Stephen F. Wessling Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stephen F. Wessling Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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% 11% 5% 9% 6% 4 4 3 5% 6% 32%	119886
	Night	14% 13% 5% 10% 5% 5% 4 3 5% 6% 30%	119429
Transparency	Day	51% 5% 11% 20% 12% 1	123444
	Night	54% 5% 11% 19% 10% 1	124012
Seeing	Day	19% 14% 30% 23% 11% 4	16847
	Night	19% 9% 29% 31% 11% 1	26709

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	11% 22% 11% 24% 6% 5% 3 2 2 3 9%	3617
	11th to 20th	Day	11% 16% 8% 20% 16% 8% 7% 3 3 2 5%	3620
	21st to end	Day	14% 19% 10% 17% 8% 5% 5% 3 3 3 12%	3982
February	1st to 10th	Day	11% 17% 6% 13% 12% 7% 5% 3 3 5% 17%	1790
	11th to 20th	Day	11% 18% 7% 16% 9% 6% 4 3 4 5% 17%	1948
	21st to end	Day	12% 18% 7% 13% 8% 5% 4 3 5% 5% 20%	1650
March	1st to 10th	Day	12% 12% 4 12% 6% 5% 5% 3 5% 5% 30%	4480
	11th to 20th	Day	12% 8% 5% 10% 5% 5% 4 3 5% 7% 35%	4796
	21st to end	Day	22% 12% 7% 9% 5% 4 4 3 4 5% 27%	5729
April	1st to 10th	Day	17% 15% 4 7% 5% 4 4 3 5% 6% 29%	2400
	11th to 20th	Day	20% 11% 2 8% 4 4 5% 3 5% 6% 33%	2427
	21st to end	Day	17% 10% 3 7% 5% 5% 4 4 5% 7% 34%	2600
May	1st to 10th	Day	18% 7% 2 6% 3 3 4 3 6% 7% 40%	5862
	11th to 20th	Day	13% 8% 2 7% 4 5% 5% 4 6% 7% 39%	5997
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 8% 41%	6935
June	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 44%	2800
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 4 6% 9% 43%	2800
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 7% 43%	2800
July	1st to 10th	Day	13% 7% 3 6% 4 4 5% 4 6% 8% 38%	4580
	11th to 20th	Day	7% 4 2 4 4 4 4 4 7% 10% 51%	4580
	21st to end	Day	8% 5% 2 6% 5% 4 5% 4 7% 9% 45%	4814
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 40%	4213
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 7% 47%	4044
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	4223
September	1st to 10th	Day	12% 5% 2 7% 5% 5% 5% 3 5% 7% 44%	2266
	11th to 20th	Day	12% 5% 2 5% 5% 4 4 4 5% 7% 47%	2090
	21st to end	Day	13% 7% 3 8% 5% 4 5% 3 5% 7% 40%	2090
October	1st to 10th	Day	17% 8% 3 7% 4 4 3 4 4 5% 42%	2090
	11th to 20th	Day	22% 11% 3 9% 6% 5% 5% 3 5% 6% 24%	1827
	21st to end	Day	24% 13% 4 9% 5% 5% 4 3 4 6% 23%	1881
November	1st to 10th	Day	16% 14% 3 7% 5% 4 4 3 5% 6% 33%	1762
	11th to 20th	Day	16% 20% 7% 11% 6% 4 4 3 5% 5% 20%	1800
	21st to end	Day	21% 18% 7% 11% 5% 4 3 3 3 5% 19%	1793
December	1st to 10th	Day	10% 24% 13% 16% 5% 5% 4 2 4 4 11%	2610
	11th to 20th	Day	15% 23% 12% 13% 10% 5% 3 2 3 3 11%	3008
	21st to end	Day	23% 23% 8% 11% 5% 4 4 3 4 3 12%	3982
January	1st to 10th	Night	10% 19% 12% 21% 6% 5% 4 2 3 4 13%	6348
	11th to 20th	Night	11% 17% 8% 23% 11% 11% 4 2 3 3 7%	6360
	21st to end	Night	14% 21% 9% 17% 7% 5% 6% 3 3 4 12%	6996
February	1st to 10th	Night	12% 17% 7% 14% 11% 6% 5% 3 4 4 18%	2606
