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 2024-06-30.

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% 7% 2 6% 5% 4 5% 4 6% 7% 40%	89583
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	89508
Transparency	Day	43% 4 10% 21% 18% 4	89357
	Night	41% 3 10% 21% 20% 5%	89338
Seeing	Day	18% 10% 22% 23% 17% 9%	14261
	Night	17% 9% 22% 29% 20% 4	24315

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% 11% 2 5% 4 4 4 3 4 6% 37%	2836
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 3 4 45%	2840
	21st to end	Day	25% 11% 2 5% 3 4 3 2 4 4 38%	3320
February	1st to 10th	Day	20% 9% 3 5% 4 3 3 3 5% 6% 39%	1900
	11th to 20th	Day	21% 9% 3 6% 4 3 3 3 4 5% 39%	1848
	21st to end	Day	22% 8% 2 7% 5% 4 3 2 4 5% 38%	1570
March	1st to 10th	Day	26% 8% 2 7% 4 3 3 3 5% 6% 33%	3142
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 5% 6% 47%	3480
	21st to end	Day	17% 6% 2 6% 5% 4 4 3 5% 7% 39%	3815
April	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 7% 44%	2223
	11th to 20th	Day	13% 6% 2 7% 5% 4 4 3 5% 5% 47%	2254
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 4 6% 7% 47%	2389
May	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 4 6% 8% 41%	4082
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 4 6% 7% 36%	4081
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 9% 43%	4476
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 6% 5% 8% 9% 34%	2470
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 6% 8% 11% 42%	2470
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 11% 37%	2457
July	1st to 10th	Day	10% 6% 3 6% 7% 6% 7% 6% 8% 10% 31%	2340
	11th to 20th	Day	8% 5% 2 7% 6% 7% 7% 7% 9% 11% 30%	2340
	21st to end	Day	7% 5% 3 8% 7% 6% 7% 6% 9% 12% 29%	2547
August	1st to 10th	Day	7% 5% 2 6% 6% 6% 8% 7% 10% 12% 31%	2306
	11th to 20th	Day	10% 5% 2 8% 6% 6% 7% 6% 8% 10% 33%	2125
	21st to end	Day	13% 7% 3 7% 7% 6% 6% 5% 7% 9% 30%	2376
September	1st to 10th	Day	9% 7% 3 7% 5% 5% 6% 4 6% 9% 38%	2030
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 7% 6% 41%	1980
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 48%	1980
October	1st to 10th	Day	11% 5% 1 6% 4 3 3 3 4 5% 54%	1980
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 5% 6% 53%	1919
	21st to end	Day	15% 5% 2 4 3 3 3 2 4 4 54%	1928
November	1st to 10th	Day	13% 5% 2 4 3 2 3 2 4 5% 56%	1672
	11th to 20th	Day	13% 5% 3 5% 4 4 3 3 4 5% 51%	1710
	21st to end	Day	16% 6% 2 5% 5% 3 4 3 5% 6% 46%	1710
December	1st to 10th	Day	21% 7% 2 6% 4 4 4 4 5% 6% 38%	1710
	11th to 20th	Day	18% 8% 2 6% 5% 3 3 3 4 5% 41%	2153
	21st to end	Day	21% 9% 2 6% 4 4 5% 4 5% 7% 35%	3124
January	1st to 10th	Night	19% 10% 2 5% 4 4 4 3 5% 6% 39%	4109
	11th to 20th	Night	16% 10% 3 5% 4 3 3 3 4 5% 44%	4120
	21st to end	Night	25% 12% 3 5% 3 3 3 2 3 4 38%	4332
February	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 5% 6% 43%	2280
