Wetzel Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wetzel Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	17% 8% 3 6% 4 4 4 4 6% 7% 37%	137678
	Night	17% 8% 3 7% 4 4 4 4 5% 7% 37%	135178
Transparency	Day	45% 5% 11% 21% 15% 2	141187
	Night	46% 5% 11% 22% 15% 2	139758
Seeing	Day	23% 16% 31% 20% 8% 2	17121
	Night	22% 12% 31% 26% 8% 1	25874

By Month

Percentage Cloud Cover NE (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% 7% 3 8% 6% 5% 5% 5% 6% 6% 38%	4661
	11th to 20th	Day	13% 11% 4 7% 4 4 4 3 5% 6% 40%	4650
	21st to end	Day	10% 9% 3 6% 4 4 4 3 5% 5% 47%	5124
February	1st to 10th	Day	15% 9% 2 7% 4 4 4 4 7% 6% 37%	1849
	11th to 20th	Day	11% 9% 3 7% 5% 5% 5% 4 6% 7% 39%	2048
	21st to end	Day	14% 10% 4 6% 5% 4 5% 4 6% 5% 37%	1730
March	1st to 10th	Day	21% 11% 2 8% 5% 5% 4 3 4 5% 31%	4576
	11th to 20th	Day	17% 11% 3 6% 4 4 4 3 5% 5% 37%	4947
	21st to end	Day	26% 9% 2 5% 3 3 3 3 5% 7% 33%	5791
April	1st to 10th	Day	19% 10% 3 6% 4 4 4 3 4 7% 35%	2520
	11th to 20th	Day	18% 6% 2 5% 5% 5% 5% 4 6% 6% 38%	2581
	21st to end	Day	22% 8% 3 7% 4 4 4 3 5% 5% 35%	2730
May	1st to 10th	Day	29% 10% 3 6% 4 4 5% 3 6% 6% 25%	7639
	11th to 20th	Day	18% 7% 3 7% 6% 5% 5% 4 6% 8% 32%	7839
	21st to end	Day	19% 7% 3 7% 5% 4 5% 5% 7% 8% 31%	9060
June	1st to 10th	Day	21% 8% 3 6% 5% 5% 4 3 6% 8% 31%	2940
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 5% 5% 8% 39%	2940
	21st to end	Day	21% 7% 3 6% 5% 5% 5% 4 6% 7% 32%	2940
July	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 4 7% 9% 32%	6500
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 8% 9% 39%	6500
	21st to end	Day	15% 6% 2 5% 4 4 4 4 6% 8% 43%	7066
August	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	4306
	11th to 20th	Day	15% 9% 3 8% 5% 4 5% 4 6% 7% 35%	4175
	21st to end	Day	10% 4 2 5% 4 4 5% 4 7% 9% 46%	4427
September	1st to 10th	Day	12% 4 2 4 3 3 4 4 6% 8% 49%	2370
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 6% 54%	2200
	21st to end	Day	16% 5% 2 5% 4 4 4 3 6% 7% 46%	2200
October	1st to 10th	Day	19% 7% 2 5% 4 4 3 3 4 6% 42%	2200
	11th to 20th	Day	18% 8% 2 5% 3 3 5% 2 5% 7% 43%	1920
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 6% 37%	1980
November	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 6% 7% 45%	1851
	11th to 20th	Day	19% 5% 1 4 4 4 3 4 6% 6% 44%	1890
	21st to end	Day	21% 9% 2 5% 3 3 3 3 5% 6% 39%	1889
December	1st to 10th	Day	16% 9% 3 6% 5% 4 4 3 5% 8% 38%	2558
	11th to 20th	Day	19% 8% 2 4 3 3 3 3 5% 6% 44%	3033
	21st to end	Day	20% 10% 3 6% 4 4 5% 3 5% 5% 35%	4048
January	1st to 10th	Night	11% 9% 3 8% 5% 4 5% 4 5% 6% 39%	8234
	11th to 20th	Night	14% 12% 4 9% 5% 5% 5% 4 5% 7% 31%	8235
	21st to end	Night	12% 8% 3 7% 4 5% 5% 4 5% 7% 40%	9057
February	1st to 10th	Night	14% 11% 3 8% 5% 5% 4 4 5% 7% 35%	2765
	11th to 20th	Night	12% 10% 2 6% 6% 4 5% 4 5% 6% 41%	2458
