Heidelberg College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heidelberg College clear sky chart from 2005-11-06 to 2026-06-01.

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	15% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	132429
	Night	15% 11% 4 9% 5% 4 4 4 5% 6% 33%	130735
Transparency	Day	50% 6% 11% 20% 12% 1	136041
	Night	50% 5% 11% 20% 12% 1	135193
Seeing	Day	19% 11% 28% 22% 14% 6%	17530
	Night	19% 10% 28% 29% 12% 1	27297

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	16% 19% 8% 17% 7% 5% 4 3 4 5% 13%	4691
	11th to 20th	Day	12% 17% 7% 15% 9% 6% 7% 3 3 3 18%	5142
	21st to end	Day	18% 16% 8% 13% 7% 6% 6% 3 3 4 14%	5780
February	1st to 10th	Day	16% 16% 7% 12% 6% 5% 4 4 4 5% 21%	1890
	11th to 20th	Day	14% 13% 6% 13% 6% 6% 5% 4 4 4 24%	2011
	21st to end	Day	16% 14% 6% 11% 5% 4 4 3 6% 6% 25%	1730
March	1st to 10th	Day	18% 14% 6% 13% 5% 4 4 3 3 5% 26%	4739
	11th to 20th	Day	11% 12% 4 8% 4 4 5% 3 5% 7% 37%	5090
	21st to end	Day	19% 14% 4 10% 6% 5% 4 3 4 5% 25%	5586
April	1st to 10th	Day	15% 12% 4 8% 6% 5% 4 4 6% 6% 28%	2400
	11th to 20th	Day	16% 11% 3 7% 5% 5% 5% 4 5% 7% 34%	2655
	21st to end	Day	19% 10% 3 6% 5% 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	23% 9% 3 8% 5% 4 4 3 5% 6% 29%	5992
	11th to 20th	Day	17% 7% 2 7% 6% 5% 5% 4 6% 8% 34%	5991
	21st to end	Day	17% 5% 2 7% 5% 5% 5% 4 6% 8% 38%	6576
June	1st to 10th	Day	13% 6% 3 6% 6% 4 5% 4 6% 8% 39%	2624
	11th to 20th	Day	11% 7% 2 8% 5% 5% 5% 4 7% 8% 39%	2800
	21st to end	Day	11% 6% 2 6% 5% 6% 6% 5% 8% 9% 38%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 10% 40%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	6140
	21st to end	Day	10% 5% 3 7% 5% 5% 5% 4 7% 10% 40%	6424
August	1st to 10th	Day	11% 6% 2 6% 4 4 6% 5% 7% 9% 41%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	4174
	21st to end	Day	8% 5% 2 7% 4 5% 5% 4 7% 10% 43%	4609
September	1st to 10th	Day	13% 6% 2 4 4 4 5% 4 5% 7% 47%	2233
	11th to 20th	Day	9% 5% 1 6% 5% 4 5% 4 6% 7% 48%	2200
	21st to end	Day	17% 8% 2 6% 4 4 4 3 4 6% 41%	2200
October	1st to 10th	Day	12% 8% 2 7% 5% 4 4 4 6% 6% 43%	2145
	11th to 20th	Day	16% 8% 3 6% 4 5% 5% 4 5% 6% 39%	2000
	21st to end	Day	18% 10% 4 8% 5% 4 5% 4 5% 7% 30%	2090
November	1st to 10th	Day	12% 10% 3 6% 5% 4 4 4 5% 7% 41%	1852
	11th to 20th	Day	17% 14% 3 7% 5% 4 4 3 4 6% 33%	1890
	21st to end	Day	18% 16% 4 9% 5% 4 3 3 5% 6% 26%	1890
December	1st to 10th	Day	16% 15% 7% 14% 6% 5% 4 3 3 5% 22%	3020
	11th to 20th	Day	19% 19% 7% 10% 5% 5% 3 3 4 5% 21%	3463
	21st to end	Day	24% 21% 7% 12% 6% 4 3 2 3 4 14%	4278
January	1st to 10th	Night	16% 18% 6% 17% 6% 4 4 3 4 4 17%	8232
	11th to 20th	Night	9% 19% 8% 17% 7% 5% 5% 4 4 4 17%	7803
	21st to end	Night	18% 15% 6% 11% 5% 5% 5% 4 5% 5% 22%	8401
February	1st to 10th	Night	18% 14% 5% 11% 7% 5% 5% 4 4 5% 23%	2730
