CF Phelps WMA
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CF Phelps WMA clear sky chart from 2005-11-06 to 2026-06-16.

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 4 4 6% 8% 42%	137555
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	134468
Transparency	Day	39% 5% 12% 23% 18% 4	141155
	Night	39% 4 11% 23% 19% 4	138938
Seeing	Day	19% 12% 26% 24% 14% 5%	17077
	Night	17% 9% 29% 30% 13% 1	26734

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	15% 5% 2 6% 4 3 4 3 5% 5% 46%	5054
	11th to 20th	Day	14% 8% 2 5% 3 3 4 2 4 4 51%	5280
	21st to end	Day	14% 7% 2 7% 4 4 4 3 5% 5% 44%	5780
February	1st to 10th	Day	12% 8% 3 7% 5% 4 4 3 5% 7% 42%	1890
	11th to 20th	Day	19% 8% 2 6% 4 3 3 4 5% 6% 41%	1877
	21st to end	Day	17% 9% 3 7% 4 4 3 3 5% 6% 38%	1740
March	1st to 10th	Day	17% 7% 2 5% 5% 5% 4 3 5% 7% 40%	5045
	11th to 20th	Day	10% 6% 2 6% 3 4 3 3 5% 6% 51%	5090
	21st to end	Day	16% 7% 2 7% 4 4 4 4 6% 6% 40%	5603
April	1st to 10th	Day	13% 6% 2 6% 4 3 5% 4 5% 6% 47%	2520
	11th to 20th	Day	13% 5% 2 6% 4 4 3 4 5% 7% 48%	2494
	21st to end	Day	16% 6% 2 6% 4 5% 4 3 6% 6% 41%	2520
May	1st to 10th	Day	14% 8% 2 7% 5% 5% 5% 5% 6% 9% 36%	7556
	11th to 20th	Day	18% 6% 2 5% 4 4 4 4 7% 9% 37%	7760
	21st to end	Day	21% 5% 2 6% 3 4 4 3 5% 7% 38%	9045
June	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 6% 9% 44%	2940
	11th to 20th	Day	11% 4 2 6% 5% 5% 5% 4 7% 10% 41%	2870
	21st to end	Day	8% 4 1 5% 5% 5% 5% 5% 8% 10% 45%	2800
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 41%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 6% 7% 6% 10% 13% 28%	5913
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	6424
August	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 5% 7% 12% 33%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 8% 9% 39%	4024
	21st to end	Day	9% 5% 1 5% 4 4 5% 4 5% 8% 50%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 4 5% 4 6% 6% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 4 4 6% 9% 47%	2390
	21st to end	Day	22% 6% 2 6% 4 4 4 4 5% 6% 37%	2090
October	1st to 10th	Day	15% 5% 2 5% 3 4 4 3 4 6% 49%	2000
	11th to 20th	Day	16% 4 2 4 3 3 4 3 4 5% 50%	2000
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 6% 51%	2200
November	1st to 10th	Day	12% 5% 2 4 4 4 3 3 4 7% 53%	2018
	11th to 20th	Day	14% 7% 1 4 3 3 4 3 6% 6% 49%	1848
	21st to end	Day	16% 5% 2 5% 4 3 3 3 4 6% 49%	1680
December	1st to 10th	Day	12% 5% 2 6% 4 3 3 3 5% 7% 50%	2680
	11th to 20th	Day	18% 8% 3 4 3 3 2 3 4 6% 46%	3070
	21st to end	Day	17% 7% 3 6% 4 4 4 4 5% 7% 39%	4048
January	1st to 10th	Night	12% 6% 2 5% 4 4 5% 4 5% 6% 48%	7836
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 53%	7680
	21st to end	Night	13% 7% 2 6% 3 3 4 3 4 5% 49%	8401
February	1st to 10th	Night	15% 7% 2 6% 3 5% 5% 4 6% 6% 42%	2730
	11th to 20th	Night	20% 7% 3 6% 4 3 3 3 4 6% 43%	2627
