Rattan Creek
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rattan Creek clear sky chart from 2005-11-06 to 2026-05-17.

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	12% 6% 2 5% 4 4 4 4 5% 7% 47%	132281
	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	130886
Transparency	Day	35% 5% 10% 21% 20% 10%	136037
	Night	34% 4 10% 20% 20% 12%	135194
Seeing	Day	14% 7% 21% 29% 23% 7%	16841
	Night	16% 7% 24% 36% 16% 1	26837

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% 8% 3 6% 3 3 3 3 4 5% 50%	5289
	11th to 20th	Day	13% 5% 2 4 3 3 3 2 5% 5% 55%	5310
	21st to end	Day	25% 7% 2 6% 4 3 3 2 3 5% 41%	6088
February	1st to 10th	Day	19% 7% 3 5% 3 4 3 2 4 5% 46%	2100
	11th to 20th	Day	17% 8% 2 5% 4 3 3 3 4 6% 45%	2049
	21st to end	Day	18% 7% 2 4 2 2 3 3 4 4 51%	1730
March	1st to 10th	Day	13% 6% 2 6% 3 3 3 2 5% 6% 50%	5040
	11th to 20th	Day	13% 8% 2 6% 3 3 3 3 4 6% 50%	5090
	21st to end	Day	14% 8% 2 6% 4 3 4 3 5% 6% 45%	5583
April	1st to 10th	Day	16% 8% 3 6% 4 4 4 4 5% 6% 41%	2310
	11th to 20th	Day	16% 8% 3 7% 5% 4 4 4 4 6% 41%	2286
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 5% 43%	2679
May	1st to 10th	Day	21% 8% 3 6% 4 4 4 4 6% 7% 33%	5992
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 6% 8% 44%	5941
	21st to end	Day	13% 6% 3 7% 5% 6% 6% 5% 8% 9% 33%	6434
June	1st to 10th	Day	6% 4 2 5% 5% 5% 5% 5% 6% 8% 50%	2600
	11th to 20th	Day	7% 4 1 5% 4 4 5% 5% 7% 9% 48%	2600
	21st to end	Day	7% 6% 2 5% 5% 5% 6% 5% 9% 10% 41%	2600
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 39%	5900
	11th to 20th	Day	5% 3 2 5% 4 5% 5% 5% 8% 12% 46%	5900
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 10% 43%	6422
August	1st to 10th	Day	1 2 1 2 2 2 3 3 5% 7% 72%	4234
	11th to 20th	Day	3 2 1 3 3 3 4 5% 6% 11% 59%	3947
	21st to end	Day	4 2 1 5% 4 4 6% 5% 8% 11% 49%	4092
September	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 3 6% 7% 46%	2200
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 6% 8% 44%	2200
	21st to end	Day	7% 4 2 5% 3 5% 5% 5% 5% 7% 53%	2200
October	1st to 10th	Day	6% 4 1 4 3 3 3 3 5% 7% 61%	2200
	11th to 20th	Day	11% 5% 2 5% 3 3 4 2 4 6% 56%	2035
	21st to end	Day	12% 4 2 4 4 3 3 2 3 5% 59%	2200
November	1st to 10th	Day	14% 7% 2 5% 3 3 4 2 5% 6% 49%	2058
	11th to 20th	Day	10% 6% 2 6% 4 3 3 3 4 6% 53%	1984
	21st to end	Day	17% 7% 2 5% 4 3 3 3 4 5% 46%	1890
December	1st to 10th	Day	21% 8% 2 6% 4 3 3 2 4 4 42%	3030
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 5% 5% 44%	3481
	21st to end	Day	15% 5% 2 4 2 2 2 2 3 4 57%	4587
January	1st to 10th	Night	15% 7% 2 4 3 2 3 2 3 5% 54%	7634
	11th to 20th	Night	10% 3 1 3 2 2 2 3 4 5% 64%	7680
	21st to end	Night	27% 6% 3 6% 3 3 2 2 4 5% 39%	8181
February	1st to 10th	Night	19% 7% 2 4 3 2 3 2 3 5% 50%	2520
	11th to 20th	Night	20% 7% 2 5% 3 2 3 2 3 5% 48%	2458
