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 2024-08-30.

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	15% 11% 4 8% 5% 5% 5% 4 5% 7% 32%	95735
	Night	16% 11% 4 8% 5% 4 4 4 5% 6% 34%	93248
Transparency	Day	50% 7% 11% 19% 11% 2	95499
	Night	49% 6% 11% 20% 11% 2	93089
Seeing	Day	18% 10% 27% 23% 15% 6%	15201
	Night	19% 9% 28% 29% 13% 1	24912

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% 21% 8% 14% 6% 5% 4 3 5% 5% 12%	2527
	11th to 20th	Day	16% 21% 7% 13% 6% 6% 10% 3 4 3 12%	2754
	21st to end	Day	26% 20% 10% 12% 6% 5% 3 3 2 3 10%	3124
February	1st to 10th	Day	17% 16% 6% 12% 6% 5% 4 4 4 5% 21%	1710
	11th to 20th	Day	14% 13% 5% 13% 6% 6% 5% 4 4 5% 24%	1814
	21st to end	Day	17% 14% 5% 10% 6% 4 4 3 5% 6% 25%	1570
March	1st to 10th	Day	21% 18% 5% 10% 5% 4 3 3 3 5% 23%	3234
	11th to 20th	Day	12% 14% 4 9% 4 4 5% 3 4 6% 34%	3480
	21st to end	Day	21% 12% 4 10% 5% 5% 4 3 4 5% 26%	3815
April	1st to 10th	Day	14% 13% 4 8% 6% 5% 4 5% 6% 6% 29%	2171
	11th to 20th	Day	16% 10% 3 6% 5% 5% 4 4 5% 7% 34%	2399
	21st to end	Day	19% 10% 2 7% 5% 4 4 3 6% 6% 34%	2470
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 7% 29%	4082
	11th to 20th	Day	18% 8% 2 6% 6% 4 5% 4 6% 7% 33%	4081
	21st to end	Day	11% 5% 2 7% 5% 5% 5% 4 6% 8% 40%	4476
June	1st to 10th	Day	13% 6% 3 6% 6% 4 5% 4 6% 9% 38%	2493
	11th to 20th	Day	10% 6% 2 8% 5% 5% 5% 4 7% 8% 40%	2660
	21st to end	Day	11% 6% 2 6% 5% 6% 5% 5% 7% 9% 38%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 10% 38%	4440
	11th to 20th	Day	8% 4 2 5% 4 4 5% 5% 7% 11% 46%	4290
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 4 7% 9% 41%	4466
August	1st to 10th	Day	9% 5% 2 5% 4 5% 6% 5% 7% 10% 43%	2457
	11th to 20th	Day	9% 5% 2 6% 5% 6% 6% 4 8% 10% 39%	2433
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 8% 10% 42%	2656
September	1st to 10th	Day	14% 6% 2 5% 4 5% 5% 4 5% 7% 43%	2011
	11th to 20th	Day	10% 6% 2 6% 5% 4 5% 4 7% 7% 45%	1980
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 6% 43%	1980
October	1st to 10th	Day	13% 9% 2 8% 5% 4 4 4 6% 6% 40%	1931
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 4 5% 7% 36%	1800
	21st to end	Day	19% 11% 4 8% 5% 4 5% 5% 5% 6% 29%	1882
November	1st to 10th	Day	12% 11% 3 6% 5% 4 4 3 5% 7% 41%	1672
	11th to 20th	Day	17% 13% 4 7% 5% 3 4 3 4 6% 34%	1710
	21st to end	Day	18% 15% 4 9% 5% 4 4 3 5% 6% 27%	1710
December	1st to 10th	Day	16% 20% 8% 12% 6% 4 4 3 3 5% 19%	1710
	11th to 20th	Day	16% 16% 6% 11% 5% 5% 4 3 5% 7% 22%	2153
	21st to end	Day	22% 22% 8% 12% 6% 5% 4 3 4 4 11%	2934
January	1st to 10th	Night	18% 19% 7% 13% 5% 4 4 3 4 4 19%	4428
	11th to 20th	Night	14% 21% 7% 17% 5% 5% 6% 5% 4 4 12%	4187
	21st to end	Night	27% 19% 8% 12% 4 4 3 2 3 3 15%	4532
February	1st to 10th	Night	19% 14% 5% 11% 6% 5% 5% 4 4 5% 23%	2470
	11th to 20th	Night	15% 15% 4 10% 7% 5% 4 3 5% 5% 27%	2250
