Delta College Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Delta College Planetarium clear sky chart from 2005-11-06 to 2026-03-03.

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% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	130124
	Night	13% 11% 4 9% 5% 5% 4 4 5% 7% 34%	131188
Transparency	Day	46% 7% 13% 21% 12% 1	133662
	Night	48% 7% 12% 20% 12% 1	135722
Seeing	Day	16% 10% 29% 25% 14% 5%	17194
	Night	18% 8% 31% 30% 12% 1	27032

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	12% 14% 6% 14% 8% 7% 5% 4 5% 6% 21%	4691
	11th to 20th	Day	10% 15% 7% 16% 11% 8% 7% 4 4 4 13%	4710
	21st to end	Day	14% 15% 6% 11% 8% 6% 5% 4 4 6% 21%	5367
February	1st to 10th	Day	11% 15% 5% 10% 7% 7% 6% 4 5% 8% 23%	1890
	11th to 20th	Day	11% 13% 4 12% 8% 7% 6% 3 5% 5% 25%	1988
	21st to end	Day	12% 14% 5% 10% 7% 6% 6% 4 5% 6% 26%	1730
March	1st to 10th	Day	14% 9% 3 8% 6% 5% 5% 4 5% 6% 35%	4541
	11th to 20th	Day	12% 11% 4 8% 5% 5% 5% 4 5% 8% 33%	4980
	21st to end	Day	16% 13% 5% 9% 5% 5% 5% 4 5% 5% 28%	5465
April	1st to 10th	Day	17% 12% 3 9% 6% 4 4 4 5% 7% 30%	2373
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 3 5% 7% 36%	2529
	21st to end	Day	15% 9% 2 7% 5% 5% 5% 4 5% 6% 35%	2600
May	1st to 10th	Day	18% 9% 3 7% 4 4 4 3 5% 8% 36%	5862
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 3 6% 8% 37%	5861
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 8% 42%	6700
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 4 6% 8% 43%	2955
	11th to 20th	Day	11% 6% 3 6% 4 4 5% 4 6% 9% 43%	3000
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 5% 8% 10% 40%	2928
July	1st to 10th	Day	9% 5% 3 7% 5% 4 5% 4 7% 10% 40%	6360
	11th to 20th	Day	7% 6% 2 6% 4 4 4 4 6% 9% 49%	6360
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 7% 9% 49%	6756
August	1st to 10th	Day	10% 5% 2 5% 5% 6% 5% 5% 7% 8% 42%	4234
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 44%	4174
	21st to end	Day	8% 6% 2 7% 4 5% 5% 5% 6% 9% 43%	4611
September	1st to 10th	Day	9% 7% 2 8% 5% 5% 4 3 6% 8% 43%	2294
	11th to 20th	Day	11% 6% 1 5% 4 4 4 3 6% 9% 48%	2200
	21st to end	Day	13% 8% 3 7% 4 5% 4 4 6% 8% 38%	2200
October	1st to 10th	Day	13% 6% 2 8% 5% 5% 4 4 5% 6% 41%	2157
	11th to 20th	Day	18% 11% 3 8% 5% 5% 5% 4 5% 7% 31%	2000
	21st to end	Day	19% 10% 3 8% 6% 4 5% 4 6% 8% 28%	1980
November	1st to 10th	Day	12% 11% 3 8% 5% 5% 5% 4 6% 7% 34%	1852
	11th to 20th	Day	13% 13% 4 8% 6% 5% 5% 4 6% 7% 26%	1890
	21st to end	Day	17% 14% 5% 10% 7% 5% 6% 4 5% 6% 23%	1737
December	1st to 10th	Day	13% 16% 6% 11% 7% 7% 4 5% 6% 7% 19%	2591
	11th to 20th	Day	15% 20% 6% 11% 6% 5% 4 3 5% 5% 19%	2695
	21st to end	Day	25% 20% 6% 11% 5% 4 4 3 4 4 13%	3863
January	1st to 10th	Night	13% 13% 6% 12% 6% 5% 4 3 5% 5% 28%	8232
	11th to 20th	Night	10% 17% 8% 17% 8% 8% 5% 4 5% 5% 14%	8280
	21st to end	Night	12% 15% 6% 11% 7% 5% 5% 4 6% 7% 22%	8820