	21st to end	Night	20% 6% 2 4 3 3 3 3 4 5% 48%	2076
March	1st to 10th	Night	17% 6% 1 5% 3 4 4 3 5% 6% 45%	5034
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 4 5% 49%	5090
	21st to end	Night	19% 9% 2 5% 4 3 3 3 3 6% 43%	5588
April	1st to 10th	Night	15% 6% 2 5% 3 4 4 3 5% 6% 45%	2310
	11th to 20th	Night	19% 8% 2 5% 4 3 4 3 4 6% 43%	2283
	21st to end	Night	17% 8% 3 4 3 2 4 3 4 6% 46%	1931
May	1st to 10th	Night	24% 8% 2 6% 3 3 3 3 4 7% 35%	4143
	11th to 20th	Night	9% 5% 2 6% 5% 4 4 4 5% 7% 48%	4107
	21st to end	Night	13% 8% 2 7% 4 5% 5% 4 6% 8% 38%	4443
June	1st to 10th	Night	5% 4 1 3 3 3 3 4 5% 11% 59%	1800
	11th to 20th	Night	6% 3 1 3 3 3 5% 3 6% 10% 56%	1800
	21st to end	Night	7% 4 1 4 4 4 4 4 7% 10% 52%	1800
July	1st to 10th	Night	9% 6% 2 6% 4 4 4 5% 7% 10% 42%	4078
	11th to 20th	Night	4 3 1 4 3 4 4 3 7% 12% 54%	4077
	21st to end	Night	7% 5% 2 5% 4 4 5% 5% 7% 9% 47%	4449
August	1st to 10th	Night	1 1 1 1 2 2 2 5% 8% 76%	2926
	11th to 20th	Night	4 2 1 3 2 3 3 4 7% 11% 61%	3107
	21st to end	Night	4 3 1 4 3 3 4 4 7% 11% 56%	3807
September	1st to 10th	Night	8% 6% 2 5% 4 4 5% 3 5% 8% 51%	2200
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 3 7% 7% 47%	2200
	21st to end	Night	8% 4 1 3 4 3 5% 3 5% 8% 55%	2200
October	1st to 10th	Night	6% 3 1 4 3 3 4 3 4 6% 64%	2200
	11th to 20th	Night	11% 4 1 4 3 3 3 2 5% 5% 58%	2345
	21st to end	Night	14% 5% 2 3 2 3 3 2 3 4 60%	2640
November	1st to 10th	Night	14% 6% 2 5% 3 2 3 3 3 6% 54%	2460
	11th to 20th	Night	11% 5% 1 4 3 3 2 2 5% 6% 57%	2630
	21st to end	Night	18% 5% 2 5% 3 3 4 3 4 5% 48%	2730
December	1st to 10th	Night	20% 8% 2 6% 4 3 3 3 4 5% 42%	4380
	11th to 20th	Night	20% 10% 2 5% 2 2 2 2 3 4 47%	4946
	21st to end	Night	18% 6% 2 4 2 2 2 2 3 4 56%	6633

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	37% 3 5% 24% 23% 8%	5290
	11th to 20th	Day	32% 2 5% 16% 27% 19%	5310
	21st to end	Day	48% 3 4 11% 23% 11%	6088
February	1st to 10th	Day	42% 4 5% 13% 22% 15%	2100
	11th to 20th	Day	42% 3 8% 18% 19% 11%	2059
	21st to end	Day	39% 3 4 19% 20% 15%	1730
March	1st to 10th	Day	36% 3 6% 18% 18% 20%	5040
	11th to 20th	Day	37% 3 6% 9% 16% 28%	5090
	21st to end	Day	40% 4 8% 19% 21% 7%	5585
April	1st to 10th	Day	44% 5% 6% 17% 15% 13%	2310
	11th to 20th	Day	45% 4 7% 15% 18% 11%	2286
	21st to end	Day	42% 4 7% 19% 19% 8%	2679
May	1st to 10th	Day	48% 6% 9% 19% 14% 4	6030
	11th to 20th	Day	37% 6% 11% 17% 23% 6%	5945
	21st to end	Day	43% 7% 11% 24% 13% 3	6447
June	1st to 10th	Day	30% 6% 13% 28% 19% 4	2600
	11th to 20th	Day	30% 7% 13% 26% 19% 5%	2600
	21st to end	Day	35% 8% 13% 24% 18% 2	2600
July	1st to 10th	Day	40% 8% 18% 29% 5% 1	5836
	11th to 20th	Day	28% 7% 18% 31% 15% 1	5831
	21st to end	Day	33% 7% 17% 26% 14% 3	6240
August	1st to 10th	Day	12% 4 11% 40% 31% 1	4220
	11th to 20th	Day	19% 5% 22% 39% 13% 2	3986
