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 2024-12-02.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	101245
	Night	14% 10% 4 8% 5% 5% 4 4 5% 7% 34%	98379
Transparency	Day	45% 7% 13% 21% 12% 1	104830
	Night	47% 7% 13% 21% 11% 1	102934
Seeing	Day	15% 10% 29% 26% 15% 6%	15593
	Night	18% 8% 31% 31% 12% 1	25292

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	14% 16% 7% 12% 7% 5% 4 3 5% 5% 22%	2527
	11th to 20th	Day	14% 17% 7% 15% 13% 8% 6% 3 4 3 9%	2530
	21st to end	Day	22% 19% 7% 12% 6% 5% 4 3 3 4 14%	2890
February	1st to 10th	Day	10% 16% 5% 11% 7% 7% 6% 4 5% 8% 21%	1710
	11th to 20th	Day	11% 13% 4 12% 8% 7% 5% 3 5% 5% 27%	1793
	21st to end	Day	12% 13% 5% 9% 6% 6% 6% 4 5% 6% 27%	1570
March	1st to 10th	Day	16% 12% 3 8% 6% 5% 4 3 5% 6% 32%	3147
	11th to 20th	Day	12% 12% 5% 9% 6% 5% 5% 4 5% 6% 31%	3480
	21st to end	Day	16% 13% 4 9% 5% 5% 5% 4 5% 5% 29%	3815
April	1st to 10th	Day	16% 12% 3 8% 6% 4 4 4 5% 7% 30%	2254
	11th to 20th	Day	16% 9% 3 8% 4 4 4 3 5% 7% 36%	2401
	21st to end	Day	16% 9% 2 7% 5% 5% 5% 4 5% 6% 35%	2470
May	1st to 10th	Day	15% 9% 3 6% 4 4 5% 4 5% 8% 37%	4082
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 4 6% 9% 38%	4081
	21st to end	Day	10% 6% 2 6% 4 4 4 4 5% 8% 47%	4659
June	1st to 10th	Day	11% 6% 2 6% 4 5% 4 4 6% 8% 43%	2807
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	2850
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 40%	2782
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 5% 4 7% 8% 38%	4440
	11th to 20th	Day	6% 5% 2 5% 4 4 4 4 6% 10% 49%	4440
	21st to end	Day	6% 5% 2 6% 5% 5% 5% 4 8% 9% 44%	4700
August	1st to 10th	Day	11% 5% 2 5% 5% 6% 5% 5% 7% 8% 41%	4104
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 45%	4044
	21st to end	Day	8% 6% 2 7% 4 5% 5% 5% 6% 9% 43%	4470
September	1st to 10th	Day	9% 7% 2 8% 5% 5% 3 4 6% 8% 43%	2180
	11th to 20th	Day	11% 6% 1 5% 4 4 4 3 6% 9% 48%	2090
	21st to end	Day	13% 7% 3 7% 5% 5% 4 5% 6% 8% 37%	2090
October	1st to 10th	Day	14% 6% 2 8% 5% 5% 4 4 5% 6% 40%	2049
	11th to 20th	Day	18% 11% 3 7% 5% 5% 5% 4 5% 8% 30%	1900
	21st to end	Day	20% 10% 3 8% 6% 4 5% 4 5% 8% 27%	1881
November	1st to 10th	Day	12% 10% 3 9% 5% 5% 4 4 6% 7% 34%	1762
	11th to 20th	Day	14% 13% 4 8% 6% 5% 5% 5% 6% 7% 27%	1800
	21st to end	Day	17% 14% 5% 9% 7% 5% 5% 4 5% 6% 23%	1655
December	1st to 10th	Day	12% 17% 7% 12% 7% 6% 4 4 5% 5% 21%	1481
	11th to 20th	Day	14% 16% 5% 11% 6% 4 4 4 6% 6% 24%	1675
	21st to end	Day	26% 19% 6% 11% 5% 4 4 3 5% 5% 11%	2636
January	1st to 10th	Night	16% 15% 7% 12% 6% 5% 4 3 4 5% 24%	4428
	11th to 20th	Night	13% 17% 8% 17% 9% 8% 4 3 4 4 14%	4440
	21st to end	Night	19% 18% 7% 12% 6% 5% 4 4 5% 5% 16%	4758
February	1st to 10th	Night	13% 15% 4 11% 7% 6% 5% 4 6% 6% 24%	2470
	11th to 20th	Night	13% 14% 6% 9% 7% 6% 4 3 5% 6% 26%	2270