February	1st to 10th	Night	12% 15% 4 11% 7% 6% 5% 3 6% 6% 25%	2730
	11th to 20th	Night	13% 13% 6% 9% 7% 5% 4 3 5% 6% 28%	2515
	21st to end	Night	15% 14% 4 10% 7% 5% 4 4 5% 6% 26%	2076
March	1st to 10th	Night	14% 9% 3 8% 5% 4 4 3 4 6% 40%	5357
	11th to 20th	Night	11% 8% 3 6% 4 4 4 4 7% 9% 39%	4980
	21st to end	Night	17% 10% 4 9% 5% 4 4 3 5% 6% 34%	5467
April	1st to 10th	Night	18% 11% 3 6% 5% 3 4 3 5% 7% 35%	2045
	11th to 20th	Night	19% 7% 2 6% 4 4 3 4 5% 6% 38%	1819
	21st to end	Night	16% 9% 3 7% 4 4 4 4 5% 6% 37%	1800
May	1st to 10th	Night	16% 9% 3 8% 5% 4 4 4 5% 6% 36%	4033
	11th to 20th	Night	15% 9% 3 8% 4 4 4 3 4 7% 38%	4032
	21st to end	Night	16% 6% 2 5% 3 3 4 3 5% 7% 47%	4162
June	1st to 10th	Night	11% 7% 3 5% 5% 3 4 4 5% 7% 46%	1440
	11th to 20th	Night	11% 5% 2 6% 5% 4 4 5% 6% 10% 42%	1400
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 7% 8% 46%	1464
July	1st to 10th	Night	9% 6% 3 6% 4 4 4 4 7% 9% 44%	3618
	11th to 20th	Night	7% 4 2 5% 3 4 4 4 6% 9% 52%	3617
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	4094
August	1st to 10th	Night	10% 5% 2 6% 4 4 3 4 6% 8% 48%	2915
	11th to 20th	Night	10% 5% 2 7% 4 3 4 3 5% 8% 47%	2875
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3283
September	1st to 10th	Night	8% 5% 2 7% 4 4 4 4 6% 6% 49%	2085
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 6% 8% 54%	2200
	21st to end	Night	13% 8% 2 7% 4 4 5% 3 5% 6% 43%	2200
October	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 7% 44%	2242
	11th to 20th	Night	17% 10% 3 9% 4 5% 4 3 4 6% 34%	2400
	21st to end	Night	20% 9% 3 9% 5% 4 4 3 5% 7% 30%	2855
November	1st to 10th	Night	14% 9% 2 7% 5% 4 4 3 5% 7% 40%	2666
	11th to 20th	Night	13% 12% 3 8% 6% 5% 5% 5% 6% 7% 28%	2730
	21st to end	Night	18% 15% 4 9% 5% 5% 5% 4 5% 6% 25%	2880
December	1st to 10th	Night	13% 15% 5% 11% 6% 5% 5% 4 6% 7% 23%	4806
	11th to 20th	Night	13% 17% 5% 11% 7% 5% 4 4 5% 6% 24%	5732
	21st to end	Night	24% 20% 6% 11% 5% 4 3 3 4 4 17%	7338

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	60% 9% 14% 13% 4	4692
	11th to 20th	Day	70% 6% 7% 9% 6%	4710
	21st to end	Day	62% 6% 11% 13% 8% 1	5367
February	1st to 10th	Day	58% 8% 12% 17% 5%	1890
	11th to 20th	Day	58% 6% 12% 15% 7% 1	1998
	21st to end	Day	56% 7% 13% 17% 7% 1	1730
March	1st to 10th	Day	48% 6% 15% 20% 8% 3	4541
	11th to 20th	Day	47% 6% 17% 19% 10% 1	4980
	21st to end	Day	55% 7% 12% 16% 9% 1	5465
April	1st to 10th	Day	53% 6% 12% 19% 10% 1	2373
	11th to 20th	Day	46% 6% 12% 20% 14% 2	2529
	21st to end	Day	47% 7% 14% 19% 12% 1	2600
May	1st to 10th	Day	45% 7% 13% 18% 15% 3	5897
	11th to 20th	Day	43% 8% 11% 23% 14% 1	5865
	21st to end	Day	39% 6% 11% 21% 19% 4	6714
June	1st to 10th	Day	36% 7% 13% 28% 14% 2	2955
	11th to 20th	Day	36% 6% 14% 24% 17% 2	3000
	21st to end	Day	35% 9% 15% 25% 15% 2	2928
July	1st to 10th	Day	36% 8% 14% 26% 14% 1	6291
	11th to 20th	Day	30% 6% 13% 34% 15% 1	6285