	11th to 20th	Night	22% 8% 3 6% 4 3 3 3 4 4 41%	2218
	21st to end	Night	22% 7% 3 6% 3 4 3 3 5% 7% 37%	1884
March	1st to 10th	Night	24% 9% 3 6% 4 4 3 3 4 6% 33%	3709
	11th to 20th	Night	12% 6% 2 5% 3 3 4 4 6% 7% 49%	3480
	21st to end	Night	16% 8% 2 7% 4 4 4 4 4 5% 40%	3817
April	1st to 10th	Night	12% 5% 2 5% 4 3 4 4 6% 8% 48%	1971
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 5% 6% 49%	1875
	21st to end	Night	10% 5% 2 5% 3 4 4 4 6% 6% 52%	1789
May	1st to 10th	Night	11% 7% 3 6% 5% 5% 5% 4 6% 7% 41%	2813
	11th to 20th	Night	17% 7% 3 6% 4 4 4 4 6% 7% 38%	2812
	21st to end	Night	6% 4 1 5% 5% 4 5% 4 7% 11% 46%	3079
June	1st to 10th	Night	7% 6% 2 7% 4 6% 6% 4 8% 11% 38%	1710
	11th to 20th	Night	4 4 2 6% 5% 4 5% 4 7% 12% 49%	1710
	21st to end	Night	6% 4 2 5% 4 4 6% 4 8% 12% 45%	1701
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 10% 10% 36%	1620
	11th to 20th	Night	6% 5% 2 6% 5% 7% 6% 5% 8% 13% 38%	1620
	21st to end	Night	5% 7% 3 6% 5% 7% 6% 5% 8% 12% 36%	1771
August	1st to 10th	Night	6% 4 2 7% 6% 5% 6% 6% 8% 13% 38%	1627
	11th to 20th	Night	8% 7% 2 6% 6% 5% 5% 4 7% 10% 40%	1776
	21st to end	Night	13% 7% 3 7% 5% 4 4 5% 7% 10% 37%	1980
September	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	1912
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 43%	1980
	21st to end	Night	11% 4 1 4 3 3 3 3 6% 7% 53%	1980
October	1st to 10th	Night	9% 5% 1 4 3 3 3 3 5% 5% 59%	1980
	11th to 20th	Night	12% 5% 2 5% 3 4 3 2 3 6% 54%	2038
	21st to end	Night	13% 5% 2 5% 4 3 3 3 3 5% 54%	2425
November	1st to 10th	Night	12% 4 1 4 3 3 3 3 4 6% 55%	2406
	11th to 20th	Night	13% 6% 2 4 3 2 3 3 4 6% 54%	2470
	21st to end	Night	16% 6% 2 5% 3 3 4 4 5% 7% 45%	2470
December	1st to 10th	Night	20% 7% 3 8% 4 3 3 3 5% 5% 38%	2470
	11th to 20th	Night	18% 8% 2 5% 3 2 3 3 4 5% 47%	3042
	21st to end	Night	19% 9% 3 6% 4 4 4 3 6% 8% 33%	4532

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	49% 2 5% 18% 23% 3	2837
	11th to 20th	Day	45% 2 6% 21% 24% 2	2840
	21st to end	Day	52% 2 4 19% 20% 3	3320
February	1st to 10th	Day	47% 2 7% 19% 21% 4	1900
	11th to 20th	Day	49% 2 5% 20% 19% 5%	1858
	21st to end	Day	51% 2 6% 14% 21% 6%	1570
March	1st to 10th	Day	54% 2 8% 12% 14% 9%	3142
	11th to 20th	Day	41% 2 6% 20% 24% 7%	3480
	21st to end	Day	47% 2 8% 21% 18% 4	3815
April	1st to 10th	Day	39% 3 10% 22% 20% 6%	2223
	11th to 20th	Day	40% 3 6% 18% 26% 8%	2254
	21st to end	Day	35% 4 9% 28% 20% 3	2389
May	1st to 10th	Day	41% 3 12% 24% 17% 2	4120
	11th to 20th	Day	48% 3 11% 25% 12% 1	4085
	21st to end	Day	36% 4 13% 24% 20% 3	4489
June	1st to 10th	Day	44% 6% 15% 22% 12% 2	2470
	11th to 20th	Day	32% 6% 19% 28% 14% 2	2470
	21st to end	Day	39% 6% 16% 23% 13% 3	2457
July	1st to 10th	Day	44% 7% 18% 19% 10% 2	2275
	11th to 20th	Day	42% 8% 20% 16% 10% 3	2271
	21st to end	Day	43% 7% 21% 19% 9%	2405
