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 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	11% 7% 3 6% 4 4 4 3 5% 7% 46%	119433
	Night	11% 7% 3 6% 4 4 4 3 5% 6% 48%	119023
Transparency	Day	37% 5% 12% 25% 18% 3	123087
	Night	35% 5% 11% 25% 20% 4	123512
Seeing	Day	14% 9% 29% 28% 16% 4	16423
	Night	14% 9% 33% 32% 12% 1	26345

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% 10% 3 9% 4 4 3 3 4 6% 37%	4077
	11th to 20th	Day	7% 9% 4 9% 7% 6% 6% 3 4 5% 41%	4080
	21st to end	Day	13% 13% 5% 8% 3 4 5% 3 4 5% 38%	4488
February	1st to 10th	Day	15% 11% 4 7% 5% 5% 3 3 4 6% 38%	1800
	11th to 20th	Day	12% 10% 3 7% 5% 4 4 3 5% 5% 41%	1754
	21st to end	Day	11% 9% 3 6% 4 4 4 3 5% 6% 45%	1710
March	1st to 10th	Day	12% 8% 2 6% 3 3 3 3 3 5% 51%	4936
	11th to 20th	Day	9% 6% 2 5% 3 3 4 3 4 6% 56%	4980
	21st to end	Day	12% 10% 4 8% 5% 4 5% 4 5% 8% 35%	5462
April	1st to 10th	Day	13% 8% 3 6% 4 3 4 3 5% 6% 45%	2306
	11th to 20th	Day	15% 8% 3 6% 4 4 5% 3 5% 6% 41%	2374
	21st to end	Day	19% 8% 2 8% 5% 4 5% 3 4 6% 36%	2538
May	1st to 10th	Day	14% 8% 2 6% 4 3 4 3 5% 6% 43%	5862
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 45%	5862
	21st to end	Day	18% 6% 2 6% 3 4 4 4 5% 7% 41%	6520
June	1st to 10th	Day	7% 4 1 5% 4 4 5% 5% 7% 10% 48%	2787
	11th to 20th	Day	7% 5% 2 5% 5% 4 4 4 7% 10% 46%	2660
	21st to end	Day	7% 3 2 5% 4 4 5% 4 6% 8% 52%	2624
July	1st to 10th	Day	10% 7% 3 7% 4 4 5% 4 6% 9% 41%	4120
	11th to 20th	Day	5% 3 2 5% 3 4 4 4 7% 10% 53%	4120
	21st to end	Day	6% 4 1 5% 4 6% 6% 5% 8% 11% 44%	4465
August	1st to 10th	Day	5% 4 2 5% 4 4 4 4 6% 9% 53%	4104
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 3 5% 8% 47%	4041
	21st to end	Day	5% 4 2 5% 3 3 4 3 6% 8% 56%	4260
September	1st to 10th	Day	12% 4 2 5% 3 3 4 3 5% 7% 53%	2280
	11th to 20th	Day	8% 5% 2 5% 5% 4 5% 4 5% 7% 50%	2103
	21st to end	Day	9% 4 2 6% 3 3 4 3 6% 6% 54%	2080
October	1st to 10th	Day	11% 6% 2 4 3 3 4 2 5% 7% 53%	1900
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 4 5% 57%	1900
	21st to end	Day	15% 6% 3 5% 4 3 3 2 4 6% 48%	2090
November	1st to 10th	Day	10% 6% 3 5% 4 3 3 3 4 6% 54%	1958
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 4 7% 51%	1744
	21st to end	Day	14% 7% 2 6% 4 3 3 3 4 5% 48%	1600
December	1st to 10th	Day	6% 6% 2 5% 5% 4 3 3 5% 7% 55%	2610
	11th to 20th	Day	10% 7% 3 7% 4 3 3 3 4 6% 49%	3008
	21st to end	Day	18% 17% 8% 11% 4 3 3 3 4 4 25%	4230
January	1st to 10th	Night	13% 9% 3 14% 5% 5% 4 3 4 5% 36%	5888
	11th to 20th	Night	9% 8% 3 9% 6% 6% 5% 3 4 5% 43%	5900
	21st to end	Night	14% 9% 4 9% 4 3 3 3 4 5% 41%	6490
February	1st to 10th	Night	12% 10% 3 7% 6% 4 4 4 5% 7% 39%	2600
	11th to 20th	Night	12% 10% 3 8% 5% 4 4 3 5% 6% 42%	2533
	21st to end	Night	12% 7% 3 7% 4 4 3 3 5% 5% 47%	1935