	11th to 20th	Night	15% 14% 5% 11% 7% 5% 4 3 5% 5% 26%	2492
	21st to end	Night	18% 14% 5% 9% 5% 5% 3 4 4 5% 27%	2076
March	1st to 10th	Night	18% 12% 4 8% 4 3 3 3 4 5% 35%	5319
	11th to 20th	Night	11% 8% 2 6% 5% 4 5% 5% 6% 8% 41%	5090
	21st to end	Night	17% 10% 4 8% 5% 4 4 3 5% 5% 36%	5588
April	1st to 10th	Night	17% 12% 3 8% 5% 5% 4 4 4 6% 33%	2237
	11th to 20th	Night	15% 8% 3 7% 5% 5% 4 4 6% 7% 37%	1913
	21st to end	Night	18% 9% 2 7% 5% 5% 4 3 5% 7% 37%	1890
May	1st to 10th	Night	21% 11% 3 8% 5% 5% 5% 3 5% 5% 27%	4123
	11th to 20th	Night	16% 8% 3 7% 4 4 4 4 6% 8% 38%	4122
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 8% 42%	4520
June	1st to 10th	Night	11% 7% 3 6% 5% 4 4 4 6% 7% 46%	1770
	11th to 20th	Night	10% 5% 2 5% 4 5% 4 5% 9% 8% 43%	1600
	21st to end	Night	9% 5% 2 5% 6% 4 5% 4 6% 9% 46%	1600
July	1st to 10th	Night	10% 4 1 6% 5% 4 5% 4 6% 10% 44%	3618
	11th to 20th	Night	10% 4 2 6% 4 4 5% 4 7% 10% 45%	3810
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 46%	4427
August	1st to 10th	Night	12% 5% 2 6% 5% 4 4 5% 7% 9% 42%	2915
	11th to 20th	Night	10% 6% 2 8% 5% 4 4 3 4 8% 46%	2875
	21st to end	Night	7% 4 1 6% 5% 5% 5% 4 6% 8% 48%	3284
September	1st to 10th	Night	11% 5% 2 4 4 3 4 3 5% 7% 50%	2145
	11th to 20th	Night	9% 4 1 4 4 3 4 3 4 7% 56%	2200
	21st to end	Night	13% 6% 3 7% 4 4 4 3 5% 6% 45%	2200
October	1st to 10th	Night	11% 6% 2 6% 5% 4 3 3 5% 6% 48%	2253
	11th to 20th	Night	18% 8% 2 7% 5% 3 3 3 5% 6% 40%	2400
	21st to end	Night	21% 8% 3 6% 4 4 4 3 5% 6% 36%	2747
November	1st to 10th	Night	13% 8% 3 7% 4 3 4 4 5% 6% 43%	2666
	11th to 20th	Night	15% 13% 3 8% 4 4 4 4 5% 7% 34%	2730
	21st to end	Night	20% 14% 5% 8% 5% 4 4 3 5% 5% 26%	2730
December	1st to 10th	Night	13% 13% 6% 13% 6% 6% 5% 4 4 5% 24%	4380
	11th to 20th	Night	18% 18% 7% 11% 5% 4 4 3 4 5% 22%	4952
	21st to end	Night	25% 19% 7% 10% 5% 4 4 3 5% 5% 14%	6897

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	73% 4 10% 11% 2	4692
	11th to 20th	Day	73% 4 6% 7% 10%	5142
	21st to end	Day	75% 3 6% 11% 5%	5780
February	1st to 10th	Day	65% 5% 10% 13% 5% 1	1890
	11th to 20th	Day	63% 5% 9% 15% 7% 1	2021
	21st to end	Day	58% 5% 13% 15% 7% 1	1730
March	1st to 10th	Day	61% 5% 8% 19% 7%	4739
	11th to 20th	Day	46% 5% 13% 25% 9% 1	5090
	21st to end	Day	60% 5% 10% 16% 8% 1	5586
April	1st to 10th	Day	54% 7% 11% 17% 11% 1	2400
	11th to 20th	Day	49% 7% 13% 19% 12% 2	2655
	21st to end	Day	49% 6% 12% 21% 10% 2	2730
May	1st to 10th	Day	55% 7% 9% 16% 12% 1	6027
	11th to 20th	Day	46% 8% 13% 21% 11% 1	5995
	21st to end	Day	43% 7% 10% 22% 15% 4	6590
June	1st to 10th	Day	41% 7% 11% 25% 13% 2	2624
	11th to 20th	Day	41% 8% 13% 21% 15% 2	2800
	21st to end	Day	38% 9% 16% 19% 15% 4	2800
July	1st to 10th	Day	40% 5% 9% 27% 16% 2	6291
	11th to 20th	Day	37% 6% 14% 26% 16% 1	6065
	21st to end	Day	39% 6% 12% 22% 19% 2	6240
August	1st to 10th	Day	36% 7% 15% 26% 14% 1	4221