	21st to end	Night	13% 9% 3 8% 5% 4 4 4 5% 7% 39%	2076
March	1st to 10th	Night	24% 11% 3 7% 4 4 4 3 5% 5% 32%	5484
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	5231
	21st to end	Night	24% 9% 3 6% 4 4 4 4 5% 6% 32%	5366
April	1st to 10th	Night	21% 10% 3 6% 4 4 4 3 4 6% 35%	2100
	11th to 20th	Night	19% 7% 3 7% 5% 4 4 3 6% 6% 36%	1988
	21st to end	Night	21% 7% 3 6% 4 5% 4 4 5% 6% 36%	1890
May	1st to 10th	Night	29% 9% 3 7% 4 4 4 3 5% 6% 27%	5259
	11th to 20th	Night	17% 7% 3 8% 5% 5% 5% 5% 6% 8% 32%	5063
	21st to end	Night	18% 8% 3 7% 5% 4 4 4 6% 8% 33%	5114
June	1st to 10th	Night	20% 8% 2 6% 5% 3 4 4 6% 6% 35%	1680
	11th to 20th	Night	17% 6% 3 5% 5% 4 4 4 5% 8% 40%	1680
	21st to end	Night	21% 6% 3 6% 4 3 5% 4 5% 8% 35%	1680
July	1st to 10th	Night	12% 6% 2 7% 4 5% 5% 5% 7% 10% 36%	3680
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 4 7% 8% 38%	3680
	21st to end	Night	12% 4 2 5% 4 5% 5% 3 6% 8% 48%	4006
August	1st to 10th	Night	21% 7% 3 7% 5% 5% 4 4 6% 7% 33%	2812
	11th to 20th	Night	15% 9% 3 8% 4 4 5% 4 5% 8% 36%	2872
	21st to end	Night	11% 5% 2 5% 4 5% 5% 4 6% 9% 44%	3454
September	1st to 10th	Night	13% 6% 2 4 4 4 4 3 5% 8% 49%	2030
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 4 7% 52%	2200
	21st to end	Night	16% 6% 2 5% 3 4 3 3 5% 7% 46%	2200
October	1st to 10th	Night	18% 6% 2 6% 4 5% 3 4 5% 6% 40%	2200
	11th to 20th	Night	17% 6% 2 5% 4 4 4 5% 6% 7% 41%	2479
	21st to end	Night	24% 7% 2 6% 3 3 3 3 5% 6% 36%	2860
November	1st to 10th	Night	16% 6% 3 6% 3 3 4 3 4 7% 44%	2667
	11th to 20th	Night	17% 7% 2 5% 3 4 4 4 5% 6% 44%	2730
	21st to end	Night	17% 8% 2 6% 4 4 4 3 6% 6% 40%	2730
December	1st to 10th	Night	16% 11% 3 7% 5% 4 4 4 6% 7% 35%	4720
	11th to 20th	Night	24% 7% 2 5% 4 3 3 3 4 5% 39%	5348
	21st to end	Night	20% 11% 3 7% 4 4 3 3 4 5% 37%	7150

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% 4 11% 24% 15% 1	4662
	11th to 20th	Day	44% 5% 10% 24% 14% 3	4680
	21st to end	Day	38% 4 9% 24% 20% 5%	5124
February	1st to 10th	Day	46% 5% 11% 20% 15% 3	1849
	11th to 20th	Day	45% 4 11% 20% 15% 4	2057
	21st to end	Day	47% 5% 10% 19% 15% 4	1730
March	1st to 10th	Day	55% 4 7% 12% 14% 8%	4579
	11th to 20th	Day	47% 5% 14% 19% 12% 3	4947
	21st to end	Day	51% 3 6% 17% 18% 5%	5791
April	1st to 10th	Day	49% 5% 8% 17% 17% 4	2520
	11th to 20th	Day	44% 6% 10% 20% 16% 3	2581
	21st to end	Day	50% 5% 8% 17% 16% 4	2730
May	1st to 10th	Day	58% 6% 11% 15% 9% 1	7651
	11th to 20th	Day	49% 6% 14% 18% 10% 2	7843
	21st to end	Day	49% 6% 12% 20% 12% 1	9061
June	1st to 10th	Day	50% 7% 10% 21% 11%	2940
	11th to 20th	Day	42% 6% 12% 20% 17% 3	2940
	21st to end	Day	51% 7% 10% 19% 12% 2	2940
July	1st to 10th	Day	47% 6% 12% 22% 12%	6430
	11th to 20th	Day	39% 7% 15% 24% 15% 1	6425
	21st to end	Day	38% 5% 12% 26% 18% 1	6868
August	1st to 10th	Day	44% 7% 15% 24% 10%	4292