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 6% 43%	2061
March	1st to 10th	Night	16% 6% 2 6% 4 3 3 3 4 5% 47%	5028
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 8% 49%	5090
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 7% 45%	5557
April	1st to 10th	Night	14% 8% 2 5% 4 4 4 3 5% 7% 44%	2100
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 49%	2075
	21st to end	Night	15% 5% 2 5% 5% 3 5% 4 6% 7% 44%	2100
May	1st to 10th	Night	16% 9% 3 8% 5% 5% 4 4 6% 8% 33%	5341
	11th to 20th	Night	19% 7% 2 6% 4 4 4 4 6% 8% 37%	5097
	21st to end	Night	21% 8% 3 7% 5% 4 5% 3 5% 7% 33%	5108
June	1st to 10th	Night	10% 5% 2 6% 4 3 4 4 8% 9% 44%	1680
	11th to 20th	Night	11% 6% 2 5% 5% 5% 5% 4 6% 9% 41%	1640
	21st to end	Night	9% 4 2 4 4 5% 5% 5% 8% 9% 44%	1600
July	1st to 10th	Night	8% 6% 2 7% 5% 6% 6% 5% 8% 9% 39%	3598
	11th to 20th	Night	9% 8% 4 9% 6% 6% 7% 6% 7% 9% 28%	4026
	21st to end	Night	7% 7% 3 7% 5% 6% 6% 5% 7% 9% 38%	4427
August	1st to 10th	Night	15% 5% 3 8% 5% 5% 6% 4 7% 9% 32%	2915
	11th to 20th	Night	10% 8% 3 9% 6% 6% 5% 4 7% 8% 35%	2991
	21st to end	Night	11% 6% 2 5% 4 3 4 4 5% 9% 47%	3586
September	1st to 10th	Night	13% 6% 2 5% 3 4 5% 3 5% 7% 46%	2000
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 48%	2005
	21st to end	Night	19% 9% 3 6% 4 4 3 3 5% 7% 37%	2309
October	1st to 10th	Night	17% 6% 2 6% 4 3 3 3 5% 5% 47%	2400
	11th to 20th	Night	16% 5% 2 5% 4 2 4 3 5% 5% 50%	2400
	21st to end	Night	14% 4 2 5% 4 3 4 3 5% 7% 50%	2640
November	1st to 10th	Night	12% 5% 2 6% 3 3 4 3 4 5% 54%	2498
	11th to 20th	Night	14% 7% 2 4 4 3 3 2 5% 6% 49%	2767
	21st to end	Night	12% 5% 1 4 4 3 4 4 5% 6% 50%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 5% 7% 54%	4720
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 5% 6% 42%	5345
	21st to end	Night	16% 6% 2 6% 4 5% 4 4 6% 7% 40%	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	39% 4 8% 23% 19% 7%	5055
	11th to 20th	Day	37% 2 5% 20% 24% 12%	5280
	21st to end	Day	42% 2 7% 29% 16% 4	5780
February	1st to 10th	Day	43% 3 11% 25% 15% 3	1890
	11th to 20th	Day	44% 4 8% 22% 19% 4	1886
	21st to end	Day	46% 3 12% 18% 17% 4	1740
March	1st to 10th	Day	44% 3 8% 20% 16% 9%	5045
	11th to 20th	Day	34% 4 7% 26% 22% 7%	5090
	21st to end	Day	44% 4 11% 17% 17% 7%	5603
April	1st to 10th	Day	37% 4 11% 21% 20% 7%	2520
	11th to 20th	Day	36% 4 11% 21% 22% 6%	2494
	21st to end	Day	43% 4 10% 25% 14% 4	2520
May	1st to 10th	Day	44% 5% 13% 21% 15% 1	7590
	11th to 20th	Day	42% 5% 11% 27% 14% 1	7764
	21st to end	Day	45% 4 12% 18% 18% 3	9061
June	1st to 10th	Day	36% 6% 13% 26% 17% 1	2940
	11th to 20th	Day	37% 6% 14% 25% 15% 2	2870
	21st to end	Day	32% 7% 16% 25% 19% 1	2800
July	1st to 10th	Day	34% 9% 20% 20% 13% 5%	6290
	11th to 20th	Day	41% 11% 19% 21% 7% 1	5843
	21st to end	Day	37% 6% 12% 23% 21% 1	6240
August	1st to 10th	Day	42% 7% 17% 26% 8%	4221
