Dry Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dry Creek Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	13% 7% 2 6% 5% 4 5% 4 6% 8% 40%	127282
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	124748
Transparency	Day	41% 4 11% 21% 18% 5%	131057
	Night	39% 4 10% 22% 20% 5%	129041
Seeing	Day	18% 10% 23% 23% 17% 9%	16358
	Night	17% 9% 22% 29% 19% 4	26421

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	18% 10% 2 4 3 3 3 3 3 5% 45%	4146
	11th to 20th	Day	14% 9% 3 6% 3 2 2 2 3 3 52%	4150
	21st to end	Day	22% 11% 2 6% 4 4 3 2 5% 5% 36%	4865
February	1st to 10th	Day	19% 9% 2 5% 4 4 4 3 5% 6% 39%	2000
	11th to 20th	Day	22% 9% 3 6% 4 3 3 3 4 5% 38%	1948
	21st to end	Day	21% 8% 2 7% 5% 4 3 2 4 5% 39%	1650
March	1st to 10th	Day	20% 8% 3 8% 5% 4 5% 4 6% 5% 33%	4516
	11th to 20th	Day	12% 6% 2 6% 4 3 3 3 5% 7% 49%	4980
	21st to end	Day	16% 6% 3 7% 5% 4 4 3 5% 6% 43%	5465
April	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 7% 44%	2340
	11th to 20th	Day	12% 6% 2 7% 5% 4 4 3 5% 6% 48%	2374
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 6% 7% 45%	2515
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 9% 34%	5862
	11th to 20th	Day	17% 8% 3 8% 6% 5% 6% 5% 6% 7% 29%	5861
	21st to end	Day	14% 7% 3 7% 4 4 5% 4 6% 7% 39%	6433
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 6% 5% 8% 10% 34%	2600
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 5% 8% 11% 41%	2600
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 11% 37%	2600
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 9% 10% 35%	5900
	11th to 20th	Day	6% 5% 2 7% 6% 6% 7% 6% 9% 13% 33%	5900
	21st to end	Day	10% 8% 4 9% 7% 7% 7% 6% 9% 11% 22%	6424
August	1st to 10th	Day	5% 4 2 5% 5% 5% 7% 6% 9% 12% 41%	4184
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 41%	3849
	21st to end	Day	8% 4 2 5% 5% 5% 5% 4 6% 8% 49%	4312
September	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 6% 10% 39%	2255
	11th to 20th	Day	14% 6% 2 7% 4 5% 5% 4 7% 7% 41%	2200
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 46%	2200
October	1st to 10th	Day	10% 5% 2 6% 4 3 4 3 4 5% 53%	2200
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 6% 56%	2131
	21st to end	Day	15% 5% 2 4 3 3 3 2 4 4 54%	2142
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 2 4 5% 54%	1852
	11th to 20th	Day	13% 5% 3 4 4 4 3 3 4 6% 51%	1890
	21st to end	Day	15% 6% 2 4 4 3 4 3 5% 7% 47%	1890
December	1st to 10th	Day	19% 6% 2 6% 4 4 3 3 5% 5% 44%	3020
	11th to 20th	Day	16% 10% 3 8% 5% 4 4 3 4 5% 39%	3463
	21st to end	Day	19% 8% 2 6% 4 3 4 3 5% 7% 38%	4565
January	1st to 10th	Night	16% 8% 3 6% 4 3 3 3 5% 5% 44%	6019
	11th to 20th	Night	15% 9% 3 5% 3 3 3 2 4 5% 48%	6030
	21st to end	Night	20% 9% 2 5% 3 3 3 3 3 4 44%	6328
February	1st to 10th	Night	16% 8% 2 6% 4 3 4 2 5% 6% 44%	2401
	11th to 20th	Night	22% 8% 3 6% 4 3 3 3 3 4 40%	2338