	21st to end	Night	16% 13% 4 10% 7% 5% 4 4 5% 6% 26%	1884
March	1st to 10th	Night	14% 10% 4 8% 4 4 3 3 4 6% 41%	3706
	11th to 20th	Night	9% 7% 3 6% 5% 5% 4 4 6% 8% 42%	3480
	21st to end	Night	15% 10% 4 10% 6% 4 4 4 5% 6% 32%	3817
April	1st to 10th	Night	18% 11% 3 6% 5% 3 4 3 5% 6% 35%	1943
	11th to 20th	Night	19% 7% 2 6% 4 4 3 5% 5% 6% 38%	1727
	21st to end	Night	17% 9% 3 7% 4 4 4 4 5% 6% 37%	1710
May	1st to 10th	Night	12% 9% 3 7% 5% 4 4 4 5% 7% 38%	2813
	11th to 20th	Night	15% 7% 2 7% 4 4 5% 3 5% 7% 41%	2812
	21st to end	Night	10% 6% 2 6% 4 4 4 3 5% 7% 48%	2898
June	1st to 10th	Night	11% 7% 3 5% 5% 3 4 4 5% 7% 46%	1368
	11th to 20th	Night	11% 5% 2 6% 5% 4 4 5% 6% 10% 43%	1330
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 8% 46%	1391
July	1st to 10th	Night	8% 5% 3 7% 5% 4 4 4 7% 9% 44%	2530
	11th to 20th	Night	8% 3 1 4 3 4 4 3 6% 8% 55%	2530
	21st to end	Night	7% 5% 2 6% 4 6% 5% 5% 7% 8% 47%	2858
August	1st to 10th	Night	10% 5% 2 6% 4 4 3 4 6% 8% 47%	2825
	11th to 20th	Night	10% 5% 2 7% 4 3 4 3 5% 8% 47%	2785
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3182
September	1st to 10th	Night	8% 5% 2 7% 4 5% 4 4 6% 6% 49%	1981
	11th to 20th	Night	10% 5% 2 4 3 3 4 2 6% 8% 53%	2090
	21st to end	Night	13% 8% 2 7% 4 4 5% 3 5% 6% 43%	2090
October	1st to 10th	Night	16% 7% 2 5% 4 3 4 3 5% 7% 43%	2130
	11th to 20th	Night	18% 10% 3 9% 4 5% 4 3 4 6% 34%	2280
	21st to end	Night	21% 9% 3 9% 5% 4 4 3 4 7% 29%	2712
November	1st to 10th	Night	14% 9% 2 7% 5% 4 3 3 5% 7% 40%	2536
	11th to 20th	Night	14% 12% 3 9% 6% 6% 5% 5% 6% 7% 28%	2600
	21st to end	Night	18% 15% 4 9% 6% 5% 5% 4 5% 6% 25%	2743
December	1st to 10th	Night	16% 16% 6% 10% 6% 4 4 4 5% 6% 22%	2728
	11th to 20th	Night	13% 14% 5% 10% 7% 6% 4 4 6% 6% 24%	3522
	21st to end	Night	26% 20% 5% 10% 5% 4 3 3 4 4 15%	5012

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	63% 8% 14% 11% 3	2528
	11th to 20th	Day	77% 6% 9% 6% 2	2530
	21st to end	Day	73% 6% 9% 8% 3	2890
February	1st to 10th	Day	60% 8% 12% 16% 5%	1710
	11th to 20th	Day	58% 6% 12% 16% 7% 1	1803
	21st to end	Day	55% 7% 13% 17% 7% 1	1570
March	1st to 10th	Day	52% 7% 12% 22% 6% 1	3147
	11th to 20th	Day	49% 7% 15% 18% 9% 1	3480
	21st to end	Day	54% 7% 12% 16% 10% 1	3815
April	1st to 10th	Day	53% 6% 13% 18% 10%	2254
	11th to 20th	Day	46% 6% 12% 20% 13% 2	2401
	21st to end	Day	47% 7% 14% 18% 13% 1	2470
May	1st to 10th	Day	42% 8% 13% 20% 15% 2	4120
	11th to 20th	Day	40% 9% 14% 25% 11% 1	4085
	21st to end	Day	34% 7% 12% 21% 22% 4	4672
June	1st to 10th	Day	37% 7% 13% 27% 14% 2	2807
	11th to 20th	Day	35% 6% 14% 24% 18% 2	2850
	21st to end	Day	35% 9% 15% 25% 15% 1	2782
July	1st to 10th	Day	40% 8% 14% 21% 15% 2	4371
	11th to 20th	Day	27% 7% 16% 35% 14% 1	4365
	21st to end	Day	32% 9% 16% 20% 21% 2	4551
August	1st to 10th	Day	36% 9% 14% 24% 16% 1	4091