	11th to 20th	Day	39% 7% 13% 25% 15% 1	4063
	21st to end	Day	32% 5% 15% 28% 18% 2	4312
September	1st to 10th	Day	38% 5% 13% 19% 21% 5%	2400
	11th to 20th	Day	34% 6% 7% 28% 22% 3	2390
	21st to end	Day	48% 4 10% 20% 16% 3	2090
October	1st to 10th	Day	37% 4 6% 19% 30% 5%	3260
	11th to 20th	Day	27% 3 6% 25% 29% 11%	3240
	21st to end	Day	22% 2 10% 28% 25% 12%	3538
November	1st to 10th	Day	32% 4 9% 26% 24% 5%	2018
	11th to 20th	Day	35% 4 10% 23% 22% 7%	1848
	21st to end	Day	38% 3 10% 25% 21% 3	1680
December	1st to 10th	Day	35% 3 11% 26% 19% 6%	2680
	11th to 20th	Day	41% 2 13% 26% 13% 4	3066
	21st to end	Day	44% 5% 13% 22% 15% 1	4048
January	1st to 10th	Night	37% 3 9% 24% 22% 5%	7837
	11th to 20th	Night	35% 3 6% 21% 24% 10%	7680
	21st to end	Night	38% 2 7% 29% 19% 4	8401
February	1st to 10th	Night	42% 4 10% 24% 17% 2	2730
	11th to 20th	Night	44% 3 8% 21% 19% 6%	2629
	21st to end	Night	42% 3 10% 21% 17% 6%	2061
March	1st to 10th	Night	39% 4 8% 23% 19% 8%	5028
	11th to 20th	Night	33% 4 9% 22% 25% 7%	5090
	21st to end	Night	39% 4 10% 22% 20% 6%	5557
April	1st to 10th	Night	39% 5% 10% 19% 22% 5%	2100
	11th to 20th	Night	36% 4 11% 23% 21% 5%	2075
	21st to end	Night	38% 5% 11% 27% 14% 5%	2100
May	1st to 10th	Night	49% 5% 13% 20% 10% 3	5368
	11th to 20th	Night	44% 5% 13% 24% 13% 1	5098
	21st to end	Night	52% 4 11% 18% 13% 2	5115
June	1st to 10th	Night	35% 6% 14% 27% 17% 2	1680
	11th to 20th	Night	39% 6% 16% 23% 14% 1	1640
	21st to end	Night	33% 7% 17% 21% 19% 3	1600
July	1st to 10th	Night	40% 9% 18% 17% 14% 2	3560
	11th to 20th	Night	50% 10% 18% 16% 6% 1	3976
	21st to end	Night	39% 8% 14% 22% 16%	4297
August	1st to 10th	Night	46% 6% 17% 22% 8% 1	2904
	11th to 20th	Night	44% 7% 12% 23% 13% 1	3018
	21st to end	Night	34% 5% 13% 29% 18% 1	3586
September	1st to 10th	Night	37% 6% 12% 22% 17% 6%	2000
	11th to 20th	Night	35% 5% 8% 27% 22% 3	2005
	21st to end	Night	47% 5% 10% 19% 16% 3	2309
October	1st to 10th	Night	41% 4 8% 20% 21% 6%	3920
	11th to 20th	Night	26% 3 8% 22% 32% 9%	3899
	21st to end	Night	23% 2 8% 31% 27% 9%	4252
November	1st to 10th	Night	34% 3 8% 24% 24% 6%	2498
	11th to 20th	Night	37% 3 7% 24% 23% 7%	2767
	21st to end	Night	34% 3 10% 26% 21% 5%	2940
December	1st to 10th	Night	32% 3 11% 28% 22% 4	4720
	11th to 20th	Night	45% 3 11% 19% 17% 5%	5348
	21st to end	Night	42% 5% 13% 23% 16% 1	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	25% 21% 31% 19% 4	291
	11th to 20th	Day	28% 28% 27% 16% 2	330
	21st to end	Day	23% 25% 36% 13% 3 1	363
February	1st to 10th	Day	24% 25% 34% 12% 5%	330
	11th to 20th	Day	34% 26% 29% 9% 1	364
	21st to end	Day	28% 19% 30% 18% 4	446
March	1st to 10th	Day	23% 19% 33% 21% 5%	540
	11th to 20th	Day	23% 22% 30% 18% 6% 1	540
	21st to end	Day	24% 18% 34% 17% 6% 1	594
April	1st to 10th	Day	24% 21% 28% 21% 5% 1	540
	11th to 20th	Day	18% 18% 30% 20% 11% 3	532