	21st to end	Night	21% 7% 3 6% 3 4 3 3 5% 7% 39%	1980
March	1st to 10th	Night	22% 9% 3 7% 4 4 4 3 5% 7% 33%	5335
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 6% 6% 51%	4980
	21st to end	Night	14% 8% 2 7% 4 4 4 4 4 5% 43%	5467
April	1st to 10th	Night	12% 5% 2 5% 4 3 4 4 6% 8% 48%	2075
	11th to 20th	Night	13% 5% 2 5% 4 3 3 2 5% 6% 50%	1975
	21st to end	Night	10% 5% 3 6% 3 4 4 4 5% 6% 50%	1883
May	1st to 10th	Night	12% 8% 3 7% 6% 5% 6% 4 6% 8% 35%	4033
	11th to 20th	Night	19% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	4032
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 9% 41%	4421
June	1st to 10th	Night	7% 6% 2 7% 4 6% 6% 4 8% 11% 39%	1800
	11th to 20th	Night	4 4 2 6% 5% 4 5% 4 7% 12% 48%	1800
	21st to end	Night	5% 4 2 5% 5% 4 6% 4 8% 12% 45%	1800
July	1st to 10th	Night	7% 5% 2 8% 6% 5% 6% 5% 9% 11% 36%	4058
	11th to 20th	Night	6% 5% 2 7% 5% 6% 6% 5% 8% 13% 37%	4057
	21st to end	Night	8% 7% 3 8% 6% 7% 6% 5% 8% 11% 30%	4427
August	1st to 10th	Night	5% 5% 2 6% 5% 5% 6% 5% 7% 12% 42%	2946
	11th to 20th	Night	7% 6% 2 6% 5% 4 4 4 6% 10% 47%	3200
	21st to end	Night	8% 5% 2 5% 4 3 4 3 6% 8% 52%	3586
September	1st to 10th	Night	9% 6% 2 7% 6% 5% 6% 4 6% 8% 43%	2125
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 5% 8% 44%	2200
	21st to end	Night	12% 5% 2 5% 3 4 4 3 6% 7% 50%	2200
October	1st to 10th	Night	9% 5% 2 4 4 3 3 3 5% 5% 58%	2200
	11th to 20th	Night	11% 4 2 4 3 4 3 2 3 6% 57%	2266
	21st to end	Night	13% 5% 2 5% 4 3 3 3 3 5% 55%	2695
November	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 4 6% 53%	2666
	11th to 20th	Night	13% 6% 2 4 3 2 3 2 4 6% 54%	2730
	21st to end	Night	16% 6% 2 5% 3 3 4 4 5% 6% 46%	2730
December	1st to 10th	Night	19% 7% 2 6% 3 2 2 3 4 5% 47%	4380
	11th to 20th	Night	19% 10% 3 6% 3 3 3 3 4 5% 43%	4952
	21st to end	Night	20% 8% 3 6% 4 4 4 3 5% 6% 38%	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	44% 2 5% 24% 23% 3	4147
	11th to 20th	Day	40% 1 4 22% 29% 3	4150
	21st to end	Day	51% 2 4 17% 22% 4	4865
February	1st to 10th	Day	47% 2 7% 20% 21% 3	2000
	11th to 20th	Day	50% 2 5% 20% 18% 5%	1958
	21st to end	Day	49% 2 5% 15% 22% 6%	1650
March	1st to 10th	Day	52% 2 7% 12% 19% 8%	4516
	11th to 20th	Day	37% 2 5% 16% 26% 14%	4980
	21st to end	Day	44% 2 7% 25% 19% 3	5465
April	1st to 10th	Day	39% 3 10% 23% 20% 5%	2340
	11th to 20th	Day	39% 3 6% 19% 26% 8%	2374
	21st to end	Day	36% 4 10% 27% 19% 3	2515
May	1st to 10th	Day	47% 4 13% 22% 13% 2	5897
	11th to 20th	Day	53% 5% 13% 20% 9% 1	5865
	21st to end	Day	43% 4 11% 22% 18% 3	6447
June	1st to 10th	Day	43% 6% 15% 22% 12% 2	2600
	11th to 20th	Day	33% 6% 19% 27% 13% 2	2600
	21st to end	Day	39% 7% 17% 23% 13% 3	2600
July	1st to 10th	Day	38% 9% 20% 23% 9% 1	5835
	11th to 20th	Day	37% 12% 29% 17% 5% 1	5831
	21st to end	Day	50% 11% 19% 16% 5%	6240
August	1st to 10th	Day	31% 7% 15% 35% 12%	4171
