1520
	21st to end	Night	7% 4 2 4 4 5% 5% 5% 7% 12% 45%	1650
July	1st to 10th	Night	6% 5% 2 7% 4 5% 6% 5% 8% 10% 41%	2840
	11th to 20th	Night	8% 7% 2 6% 5% 5% 6% 4 8% 11% 38%	2840
	21st to end	Night	11% 9% 3 9% 5% 7% 6% 4 8% 8% 30%	3087
August	1st to 10th	Night	16% 6% 3 7% 6% 5% 5% 4 7% 10% 30%	2825
	11th to 20th	Night	8% 6% 3 7% 6% 5% 5% 5% 7% 10% 37%	2785
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 45%	3476
September	1st to 10th	Night	12% 5% 2 5% 4 5% 4 4 5% 8% 45%	1900
	11th to 20th	Night	15% 6% 2 5% 4 5% 5% 4 5% 7% 43%	2038
	21st to end	Night	24% 8% 3 6% 4 4 3 3 4 5% 37%	2120
October	1st to 10th	Night	17% 6% 2 6% 3 3 3 2 4 6% 48%	2280
	11th to 20th	Night	14% 5% 2 5% 3 4 3 2 5% 5% 51%	2280
	21st to end	Night	14% 4 1 4 3 4 3 3 4 6% 55%	2508
November	1st to 10th	Night	14% 6% 3 5% 3 3 4 3 3 4 53%	2341
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 4 6% 51%	2521
	21st to end	Night	12% 4 2 5% 3 4 3 4 5% 7% 50%	2800
December	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 6% 53%	4580
	11th to 20th	Night	19% 7% 2 5% 4 4 4 3 4 6% 40%	5205
	21st to end	Night	17% 5% 2 5% 4 4 4 4 6% 7% 42%	6605

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	34% 3 7% 23% 26% 7%	4057
	11th to 20th	Day	33% 4 6% 20% 27% 11%	4060
	21st to end	Day	40% 4 7% 25% 20% 4	4466
February	1st to 10th	Day	40% 5% 12% 24% 16% 3	1800
	11th to 20th	Day	42% 5% 9% 21% 18% 5%	1762
	21st to end	Day	45% 5% 11% 16% 17% 6%	1754
March	1st to 10th	Day	42% 5% 9% 19% 17% 9%	4935
	11th to 20th	Day	32% 5% 8% 22% 23% 10%	4980
	21st to end	Day	44% 5% 10% 18% 18% 6%	5465
April	1st to 10th	Day	35% 5% 11% 23% 18% 8%	2321
	11th to 20th	Day	35% 5% 11% 23% 20% 5%	2374
	21st to end	Day	41% 6% 11% 23% 14% 4	2464
May	1st to 10th	Day	41% 9% 15% 22% 11% 2	5897
	11th to 20th	Day	49% 7% 11% 19% 12% 1	5865
	21st to end	Day	37% 8% 15% 20% 18% 3	6447
June	1st to 10th	Day	33% 9% 16% 26% 15% 1	2731
	11th to 20th	Day	35% 11% 17% 21% 15% 1	2660
	21st to end	Day	34% 10% 17% 22% 15% 2	2520
July	1st to 10th	Day	33% 11% 22% 18% 10% 5%	4055
	11th to 20th	Day	37% 8% 20% 24% 10% 1	4051
	21st to end	Day	48% 10% 13% 19% 10%	4324
August	1st to 10th	Day	44% 9% 17% 23% 6% 1	4091
	11th to 20th	Day	35% 9% 17% 28% 11%	4036
	21st to end	Day	30% 7% 20% 27% 15% 1	4180
September	1st to 10th	Day	35% 9% 12% 20% 21% 4	2280
	11th to 20th	Day	38% 7% 10% 24% 18% 3	2138
	21st to end	Day	48% 6% 10% 17% 17% 2	2058
October	1st to 10th	Day	35% 3 8% 19% 29% 6%	3160
	11th to 20th	Day	24% 3 7% 24% 27% 14%	3140
	21st to end	Day	22% 3 12% 27% 24% 12%	3428
November	1st to 10th	Day	32% 4 9% 25% 26% 5%	1957
	11th to 20th	Day	35% 4 10% 22% 21% 8%	1873