August	1st to 10th	Day	40% 8% 20% 23% 8% 1	2293
	11th to 20th	Day	45% 6% 17% 20% 10% 2	2125
	21st to end	Day	49% 7% 14% 16% 11% 4	2376
September	1st to 10th	Day	41% 5% 15% 18% 16% 5%	2030
	11th to 20th	Day	42% 5% 10% 21% 19% 3	1980
	21st to end	Day	37% 4 9% 18% 24% 7%	1980
October	1st to 10th	Day	33% 4 8% 19% 27% 8%	1980
	11th to 20th	Day	33% 3 7% 24% 24% 9%	1919
	21st to end	Day	36% 2 4 20% 30% 9%	1928
November	1st to 10th	Day	33% 2 6% 19% 32% 8%	1672
	11th to 20th	Day	37% 2 6% 19% 27% 9%	1710
	21st to end	Day	40% 2 7% 19% 27% 5%	1710
December	1st to 10th	Day	49% 2 7% 20% 18% 5%	1710
	11th to 20th	Day	46% 2 5% 20% 22% 5%	2150
	21st to end	Day	50% 2 7% 22% 17% 2	3124
January	1st to 10th	Night	47% 2 4 21% 23% 3	4109
	11th to 20th	Night	45% 2 5% 23% 19% 6%	4120
	21st to end	Night	53% 1 4 16% 23% 3	4332
February	1st to 10th	Night	44% 2 7% 20% 21% 6%	2280
	11th to 20th	Night	49% 2 5% 19% 19% 7%	2219
	21st to end	Night	48% 2 6% 17% 19% 7%	1884
March	1st to 10th	Night	53% 2 9% 14% 16% 6%	3709
	11th to 20th	Night	35% 2 8% 19% 26% 9%	3480
	21st to end	Night	47% 2 5% 22% 21% 4	3817
April	1st to 10th	Night	35% 4 10% 21% 23% 7%	1971
	11th to 20th	Night	37% 2 5% 23% 23% 9%	1875
	21st to end	Night	34% 3 10% 29% 20% 4	1789
May	1st to 10th	Night	44% 3 12% 23% 16% 2	2840
	11th to 20th	Night	45% 3 14% 26% 11%	2813
	21st to end	Night	32% 3 14% 29% 20% 2	3087
June	1st to 10th	Night	38% 7% 18% 22% 14% 2	1710
	11th to 20th	Night	28% 5% 21% 29% 16% 1	1710
	21st to end	Night	32% 6% 21% 22% 14% 5%	1701
July	1st to 10th	Night	38% 8% 18% 22% 12% 2	1575
	11th to 20th	Night	35% 10% 23% 19% 11% 3	1566
	21st to end	Night	39% 9% 20% 21% 10% 2	1672
August	1st to 10th	Night	36% 8% 20% 25% 9% 1	1616
	11th to 20th	Night	37% 9% 20% 18% 14% 2	1776
	21st to end	Night	42% 7% 16% 19% 13% 3	1980
September	1st to 10th	Night	37% 6% 17% 19% 16% 6%	1912
	11th to 20th	Night	38% 6% 12% 22% 19% 2	1980
	21st to end	Night	30% 4 11% 23% 23% 9%	1980
October	1st to 10th	Night	28% 3 8% 22% 29% 9%	1980
	11th to 20th	Night	33% 2 7% 22% 25% 10%	2038
	21st to end	Night	34% 2 4 20% 29% 11%	2425
November	1st to 10th	Night	32% 3 7% 20% 27% 11%	2406
	11th to 20th	Night	34% 2 5% 21% 28% 11%	2470
	21st to end	Night	39% 3 6% 22% 25% 5%	2470
December	1st to 10th	Night	47% 3 7% 20% 19% 3	2470
	11th to 20th	Night	41% 2 6% 19% 27% 4	3044
	21st to end	Night	51% 2 6% 23% 16% 2	4532

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	29% 22% 35% 11% 2	270
	11th to 20th	Day	32% 19% 30% 13% 5% 1	270
	21st to end	Day	27% 24% 35% 10% 3 1	297
February	1st to 10th	Day	31% 19% 32% 17% 2	270
	11th to 20th	Day	40% 23% 26% 10% 1	270
	21st to end	Day	35% 22% 34% 8% 1	225
March	1st to 10th	Day	32% 19% 29% 15% 4 1	279
	11th to 20th	Day	26% 18% 30% 19% 5% 1	460
	21st to end	Day	24% 16% 36% 17% 6% 1	506
April	1st to 10th	Day	24% 15% 31% 16% 12% 2	460
	11th to 20th	Day	19% 16% 32% 20% 12% 2	452