	11th to 20th	Day	34% 6% 17% 26% 14% 2	3885
	21st to end	Day	33% 6% 12% 25% 22% 3	4311
September	1st to 10th	Day	40% 5% 15% 19% 17% 5%	2255
	11th to 20th	Day	42% 5% 10% 23% 19% 2	2200
	21st to end	Day	37% 4 11% 18% 23% 7%	2200
October	1st to 10th	Day	26% 3 6% 14% 38% 12%	3590
	11th to 20th	Day	21% 2 4 19% 30% 24%	3449
	21st to end	Day	27% 3 5% 24% 30% 11%	3444
November	1st to 10th	Day	35% 2 6% 19% 30% 8%	1852
	11th to 20th	Day	36% 2 6% 20% 27% 9%	1890
	21st to end	Day	39% 2 7% 21% 26% 5%	1890
December	1st to 10th	Day	43% 2 5% 19% 26% 5%	3020
	11th to 20th	Day	49% 2 7% 19% 18% 5%	3460
	21st to end	Day	47% 2 6% 28% 15% 2	4565
January	1st to 10th	Night	43% 2 5% 25% 22% 4	6019
	11th to 20th	Night	42% 2 4 21% 26% 6%	6030
	21st to end	Night	46% 1 4 19% 25% 5%	6328
February	1st to 10th	Night	43% 2 7% 21% 21% 6%	2401
	11th to 20th	Night	50% 2 5% 19% 18% 6%	2339
	21st to end	Night	46% 2 6% 17% 20% 8%	1980
March	1st to 10th	Night	52% 2 7% 15% 19% 4	5335
	11th to 20th	Night	34% 2 7% 17% 25% 14%	4980
	21st to end	Night	44% 2 5% 23% 21% 5%	5467
April	1st to 10th	Night	35% 4 11% 22% 22% 7%	2075
	11th to 20th	Night	36% 2 5% 24% 24% 9%	1975
	21st to end	Night	35% 3 10% 28% 19% 4	1883
May	1st to 10th	Night	47% 5% 14% 20% 12% 2	4060
	11th to 20th	Night	53% 4 15% 20% 8%	4033
	21st to end	Night	40% 4 12% 26% 17% 1	4429
June	1st to 10th	Night	37% 7% 18% 22% 14% 2	1800
	11th to 20th	Night	29% 6% 21% 28% 16% 2	1800
	21st to end	Night	31% 7% 21% 22% 15% 5%	1800
July	1st to 10th	Night	38% 10% 19% 23% 8% 1	4015
	11th to 20th	Night	36% 11% 27% 20% 5% 1	4006
	21st to end	Night	47% 11% 19% 18% 5% 1	4297
August	1st to 10th	Night	32% 7% 16% 34% 10% 1	2935
	11th to 20th	Night	32% 8% 16% 29% 15% 1	3230
	21st to end	Night	30% 6% 13% 19% 28% 3	3586
September	1st to 10th	Night	38% 6% 16% 18% 15% 6%	2125
	11th to 20th	Night	38% 6% 12% 23% 19% 2	2200
	21st to end	Night	32% 5% 12% 22% 22% 8%	2200
October	1st to 10th	Night	24% 4 7% 15% 37% 13%	3590
	11th to 20th	Night	19% 1 4 18% 35% 22%	3687
	21st to end	Night	24% 2 3 26% 31% 14%	4343
November	1st to 10th	Night	34% 3 7% 20% 26% 10%	2666
	11th to 20th	Night	33% 2 5% 22% 29% 10%	2730
	21st to end	Night	39% 3 6% 22% 25% 6%	2730
December	1st to 10th	Night	42% 2 5% 21% 27% 3	4380
	11th to 20th	Night	46% 2 7% 18% 23% 4	4954
	21st to end	Night	47% 3 6% 28% 15% 1	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	31% 23% 33% 11% 2	330
	11th to 20th	Day	30% 22% 30% 13% 4 1	330
	21st to end	Day	29% 23% 33% 10% 3 1	363
February	1st to 10th	Day	28% 18% 34% 18% 2	300
	11th to 20th	Day	43% 22% 25% 9% 1	300
	21st to end	Day	33% 21% 35% 10% 2	249
March	1st to 10th	Day	32% 21% 27% 15% 4 1	310
	11th to 20th	Day	26% 19% 30% 19% 6% 1	500
	21st to end	Day	24% 17% 37% 17% 5% 1	550
April	1st to 10th	Day	24% 16% 31% 16% 11% 2	500
	11th to 20th	Day	18% 15% 33% 21% 12% 1	492
	21st to end	Day	17% 9% 32% 26% 14% 2	500