March	1st to 10th	Night	12% 9% 3 5% 3 3 3 3 3 5% 51%	4919
	11th to 20th	Night	7% 7% 2 4 3 3 3 3 5% 7% 56%	4980
	21st to end	Night	13% 8% 3 7% 5% 4 5% 3 5% 8% 39%	5467
April	1st to 10th	Night	10% 7% 3 6% 3 3 4 3 6% 7% 49%	2092
	11th to 20th	Night	13% 7% 3 6% 3 3 3 2 5% 7% 47%	1975
	21st to end	Night	16% 8% 2 7% 4 3 4 4 5% 5% 43%	1858
May	1st to 10th	Night	15% 7% 2 6% 4 3 3 4 5% 7% 45%	4053
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 4 6% 50%	4053
	21st to end	Night	17% 5% 2 4 3 3 3 3 4 6% 50%	4352
June	1st to 10th	Night	6% 3 1 4 3 5% 5% 5% 6% 9% 54%	1597
	11th to 20th	Night	5% 3 2 4 4 4 4 4 7% 11% 52%	1520
	21st to end	Night	5% 2 1 4 3 3 4 3 6% 10% 60%	1548
July	1st to 10th	Night	10% 6% 3 6% 4 3 4 3 6% 7% 47%	2850
	11th to 20th	Night	2 2 1 4 3 4 4 4 6% 10% 59%	2850
	21st to end	Night	5% 3 2 4 3 4 6% 5% 8% 11% 48%	3098
August	1st to 10th	Night	2 2 1 3 3 4 5% 4 6% 10% 61%	2836
	11th to 20th	Night	10% 7% 2 5% 3 3 4 3 4 7% 52%	2798
	21st to end	Night	3 3 2 4 4 4 4 3 5% 8% 60%	3396
September	1st to 10th	Night	9% 5% 3 5% 3 3 3 2 5% 5% 58%	1900
	11th to 20th	Night	9% 4 2 4 3 3 4 4 6% 7% 54%	2074
	21st to end	Night	6% 5% 2 5% 3 4 3 4 5% 5% 58%	2090
October	1st to 10th	Night	10% 4 2 4 3 3 3 3 4 5% 59%	2270
	11th to 20th	Night	12% 4 2 5% 3 3 3 3 4 5% 58%	2280
	21st to end	Night	15% 7% 2 5% 3 3 3 2 3 5% 51%	2508
November	1st to 10th	Night	9% 6% 2 4 2 3 2 2 4 5% 61%	2340
	11th to 20th	Night	10% 7% 2 4 3 3 3 3 4 7% 54%	2651
	21st to end	Night	14% 7% 2 4 3 3 3 3 3 5% 53%	2800
December	1st to 10th	Night	7% 5% 2 4 4 3 4 3 5% 5% 57%	4580
	11th to 20th	Night	11% 8% 3 7% 4 4 4 4 5% 6% 45%	5199
	21st to end	Night	18% 16% 6% 9% 5% 4 3 3 3 4 30%	6743

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	48% 5% 9% 25% 12% 1	4078
	11th to 20th	Day	46% 4 9% 22% 15% 4	4080
	21st to end	Day	47% 4 10% 16% 18% 4	4488
February	1st to 10th	Day	48% 4 12% 23% 11% 3	1800
	11th to 20th	Day	45% 5% 12% 21% 15% 3	1763
	21st to end	Day	40% 4 11% 24% 17% 4	1710
March	1st to 10th	Day	37% 3 8% 21% 22% 8%	4936
	11th to 20th	Day	30% 5% 9% 21% 27% 8%	4980
	21st to end	Day	46% 7% 16% 25% 6% 1	5464
April	1st to 10th	Day	40% 4 12% 24% 18% 2	2306
	11th to 20th	Day	44% 5% 11% 23% 16% 1	2374
	21st to end	Day	50% 4 11% 23% 11% 1	2538
May	1st to 10th	Day	42% 6% 10% 24% 16% 1	5900
	11th to 20th	Day	37% 7% 11% 24% 19% 3	5866
	21st to end	Day	42% 6% 11% 27% 13% 1	6534
June	1st to 10th	Day	29% 9% 18% 27% 15% 2	2787
	11th to 20th	Day	31% 8% 15% 28% 16% 2	2660
	21st to end	Day	27% 8% 14% 29% 17% 4	2624
July	1st to 10th	Day	38% 8% 13% 21% 17% 3	4056
	11th to 20th	Day	24% 7% 14% 35% 17% 2	4051
	21st to end	Day	28% 10% 18% 30% 12% 1	4325
August	1st to 10th	Day	26% 7% 14% 29% 22% 2	4090
	11th to 20th	Day	35% 7% 18% 25% 15% 1	4080
	21st to end	Day	24% 7% 15% 35% 15% 4	4259