	21st to end	Day	25% 10% 19% 26% 18% 2	4091
September	1st to 10th	Day	36% 7% 9% 22% 16% 10%	2200
	11th to 20th	Day	36% 8% 12% 21% 18% 5%	2200
	21st to end	Day	29% 6% 10% 22% 19% 15%	2200
October	1st to 10th	Day	16% 2 7% 13% 25% 37%	3590
	11th to 20th	Day	21% 3 7% 19% 30% 20%	3288
	21st to end	Day	26% 2 5% 20% 32% 14%	3538
November	1st to 10th	Day	36% 5% 8% 18% 22% 11%	2058
	11th to 20th	Day	35% 3 7% 19% 21% 15%	1984
	21st to end	Day	42% 3 6% 17% 20% 13%	1890
December	1st to 10th	Day	47% 2 6% 16% 19% 9%	3030
	11th to 20th	Day	42% 4 7% 11% 23% 13%	3479
	21st to end	Day	33% 2 5% 17% 29% 15%	4587
January	1st to 10th	Night	35% 3 5% 19% 24% 13%	7635
	11th to 20th	Night	24% 2 4 18% 31% 21%	7680
	21st to end	Night	49% 2 3 11% 22% 13%	8181
February	1st to 10th	Night	39% 3 6% 13% 21% 18%	2520
	11th to 20th	Night	40% 3 7% 17% 19% 14%	2458
	21st to end	Night	40% 3 5% 15% 20% 16%	2076
March	1st to 10th	Night	39% 5% 6% 16% 20% 15%	5034
	11th to 20th	Night	38% 4 7% 10% 15% 25%	5090
	21st to end	Night	44% 4 7% 17% 19% 9%	5588
April	1st to 10th	Night	38% 5% 8% 18% 16% 14%	2310
	11th to 20th	Night	42% 5% 9% 14% 16% 14%	2283
	21st to end	Night	41% 4 10% 18% 18% 9%	1931
May	1st to 10th	Night	49% 6% 8% 19% 14% 5%	4170
	11th to 20th	Night	34% 6% 10% 21% 19% 9%	4108
	21st to end	Night	42% 6% 13% 25% 10% 3	4452
June	1st to 10th	Night	21% 6% 17% 30% 23% 3	1800
	11th to 20th	Night	24% 6% 15% 30% 20% 5%	1800
	21st to end	Night	27% 7% 18% 28% 17% 3	1800
July	1st to 10th	Night	36% 9% 24% 26% 5%	4035
	11th to 20th	Night	23% 7% 23% 34% 12% 1	4026
	21st to end	Night	31% 8% 20% 25% 12% 4	4320
August	1st to 10th	Night	8% 4 14% 44% 29% 1	2916
	11th to 20th	Night	18% 5% 30% 33% 12% 2	3134
	21st to end	Night	22% 11% 21% 28% 16% 2	3807
September	1st to 10th	Night	30% 7% 14% 24% 16% 8%	2200
	11th to 20th	Night	35% 7% 14% 24% 15% 6%	2200
	21st to end	Night	27% 5% 14% 24% 18% 13%	2200
October	1st to 10th	Night	16% 3 6% 16% 20% 40%	3590
	11th to 20th	Night	19% 3 8% 17% 32% 21%	3818
	21st to end	Night	25% 2 5% 20% 31% 17%	4252
November	1st to 10th	Night	34% 4 8% 18% 23% 14%	2460
	11th to 20th	Night	30% 4 7% 21% 21% 16%	2630
	21st to end	Night	40% 3 7% 17% 18% 15%	2730
December	1st to 10th	Night	46% 3 6% 16% 19% 10%	4380
	11th to 20th	Night	43% 3 6% 9% 23% 17%	4947
	21st to end	Night	35% 3 3 15% 28% 16%	6633

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% 14% 36% 23% 7% 1	330
	11th to 20th	Day	23% 13% 31% 25% 6% 1	330
	21st to end	Day	23% 14% 34% 21% 7% 1	363
February	1st to 10th	Day	24% 15% 34% 19% 8%	330
	11th to 20th	Day	24% 14% 37% 20% 4 1	330
	21st to end	Day	28% 11% 41% 18% 3	273
March	1st to 10th	Day	21% 15% 38% 21% 5%	330
	11th to 20th	Day	18% 17% 39% 20% 5% 1	330
	21st to end	Day	20% 10% 38% 23% 7% 2	363
April	1st to 10th	Day	23% 15% 37% 19% 5% 1	330
	11th to 20th	Day	20% 17% 32% 23% 8%	324