May	1st to 10th	Day	18% 11% 28% 26% 11% 6%	503
	11th to 20th	Day	15% 7% 21% 34% 13% 10%	527
	21st to end	Day	12% 6% 23% 29% 19% 11%	573
June	1st to 10th	Day	12% 4 22% 28% 23% 11%	530
	11th to 20th	Day	7% 4 16% 27% 25% 21%	530
	21st to end	Day	12% 5% 15% 26% 25% 18%	530
July	1st to 10th	Day	11% 2 7% 28% 30% 22%	525
	11th to 20th	Day	8% 4 8% 23% 30% 27%	524
	21st to end	Day	10% 3 8% 24% 36% 18%	576
August	1st to 10th	Day	11% 3 12% 26% 30% 17%	526
	11th to 20th	Day	13% 5% 10% 30% 26% 17%	519
	21st to end	Day	13% 3 8% 27% 28% 21%	583
September	1st to 10th	Day	9% 2 13% 31% 29% 15%	530
	11th to 20th	Day	18% 3 12% 25% 29% 12%	530
	21st to end	Day	12% 4 19% 31% 26% 8%	530
October	1st to 10th	Day	14% 5% 23% 30% 20% 8%	530
	11th to 20th	Day	14% 10% 22% 32% 20% 2	530
	21st to end	Day	20% 10% 26% 25% 17% 3	525
November	1st to 10th	Day	21% 12% 30% 23% 12% 2	330
	11th to 20th	Day	18% 15% 38% 20% 8% 1	330
	21st to end	Day	17% 21% 36% 15% 9% 2	330
December	1st to 10th	Day	32% 18% 29% 14% 4 2	330
	11th to 20th	Day	25% 17% 31% 21% 5% 1	330
	21st to end	Day	30% 17% 30% 17% 5% 1	363
January	1st to 10th	Night	25% 19% 29% 22% 5%	840
	11th to 20th	Night	24% 17% 29% 24% 6%	840
	21st to end	Night	24% 16% 33% 24% 3	924
February	1st to 10th	Night	21% 14% 33% 28% 5%	801
	11th to 20th	Night	29% 17% 31% 22% 1	783
	21st to end	Night	25% 19% 35% 18% 3	660
March	1st to 10th	Night	22% 14% 32% 28% 4	800
	11th to 20th	Night	19% 15% 32% 25% 8% 1	800
	21st to end	Night	20% 13% 31% 28% 8%	880
April	1st to 10th	Night	15% 13% 32% 28% 11%	675
	11th to 20th	Night	16% 11% 34% 27% 11% 1	593
	21st to end	Night	12% 9% 26% 37% 15% 1	600
May	1st to 10th	Night	13% 6% 25% 36% 20% 1	600
	11th to 20th	Night	16% 5% 19% 33% 21% 5%	591
	21st to end	Night	11% 2 13% 36% 31% 7%	650
June	1st to 10th	Night	10% 2 12% 39% 32% 5%	600
	11th to 20th	Night	4 3 13% 32% 35% 13%	600
	21st to end	Night	7% 3 8% 32% 41% 9%	600
July	1st to 10th	Night	10% 4 6% 24% 43% 13%	585
	11th to 20th	Night	8% 1 4 26% 46% 15%	582
	21st to end	Night	9% 2 6% 24% 47% 11%	657
August	1st to 10th	Night	10% 2 6% 31% 40% 11%	597
	11th to 20th	Night	11% 2 6% 32% 40% 9%	594
	21st to end	Night	14% 3 7% 28% 38% 10%	660
September	1st to 10th	Night	10% 3 10% 31% 38% 9%	725
	11th to 20th	Night	16% 3 12% 29% 37% 4	800
	21st to end	Night	15% 3 14% 33% 32% 3	800
October	1st to 10th	Night	12% 5% 21% 33% 23% 6%	800
	11th to 20th	Night	13% 6% 26% 33% 20% 2	800
	21st to end	Night	15% 9% 21% 35% 17% 2	880
November	1st to 10th	Night	16% 8% 24% 32% 18% 2	820
	11th to 20th	Night	15% 10% 32% 29% 13% 1	840
	21st to end	Night	19% 14% 28% 28% 11%	840
December	1st to 10th	Night	24% 16% 28% 24% 8%	840
	11th to 20th	Night	26% 11% 28% 27% 7% 1	840
	21st to end	Night	27% 12% 28% 27% 7%	924

Page generated at 2026-01-29 01:21:43. Version ac07

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