	21st to end	Day	22% 13% 32% 23% 10% 1	540
May	1st to 10th	Day	21% 16% 32% 17% 11% 3	543
	11th to 20th	Day	21% 11% 28% 25% 13% 3	567
	21st to end	Day	22% 6% 25% 24% 18% 5%	617
June	1st to 10th	Day	11% 7% 27% 31% 17% 7%	570
	11th to 20th	Day	13% 6% 26% 31% 18% 6%	550
	21st to end	Day	9% 4 23% 36% 22% 6%	530
July	1st to 10th	Day	12% 3 17% 33% 25% 11%	525
	11th to 20th	Day	7% 1 14% 36% 29% 13%	524
	21st to end	Day	12% 3 17% 31% 24% 12%	576
August	1st to 10th	Day	12% 3 18% 29% 28% 10%	526
	11th to 20th	Day	13% 4 17% 30% 24% 13%	519
	21st to end	Day	10% 4 20% 29% 22% 16%	583
September	1st to 10th	Day	16% 5% 19% 27% 18% 15%	530
	11th to 20th	Day	15% 3 19% 28% 23% 12%	530
	21st to end	Day	28% 9% 21% 25% 12% 4	530
October	1st to 10th	Day	20% 5% 23% 27% 17% 8%	530
	11th to 20th	Day	17% 14% 26% 25% 15% 3	530
	21st to end	Day	18% 12% 32% 26% 9% 3	583
November	1st to 10th	Day	15% 12% 31% 25% 14% 2	514
	11th to 20th	Day	20% 11% 35% 22% 9% 2	388
	21st to end	Day	23% 17% 32% 19% 9%	220
December	1st to 10th	Day	24% 17% 36% 16% 5% 1	220
	11th to 20th	Day	20% 17% 34% 21% 8%	220
	21st to end	Day	28% 17% 29% 19% 7%	242
January	1st to 10th	Night	21% 16% 39% 21% 3	840
	11th to 20th	Night	21% 15% 34% 26% 4	840
	21st to end	Night	19% 15% 39% 26% 2	924
February	1st to 10th	Night	20% 13% 39% 25% 3	840
	11th to 20th	Night	27% 18% 36% 19% 1	823
	21st to end	Night	22% 16% 38% 21% 2	692
March	1st to 10th	Night	20% 16% 37% 25% 3	840
	11th to 20th	Night	22% 17% 34% 22% 5%	840
	21st to end	Night	18% 16% 38% 23% 5%	924
April	1st to 10th	Night	18% 17% 38% 23% 5%	840
	11th to 20th	Night	14% 14% 34% 29% 8%	831
	21st to end	Night	17% 9% 36% 31% 7%	840
May	1st to 10th	Night	21% 9% 31% 27% 11% 1	669
	11th to 20th	Night	20% 5% 28% 33% 13%	575
	21st to end	Night	16% 4 20% 37% 21% 2	455
June	1st to 10th	Night	11% 3 24% 42% 20% 1	420
	11th to 20th	Night	12% 3 17% 46% 20% 1	410
	21st to end	Night	10% 2 19% 48% 20% 2	400
July	1st to 10th	Night	11% 4 17% 38% 28% 3	390
	11th to 20th	Night	7% 2 13% 41% 32% 5%	565
	21st to end	Night	9% 3 14% 41% 30% 4	657
August	1st to 10th	Night	12% 2 11% 41% 31% 3	597
	11th to 20th	Night	12% 3 11% 40% 30% 3	658
	21st to end	Night	13% 3 12% 38% 30% 5%	880
September	1st to 10th	Night	16% 3 15% 33% 28% 5%	800
	11th to 20th	Night	12% 3 14% 37% 31% 3	800
	21st to end	Night	25% 4 19% 31% 20% 1	800
October	1st to 10th	Night	20% 4 20% 31% 21% 4	800
	11th to 20th	Night	19% 8% 26% 30% 16% 1	800
	21st to end	Night	16% 8% 34% 29% 12% 1	880
November	1st to 10th	Night	15% 9% 30% 33% 12% 1	820
	11th to 20th	Night	18% 11% 34% 29% 8% 1	840
	21st to end	Night	16% 12% 33% 30% 9% 1	840
December	1st to 10th	Night	20% 11% 36% 25% 7%	840
	11th to 20th	Night	20% 10% 35% 30% 5%	840
	21st to end	Night	21% 10% 33% 32% 4	924

Page generated at 2026-06-13 02:31:43. Version ac07

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