	11th to 20th	Day	36% 7% 13% 32% 10% 1	4213
	21st to end	Day	35% 6% 13% 31% 12% 2	4608
September	1st to 10th	Day	35% 7% 13% 25% 17% 4	2233
	11th to 20th	Day	33% 7% 11% 28% 19% 2	2200
	21st to end	Day	43% 5% 11% 22% 18%	2200
October	1st to 10th	Day	29% 4 12% 32% 22% 1	3496
	11th to 20th	Day	39% 4 9% 26% 20% 1	3240
	21st to end	Day	43% 5% 10% 23% 18% 1	3353
November	1st to 10th	Day	42% 5% 10% 26% 15% 1	1852
	11th to 20th	Day	53% 5% 11% 19% 11% 2	1890
	21st to end	Day	59% 5% 11% 17% 8%	1890
December	1st to 10th	Day	66% 4 13% 13% 3	3020
	11th to 20th	Day	66% 4 10% 16% 4	3460
	21st to end	Day	77% 4 6% 9% 4	4278
January	1st to 10th	Night	70% 4 7% 15% 3	8233
	11th to 20th	Night	70% 4 7% 9% 8% 1	7803
	21st to end	Night	64% 4 12% 14% 7%	8401
February	1st to 10th	Night	62% 5% 11% 14% 6% 1	2730
	11th to 20th	Night	59% 6% 9% 18% 7% 1	2493
	21st to end	Night	59% 5% 10% 16% 9% 1	2076
March	1st to 10th	Night	53% 4 11% 23% 9% 1	5319
	11th to 20th	Night	38% 6% 15% 28% 12% 1	5090
	21st to end	Night	49% 5% 10% 22% 14% 1	5588
April	1st to 10th	Night	52% 5% 11% 17% 13% 2	2237
	11th to 20th	Night	45% 7% 12% 17% 17% 2	1913
	21st to end	Night	47% 5% 11% 22% 13% 2	1890
May	1st to 10th	Night	58% 6% 8% 14% 13% 2	4150
	11th to 20th	Night	44% 8% 11% 22% 14% 1	4123
	21st to end	Night	39% 6% 12% 25% 16% 2	4528
June	1st to 10th	Night	37% 6% 10% 25% 17% 3	1770
	11th to 20th	Night	34% 9% 14% 24% 16% 3	1600
	21st to end	Night	33% 8% 15% 26% 16% 3	1600
July	1st to 10th	Night	35% 6% 12% 26% 17% 4	3580
	11th to 20th	Night	35% 5% 16% 28% 15% 2	3765
	21st to end	Night	35% 5% 14% 22% 22% 3	4297
August	1st to 10th	Night	38% 6% 13% 26% 15% 2	2904
	11th to 20th	Night	37% 5% 10% 30% 16% 1	2902
	21st to end	Night	33% 6% 13% 30% 16% 3	3284
September	1st to 10th	Night	32% 7% 12% 26% 18% 5%	2145
	11th to 20th	Night	28% 6% 10% 28% 26% 2	2200
	21st to end	Night	39% 7% 10% 22% 19% 2	2200
October	1st to 10th	Night	27% 4 8% 31% 30% 1	3682
	11th to 20th	Night	37% 4 9% 28% 21% 1	3899
	21st to end	Night	37% 3 9% 31% 17% 3	4434
November	1st to 10th	Night	40% 5% 11% 25% 16% 2	2666
	11th to 20th	Night	49% 5% 10% 22% 12% 2	2730
	21st to end	Night	59% 5% 12% 17% 7%	2730
December	1st to 10th	Night	62% 4 10% 17% 6% 1	4380
	11th to 20th	Night	64% 4 13% 13% 5%	4954
	21st to end	Night	72% 6% 9% 9% 4	6897

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	23% 21% 30% 20% 5% 1	330
	11th to 20th	Day	28% 19% 37% 11% 3 1	330
	21st to end	Day	28% 20% 29% 18% 4	363
February	1st to 10th	Day	25% 19% 38% 14% 3 1	330
	11th to 20th	Day	25% 19% 34% 19% 3	330
	21st to end	Day	22% 21% 44% 11% 3	273
March	1st to 10th	Day	24% 16% 38% 17% 5%	398
	11th to 20th	Day	26% 15% 35% 17% 5% 1	540
	21st to end	Day	31% 14% 31% 19% 5% 1	594
April	1st to 10th	Day	22% 16% 32% 19% 10%	540
	11th to 20th	Day	23% 17% 31% 16% 10% 3	532