	11th to 20th	Day	35% 7% 14% 32% 11% 1	4083
	21st to end	Day	35% 8% 16% 28% 13% 1	4469
September	1st to 10th	Day	39% 7% 16% 25% 12% 2	2180
	11th to 20th	Day	33% 7% 16% 29% 15% 1	2090
	21st to end	Day	43% 7% 14% 24% 12%	2090
October	1st to 10th	Day	32% 6% 10% 32% 17% 3	3400
	11th to 20th	Day	40% 6% 14% 26% 14%	3140
	21st to end	Day	49% 7% 14% 23% 7%	3081
November	1st to 10th	Day	48% 7% 13% 21% 10%	1762
	11th to 20th	Day	53% 7% 15% 16% 8%	1800
	21st to end	Day	61% 7% 14% 14% 4 1	1655
December	1st to 10th	Day	64% 7% 13% 12% 3	1481
	11th to 20th	Day	58% 7% 14% 16% 3 1	1671
	21st to end	Day	75% 7% 9% 7% 3	2636
January	1st to 10th	Night	61% 7% 13% 13% 6% 1	4428
	11th to 20th	Night	72% 6% 10% 8% 3	4440
	21st to end	Night	68% 7% 11% 11% 3	4758
February	1st to 10th	Night	59% 7% 11% 17% 6% 1	2470
	11th to 20th	Night	58% 6% 10% 17% 8% 1	2271
	21st to end	Night	58% 5% 11% 17% 9%	1884
March	1st to 10th	Night	45% 5% 15% 27% 8% 1	3706
	11th to 20th	Night	37% 7% 19% 22% 14% 1	3480
	21st to end	Night	50% 8% 14% 19% 8% 1	3817
April	1st to 10th	Night	48% 6% 11% 21% 12% 2	1943
	11th to 20th	Night	44% 6% 11% 20% 17% 2	1727
	21st to end	Night	47% 5% 11% 18% 15% 3	1710
May	1st to 10th	Night	41% 8% 13% 19% 16% 3	2840
	11th to 20th	Night	40% 7% 11% 27% 12% 3	2813
	21st to end	Night	35% 7% 10% 23% 19% 6%	2906
June	1st to 10th	Night	37% 7% 11% 23% 20% 2	1368
	11th to 20th	Night	35% 8% 13% 21% 22% 2	1330
	21st to end	Night	34% 8% 14% 26% 17% 2	1391
July	1st to 10th	Night	34% 7% 13% 25% 15% 7%	2490
	11th to 20th	Night	25% 7% 14% 35% 17% 1	2482
	21st to end	Night	32% 10% 17% 21% 19% 1	2767
August	1st to 10th	Night	33% 8% 14% 29% 16% 1	2814
	11th to 20th	Night	34% 6% 16% 30% 13% 1	2812
	21st to end	Night	32% 9% 14% 31% 15% 1	3182
September	1st to 10th	Night	35% 7% 15% 26% 17% 1	1981
	11th to 20th	Night	29% 7% 14% 32% 18% 1	2090
	21st to end	Night	41% 6% 14% 26% 12% 1	2090
October	1st to 10th	Night	27% 6% 12% 33% 22% 1	3559
	11th to 20th	Night	45% 6% 9% 25% 15%	3779
	21st to end	Night	45% 6% 11% 28% 9%	4462
November	1st to 10th	Night	43% 7% 13% 24% 13% 1	2536
	11th to 20th	Night	52% 9% 13% 17% 8% 1	2600
	21st to end	Night	58% 7% 15% 14% 4 1	2743
December	1st to 10th	Night	62% 6% 13% 13% 5%	2728
	11th to 20th	Night	58% 8% 15% 12% 6% 1	3525
	21st to end	Night	72% 7% 11% 7% 4	5012

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% 11% 34% 26% 9% 2	270
	11th to 20th	Day	20% 11% 37% 24% 8% 1	270
	21st to end	Day	21% 11% 29% 30% 7% 1	297
February	1st to 10th	Day	18% 15% 40% 22% 3 1	270
	11th to 20th	Day	17% 14% 36% 27% 6%	270
	21st to end	Day	20% 18% 41% 19% 3	225
March	1st to 10th	Day	21% 12% 40% 23% 4 1	284
	11th to 20th	Day	15% 13% 32% 26% 12% 1	460
	21st to end	Day	23% 11% 31% 25% 8% 1	506
April	1st to 10th	Day	19% 11% 34% 27% 9% 1	460
	11th to 20th	Day	23% 14% 31% 20% 8% 4	452