	21st to end	Night	19% 14% 5% 9% 5% 5% 4 4 4 6% 26%	1884
March	1st to 10th	Night	19% 14% 4 7% 4 3 3 3 3 5% 34%	3618
	11th to 20th	Night	13% 8% 2 6% 5% 4 4 4 5% 7% 41%	3480
	21st to end	Night	17% 10% 4 8% 4 4 4 3 5% 6% 36%	3817
April	1st to 10th	Night	15% 12% 3 7% 5% 5% 4 4 4 6% 33%	2024
	11th to 20th	Night	17% 8% 2 7% 5% 5% 4 3 6% 7% 37%	1729
	21st to end	Night	18% 8% 2 7% 5% 5% 4 3 5% 7% 37%	1710
May	1st to 10th	Night	20% 8% 3 9% 6% 5% 5% 3 5% 6% 29%	2813
	11th to 20th	Night	17% 8% 2 7% 4 4 4 3 5% 7% 39%	2812
	21st to end	Night	10% 5% 1 5% 4 4 4 4 5% 9% 48%	3079
June	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 7% 45%	1682
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 9% 8% 44%	1520
	21st to end	Night	9% 5% 2 5% 6% 4 5% 4 6% 9% 46%	1520
July	1st to 10th	Night	10% 4 1 6% 5% 4 5% 4 6% 9% 45%	2530
	11th to 20th	Night	9% 3 1 4 3 4 4 4 7% 10% 51%	2671
	21st to end	Night	7% 5% 2 6% 4 5% 5% 5% 6% 10% 45%	3087
August	1st to 10th	Night	8% 3 2 5% 5% 5% 5% 5% 7% 9% 46%	1701
	11th to 20th	Night	8% 5% 2 7% 5% 4 4 4 5% 9% 47%	1688
	21st to end	Night	7% 4 1 6% 4 4 4 4 6% 8% 51%	1904
September	1st to 10th	Night	11% 6% 2 5% 4 3 4 3 5% 8% 48%	1930
	11th to 20th	Night	10% 5% 1 5% 4 4 4 3 4 7% 53%	1980
	21st to end	Night	12% 6% 3 7% 4 4 3 3 5% 6% 47%	1980
October	1st to 10th	Night	12% 6% 2 6% 5% 5% 3 3 5% 6% 46%	2027
	11th to 20th	Night	19% 9% 2 7% 5% 3 3 3 5% 6% 38%	2160
	21st to end	Night	22% 9% 3 7% 4 4 4 3 5% 6% 34%	2471
November	1st to 10th	Night	14% 8% 3 7% 4 3 4 4 5% 7% 42%	2406
	11th to 20th	Night	15% 13% 3 8% 4 4 4 4 5% 7% 33%	2470
	21st to end	Night	19% 14% 4 8% 6% 4 4 4 5% 5% 27%	2470
December	1st to 10th	Night	18% 16% 6% 10% 6% 4 4 4 4 5% 23%	2470
	11th to 20th	Night	19% 15% 5% 10% 4 4 4 3 5% 6% 26%	3042
	21st to end	Night	25% 19% 8% 10% 5% 4 4 3 5% 5% 12%	4706

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% 6% 9% 10% 2	2528
	11th to 20th	Day	76% 5% 8% 8% 4	2754
	21st to end	Day	82% 3 5% 7% 3 1	3124
February	1st to 10th	Day	65% 5% 11% 12% 6% 1	1710
	11th to 20th	Day	62% 6% 9% 16% 7% 1	1824
	21st to end	Day	59% 6% 12% 15% 7% 1	1570
March	1st to 10th	Day	65% 5% 9% 17% 5%	3234
	11th to 20th	Day	49% 6% 14% 21% 8% 1	3480
	21st to end	Day	58% 7% 13% 16% 6%	3815
April	1st to 10th	Day	53% 8% 11% 17% 11% 1	2171
	11th to 20th	Day	48% 7% 12% 19% 12% 2	2399
	21st to end	Day	49% 6% 13% 20% 10% 2	2470
May	1st to 10th	Day	51% 9% 10% 21% 8% 1	4120
	11th to 20th	Day	45% 10% 13% 23% 9% 1	4085
	21st to end	Day	37% 9% 12% 20% 17% 5%	4489
June	1st to 10th	Day	41% 8% 11% 26% 12% 2	2493
	11th to 20th	Day	40% 8% 13% 22% 15% 2	2660
	21st to end	Day	39% 9% 16% 19% 15% 4	2660
July	1st to 10th	Day	42% 5% 11% 22% 17% 3	4371
	11th to 20th	Day	31% 7% 13% 28% 19% 2	4215
	21st to end	Day	38% 7% 14% 18% 20% 3	4324
August	1st to 10th	Day	33% 9% 16% 27% 14% 1	2444