	21st to end	Day	22% 12% 35% 22% 8% 1	699
May	1st to 10th	Day	19% 9% 36% 28% 8%	753
	11th to 20th	Day	19% 7% 25% 33% 13% 3	756
	21st to end	Day	17% 5% 24% 34% 16% 3	789
June	1st to 10th	Day	7% 3 14% 41% 28% 7%	730
	11th to 20th	Day	9% 2 16% 45% 23% 5%	730
	21st to end	Day	7% 2 11% 34% 35% 11%	730
July	1st to 10th	Day	9% 4 7% 34% 36% 10%	721
	11th to 20th	Day	5% 1 6% 33% 47% 9%	718
	21st to end	Day	5% 1 4 32% 47% 11%	794
August	1st to 10th	Day	2 2 2 29% 49% 15%	726
	11th to 20th	Day	5% 1 3 31% 46% 14%	582
	21st to end	Day	6% 1 5% 23% 37% 28%	454
September	1st to 10th	Day	9% 2 5% 22% 41% 21%	330
	11th to 20th	Day	8% 2 4 28% 41% 19%	330
	21st to end	Day	9% 2 6% 30% 33% 20%	330
October	1st to 10th	Day	4 3 10% 31% 39% 12%	330
	11th to 20th	Day	10% 3 24% 35% 22% 7%	330
	21st to end	Day	18% 10% 22% 27% 19% 4	363
November	1st to 10th	Day	18% 8% 20% 35% 15% 5%	330
	11th to 20th	Day	18% 7% 28% 29% 16% 3	330
	21st to end	Day	20% 15% 30% 22% 11% 2	330
December	1st to 10th	Day	18% 8% 34% 27% 10% 3	330
	11th to 20th	Day	21% 12% 32% 23% 11% 1	330
	21st to end	Day	19% 11% 32% 27% 9% 2	363
January	1st to 10th	Night	21% 9% 30% 32% 8% 1	1050
	11th to 20th	Night	20% 11% 30% 32% 7%	1050
	21st to end	Night	23% 11% 30% 30% 6%	1076
February	1st to 10th	Night	24% 12% 29% 29% 5%	840
	11th to 20th	Night	25% 12% 35% 25% 3	824
	21st to end	Night	26% 12% 39% 21% 2	692
March	1st to 10th	Night	25% 11% 35% 26% 3	635
	11th to 20th	Night	21% 11% 32% 31% 4	630
	21st to end	Night	17% 11% 41% 27% 3	693
April	1st to 10th	Night	17% 10% 38% 31% 2	630
	11th to 20th	Night	23% 10% 37% 25% 4	623
	21st to end	Night	21% 9% 39% 24% 7%	630
May	1st to 10th	Night	18% 9% 37% 30% 6%	630
	11th to 20th	Night	14% 6% 32% 35% 12%	609
	21st to end	Night	14% 6% 31% 36% 13%	650
June	1st to 10th	Night	7% 3 19% 47% 22% 2	600
	11th to 20th	Night	6% 2 28% 48% 15% 1	600
	21st to end	Night	7% 2 14% 47% 27% 3	600
July	1st to 10th	Night	8% 2 10% 46% 29% 4	585
	11th to 20th	Night	5% 1 8% 54% 31% 2	582
	21st to end	Night	6% 1 8% 47% 35% 3	657
August	1st to 10th	Night	1 6% 54% 35% 4	597
	11th to 20th	Night	6% 1 6% 51% 32% 5%	594
	21st to end	Night	5% 1 3 43% 42% 5%	660
September	1st to 10th	Night	10% 2 7% 41% 35% 5%	600
	11th to 20th	Night	11% 1 5% 41% 38% 4	600
	21st to end	Night	11% 1 10% 38% 37% 4	600
October	1st to 10th	Night	7% 1 11% 48% 30% 5%	600
	11th to 20th	Night	12% 4 20% 39% 22% 2	745
	21st to end	Night	16% 7% 24% 30% 20% 1	880
November	1st to 10th	Night	16% 5% 20% 40% 19% 1	820
	11th to 20th	Night	14% 7% 25% 37% 15% 1	950
	21st to end	Night	20% 10% 28% 30% 12% 1	1050
December	1st to 10th	Night	20% 8% 29% 33% 10%	1050
	11th to 20th	Night	23% 9% 27% 33% 7%	1050
	21st to end	Night	21% 9% 27% 35% 8%	1155

Page generated at 2026-05-14 00:58:56. Version ac07

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