Coffee Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coffee Creek Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	11% 7% 3 6% 4 4 4 3 5% 7% 46%	148771
	Night	10% 7% 3 6% 4 4 4 3 5% 7% 48%	143352
Transparency	Day	36% 6% 12% 26% 17% 3	152383
	Night	35% 5% 11% 26% 19% 4	147820
Seeing	Day	14% 10% 29% 28% 16% 4	18344
	Night	14% 9% 33% 32% 12% 1	28079

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	13% 11% 3 8% 4 4 3 3 4 5% 41%	5289
	11th to 20th	Day	5% 7% 3 9% 7% 6% 6% 3 5% 6% 43%	5310
	21st to end	Day	12% 12% 4 8% 4 4 4 3 3 5% 41%	5812
February	1st to 10th	Day	14% 11% 4 7% 5% 5% 3 2 4 6% 40%	1890
	11th to 20th	Day	12% 10% 3 7% 5% 5% 4 3 5% 5% 41%	1844
	21st to end	Day	10% 9% 3 6% 4 4 4 3 5% 7% 46%	1794
March	1st to 10th	Day	12% 9% 2 6% 3 3 3 3 3 5% 51%	5046
	11th to 20th	Day	9% 6% 2 5% 3 3 3 3 4 6% 56%	5090
	21st to end	Day	12% 10% 4 8% 5% 4 5% 4 5% 8% 35%	5583
April	1st to 10th	Day	14% 9% 3 6% 4 3 4 3 5% 6% 44%	2421
	11th to 20th	Day	15% 8% 3 6% 4 4 5% 4 5% 6% 41%	2494
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 4 6% 36%	2665
May	1st to 10th	Day	15% 7% 2 6% 4 3 4 3 5% 6% 44%	7617
	11th to 20th	Day	13% 6% 2 5% 4 3 4 4 5% 8% 46%	7627
	21st to end	Day	20% 7% 2 6% 4 4 4 4 5% 7% 38%	8506
June	1st to 10th	Day	7% 5% 1 5% 4 4 5% 4 7% 10% 47%	2940
	11th to 20th	Day	7% 5% 2 5% 5% 4 4 4 6% 10% 46%	2940
	21st to end	Day	7% 4 2 5% 4 4 5% 4 6% 9% 50%	2900
July	1st to 10th	Day	7% 5% 2 7% 4 5% 5% 4 6% 9% 46%	7680
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 9% 53%	7680
	21st to end	Day	9% 5% 2 7% 4 5% 5% 5% 8% 10% 40%	8337
August	1st to 10th	Day	5% 4 2 6% 4 4 5% 4 6% 9% 50%	6014
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 6% 9% 46%	5948
	21st to end	Day	5% 4 2 5% 3 3 4 3 6% 10% 54%	6202
September	1st to 10th	Day	12% 4 2 5% 3 3 4 3 5% 7% 53%	2520
	11th to 20th	Day	8% 5% 2 5% 5% 4 4 4 5% 8% 49%	2324
	21st to end	Day	9% 4 2 7% 4 4 4 3 6% 6% 52%	2298
October	1st to 10th	Day	11% 6% 2 4 3 3 4 2 5% 7% 55%	2100
	11th to 20th	Day	11% 6% 2 5% 3 2 3 2 4 6% 57%	2030
	21st to end	Day	16% 6% 3 5% 4 3 3 2 4 6% 48%	2200
November	1st to 10th	Day	9% 5% 2 5% 4 3 3 3 4 6% 55%	2058
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 7% 51%	1831
	21st to end	Day	15% 8% 2 7% 4 3 3 3 4 5% 47%	1680
December	1st to 10th	Day	6% 6% 2 6% 5% 4 3 3 5% 7% 54%	2690
	11th to 20th	Day	10% 7% 3 7% 4 3 3 3 4 6% 50%	3088
	21st to end	Day	17% 16% 8% 11% 4 3 3 3 4 4 26%	4323
January	1st to 10th	Night	12% 10% 3 12% 5% 4 3 3 3 4 39%	7634
	11th to 20th	Night	7% 7% 3 8% 5% 5% 4 3 4 5% 48%	7680
	21st to end	Night	13% 10% 4 9% 4 5% 4 3 4 5% 40%	8402
February	1st to 10th	Night	11% 10% 3 7% 6% 4 4 3 5% 7% 40%	2730
	11th to 20th	Night	12% 10% 3 8% 5% 4 4 3 5% 6% 42%	2663
	21st to end	Night	12% 6% 3 7% 4 4 3 3 5% 5% 48%	2028