September	1st to 10th	Day	32% 4 12% 26% 20% 6%	2280
	11th to 20th	Day	32% 7% 10% 25% 23% 2	2103
	21st to end	Day	30% 4 10% 27% 26% 3	2080
October	1st to 10th	Day	19% 3 7% 23% 41% 8%	3160
	11th to 20th	Day	19% 2 5% 26% 39% 9%	3140
	21st to end	Day	32% 4 10% 24% 26% 4	3428
November	1st to 10th	Day	32% 5% 11% 29% 19% 5%	1958
	11th to 20th	Day	35% 3 10% 26% 22% 4	1744
	21st to end	Day	39% 4 8% 28% 20% 1	1600
December	1st to 10th	Day	29% 4 12% 36% 17% 1	2610
	11th to 20th	Day	37% 4 11% 32% 15% 1	3005
	21st to end	Day	62% 4 9% 17% 7% 1	4230
January	1st to 10th	Night	50% 4 12% 21% 11% 1	5888
	11th to 20th	Night	44% 4 9% 22% 16% 4	5900
	21st to end	Night	45% 3 10% 19% 17% 6%	6490
February	1st to 10th	Night	45% 4 11% 22% 14% 3	2600
	11th to 20th	Night	43% 4 11% 23% 17% 3	2534
	21st to end	Night	39% 4 8% 23% 20% 7%	1935
March	1st to 10th	Night	36% 4 8% 22% 25% 5%	4919
	11th to 20th	Night	26% 4 8% 22% 32% 8%	4980
	21st to end	Night	42% 7% 16% 23% 10% 2	5467
April	1st to 10th	Night	35% 5% 11% 23% 23% 4	2092
	11th to 20th	Night	39% 3 8% 28% 18% 4	1975
	21st to end	Night	43% 5% 12% 26% 14% 1	1858
May	1st to 10th	Night	40% 6% 13% 24% 16% 1	4080
	11th to 20th	Night	35% 5% 12% 24% 18% 6%	4054
	21st to end	Night	36% 6% 9% 28% 21% 1	4360
June	1st to 10th	Night	24% 8% 17% 31% 16% 3	1597
	11th to 20th	Night	25% 7% 14% 32% 19% 2	1520
	21st to end	Night	20% 5% 13% 34% 23% 4	1548
July	1st to 10th	Night	35% 6% 13% 24% 19% 4	2805
	11th to 20th	Night	19% 6% 15% 37% 20% 3	2796
	21st to end	Night	27% 9% 20% 30% 14% 1	2999
August	1st to 10th	Night	18% 7% 19% 28% 22% 6%	2826
	11th to 20th	Night	33% 6% 12% 28% 20% 2	2825
	21st to end	Night	23% 6% 12% 38% 17% 4	3396
September	1st to 10th	Night	29% 4 10% 28% 21% 9%	1900
	11th to 20th	Night	29% 6% 10% 26% 25% 4	2074
	21st to end	Night	27% 5% 7% 27% 30% 5%	2090
October	1st to 10th	Night	17% 3 6% 20% 45% 9%	3790
	11th to 20th	Night	22% 3 6% 25% 31% 14%	3779
	21st to end	Night	32% 4 7% 29% 25% 3	4120
November	1st to 10th	Night	28% 3 11% 28% 24% 6%	2340
	11th to 20th	Night	31% 4 9% 26% 25% 5%	2651
	21st to end	Night	36% 3 9% 29% 20% 3	2800
December	1st to 10th	Night	28% 5% 13% 34% 18% 1	4580
	11th to 20th	Night	38% 4 11% 26% 17% 2	5201
	21st to end	Night	58% 5% 9% 20% 7% 1	6743

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% 16% 39% 21% 4	300
	11th to 20th	Day	22% 18% 40% 18% 3	300
	21st to end	Day	15% 11% 48% 23% 3	330
February	1st to 10th	Day	24% 15% 40% 18% 3	300
	11th to 20th	Day	20% 19% 41% 16% 4 1	300
	21st to end	Day	14% 23% 47% 13% 3	249
March	1st to 10th	Day	18% 16% 44% 19% 3	300
	11th to 20th	Day	24% 14% 35% 21% 5%	300
	21st to end	Day	27% 21% 32% 17% 3	330
April	1st to 10th	Day	23% 17% 36% 18% 6% 1	406
	11th to 20th	Day	22% 14% 39% 19% 5%	492
	21st to end	Day	21% 14% 37% 20% 7% 1	638