	21st to end	Day	30% 8% 15% 26% 20% 1	6562
August	1st to 10th	Day	35% 9% 14% 25% 15% 1	4221
	11th to 20th	Day	35% 7% 14% 32% 11% 1	4213
	21st to end	Day	34% 8% 16% 28% 12% 1	4610
September	1st to 10th	Day	38% 7% 16% 26% 12% 2	2294
	11th to 20th	Day	32% 7% 16% 30% 15% 1	2200
	21st to end	Day	43% 7% 14% 24% 13%	2200
October	1st to 10th	Day	32% 6% 10% 32% 17% 3	3508
	11th to 20th	Day	40% 6% 13% 27% 14%	3240
	21st to end	Day	48% 7% 14% 23% 8%	3180
November	1st to 10th	Day	48% 7% 13% 21% 10%	1852
	11th to 20th	Day	54% 7% 15% 16% 8%	1890
	21st to end	Day	61% 7% 14% 14% 4 1	1737
December	1st to 10th	Day	62% 9% 17% 9% 2	2591
	11th to 20th	Day	65% 6% 11% 13% 4 1	2691
	21st to end	Day	74% 6% 7% 9% 5%	3863
January	1st to 10th	Night	56% 6% 11% 17% 8% 2	8233
	11th to 20th	Night	68% 6% 10% 9% 6% 1	8280
	21st to end	Night	58% 6% 11% 16% 8%	8820
February	1st to 10th	Night	58% 6% 11% 18% 6% 1	2730
	11th to 20th	Night	57% 6% 10% 19% 8% 1	2516
	21st to end	Night	57% 5% 12% 17% 8%	2076
March	1st to 10th	Night	45% 5% 15% 24% 10% 1	5357
	11th to 20th	Night	39% 6% 20% 23% 11% 1	4980
	21st to end	Night	50% 7% 12% 18% 11% 1	5467
April	1st to 10th	Night	48% 6% 11% 21% 13% 2	2045
	11th to 20th	Night	44% 6% 11% 20% 17% 2	1819
	21st to end	Night	47% 5% 11% 18% 16% 3	1800
May	1st to 10th	Night	46% 7% 12% 15% 17% 3	4060
	11th to 20th	Night	44% 7% 10% 25% 12% 3	4033
	21st to end	Night	37% 6% 8% 27% 17% 5%	4170
June	1st to 10th	Night	37% 6% 11% 23% 21% 2	1440
	11th to 20th	Night	36% 8% 13% 21% 20% 2	1400
	21st to end	Night	33% 8% 14% 27% 17% 2	1464
July	1st to 10th	Night	34% 7% 13% 23% 18% 5%	3580
	11th to 20th	Night	27% 7% 14% 34% 17% 1	3572
	21st to end	Night	32% 9% 14% 25% 19% 1	3976
August	1st to 10th	Night	32% 7% 14% 29% 16% 1	2904
	11th to 20th	Night	34% 6% 16% 30% 13% 1	2902
	21st to end	Night	31% 9% 14% 31% 15% 1	3283
September	1st to 10th	Night	34% 6% 15% 27% 17% 1	2085
	11th to 20th	Night	28% 7% 14% 33% 17% 1	2200
	21st to end	Night	41% 7% 14% 25% 12% 1	2200
October	1st to 10th	Night	27% 6% 11% 33% 22% 1	3671
	11th to 20th	Night	45% 6% 9% 24% 16%	3899
	21st to end	Night	44% 6% 11% 28% 10%	4605
November	1st to 10th	Night	44% 7% 13% 24% 13% 1	2666
	11th to 20th	Night	51% 9% 14% 17% 8% 1	2730
	21st to end	Night	59% 7% 15% 14% 4 1	2880
December	1st to 10th	Night	58% 8% 18% 12% 4	4806
	11th to 20th	Night	60% 7% 12% 12% 9% 1	5735
	21st to end	Night	70% 6% 9% 7% 7%	7338

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	18% 12% 38% 23% 8% 2	330
	11th to 20th	Day	20% 16% 37% 20% 6% 1	330
	21st to end	Day	21% 15% 29% 27% 6% 1	363
February	1st to 10th	Day	20% 17% 38% 21% 4 1	330
	11th to 20th	Day	17% 16% 37% 24% 5%	330
	21st to end	Day	21% 19% 41% 16% 3	273
March	1st to 10th	Day	21% 13% 41% 21% 4 1	321
	11th to 20th	Day	16% 14% 32% 25% 11% 1	500
	21st to end	Day	24% 12% 30% 25% 8% 1	550
April	1st to 10th	Day	21% 12% 32% 26% 9% 1	500
	11th to 20th	Day	23% 15% 31% 20% 8% 4	492