	21st to end	Day	33% 4 11% 24% 24% 4	1600
December	1st to 10th	Day	35% 5% 10% 23% 20% 7%	2600
	11th to 20th	Day	39% 3 13% 21% 19% 5%	2986
	21st to end	Day	44% 5% 12% 23% 14% 1	4346
January	1st to 10th	Night	31% 3 6% 22% 26% 11%	5889
	11th to 20th	Night	32% 4 6% 20% 30% 9%	5900
	21st to end	Night	37% 3 6% 27% 20% 6%	6490
February	1st to 10th	Night	38% 5% 12% 24% 18% 3	2600
	11th to 20th	Night	43% 4 9% 19% 18% 6%	2535
	21st to end	Night	40% 5% 10% 23% 16% 7%	1872
March	1st to 10th	Night	39% 5% 8% 20% 20% 9%	4918
	11th to 20th	Night	27% 5% 9% 19% 29% 10%	4980
	21st to end	Night	39% 5% 9% 22% 20% 6%	5467
April	1st to 10th	Night	38% 6% 12% 18% 21% 6%	2077
	11th to 20th	Night	34% 5% 12% 22% 21% 6%	1975
	21st to end	Night	37% 7% 12% 23% 17% 4	1952
May	1st to 10th	Night	45% 8% 15% 18% 11% 3	4060
	11th to 20th	Night	51% 7% 11% 19% 12% 1	4033
	21st to end	Night	38% 8% 15% 25% 11% 2	4429
June	1st to 10th	Night	33% 8% 16% 26% 16% 1	1643
	11th to 20th	Night	36% 8% 17% 21% 16% 2	1520
	21st to end	Night	30% 11% 17% 21% 17% 4	1650
July	1st to 10th	Night	35% 11% 21% 17% 13% 3	2795
	11th to 20th	Night	38% 9% 21% 22% 8% 1	2786
	21st to end	Night	49% 9% 13% 20% 9%	2988
August	1st to 10th	Night	47% 9% 16% 20% 7%	2814
	11th to 20th	Night	39% 9% 16% 25% 11%	2812
	21st to end	Night	35% 7% 15% 32% 9% 2	3476
September	1st to 10th	Night	36% 8% 14% 18% 18% 6%	1900
	11th to 20th	Night	39% 7% 10% 22% 19% 2	2038
	21st to end	Night	50% 5% 8% 18% 16% 3	2120
October	1st to 10th	Night	40% 4 9% 18% 23% 6%	3800
	11th to 20th	Night	26% 3 8% 21% 29% 13%	3779
	21st to end	Night	20% 3 7% 30% 28% 12%	4120
November	1st to 10th	Night	35% 5% 8% 21% 25% 7%	2341
	11th to 20th	Night	34% 5% 9% 21% 24% 8%	2521
	21st to end	Night	33% 5% 9% 25% 22% 6%	2800
December	1st to 10th	Night	32% 4 7% 29% 24% 4	4580
	11th to 20th	Night	45% 4 10% 19% 15% 7%	5208
	21st to end	Night	39% 6% 11% 25% 17% 2	6605

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% 28% 25% 16% 3 1	300
	11th to 20th	Day	28% 23% 34% 14% 1	300
	21st to end	Day	27% 24% 33% 13% 3	330
February	1st to 10th	Day	23% 24% 37% 12% 5%	300
	11th to 20th	Day	42% 21% 28% 6% 3	300
	21st to end	Day	30% 20% 30% 17% 3	378
March	1st to 10th	Day	26% 18% 30% 21% 5%	500
	11th to 20th	Day	21% 22% 33% 19% 5%	500
	21st to end	Day	23% 17% 35% 20% 5% 1	550
April	1st to 10th	Day	23% 20% 29% 20% 7% 1	500
	11th to 20th	Day	20% 16% 29% 21% 11% 3	492
	21st to end	Day	21% 11% 31% 24% 11% 1	500
May	1st to 10th	Day	20% 14% 32% 22% 10% 3	503
	11th to 20th	Day	21% 11% 25% 25% 15% 4	527
	21st to end	Day	19% 6% 22% 28% 17% 7%	573
June	1st to 10th	Day	11% 6% 26% 32% 20% 6%	526