	11th to 20th	Day	47% 6% 14% 24% 8%	4214
	21st to end	Day	33% 6% 15% 25% 19% 2	4427
September	1st to 10th	Day	31% 8% 13% 28% 19% 1	2370
	11th to 20th	Day	32% 4 9% 30% 23% 3	2200
	21st to end	Day	37% 5% 10% 28% 19% 2	2200
October	1st to 10th	Day	44% 5% 8% 25% 16% 1	3590
	11th to 20th	Day	35% 4 10% 29% 21% 1	3097
	21st to end	Day	37% 3 10% 26% 21% 4	3180
November	1st to 10th	Day	38% 5% 13% 25% 17% 3	1851
	11th to 20th	Day	39% 5% 10% 21% 21% 4	1890
	21st to end	Day	45% 4 15% 22% 12% 1	1889
December	1st to 10th	Day	45% 4 12% 27% 11% 1	2558
	11th to 20th	Day	40% 4 11% 19% 17% 8%	3033
	21st to end	Day	50% 5% 9% 18% 12% 6%	4048
January	1st to 10th	Night	44% 4 11% 27% 13% 2	8234
	11th to 20th	Night	52% 5% 10% 17% 13% 3	8280
	21st to end	Night	43% 4 7% 22% 19% 6%	9057
February	1st to 10th	Night	48% 4 11% 19% 15% 3	2765
	11th to 20th	Night	45% 4 9% 22% 17% 3	2460
	21st to end	Night	44% 5% 9% 22% 15% 4	2076
March	1st to 10th	Night	55% 4 7% 14% 14% 6%	5494
	11th to 20th	Night	47% 6% 17% 18% 10% 2	5232
	21st to end	Night	52% 3 6% 14% 20% 5%	5366
April	1st to 10th	Night	50% 5% 7% 19% 16% 3	2100
	11th to 20th	Night	47% 5% 9% 20% 18% 1	1988
	21st to end	Night	48% 5% 10% 17% 16% 4	1890
May	1st to 10th	Night	58% 5% 11% 14% 11% 1	5265
	11th to 20th	Night	49% 6% 11% 25% 7% 1	5064
	21st to end	Night	48% 6% 13% 22% 10% 1	5115
June	1st to 10th	Night	49% 6% 10% 22% 12% 1	1680
	11th to 20th	Night	42% 6% 14% 21% 15% 2	1680
	21st to end	Night	47% 8% 13% 21% 9% 2	1680
July	1st to 10th	Night	42% 6% 17% 23% 12%	3640
	11th to 20th	Night	44% 6% 13% 23% 14%	3632
	21st to end	Night	34% 6% 14% 33% 12% 1	3889
August	1st to 10th	Night	51% 6% 13% 21% 8%	2802
	11th to 20th	Night	46% 6% 15% 22% 11% 1	2899
	21st to end	Night	34% 6% 15% 26% 17% 1	3454
September	1st to 10th	Night	34% 6% 12% 32% 16%	2030
	11th to 20th	Night	33% 4 11% 30% 20% 2	2200
	21st to end	Night	37% 5% 10% 29% 16% 2	2200
October	1st to 10th	Night	44% 5% 12% 22% 16% 1	3590
	11th to 20th	Night	33% 4 10% 27% 23% 2	4041
	21st to end	Night	40% 4 9% 28% 17% 3	4610
November	1st to 10th	Night	41% 4 11% 26% 17% 2	2667
	11th to 20th	Night	40% 4 11% 24% 18% 3	2730
	21st to end	Night	43% 5% 14% 22% 14% 1	2730
December	1st to 10th	Night	47% 4 9% 28% 11% 1	4720
	11th to 20th	Night	48% 4 10% 16% 19% 4	5348
	21st to end	Night	51% 4 8% 19% 16% 2	7150

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	26% 21% 26% 22% 4	220
	11th to 20th	Day	25% 22% 30% 19% 2 1	220
	21st to end	Day	19% 23% 35% 18% 5%	242
February	1st to 10th	Day	26% 18% 35% 19% 2	377
	11th to 20th	Day	25% 22% 36% 15% 2	536
	21st to end	Day	26% 18% 36% 16% 4	446
March	1st to 10th	Day	27% 20% 36% 14% 3	540
	11th to 20th	Day	27% 24% 31% 14% 3 1	540
	21st to end	Day	31% 20% 30% 12% 5% 2	594
April	1st to 10th	Day	31% 22% 28% 13% 5% 1	540
	11th to 20th	Day	30% 20% 30% 14% 5% 2	532