	21st to end	Day	20% 12% 31% 25% 10% 2	500
May	1st to 10th	Day	19% 15% 31% 20% 11% 3	503
	11th to 20th	Day	17% 9% 25% 25% 16% 7%	527
	21st to end	Day	14% 8% 23% 27% 21% 7%	684
June	1st to 10th	Day	12% 9% 30% 30% 14% 5%	730
	11th to 20th	Day	14% 9% 29% 28% 14% 6%	730
	21st to end	Day	12% 7% 30% 30% 17% 5%	730
July	1st to 10th	Day	7% 5% 24% 31% 22% 11%	721
	11th to 20th	Day	9% 6% 26% 31% 19% 9%	718
	21st to end	Day	7% 3 27% 33% 23% 7%	726
August	1st to 10th	Day	8% 4 19% 28% 27% 14%	526
	11th to 20th	Day	10% 4 17% 29% 29% 12%	519
	21st to end	Day	11% 7% 21% 27% 25% 10%	583
September	1st to 10th	Day	9% 8% 25% 26% 19% 12%	530
	11th to 20th	Day	9% 4 18% 24% 29% 16%	530
	21st to end	Day	17% 9% 22% 25% 18% 9%	530
October	1st to 10th	Day	17% 10% 26% 23% 17% 8%	530
	11th to 20th	Day	21% 14% 31% 26% 8% 1	530
	21st to end	Day	21% 15% 37% 20% 7% 1	363
November	1st to 10th	Day	18% 16% 35% 22% 8% 2	330
	11th to 20th	Day	19% 22% 35% 18% 5% 2	330
	21st to end	Day	26% 10% 39% 19% 5%	249
December	1st to 10th	Day	25% 12% 37% 18% 7% 1	220
	11th to 20th	Day	24% 17% 36% 18% 4 1	220
	21st to end	Day	27% 16% 34% 20% 3	316
January	1st to 10th	Night	19% 12% 37% 25% 6% 1	1050
	11th to 20th	Night	22% 13% 35% 23% 6% 1	1050
	21st to end	Night	17% 10% 34% 31% 8%	1073
February	1st to 10th	Night	19% 9% 38% 30% 4	840
	11th to 20th	Night	19% 10% 39% 26% 6%	823
	21st to end	Night	23% 11% 40% 23% 4	692
March	1st to 10th	Night	18% 10% 35% 30% 8%	809
	11th to 20th	Night	16% 10% 37% 29% 8% 1	800
	21st to end	Night	22% 9% 32% 30% 6% 1	880
April	1st to 10th	Night	19% 12% 36% 26% 7% 1	645
	11th to 20th	Night	23% 7% 32% 27% 9% 2	593
	21st to end	Night	22% 8% 29% 32% 9% 1	600
May	1st to 10th	Night	15% 8% 32% 37% 8% 1	600
	11th to 20th	Night	18% 7% 26% 34% 13% 1	591
	21st to end	Night	14% 4 26% 35% 19% 2	650
June	1st to 10th	Night	12% 6% 30% 38% 15%	440
	11th to 20th	Night	12% 5% 26% 41% 16%	400
	21st to end	Night	10% 4 24% 41% 19% 2	464
July	1st to 10th	Night	9% 3 18% 42% 26% 3	585
	11th to 20th	Night	8% 3 18% 42% 26% 3	582
	21st to end	Night	9% 2 19% 46% 21% 2	657
August	1st to 10th	Night	9% 2 16% 43% 27% 3	597
	11th to 20th	Night	9% 3 18% 37% 30% 4	594
	21st to end	Night	11% 2 21% 36% 27% 3	660
September	1st to 10th	Night	9% 4 25% 32% 24% 5%	685
	11th to 20th	Night	11% 4 17% 35% 29% 5%	800
	21st to end	Night	16% 6% 24% 31% 19% 4	800
October	1st to 10th	Night	18% 8% 25% 31% 17% 2	800
	11th to 20th	Night	22% 12% 29% 28% 9% 1	800
	21st to end	Night	24% 12% 33% 26% 5%	880
November	1st to 10th	Night	20% 10% 34% 27% 9%	820
	11th to 20th	Night	19% 13% 38% 21% 9%	840
	21st to end	Night	24% 13% 37% 22% 4	840
December	1st to 10th	Night	22% 12% 36% 26% 4 1	887
	11th to 20th	Night	22% 11% 40% 24% 4 1	1050
	21st to end	Night	30% 11% 35% 22% 3	1155

Page generated at 2026-02-28 01:40:19. Version ac07

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