	11th to 20th	Day	36% 10% 15% 26% 11% 2	2433
	21st to end	Day	34% 9% 14% 22% 17% 4	2656
September	1st to 10th	Day	38% 7% 13% 22% 16% 4	2011
	11th to 20th	Day	36% 7% 12% 26% 18% 2	1980
	21st to end	Day	41% 5% 12% 23% 18%	1980
October	1st to 10th	Day	43% 7% 11% 22% 16% 1	1931
	11th to 20th	Day	46% 7% 11% 22% 13% 1	1800
	21st to end	Day	53% 6% 12% 18% 10% 1	1882
November	1st to 10th	Day	42% 5% 11% 24% 16% 1	1672
	11th to 20th	Day	52% 5% 11% 19% 11% 2	1710
	21st to end	Day	57% 5% 11% 18% 8%	1710
December	1st to 10th	Day	69% 5% 9% 13% 3	1710
	11th to 20th	Day	62% 6% 11% 17% 5%	2150
	21st to end	Day	77% 6% 7% 7% 3	2934
January	1st to 10th	Night	66% 6% 8% 15% 4	4428
	11th to 20th	Night	76% 5% 8% 9% 2	4187
	21st to end	Night	75% 4 8% 9% 3	4532
February	1st to 10th	Night	63% 6% 11% 13% 6% 1	2470
	11th to 20th	Night	58% 6% 9% 18% 8% 1	2251
	21st to end	Night	60% 6% 9% 15% 9% 2	1884
March	1st to 10th	Night	53% 5% 12% 22% 8% 1	3618
	11th to 20th	Night	38% 8% 16% 23% 14% 1	3480
	21st to end	Night	48% 6% 11% 26% 8% 1	3817
April	1st to 10th	Night	50% 6% 12% 17% 13% 2	2024
	11th to 20th	Night	44% 7% 12% 17% 17% 2	1729
	21st to end	Night	47% 6% 11% 21% 14% 3	1710
May	1st to 10th	Night	53% 8% 9% 18% 11% 2	2840
	11th to 20th	Night	42% 10% 12% 23% 13% 1	2813
	21st to end	Night	31% 8% 13% 27% 19% 2	3087
June	1st to 10th	Night	37% 7% 11% 26% 16% 4	1682
	11th to 20th	Night	33% 9% 14% 25% 16% 3	1520
	21st to end	Night	33% 8% 14% 26% 15% 3	1520
July	1st to 10th	Night	35% 6% 11% 26% 16% 5%	2490
	11th to 20th	Night	28% 6% 16% 31% 18% 2	2623
	21st to end	Night	34% 6% 17% 20% 20% 4	2988
August	1st to 10th	Night	31% 8% 15% 29% 17% 1	1690
	11th to 20th	Night	33% 7% 14% 30% 14% 2	1688
	21st to end	Night	28% 8% 14% 24% 21% 4	1904
September	1st to 10th	Night	34% 8% 13% 24% 17% 5%	1930
	11th to 20th	Night	30% 7% 11% 27% 24% 1	1980
	21st to end	Night	37% 7% 10% 24% 20% 2	1980
October	1st to 10th	Night	39% 6% 11% 23% 19% 2	2027
	11th to 20th	Night	47% 6% 9% 23% 15% 1	2160
	21st to end	Night	51% 5% 10% 22% 10% 2	2471
November	1st to 10th	Night	41% 6% 11% 23% 16% 2	2406
	11th to 20th	Night	49% 5% 11% 21% 12% 1	2470
	21st to end	Night	58% 5% 12% 17% 7%	2470
December	1st to 10th	Night	62% 6% 11% 14% 7%	2470
	11th to 20th	Night	58% 5% 14% 17% 5%	3044
	21st to end	Night	73% 7% 9% 8% 2	4706

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% 23% 27% 20% 6% 1	270
	11th to 20th	Day	28% 15% 39% 12% 4 1	270
	21st to end	Day	31% 16% 28% 20% 5%	297
February	1st to 10th	Day	28% 19% 37% 14% 2 1	270
	11th to 20th	Day	24% 19% 33% 21% 4	270
	21st to end	Day	24% 20% 42% 11% 3	225
March	1st to 10th	Day	24% 16% 35% 19% 6%	332
	11th to 20th	Day	23% 15% 36% 18% 6% 2	460
	21st to end	Day	32% 13% 29% 20% 6% 1	506
April	1st to 10th	Day	20% 15% 31% 22% 11%	460
	11th to 20th	Day	22% 15% 31% 17% 11% 3	452