March	1st to 10th	Night	13% 9% 3 5% 3 2 3 3 3 5% 50%	5029
	11th to 20th	Night	7% 7% 2 4 3 3 3 3 5% 7% 56%	5090
	21st to end	Night	13% 8% 3 7% 4 4 5% 3 5% 8% 39%	5588
April	1st to 10th	Night	11% 7% 3 6% 3 3 4 3 5% 6% 48%	2197
	11th to 20th	Night	13% 7% 3 7% 3 4 3 2 5% 7% 47%	2075
	21st to end	Night	16% 8% 2 7% 4 3 4 4 5% 5% 43%	1951
May	1st to 10th	Night	14% 7% 2 6% 4 3 4 3 5% 6% 46%	5266
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 50%	5265
	21st to end	Night	17% 6% 2 6% 4 3 3 3 5% 6% 44%	5674
June	1st to 10th	Night	6% 3 1 4 3 5% 5% 5% 6% 9% 54%	1680
	11th to 20th	Night	6% 4 2 4 4 4 4 4 7% 10% 51%	1680
	21st to end	Night	5% 3 1 4 3 3 4 3 6% 10% 58%	1712
July	1st to 10th	Night	6% 5% 2 5% 3 4 4 4 7% 9% 51%	5308
	11th to 20th	Night	4 4 1 4 3 4 4 4 5% 9% 56%	5307
	21st to end	Night	7% 5% 2 6% 4 5% 6% 4 8% 12% 42%	5770
August	1st to 10th	Night	4 2 1 4 3 4 5% 4 6% 9% 59%	4156
	11th to 20th	Night	7% 5% 2 5% 3 3 4 3 6% 9% 52%	4118
	21st to end	Night	4 3 2 4 4 3 4 3 5% 7% 61%	4927
September	1st to 10th	Night	9% 5% 3 5% 3 3 3 3 4 5% 58%	2100
	11th to 20th	Night	9% 4 2 4 4 3 4 4 6% 7% 53%	2292
	21st to end	Night	6% 5% 2 5% 4 4 4 4 5% 6% 56%	2310
October	1st to 10th	Night	10% 4 2 4 3 3 3 3 4 5% 60%	2509
	11th to 20th	Night	11% 4 2 5% 3 3 3 3 4 5% 57%	2437
	21st to end	Night	16% 7% 2 5% 3 2 3 2 3 4 51%	2640
November	1st to 10th	Night	9% 5% 2 4 2 3 2 2 4 5% 61%	2460
	11th to 20th	Night	9% 7% 2 4 3 3 3 3 4 8% 54%	2784
	21st to end	Night	15% 7% 2 4 3 3 3 2 3 5% 52%	2940
December	1st to 10th	Night	7% 6% 2 5% 4 4 4 3 5% 6% 56%	4720
	11th to 20th	Night	11% 7% 3 7% 4 4 4 3 5% 6% 46%	5339
	21st to end	Night	18% 16% 5% 9% 4 4 3 3 3 4 31%	6891

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	46% 5% 9% 27% 12% 2	5290
	11th to 20th	Day	42% 4 10% 24% 16% 4	5310
	21st to end	Day	47% 3 11% 19% 17% 3	5812
February	1st to 10th	Day	47% 4 12% 23% 11% 2	1890
	11th to 20th	Day	45% 4 12% 22% 15% 3	1853
	21st to end	Day	39% 4 11% 24% 19% 4	1794
March	1st to 10th	Day	38% 3 8% 21% 22% 8%	5046
	11th to 20th	Day	30% 5% 9% 22% 27% 8%	5090
	21st to end	Day	45% 7% 16% 25% 6% 1	5585
April	1st to 10th	Day	41% 4 12% 23% 18% 2	2421
	11th to 20th	Day	43% 5% 12% 22% 17% 1	2494
	21st to end	Day	51% 4 11% 24% 10% 1	2665
May	1st to 10th	Day	41% 6% 10% 26% 17% 1	7655
	11th to 20th	Day	35% 6% 12% 24% 20% 2	7631
	21st to end	Day	46% 6% 11% 24% 13% 1	8520
June	1st to 10th	Day	30% 8% 18% 26% 16% 2	2940
	11th to 20th	Day	31% 8% 14% 28% 17% 2	2940
	21st to end	Day	28% 8% 15% 28% 17% 4	2900
July	1st to 10th	Day	34% 8% 14% 25% 18% 2	7616
	11th to 20th	Day	26% 7% 15% 29% 17% 5%	7611
	21st to end	Day	35% 8% 16% 32% 8% 1	8155
August	1st to 10th	Day	29% 7% 13% 28% 22% 2	6000
	11th to 20th	Day	32% 7% 17% 30% 12% 1	5987
	21st to end	Day	25% 6% 15% 37% 14% 3	6201