	21st to end	Day	17% 10% 32% 24% 15% 2	460
May	1st to 10th	Day	17% 10% 28% 26% 12% 7%	463
	11th to 20th	Day	15% 6% 19% 35% 14% 11%	487
	21st to end	Day	11% 4 23% 30% 21% 12%	529
June	1st to 10th	Day	12% 4 21% 28% 23% 12%	490
	11th to 20th	Day	7% 4 16% 27% 24% 22%	490
	21st to end	Day	12% 4 15% 26% 24% 18%	486
July	1st to 10th	Day	12% 2 6% 27% 29% 24%	445
	11th to 20th	Day	8% 5% 8% 23% 28% 28%	444
	21st to end	Day	10% 3 7% 22% 37% 21%	488
August	1st to 10th	Day	11% 3 11% 25% 32% 20%	446
	11th to 20th	Day	13% 3 8% 32% 27% 18%	439
	21st to end	Day	14% 3 7% 25% 29% 21%	495
September	1st to 10th	Day	10% 3 11% 29% 30% 18%	450
	11th to 20th	Day	18% 2 12% 26% 28% 13%	450
	21st to end	Day	11% 3 18% 32% 27% 9%	450
October	1st to 10th	Day	15% 5% 22% 28% 21% 9%	450
	11th to 20th	Day	16% 10% 24% 30% 18% 3	450
	21st to end	Day	21% 10% 26% 25% 16% 3	443
November	1st to 10th	Day	22% 12% 29% 24% 11% 3	270
	11th to 20th	Day	19% 16% 36% 22% 7% 1	270
	21st to end	Day	17% 17% 37% 17% 10% 1	270
December	1st to 10th	Day	32% 17% 29% 16% 4 2	270
	11th to 20th	Day	26% 17% 31% 20% 5% 1	270
	21st to end	Day	31% 18% 28% 17% 4 1	297
January	1st to 10th	Night	24% 18% 30% 22% 6%	760
	11th to 20th	Night	25% 15% 29% 24% 7%	760
	21st to end	Night	24% 16% 32% 25% 3	836
February	1st to 10th	Night	22% 13% 33% 28% 5%	760
	11th to 20th	Night	29% 16% 31% 23% 1	743
	21st to end	Night	26% 19% 35% 17% 3	628
March	1st to 10th	Night	21% 14% 32% 28% 4	760
	11th to 20th	Night	19% 15% 32% 24% 9% 1	760
	21st to end	Night	20% 13% 30% 29% 9%	836
April	1st to 10th	Night	15% 13% 31% 28% 12%	641
	11th to 20th	Night	17% 12% 33% 26% 12% 1	563
	21st to end	Night	12% 9% 26% 36% 16% 1	570
May	1st to 10th	Night	12% 6% 25% 36% 21% 1	570
	11th to 20th	Night	15% 5% 19% 34% 23% 5%	561
	21st to end	Night	10% 2 13% 36% 32% 7%	617
June	1st to 10th	Night	11% 2 12% 38% 32% 5%	570
	11th to 20th	Night	4 2 13% 32% 36% 13%	570
	21st to end	Night	8% 2 8% 33% 40% 9%	567
July	1st to 10th	Night	10% 4 5% 23% 43% 15%	525
	11th to 20th	Night	8% 1 4 27% 46% 15%	522
	21st to end	Night	8% 2 6% 24% 47% 12%	591
August	1st to 10th	Night	9% 1 5% 31% 42% 12%	537
	11th to 20th	Night	11% 2 4 33% 40% 9%	534
	21st to end	Night	15% 3 5% 27% 39% 11%	594
September	1st to 10th	Night	10% 3 8% 29% 41% 10%	652
	11th to 20th	Night	16% 3 12% 29% 37% 3	720
	21st to end	Night	13% 2 14% 34% 33% 3	720
October	1st to 10th	Night	12% 5% 21% 33% 23% 7%	720
	11th to 20th	Night	14% 6% 27% 31% 20% 2	720
	21st to end	Night	16% 9% 20% 35% 17% 2	792
November	1st to 10th	Night	16% 7% 23% 33% 19% 2	740
	11th to 20th	Night	15% 10% 32% 29% 13% 1	760
	21st to end	Night	19% 12% 29% 29% 11%	760
December	1st to 10th	Night	23% 16% 29% 24% 9%	760
	11th to 20th	Night	25% 11% 28% 27% 7% 1	760
	21st to end	Night	27% 12% 28% 26% 7%	836

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