	21st to end	Day	19% 13% 30% 25% 11% 2	460
May	1st to 10th	Day	19% 14% 31% 21% 11% 4	463
	11th to 20th	Day	16% 8% 27% 25% 17% 7%	487
	21st to end	Day	14% 6% 22% 28% 22% 7%	634
June	1st to 10th	Day	12% 8% 29% 31% 15% 6%	680
	11th to 20th	Day	13% 8% 28% 30% 15% 6%	680
	21st to end	Day	12% 6% 28% 30% 18% 5%	680
July	1st to 10th	Day	7% 5% 23% 30% 23% 12%	671
	11th to 20th	Day	9% 5% 26% 30% 19% 10%	668
	21st to end	Day	7% 3 26% 33% 24% 7%	675
August	1st to 10th	Day	8% 4 20% 27% 26% 14%	486
	11th to 20th	Day	10% 3 16% 28% 29% 13%	479
	21st to end	Day	11% 7% 20% 27% 26% 10%	539
September	1st to 10th	Day	9% 8% 26% 27% 19% 11%	490
	11th to 20th	Day	10% 4 19% 24% 28% 15%	490
	21st to end	Day	16% 9% 22% 26% 18% 9%	490
October	1st to 10th	Day	18% 10% 27% 22% 16% 8%	490
	11th to 20th	Day	21% 15% 29% 26% 8% 1	490
	21st to end	Day	22% 15% 36% 19% 6% 1	330
November	1st to 10th	Day	17% 15% 34% 23% 9% 2	300
	11th to 20th	Day	17% 21% 37% 18% 5% 2	300
	21st to end	Day	25% 10% 40% 20% 5%	227
December	1st to 10th	Day	22% 13% 38% 19% 7% 1	182
	11th to 20th	Day	22% 16% 37% 20% 4 1	180
	21st to end	Day	23% 16% 36% 21% 3	258
January	1st to 10th	Night	19% 11% 37% 25% 7% 1	950
	11th to 20th	Night	22% 11% 35% 24% 7% 1	950
	21st to end	Night	17% 8% 33% 33% 8% 1	971
February	1st to 10th	Night	19% 8% 38% 31% 4 1	760
	11th to 20th	Night	19% 9% 40% 26% 6%	743
	21st to end	Night	23% 10% 39% 23% 4	628
March	1st to 10th	Night	18% 8% 35% 31% 8%	760
	11th to 20th	Night	16% 9% 37% 30% 8% 1	760
	21st to end	Night	21% 9% 32% 31% 6% 1	836
April	1st to 10th	Night	19% 10% 36% 27% 7% 1	613
	11th to 20th	Night	23% 7% 33% 28% 9% 2	563
	21st to end	Night	22% 6% 29% 32% 9% 1	570
May	1st to 10th	Night	15% 7% 32% 37% 9% 1	570
	11th to 20th	Night	18% 6% 26% 36% 13% 1	561
	21st to end	Night	14% 3 25% 36% 20% 2	617
June	1st to 10th	Night	12% 5% 29% 39% 15%	418
	11th to 20th	Night	12% 5% 24% 42% 17%	380
	21st to end	Night	10% 4 24% 41% 20% 2	441
July	1st to 10th	Night	8% 3 18% 42% 27% 3	555
	11th to 20th	Night	8% 3 17% 42% 26% 3	552
	21st to end	Night	10% 2 19% 46% 22% 2	624
August	1st to 10th	Night	9% 2 16% 44% 27% 2	567
	11th to 20th	Night	8% 2 18% 36% 31% 4	564
	21st to end	Night	12% 3 20% 36% 27% 3	627
September	1st to 10th	Night	10% 4 25% 33% 24% 5%	651
	11th to 20th	Night	12% 4 17% 34% 28% 5%	760
	21st to end	Night	16% 6% 24% 31% 19% 4	760
October	1st to 10th	Night	18% 8% 25% 30% 16% 2	760
	11th to 20th	Night	22% 12% 29% 27% 9% 1	760
	21st to end	Night	25% 12% 33% 25% 5%	836
November	1st to 10th	Night	19% 9% 34% 27% 9% 1	780
	11th to 20th	Night	20% 13% 38% 21% 9%	800
	21st to end	Night	23% 13% 37% 23% 4	800
December	1st to 10th	Night	23% 10% 36% 26% 5% 1	810
	11th to 20th	Night	21% 9% 40% 25% 4 1	950
	21st to end	Night	29% 10% 35% 23% 3	1045

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