September	1st to 10th	Day	32% 5% 12% 26% 20% 6%	2520
	11th to 20th	Day	32% 7% 11% 26% 22% 2	2324
	21st to end	Day	32% 4 11% 26% 25% 3	2298
October	1st to 10th	Day	19% 3 7% 23% 41% 8%	3360
	11th to 20th	Day	20% 2 5% 26% 38% 9%	3270
	21st to end	Day	32% 4 10% 24% 25% 4	3538
November	1st to 10th	Day	31% 5% 11% 30% 19% 5%	2058
	11th to 20th	Day	34% 3 10% 27% 21% 4	1831
	21st to end	Day	41% 4 8% 27% 19% 1	1680
December	1st to 10th	Day	30% 4 13% 36% 17% 1	2690
	11th to 20th	Day	36% 4 11% 32% 15% 2	3085
	21st to end	Day	62% 4 10% 17% 7% 1	4323
January	1st to 10th	Night	49% 4 12% 23% 11% 1	7635
	11th to 20th	Night	38% 4 9% 25% 20% 4	7680
	21st to end	Night	46% 4 11% 19% 16% 5%	8402
February	1st to 10th	Night	44% 4 12% 22% 15% 3	2730
	11th to 20th	Night	42% 4 11% 24% 16% 3	2664
	21st to end	Night	38% 4 8% 23% 21% 7%	2028
March	1st to 10th	Night	37% 4 8% 22% 25% 5%	5029
	11th to 20th	Night	26% 4 8% 22% 32% 8%	5090
	21st to end	Night	42% 7% 16% 23% 10% 2	5588
April	1st to 10th	Night	36% 5% 10% 22% 23% 4	2197
	11th to 20th	Night	39% 3 9% 27% 18% 4	2075
	21st to end	Night	43% 5% 11% 26% 14% 1	1951
May	1st to 10th	Night	39% 6% 11% 26% 17% 2	5293
	11th to 20th	Night	35% 5% 11% 23% 20% 5%	5266
	21st to end	Night	40% 7% 9% 25% 18% 1	5685
June	1st to 10th	Night	25% 8% 17% 31% 16% 3	1680
	11th to 20th	Night	26% 7% 14% 31% 20% 2	1680
	21st to end	Night	21% 5% 14% 33% 22% 4	1712
July	1st to 10th	Night	28% 6% 16% 26% 22% 2	5265
	11th to 20th	Night	23% 7% 16% 29% 18% 6%	5256
	21st to end	Night	31% 8% 16% 36% 9%	5641
August	1st to 10th	Night	21% 6% 16% 28% 24% 5%	4146
	11th to 20th	Night	28% 7% 15% 33% 16% 1	4145
	21st to end	Night	22% 6% 13% 40% 17% 3	4927
September	1st to 10th	Night	29% 4 10% 28% 21% 8%	2100
	11th to 20th	Night	29% 6% 11% 27% 23% 4	2292
	21st to end	Night	29% 5% 8% 25% 28% 4	2310
October	1st to 10th	Night	18% 3 5% 21% 44% 9%	4029
	11th to 20th	Night	22% 3 6% 25% 30% 14%	3936
	21st to end	Night	33% 4 7% 29% 25% 3	4252
November	1st to 10th	Night	28% 3 11% 29% 24% 6%	2460
	11th to 20th	Night	31% 4 10% 27% 24% 5%	2784
	21st to end	Night	37% 3 9% 29% 19% 3	2940
December	1st to 10th	Night	29% 5% 14% 34% 18% 1	4720
	11th to 20th	Night	38% 4 11% 26% 18% 2	5341
	21st to end	Night	57% 5% 9% 20% 7% 1	6891

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	19% 15% 40% 21% 4	330
	11th to 20th	Day	20% 19% 41% 17% 2	330
	21st to end	Day	14% 13% 48% 22% 2	363
February	1st to 10th	Day	22% 16% 41% 18% 3	330
	11th to 20th	Day	19% 19% 41% 17% 4 1	330
	21st to end	Day	15% 22% 46% 14% 3	273
March	1st to 10th	Day	20% 17% 44% 18% 2	330
	11th to 20th	Day	26% 13% 35% 20% 5%	330
	21st to end	Day	25% 22% 33% 17% 3	363
April	1st to 10th	Day	23% 18% 35% 18% 5% 1	441
	11th to 20th	Day	21% 16% 39% 18% 5%	532
	21st to end	Day	21% 15% 38% 19% 6% 1	685
