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 2025-02-06.

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 9% 5% 5% 5% 4 5% 7% 32%	109079
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 33%	109191
Transparency	Day	50% 6% 11% 20% 12% 1	112735
	Night	50% 6% 11% 20% 12% 1	113674
Seeing	Day	18% 11% 28% 23% 15% 6%	15737
	Night	19% 9% 28% 29% 13% 1	25564

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% 19% 9% 19% 6% 5% 3 3 4 5% 12%	3617
	11th to 20th	Day	14% 19% 7% 14% 9% 5% 8% 3 3 3 15%	3948
	21st to end	Day	21% 18% 9% 13% 6% 5% 4 3 3 3 14%	4466
February	1st to 10th	Day	17% 16% 6% 12% 6% 5% 4 4 4 5% 21%	1755
	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	11% 6% 2 6% 4 4 6% 4 7% 9% 41%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 11% 39%	4044
	21st to end	Day	8% 5% 2 7% 4 5% 5% 4 7% 10% 43%	4468
September	1st to 10th	Day	14% 6% 2 5% 4 4 5% 3 5% 7% 45%	2122
	11th to 20th	Day	10% 5% 1 6% 5% 4 4 4 6% 7% 47%	2090
	21st to end	Day	17% 7% 2 6% 4 4 4 3 4 6% 41%	2090
October	1st to 10th	Day	12% 9% 2 8% 5% 4 4 4 6% 6% 42%	2038
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 4 5% 6% 37%	1900
	21st to end	Day	18% 10% 4 8% 5% 4 5% 4 5% 7% 30%	1986
November	1st to 10th	Day	12% 10% 3 6% 5% 4 4 4 5% 7% 41%	1762
	11th to 20th	Day	18% 13% 4 7% 5% 4 4 3 4 6% 33%	1800
	21st to end	Day	18% 16% 4 9% 5% 4 4 3 5% 6% 26%	1800
December	1st to 10th	Day	16% 15% 7% 14% 6% 5% 4 3 3 5% 22%	2930
	11th to 20th	Day	19% 19% 7% 10% 5% 5% 3 3 4 5% 21%	3373
	21st to end	Day	24% 21% 7% 12% 6% 4 3 2 3 4 14%	4185
January	1st to 10th	Night	16% 18% 7% 18% 6% 4 4 3 4 4 17%	6348
	11th to 20th	Night	11% 20% 8% 17% 8% 6% 6% 4 3 4 13%	5995
	21st to end	Night	21% 16% 7% 12% 5% 4 4 3 3 4 22%	6490
February	1st to 10th	Night	19% 14% 5% 11% 6% 5% 5% 4 4 5% 22%	2536
	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	12% 5% 2 6% 5% 4 4 5% 7% 9% 41%	2825
	11th to 20th	Night	10% 6% 3 8% 5% 4 4 3 4 8% 45%	2785
	21st to end	Night	7% 4 2 6% 5% 5% 5% 4 6% 8% 48%	3183
September	1st to 10th	Night	11% 5% 2 4 4 3 4 3 5% 7% 49%	2038
	11th to 20th	Night	10% 5% 1 4 4 3 4 3 4 7% 55%	2090
	21st to end	Night	14% 6% 3 7% 4 4 3 3 5% 6% 45%	2090
October	1st to 10th	Night	12% 6% 2 6% 5% 4 3 3 5% 6% 48%	2140
	11th to 20th	Night	18% 8% 2 7% 5% 3 3 3 5% 6% 39%	2280
	21st to end	Night	21% 8% 3 6% 4 4 4 3 5% 6% 35%	2609
November	1st to 10th	Night	14% 8% 3 7% 4 3 4 4 5% 6% 43%	2536
	11th to 20th	Night	15% 13% 3 8% 4 4 4 4 5% 7% 33%	2600
	21st to end	Night	19% 14% 4 8% 5% 4 4 3 5% 5% 27%	2600
December	1st to 10th	Night	13% 13% 6% 13% 6% 6% 5% 4 4 5% 25%	4250
	11th to 20th	Night	19% 18% 7% 11% 5% 4 4 3 4 5% 22%	4822
	21st to end	Night	25% 19% 7% 10% 5% 4 4 3 5% 5% 14%	6748