	11th to 20th	Night	13% 14% 6% 13% 9% 5% 5% 4 5% 6% 20%	2338
	21st to end	Night	15% 16% 4 11% 6% 5% 5% 3 5% 7% 23%	1980
March	1st to 10th	Night	16% 8% 4 9% 5% 4 4 3 5% 7% 35%	5373
	11th to 20th	Night	11% 9% 3 7% 5% 4 4 3 5% 8% 40%	5150
	21st to end	Night	17% 14% 5% 8% 4 4 5% 3 5% 6% 30%	5243
April	1st to 10th	Night	17% 11% 3 7% 5% 4 4 3 4 7% 34%	2000
	11th to 20th	Night	19% 10% 4 6% 3 3 4 3 5% 6% 36%	1920
	21st to end	Night	16% 10% 3 8% 4 4 4 4 5% 7% 37%	1800
May	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 5% 8% 37%	4033
	11th to 20th	Night	14% 8% 3 7% 4 4 5% 3 6% 7% 40%	3889
	21st to end	Night	19% 7% 3 5% 3 3 3 3 4 5% 45%	3942
June	1st to 10th	Night	12% 6% 3 5% 3 4 4 4 7% 8% 44%	1600
	11th to 20th	Night	11% 5% 3 5% 4 4 4 4 7% 8% 45%	1600
	21st to end	Night	10% 7% 2 5% 4 4 5% 4 7% 8% 45%	1600
July	1st to 10th	Night	10% 8% 3 6% 4 5% 5% 5% 7% 7% 41%	2610
	11th to 20th	Night	6% 3 2 4 4 4 4 3 5% 8% 56%	2610
	21st to end	Night	8% 5% 2 6% 4 3 4 3 6% 7% 51%	2750
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	2719
	11th to 20th	Night	12% 4 2 5% 3 4 4 3 6% 7% 51%	2785
	21st to end	Night	8% 5% 3 7% 4 4 5% 4 5% 8% 47%	3430
September	1st to 10th	Night	9% 6% 2 6% 5% 3 4 3 5% 6% 52%	1900
	11th to 20th	Night	11% 5% 1 4 3 2 4 2 5% 6% 55%	2083
	21st to end	Night	14% 6% 2 7% 4 5% 3 4 5% 7% 43%	2090
October	1st to 10th	Night	18% 6% 2 5% 3 3 3 2 4 8% 46%	2090
	11th to 20th	Night	20% 9% 3 8% 5% 4 4 3 5% 6% 33%	2351
	21st to end	Night	24% 11% 3 7% 4 4 4 2 4 7% 30%	2717
November	1st to 10th	Night	15% 11% 3 7% 4 5% 4 3 5% 6% 36%	2536
	11th to 20th	Night	15% 18% 4 10% 5% 5% 4 5% 6% 6% 22%	2600
	21st to end	Night	18% 20% 5% 9% 5% 4 4 3 4 6% 21%	2605
December	1st to 10th	Night	12% 18% 7% 16% 7% 6% 5% 4 5% 5% 15%	4580
	11th to 20th	Night	17% 24% 7% 14% 10% 5% 4 3 3 4 11%	5199
	21st to end	Night	23% 21% 7% 11% 5% 4 4 3 4 5% 14%	6996

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	82% 4 6% 8% 1	3618
	11th to 20th	Day	86% 4 5% 4 1	3620
	21st to end	Day	79% 3 8% 8% 1	3982
February	1st to 10th	Day	70% 5% 8% 14% 3	1790
	11th to 20th	Day	70% 4 9% 12% 5%	1958
	21st to end	Day	66% 5% 12% 12% 5%	1650
March	1st to 10th	Day	54% 4 14% 17% 9% 1	4480
	11th to 20th	Day	48% 5% 16% 19% 10% 1	4796
	21st to end	Day	60% 4 9% 16% 10% 1	5731
April	1st to 10th	Day	55% 5% 10% 20% 10% 1	2400
	11th to 20th	Day	52% 4 9% 20% 13% 2	2427
	21st to end	Day	49% 5% 12% 18% 13% 2	2600
May	1st to 10th	Day	41% 5% 10% 24% 16% 3	5897
	11th to 20th	Day	42% 6% 11% 20% 18% 2	6001
	21st to end	Day	40% 5% 10% 24% 20% 2	6949
June	1st to 10th	Day	38% 5% 11% 28% 17% 2	2800
	11th to 20th	Day	38% 5% 13% 23% 20% 1	2800
	21st to end	Day	40% 6% 13% 23% 17% 2	2800
July	1st to 10th	Day	43% 5% 13% 22% 15% 3	4511
	11th to 20th	Day	26% 6% 15% 31% 19% 2	4505
	21st to end	Day	33% 7% 13% 24% 21% 2	4663