	21st to end	Day	24% 13% 31% 22% 8% 1	460
May	1st to 10th	Day	26% 9% 33% 20% 10% 3	463
	11th to 20th	Day	22% 7% 28% 23% 13% 7%	487
	21st to end	Day	12% 7% 25% 26% 19% 10%	529
June	1st to 10th	Day	14% 5% 27% 30% 18% 6%	513
	11th to 20th	Day	12% 7% 29% 27% 20% 6%	680
	21st to end	Day	14% 7% 28% 28% 18% 5%	680
July	1st to 10th	Day	11% 4 21% 29% 21% 14%	671
	11th to 20th	Day	10% 4 24% 28% 21% 13%	583
	21st to end	Day	10% 5% 19% 26% 28% 12%	532
August	1st to 10th	Day	12% 5% 15% 28% 24% 16%	486
	11th to 20th	Day	11% 4 17% 31% 25% 12%	479
	21st to end	Day	9% 4 18% 26% 27% 15%	532
September	1st to 10th	Day	16% 4 16% 28% 24% 12%	450
	11th to 20th	Day	10% 2 17% 24% 29% 18%	450
	21st to end	Day	17% 11% 22% 22% 19% 10%	450
October	1st to 10th	Day	15% 7% 26% 24% 21% 8%	450
	11th to 20th	Day	18% 16% 32% 23% 10% 2	450
	21st to end	Day	23% 15% 36% 19% 5% 1	397
November	1st to 10th	Day	19% 12% 33% 21% 11% 3	270
	11th to 20th	Day	26% 18% 29% 17% 10%	270
	21st to end	Day	26% 16% 34% 21% 3	270
December	1st to 10th	Day	21% 19% 35% 16% 7% 1	270
	11th to 20th	Day	23% 19% 37% 17% 3	270
	21st to end	Day	31% 18% 32% 15% 4	297
January	1st to 10th	Night	25% 16% 35% 20% 4	950
	11th to 20th	Night	29% 14% 35% 17% 3 1	801
	21st to end	Night	23% 15% 35% 23% 4	836
February	1st to 10th	Night	27% 12% 34% 24% 3	760
	11th to 20th	Night	24% 12% 35% 26% 3	743
	21st to end	Night	27% 15% 37% 19% 3	628
March	1st to 10th	Night	22% 11% 34% 27% 7%	760
	11th to 20th	Night	18% 15% 35% 25% 5% 1	760
	21st to end	Night	22% 12% 31% 26% 8%	836
April	1st to 10th	Night	21% 12% 36% 26% 5%	694
	11th to 20th	Night	20% 10% 37% 23% 9% 1	563
	21st to end	Night	21% 8% 34% 29% 7% 1	570
May	1st to 10th	Night	23% 6% 28% 32% 9% 1	570
	11th to 20th	Night	22% 6% 26% 32% 13% 1	561
	21st to end	Night	12% 3 22% 41% 20% 3	617
June	1st to 10th	Night	11% 3 25% 43% 18%	570
	11th to 20th	Night	11% 3 23% 44% 19% 1	570
	21st to end	Night	12% 3 25% 38% 22% 1	570
July	1st to 10th	Night	9% 3 15% 40% 30% 3	555
	11th to 20th	Night	9% 4 18% 39% 26% 4	552
	21st to end	Night	8% 3 15% 42% 30% 2	624
August	1st to 10th	Night	9% 2 12% 44% 30% 3	567
	11th to 20th	Night	11% 2 13% 41% 29% 5%	564
	21st to end	Night	8% 3 12% 37% 36% 4	624
September	1st to 10th	Night	14% 4 14% 36% 28% 5%	670
	11th to 20th	Night	10% 3 14% 36% 32% 4	720
	21st to end	Night	13% 6% 22% 34% 23% 2	720
October	1st to 10th	Night	16% 7% 22% 34% 19% 3	720
	11th to 20th	Night	23% 12% 28% 28% 8% 2	720
	21st to end	Night	26% 11% 31% 26% 6% 1	792
November	1st to 10th	Night	17% 10% 32% 28% 11% 1	740
	11th to 20th	Night	21% 14% 33% 22% 8% 1	760
	21st to end	Night	26% 12% 32% 24% 6% 1	760
December	1st to 10th	Night	27% 13% 35% 21% 5%	760
	11th to 20th	Night	26% 13% 36% 21% 4	760
	21st to end	Night	29% 14% 34% 19% 3	945

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