	21st to end	Day	32% 19% 29% 14% 5% 2	540
May	1st to 10th	Day	34% 19% 25% 16% 4 1	543
	11th to 20th	Day	22% 19% 34% 17% 7% 2	567
	21st to end	Day	23% 16% 33% 21% 7% 1	617
June	1st to 10th	Day	21% 12% 36% 19% 9% 2	570
	11th to 20th	Day	20% 16% 33% 20% 9% 2	570
	21st to end	Day	23% 14% 32% 21% 8% 1	570
July	1st to 10th	Day	18% 10% 32% 27% 11% 3	565
	11th to 20th	Day	16% 7% 37% 26% 11% 4	564
	21st to end	Day	16% 9% 34% 27% 12% 3	616
August	1st to 10th	Day	11% 10% 29% 30% 17% 3	526
	11th to 20th	Day	17% 10% 25% 28% 15% 6%	519
	21st to end	Day	12% 10% 29% 29% 14% 6%	583
September	1st to 10th	Day	14% 10% 31% 23% 17% 5%	530
	11th to 20th	Day	16% 12% 26% 22% 14% 10%	530
	21st to end	Day	18% 12% 31% 23% 10% 6%	530
October	1st to 10th	Day	23% 13% 23% 23% 13% 4	530
	11th to 20th	Day	20% 16% 29% 20% 11% 3	436
	21st to end	Day	30% 17% 26% 21% 7%	462
November	1st to 10th	Day	24% 18% 29% 20% 8% 2	425
	11th to 20th	Day	27% 18% 30% 19% 5% 1	430
	21st to end	Day	31% 17% 26% 19% 6%	429
December	1st to 10th	Day	29% 21% 27% 17% 6%	220
	11th to 20th	Day	34% 16% 31% 15% 4	220
	21st to end	Day	30% 21% 25% 19% 5%	242
January	1st to 10th	Night	21% 18% 36% 21% 3	840
	11th to 20th	Night	26% 16% 33% 21% 4	840
	21st to end	Night	19% 18% 38% 21% 4	924
February	1st to 10th	Night	23% 15% 35% 23% 3	840
	11th to 20th	Night	20% 18% 38% 21% 3	823
	21st to end	Night	21% 15% 36% 24% 4	692
March	1st to 10th	Night	26% 16% 35% 20% 2	840
	11th to 20th	Night	24% 17% 33% 20% 6%	840
	21st to end	Night	26% 15% 34% 19% 5% 1	924
April	1st to 10th	Night	28% 15% 30% 23% 3	840
	11th to 20th	Night	23% 14% 36% 22% 4	744
	21st to end	Night	26% 15% 33% 21% 5%	630
May	1st to 10th	Night	26% 11% 31% 26% 5% 1	630
	11th to 20th	Night	23% 12% 32% 27% 6%	567
	21st to end	Night	23% 9% 25% 34% 9% 1	455
June	1st to 10th	Night	23% 7% 29% 30% 10%	420
	11th to 20th	Night	20% 6% 27% 33% 13%	420
	21st to end	Night	23% 3 25% 38% 10%	420
July	1st to 10th	Night	16% 3 26% 39% 15%	410
	11th to 20th	Night	15% 4 24% 36% 21%	408
	21st to end	Night	15% 3 23% 41% 16% 1	458
August	1st to 10th	Night	15% 3 22% 43% 17%	533
	11th to 20th	Night	16% 4 27% 35% 16% 2	592
	21st to end	Night	14% 6% 25% 37% 17% 2	800
September	1st to 10th	Night	16% 4 30% 34% 16% 1	800
	11th to 20th	Night	17% 8% 25% 27% 20% 4	800
	21st to end	Night	19% 8% 28% 29% 16% 1	800
October	1st to 10th	Night	22% 10% 22% 30% 15% 1	800
	11th to 20th	Night	22% 10% 32% 25% 11% 1	800
	21st to end	Night	28% 14% 28% 25% 5%	880
November	1st to 10th	Night	22% 12% 32% 27% 5% 1	820
	11th to 20th	Night	22% 15% 31% 24% 6% 1	840
	21st to end	Night	22% 20% 31% 22% 5%	840
December	1st to 10th	Night	26% 12% 37% 20% 5%	840
	11th to 20th	Night	25% 18% 32% 21% 4	840
	21st to end	Night	28% 15% 31% 23% 3	924

Page generated at 2026-07-28 03:25:26. Version ac07

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