May	1st to 10th	Day	18% 11% 38% 23% 8% 1	703
	11th to 20th	Day	20% 10% 28% 26% 13% 3	725
	21st to end	Day	17% 7% 30% 26% 14% 5%	789
June	1st to 10th	Day	8% 5% 21% 32% 26% 7%	726
	11th to 20th	Day	9% 4 26% 35% 21% 5%	680
	21st to end	Day	6% 5% 34% 35% 18% 2	680
July	1st to 10th	Day	12% 4 14% 29% 30% 12%	671
	11th to 20th	Day	5% 2 16% 39% 30% 8%	668
	21st to end	Day	9% 2 15% 36% 31% 8%	739
August	1st to 10th	Day	8% 3 13% 40% 31% 6%	676
	11th to 20th	Day	7% 3 18% 37% 29% 6%	665
	21st to end	Day	8% 2 13% 29% 36% 10%	593
September	1st to 10th	Day	15% 1 14% 35% 27% 8%	490
	11th to 20th	Day	5% 3 19% 39% 26% 7%	313
	21st to end	Day	10% 5% 24% 37% 18% 5%	300
October	1st to 10th	Day	12% 5% 31% 26% 18% 8%	300
	11th to 20th	Day	11% 14% 41% 25% 8% 1	300
	21st to end	Day	22% 13% 37% 23% 5% 1	330
November	1st to 10th	Day	16% 14% 35% 26% 6% 3	300
	11th to 20th	Day	20% 15% 32% 23% 8% 2	300
	21st to end	Day	20% 14% 36% 23% 7%	300
December	1st to 10th	Day	10% 15% 41% 28% 3 3	300
	11th to 20th	Day	19% 17% 37% 19% 8% 1	300
	21st to end	Day	24% 22% 35% 18% 1	330
January	1st to 10th	Night	13% 12% 41% 27% 6%	1000
	11th to 20th	Night	14% 14% 41% 26% 5%	1000
	21st to end	Night	14% 12% 42% 28% 3	1100
February	1st to 10th	Night	17% 12% 43% 24% 4	1000
	11th to 20th	Night	18% 14% 40% 25% 2	979
	21st to end	Night	17% 13% 46% 20% 3	615
March	1st to 10th	Night	15% 12% 44% 24% 5%	600
	11th to 20th	Night	20% 12% 36% 27% 6% 1	600
	21st to end	Night	25% 11% 37% 24% 4	660
April	1st to 10th	Night	16% 12% 41% 27% 5%	600
	11th to 20th	Night	17% 13% 36% 28% 6%	593
	21st to end	Night	20% 11% 37% 26% 6%	600
May	1st to 10th	Night	17% 7% 36% 30% 10%	600
	11th to 20th	Night	16% 6% 29% 34% 14% 1	591
	21st to end	Night	12% 7% 28% 35% 16% 2	650
June	1st to 10th	Night	6% 4 18% 45% 25% 3	599
	11th to 20th	Night	5% 4 25% 45% 20% 1	570
	21st to end	Night	5% 3 32% 46% 14% 1	570
July	1st to 10th	Night	9% 2 12% 40% 34% 3	555
	11th to 20th	Night	2 2 15% 48% 30% 3	552
	21st to end	Night	8% 1 13% 42% 33% 3	624
August	1st to 10th	Night	4 1 14% 45% 34% 1	567
	11th to 20th	Night	7% 3 12% 45% 31% 2	564
	21st to end	Night	6% 2 12% 40% 37% 3	627
September	1st to 10th	Night	11% 3 19% 40% 23% 4	570
	11th to 20th	Night	10% 2 19% 39% 26% 4	570
	21st to end	Night	8% 4 22% 43% 19% 4	570
October	1st to 10th	Night	11% 5% 27% 36% 19% 3	750
	11th to 20th	Night	13% 9% 39% 31% 8%	760
	21st to end	Night	19% 12% 33% 27% 9%	836
November	1st to 10th	Night	11% 9% 36% 32% 11% 1	780
	11th to 20th	Night	13% 13% 38% 28% 8%	993
	21st to end	Night	18% 12% 37% 27% 6%	1000
December	1st to 10th	Night	16% 11% 40% 27% 5% 1	1000
	11th to 20th	Night	18% 10% 41% 26% 5%	1000
	21st to end	Night	19% 15% 37% 26% 3	1100

Page generated at 2025-06-07 09:28:20. Version ac07

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