	21st to end	Day	23% 13% 31% 22% 9% 1	540
May	1st to 10th	Day	28% 10% 33% 18% 9% 2	543
	11th to 20th	Day	22% 9% 29% 23% 11% 6%	567
	21st to end	Day	15% 8% 26% 24% 18% 9%	617
June	1st to 10th	Day	14% 6% 28% 29% 17% 5%	554
	11th to 20th	Day	13% 7% 29% 27% 19% 5%	730
	21st to end	Day	13% 7% 27% 28% 19% 5%	730
July	1st to 10th	Day	11% 4 21% 30% 21% 14%	721
	11th to 20th	Day	10% 4 24% 28% 21% 13%	628
	21st to end	Day	11% 5% 18% 26% 28% 13%	576
August	1st to 10th	Day	11% 5% 15% 28% 26% 16%	526
	11th to 20th	Day	11% 3 17% 32% 25% 12%	519
	21st to end	Day	9% 4 19% 27% 26% 15%	583
September	1st to 10th	Day	14% 5% 17% 27% 23% 13%	530
	11th to 20th	Day	8% 2 16% 25% 31% 18%	530
	21st to end	Day	19% 10% 21% 23% 18% 9%	530
October	1st to 10th	Day	13% 7% 26% 25% 22% 8%	530
	11th to 20th	Day	17% 15% 32% 23% 11% 2	530
	21st to end	Day	22% 16% 36% 19% 5% 1	473
November	1st to 10th	Day	20% 13% 33% 21% 10% 3	330
	11th to 20th	Day	27% 19% 30% 15% 8%	330
	21st to end	Day	29% 17% 33% 18% 2	330
December	1st to 10th	Day	25% 18% 34% 16% 6% 1	330
	11th to 20th	Day	25% 21% 34% 17% 2	330
	21st to end	Day	34% 16% 31% 14% 4	363
January	1st to 10th	Night	25% 16% 35% 19% 4	1050
	11th to 20th	Night	27% 17% 36% 16% 3 1	885
	21st to end	Night	22% 18% 35% 21% 4	924
February	1st to 10th	Night	27% 13% 33% 24% 3	840
	11th to 20th	Night	24% 13% 36% 25% 2	823
	21st to end	Night	26% 15% 39% 18% 2	692
March	1st to 10th	Night	23% 12% 34% 25% 6%	840
	11th to 20th	Night	20% 17% 35% 23% 5% 1	840
	21st to end	Night	23% 13% 32% 25% 8%	924
April	1st to 10th	Night	23% 13% 35% 24% 4	767
	11th to 20th	Night	19% 13% 36% 22% 9% 1	623
	21st to end	Night	22% 8% 33% 29% 7% 1	630
May	1st to 10th	Night	24% 7% 29% 32% 8% 1	630
	11th to 20th	Night	21% 7% 28% 31% 12% 1	621
	21st to end	Night	13% 3 23% 40% 18% 3	683
June	1st to 10th	Night	11% 3 27% 42% 17%	600
	11th to 20th	Night	11% 3 24% 43% 18% 1	600
	21st to end	Night	11% 3 24% 38% 23% 1	600
July	1st to 10th	Night	9% 3 15% 41% 29% 3	585
	11th to 20th	Night	9% 4 18% 39% 26% 4	582
	21st to end	Night	9% 3 14% 42% 30% 2	657
August	1st to 10th	Night	8% 2 12% 45% 30% 3	597
	11th to 20th	Night	11% 2 12% 41% 29% 4	594
	21st to end	Night	7% 3 13% 38% 35% 4	660
September	1st to 10th	Night	14% 4 15% 36% 27% 5%	745
	11th to 20th	Night	9% 3 15% 36% 33% 4	800
	21st to end	Night	15% 6% 21% 34% 22% 2	800
October	1st to 10th	Night	14% 7% 23% 34% 20% 3	800
	11th to 20th	Night	22% 12% 28% 29% 9% 2	800
	21st to end	Night	24% 11% 32% 27% 6%	880
November	1st to 10th	Night	18% 10% 33% 28% 10% 1	820
	11th to 20th	Night	22% 14% 34% 21% 7% 1	840
	21st to end	Night	27% 13% 31% 23% 5%	840
December	1st to 10th	Night	26% 14% 35% 20% 5%	840
	11th to 20th	Night	26% 14% 36% 20% 4	840
	21st to end	Night	30% 15% 33% 19% 3	1045

Page generated at 2026-05-29 01:53:31. Version ac07

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