August	1st to 10th	Day	38% 6% 11% 28% 15% 1	4198
	11th to 20th	Day	34% 7% 12% 28% 18% 1	4083
	21st to end	Day	36% 7% 14% 25% 17% 2	4222
September	1st to 10th	Day	39% 6% 13% 26% 14% 2	2266
	11th to 20th	Day	36% 5% 13% 30% 14% 2	2090
	21st to end	Day	43% 4 12% 27% 13%	2090
October	1st to 10th	Day	30% 4 9% 36% 19% 2	3480
	11th to 20th	Day	48% 5% 12% 24% 12%	3004
	21st to end	Day	54% 5% 15% 18% 7%	3081
November	1st to 10th	Day	51% 6% 12% 22% 9%	1762
	11th to 20th	Day	67% 4 12% 12% 5%	1800
	21st to end	Day	69% 4 13% 10% 3	1793
December	1st to 10th	Day	77% 7% 7% 7% 1	2610
	11th to 20th	Day	80% 3 9% 8% 1	3005
	21st to end	Day	78% 4 5% 9% 4	3982
January	1st to 10th	Night	76% 4 7% 11% 3	6348
	11th to 20th	Night	85% 4 5% 4 2	6360
	21st to end	Night	77% 4 9% 8% 2	6996
February	1st to 10th	Night	71% 4 8% 12% 6% 1	2606
	11th to 20th	Night	65% 5% 10% 14% 6%	2339
	21st to end	Night	60% 5% 14% 14% 6%	1980
March	1st to 10th	Night	49% 5% 15% 22% 8% 1	5373
	11th to 20th	Night	43% 4 18% 24% 10% 1	5150
	21st to end	Night	55% 4 10% 19% 10% 1	5243
April	1st to 10th	Night	51% 4 10% 21% 13% 1	2000
	11th to 20th	Night	49% 4 8% 22% 15% 3	1920
	21st to end	Night	47% 5% 10% 20% 14% 3	1800
May	1st to 10th	Night	44% 6% 12% 19% 17% 3	4060
	11th to 20th	Night	43% 6% 10% 24% 15% 2	3890
	21st to end	Night	43% 4 8% 23% 18% 4	3950
June	1st to 10th	Night	35% 6% 11% 29% 15% 3	1600
	11th to 20th	Night	35% 7% 13% 24% 19% 2	1600
	21st to end	Night	35% 8% 14% 25% 15% 2	1600
July	1st to 10th	Night	41% 7% 11% 23% 12% 6%	2570
	11th to 20th	Night	26% 5% 13% 33% 20% 2	2562
	21st to end	Night	32% 6% 15% 22% 24% 2	2662
August	1st to 10th	Night	34% 5% 13% 29% 19% 1	2710
	11th to 20th	Night	32% 5% 10% 34% 17% 1	2812
	21st to end	Night	34% 6% 11% 25% 21% 2	3430
September	1st to 10th	Night	32% 8% 11% 29% 18% 2	1900
	11th to 20th	Night	29% 4 12% 37% 17% 2	2083
	21st to end	Night	40% 5% 14% 28% 12% 1	2090
October	1st to 10th	Night	32% 4 9% 37% 17% 1	3480
	11th to 20th	Night	40% 5% 14% 26% 14%	3913
	21st to end	Night	46% 4 12% 28% 10%	4467
November	1st to 10th	Night	48% 6% 13% 22% 11% 1	2536
	11th to 20th	Night	62% 5% 14% 13% 6% 1	2600
	21st to end	Night	66% 4 16% 13% 2	2605
December	1st to 10th	Night	70% 6% 14% 8% 2	4580
	11th to 20th	Night	77% 4 8% 9% 1	5201
	21st to end	Night	74% 4 9% 9% 5%	6996

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	20% 15% 40% 18% 6% 1	300
	11th to 20th	Day	23% 19% 38% 16% 4	300
	21st to end	Day	28% 15% 34% 19% 4	330
February	1st to 10th	Day	21% 27% 30% 17% 4 2	300
	11th to 20th	Day	21% 26% 36% 15% 3	300
	21st to end	Day	23% 31% 35% 9% 2	249
March	1st to 10th	Day	22% 26% 38% 10% 4	300
	11th to 20th	Day	24% 18% 29% 19% 9% 1	420
	21st to end	Day	29% 14% 30% 22% 5% 2	550
April	1st to 10th	Day	26% 18% 32% 17% 6% 1	500
	11th to 20th	Day	29% 23% 23% 16% 7% 2	492
	21st to end	Day	25% 15% 28% 22% 10% 1	500