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	74% 5% 7% 11% 2	3618
	11th to 20th	Day	74% 4 7% 6% 8% 1	3948
	21st to end	Day	77% 3 5% 9% 5%	4466
February	1st to 10th	Day	66% 5% 11% 12% 5% 1	1755
	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	36% 7% 15% 26% 14% 1	4091
	11th to 20th	Day	36% 8% 13% 32% 11% 1	4083
	21st to end	Day	35% 6% 13% 31% 12% 2	4467
September	1st to 10th	Day	37% 7% 13% 23% 17% 4	2122
	11th to 20th	Day	34% 7% 11% 26% 19% 2	2090
	21st to end	Day	43% 5% 11% 22% 18%	2090
October	1st to 10th	Day	29% 5% 12% 32% 21% 1	3389
	11th to 20th	Day	40% 5% 9% 26% 20% 1	3140
	21st to end	Day	43% 5% 10% 23% 18% 1	3249
November	1st to 10th	Day	42% 5% 11% 26% 15% 1	1762
	11th to 20th	Day	53% 5% 11% 19% 11% 1	1800
	21st to end	Day	58% 5% 11% 17% 8%	1800
December	1st to 10th	Day	66% 4 13% 13% 3	2930
	11th to 20th	Day	66% 4 10% 16% 4	3370
	21st to end	Day	77% 4 6% 9% 4	4185
January	1st to 10th	Night	70% 5% 6% 16% 3	6348
	11th to 20th	Night	75% 4 7% 7% 6%	5995
	21st to end	Night	67% 4 12% 10% 7%	6490
February	1st to 10th	Night	63% 6% 11% 13% 6% 1	2536
	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	39% 6% 13% 26% 15% 2	2814
	11th to 20th	Night	38% 5% 10% 30% 16% 1	2812
	21st to end	Night	33% 6% 13% 29% 17% 3	3183
September	1st to 10th	Night	33% 7% 12% 24% 18% 5%	2038
	11th to 20th	Night	29% 6% 11% 27% 25% 2	2090
	21st to end	Night	39% 7% 10% 23% 19% 2	2090
October	1st to 10th	Night	27% 4 8% 31% 30% 1	3569
	11th to 20th	Night	37% 4 9% 28% 21% 1	3779
	21st to end	Night	37% 4 9% 31% 17% 3	4296
November	1st to 10th	Night	41% 6% 11% 24% 16% 2	2536
	11th to 20th	Night	50% 5% 11% 21% 12% 2	2600
	21st to end	Night	59% 5% 12% 17% 7% 1	2600
December	1st to 10th	Night	61% 5% 10% 17% 6%	4250
	11th to 20th	Night	64% 4 13% 13% 5%	4824
	21st to end	Night	72% 6% 9% 9% 4	6748

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	22% 22% 29% 20% 5% 1	300
	11th to 20th	Day	27% 18% 38% 12% 4 1	300
	21st to end	Day	28% 19% 30% 18% 4	330
February	1st to 10th	Day	28% 19% 36% 13% 2 1	285
	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% 27% 27% 15%	539
September	1st to 10th	Day	15% 5% 17% 28% 24% 12%	490
	11th to 20th	Day	9% 2 17% 25% 30% 18%	490
	21st to end	Day	19% 11% 22% 22% 18% 9%	490
October	1st to 10th	Day	14% 7% 26% 24% 22% 7%	490
	11th to 20th	Day	17% 16% 31% 22% 11% 2	490
	21st to end	Day	22% 16% 36% 20% 6% 1	435
November	1st to 10th	Day	18% 13% 33% 21% 11% 3	300
	11th to 20th	Day	26% 18% 30% 17% 9%	300
	21st to end	Day	28% 16% 34% 19% 2	300
December	1st to 10th	Day	23% 18% 34% 17% 7% 1	300
	11th to 20th	Day	24% 20% 35% 17% 3	300
	21st to end	Day	32% 17% 32% 15% 4	330
January	1st to 10th	Night	25% 17% 35% 19% 4	1000
	11th to 20th	Night	28% 16% 36% 16% 3 1	843
	21st to end	Night	23% 16% 35% 22% 4	880
February	1st to 10th	Night	28% 12% 34% 23% 3	781
	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	7% 3 12% 37% 36% 4	627
September	1st to 10th	Night	14% 4 15% 36% 27% 5%	708
	11th to 20th	Night	10% 3 15% 36% 32% 4	760
	21st to end	Night	15% 6% 21% 34% 22% 2	760
October	1st to 10th	Night	15% 7% 23% 34% 19% 3	760
	11th to 20th	Night	22% 12% 28% 28% 9% 2	760
	21st to end	Night	24% 11% 31% 27% 7%	836
November	1st to 10th	Night	17% 10% 33% 28% 11% 1	780
	11th to 20th	Night	22% 14% 34% 22% 8% 1	800
	21st to end	Night	26% 13% 32% 23% 6% 1	800
December	1st to 10th	Night	26% 14% 35% 20% 5%	800
	11th to 20th	Night	27% 13% 36% 20% 4	800
	21st to end	Night	29% 14% 34% 20% 3	995

Page generated at 2025-02-03 03:05:56. Version ac07

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