	11th to 20th	Day	10% 6% 21% 33% 19% 10%	490
	21st to end	Day	10% 4 24% 35% 21% 7%	490
July	1st to 10th	Day	9% 4 16% 28% 30% 13%	485
	11th to 20th	Day	7% 1 14% 36% 29% 13%	484
	21st to end	Day	11% 2 18% 31% 27% 11%	532
August	1st to 10th	Day	14% 6% 16% 27% 24% 12%	486
	11th to 20th	Day	11% 3 16% 29% 27% 14%	479
	21st to end	Day	12% 2 12% 27% 27% 19%	539
September	1st to 10th	Day	13% 4 14% 27% 24% 17%	490
	11th to 20th	Day	17% 3 14% 29% 25% 12%	490
	21st to end	Day	28% 8% 19% 24% 16% 5%	490
October	1st to 10th	Day	21% 6% 17% 32% 18% 6%	490
	11th to 20th	Day	16% 12% 22% 28% 20% 3	490
	21st to end	Day	16% 10% 29% 30% 11% 4	539
November	1st to 10th	Day	18% 8% 30% 29% 12% 3	495
	11th to 20th	Day	19% 14% 30% 25% 11% 2	385
	21st to end	Day	19% 23% 25% 23% 8% 3	200
December	1st to 10th	Day	25% 16% 35% 18% 6% 1	200
	11th to 20th	Day	25% 14% 30% 25% 5% 2	200
	21st to end	Day	26% 14% 32% 19% 7% 2	303
January	1st to 10th	Night	20% 17% 37% 24% 3	800
	11th to 20th	Night	18% 18% 33% 29% 2	800
	21st to end	Night	18% 14% 42% 23% 3	880
February	1st to 10th	Night	17% 15% 39% 26% 3	800
	11th to 20th	Night	26% 17% 37% 20% 1	783
	21st to end	Night	20% 14% 39% 24% 3	660
March	1st to 10th	Night	17% 15% 37% 27% 4	800
	11th to 20th	Night	22% 13% 34% 25% 6% 1	800
	21st to end	Night	18% 13% 37% 28% 5%	880
April	1st to 10th	Night	19% 16% 34% 25% 6%	800
	11th to 20th	Night	14% 13% 32% 30% 11%	791
	21st to end	Night	16% 7% 32% 36% 9%	752
May	1st to 10th	Night	16% 9% 30% 30% 14% 1	600
	11th to 20th	Night	21% 6% 21% 34% 16% 1	591
	21st to end	Night	18% 2 15% 33% 29% 4	650
June	1st to 10th	Night	8% 2 18% 42% 28% 2	445
	11th to 20th	Night	11% 2 16% 41% 26% 3	380
	21st to end	Night	8% 2 12% 45% 29% 3	510
July	1st to 10th	Night	11% 2 10% 37% 36% 4	555
	11th to 20th	Night	7% 1 9% 33% 46% 4	552
	21st to end	Night	9% 1 8% 44% 34% 4	624
August	1st to 10th	Night	13% 3 8% 39% 31% 5%	567
	11th to 20th	Night	14% 2 6% 41% 33% 4	564
	21st to end	Night	12% 2 10% 35% 36% 5%	836
September	1st to 10th	Night	14% 2 9% 33% 36% 6%	760
	11th to 20th	Night	18% 3 11% 34% 31% 5%	760
	21st to end	Night	27% 5% 17% 29% 22% 1	760
October	1st to 10th	Night	19% 4 19% 28% 25% 5%	760
	11th to 20th	Night	16% 8% 23% 35% 17% 2	760
	21st to end	Night	16% 7% 26% 36% 14% 2	836
November	1st to 10th	Night	16% 6% 29% 35% 13% 2	780
	11th to 20th	Night	17% 11% 30% 31% 12% 1	800
	21st to end	Night	14% 11% 33% 33% 9% 1	800
December	1st to 10th	Night	22% 10% 36% 26% 6% 1	800
	11th to 20th	Night	18% 10% 35% 32% 5%	800
	21st to end	Night	21% 11% 32% 32% 4	880

Page generated at 2025-06-07 10:28:13. Version ac07

Copyright © 2025 Attilla Danko.