May	1st to 10th	Day	18% 12% 38% 23% 8% 1	753
	11th to 20th	Day	19% 11% 29% 26% 12% 3	775
	21st to end	Day	18% 7% 30% 26% 13% 5%	844
June	1st to 10th	Day	8% 5% 22% 32% 25% 7%	780
	11th to 20th	Day	9% 6% 27% 34% 20% 5%	780
	21st to end	Day	7% 5% 35% 34% 17% 2	780
July	1st to 10th	Day	10% 4 15% 30% 30% 12%	771
	11th to 20th	Day	5% 2 16% 39% 30% 8%	768
	21st to end	Day	9% 3 16% 36% 29% 7%	849
August	1st to 10th	Day	7% 4 16% 39% 29% 5%	776
	11th to 20th	Day	7% 2 19% 38% 28% 6%	765
	21st to end	Day	8% 3 14% 32% 33% 9%	687
September	1st to 10th	Day	14% 2 14% 38% 26% 6%	570
	11th to 20th	Day	5% 3 20% 39% 26% 7%	374
	21st to end	Day	11% 5% 24% 37% 18% 6%	360
October	1st to 10th	Day	12% 4 29% 29% 18% 8%	360
	11th to 20th	Day	11% 14% 41% 25% 9% 1	339
	21st to end	Day	23% 13% 37% 22% 5% 1	363
November	1st to 10th	Day	16% 15% 36% 25% 6% 2	330
	11th to 20th	Day	18% 15% 35% 23% 8% 2	330
	21st to end	Day	21% 15% 35% 22% 7%	330
December	1st to 10th	Day	10% 16% 42% 27% 3 2	330
	11th to 20th	Day	18% 16% 38% 19% 8% 1	330
	21st to end	Day	23% 21% 37% 18% 2	363
January	1st to 10th	Night	13% 13% 42% 27% 5%	1050
	11th to 20th	Night	14% 14% 42% 25% 4	1050
	21st to end	Night	14% 13% 42% 27% 3	1155
February	1st to 10th	Night	17% 11% 43% 24% 4	1050
	11th to 20th	Night	18% 14% 41% 25% 2	1029
	21st to end	Night	17% 14% 46% 20% 3	644
March	1st to 10th	Night	16% 13% 43% 23% 4	630
	11th to 20th	Night	19% 12% 36% 26% 6% 1	630
	21st to end	Night	24% 11% 38% 23% 3	693
April	1st to 10th	Night	17% 12% 40% 26% 5%	630
	11th to 20th	Night	18% 14% 36% 27% 5%	623
	21st to end	Night	20% 11% 38% 25% 6%	630
May	1st to 10th	Night	17% 8% 37% 29% 9%	630
	11th to 20th	Night	16% 6% 29% 34% 14% 1	621
	21st to end	Night	13% 7% 27% 36% 16% 2	683
June	1st to 10th	Night	7% 4 18% 44% 23% 2	630
	11th to 20th	Night	6% 4 25% 45% 19% 1	630
	21st to end	Night	6% 3 33% 45% 13% 1	630
July	1st to 10th	Night	8% 2 12% 41% 33% 3	615
	11th to 20th	Night	2 2 15% 47% 30% 3	612
	21st to end	Night	8% 1 13% 43% 32% 2	690
August	1st to 10th	Night	5% 1 16% 45% 31% 2	627
	11th to 20th	Night	7% 3 12% 44% 31% 3	624
	21st to end	Night	6% 2 11% 43% 35% 3	693
September	1st to 10th	Night	11% 3 18% 42% 23% 3	630
	11th to 20th	Night	9% 2 19% 40% 26% 3	630
	21st to end	Night	8% 4 22% 43% 20% 4	630
October	1st to 10th	Night	10% 5% 27% 37% 19% 2	829
	11th to 20th	Night	13% 9% 39% 31% 8%	813
	21st to end	Night	19% 12% 34% 26% 8%	880
November	1st to 10th	Night	11% 9% 37% 32% 10% 1	820
	11th to 20th	Night	12% 12% 39% 28% 8%	1043
	21st to end	Night	19% 12% 36% 27% 5%	1050
December	1st to 10th	Night	16% 11% 40% 27% 5% 1	1050
	11th to 20th	Night	18% 11% 41% 26% 4	1050
	21st to end	Night	18% 14% 37% 26% 3	1155

Page generated at 2026-10-11 04:06:28. Version ac07

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