May	1st to 10th	Day	22% 17% 31% 20% 7% 3	503
	11th to 20th	Day	19% 13% 29% 24% 10% 4	585
	21st to end	Day	16% 15% 28% 25% 13% 4	789
June	1st to 10th	Day	13% 11% 37% 27% 8% 4	730
	11th to 20th	Day	16% 11% 32% 27% 10% 3	730
	21st to end	Day	17% 9% 34% 27% 9% 4	730
July	1st to 10th	Day	9% 6% 28% 35% 15% 7%	721
	11th to 20th	Day	9% 8% 29% 34% 15% 6%	718
	21st to end	Day	9% 6% 30% 32% 16% 8%	740
August	1st to 10th	Day	10% 6% 27% 26% 22% 10%	557
	11th to 20th	Day	14% 5% 17% 29% 22% 13%	479
	21st to end	Day	14% 6% 22% 27% 22% 9%	539
September	1st to 10th	Day	16% 7% 23% 28% 19% 7%	490
	11th to 20th	Day	9% 6% 22% 31% 24% 8%	490
	21st to end	Day	16% 12% 24% 26% 16% 6%	490
October	1st to 10th	Day	19% 13% 25% 25% 16% 2	490
	11th to 20th	Day	29% 13% 31% 23% 4	365
	21st to end	Day	34% 14% 33% 12% 4 2	330
November	1st to 10th	Day	26% 21% 30% 15% 6% 2	300
	11th to 20th	Day	25% 22% 37% 12% 4	300
	21st to end	Day	31% 17% 36% 12% 4	300
December	1st to 10th	Day	26% 28% 25% 15% 4	300
	11th to 20th	Day	23% 27% 34% 12% 3	300
	21st to end	Day	32% 22% 31% 10% 3 2	330
January	1st to 10th	Night	21% 13% 35% 26% 6% 1	1000
	11th to 20th	Night	24% 13% 33% 23% 6%	1000
	21st to end	Night	22% 10% 33% 29% 6%	1100
February	1st to 10th	Night	19% 14% 35% 29% 4	874
	11th to 20th	Night	21% 11% 35% 27% 5% 1	783
	21st to end	Night	23% 14% 37% 22% 3	660
March	1st to 10th	Night	20% 11% 35% 26% 8% 1	800
	11th to 20th	Night	18% 14% 34% 27% 7% 1	800
	21st to end	Night	21% 11% 30% 31% 8% 1	785
April	1st to 10th	Night	22% 11% 34% 27% 5% 1	600
	11th to 20th	Night	26% 9% 30% 26% 7% 1	593
	21st to end	Night	20% 10% 32% 30% 8% 1	600
May	1st to 10th	Night	18% 11% 31% 34% 7% 1	600
	11th to 20th	Night	17% 7% 31% 33% 11%	591
	21st to end	Night	17% 5% 25% 34% 18% 1	650
June	1st to 10th	Night	12% 5% 28% 41% 13% 1	600
	11th to 20th	Night	14% 6% 29% 36% 15% 1	600
	21st to end	Night	11% 4 26% 41% 17% 1	600
July	1st to 10th	Night	6% 5% 18% 42% 25% 3	585
	11th to 20th	Night	7% 4 19% 44% 23% 2	582
	21st to end	Night	9% 3 19% 44% 24% 1	624
August	1st to 10th	Night	9% 2 16% 46% 26% 2	567
	11th to 20th	Night	10% 2 15% 43% 27% 3	564
	21st to end	Night	12% 2 20% 41% 22% 2	627
September	1st to 10th	Night	10% 4 19% 41% 22% 4	570
	11th to 20th	Night	12% 5% 18% 36% 25% 3	753
	21st to end	Night	16% 7% 24% 32% 18% 3	760
October	1st to 10th	Night	21% 6% 23% 34% 15% 2	760
	11th to 20th	Night	26% 9% 27% 29% 8% 1	760
	21st to end	Night	30% 10% 29% 26% 6%	836
November	1st to 10th	Night	23% 10% 29% 29% 8% 1	780
	11th to 20th	Night	24% 15% 31% 24% 6%	800
	21st to end	Night	27% 13% 31% 25% 4	805
December	1st to 10th	Night	24% 14% 33% 24% 4	1000
	11th to 20th	Night	26% 15% 34% 20% 4 1	1000
	21st to end	Night	31% 12% 34% 22% 1	1100

Page generated at 2025-07-18 01:10:05. Version ac07

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