Blackhawk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackhawk Observatory 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% 4 4 4 6% 7% 37%	101635
	Night	14% 10% 3 7% 4 4 4 3 5% 6% 38%	98091
Transparency	Day	44% 6% 12% 24% 13% 1	105175
	Night	45% 5% 11% 23% 15% 2	102692
Seeing	Day	17% 10% 29% 25% 14% 5%	15937
	Night	18% 9% 31% 30% 12% 1	25811

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% 10% 14% 6% 4 5% 3 4 4 25%	2527
	11th to 20th	Day	12% 12% 6% 16% 5% 4 4 9% 7% 5% 19%	2530
	21st to end	Day	17% 18% 8% 12% 5% 4 4 3 5% 5% 19%	2910
February	1st to 10th	Day	12% 14% 5% 10% 6% 6% 6% 4 5% 5% 28%	1710
	11th to 20th	Day	13% 10% 4 7% 6% 5% 4 3 4 6% 37%	1830
	21st to end	Day	14% 13% 4 11% 4 4 4 3 5% 6% 32%	1570
March	1st to 10th	Day	13% 10% 4 7% 5% 3 4 3 5% 5% 40%	3120
	11th to 20th	Day	11% 6% 3 7% 5% 5% 4 4 6% 7% 41%	3306
	21st to end	Day	21% 13% 5% 11% 6% 4 4 4 5% 6% 22%	4155
April	1st to 10th	Day	18% 12% 4 8% 5% 4 5% 4 4 6% 31%	2280
	11th to 20th	Day	17% 9% 3 6% 5% 4 5% 4 5% 6% 34%	2254
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 6% 6% 32%	2356
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 5% 7% 36%	4107
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 5% 7% 43%	4399
	21st to end	Day	12% 6% 2 6% 5% 4 5% 5% 7% 9% 39%	4824
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 45%	2660
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 7% 9% 45%	2660
	21st to end	Day	11% 7% 3 7% 5% 5% 4 4 8% 10% 37%	2660
July	1st to 10th	Day	11% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	4440
	11th to 20th	Day	6% 4 1 5% 4 4 5% 4 6% 11% 50%	4440
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 6% 8% 41%	4813
August	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 5% 6% 9% 43%	4374
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 45%	3735
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 5% 8% 48%	4180
September	1st to 10th	Day	13% 6% 1 5% 4 4 4 3 5% 8% 48%	2280
	11th to 20th	Day	13% 3 2 4 3 3 4 3 5% 8% 52%	2193
	21st to end	Day	12% 7% 2 6% 5% 4 4 3 6% 7% 44%	2090
October	1st to 10th	Day	14% 6% 2 5% 4 4 3 3 5% 6% 47%	1985
	11th to 20th	Day	20% 8% 3 7% 3 4 4 3 6% 6% 36%	1900
	21st to end	Day	19% 10% 3 7% 5% 4 4 2 4 5% 37%	1897
November	1st to 10th	Day	13% 9% 3 6% 4 3 4 4 5% 6% 44%	1762
	11th to 20th	Day	15% 13% 4 9% 6% 4 4 4 4 7% 31%	1800
	21st to end	Day	17% 13% 4 9% 5% 3 2 2 4 5% 35%	1659
December	1st to 10th	Day	15% 14% 4 9% 6% 4 5% 4 4 5% 30%	1528
	11th to 20th	Day	12% 14% 5% 11% 6% 5% 4 4 6% 6% 27%	1918
	21st to end	Day	21% 20% 9% 14% 6% 4 4 2 3 4 14%	2783
January	1st to 10th	Night	12% 14% 8% 12% 5% 4 3 2 4 4 30%	4428
	11th to 20th	Night	14% 17% 6% 14% 6% 4 4 6% 7% 3 20%	4440
	21st to end	Night	19% 17% 8% 11% 5% 4 4 3 4 6% 20%	4756
February	1st to 10th	Night	14% 15% 4 8% 5% 4 4 3 5% 5% 32%	2470
	11th to 20th	Night	13% 11% 4 7% 4 5% 4 4 5% 6% 39%	2234
	21st to end	Night	17% 13% 4 7% 4 3 3 3 5% 6% 35%	1884
March	1st to 10th	Night	18% 11% 3 5% 3 4 3 3 4 5% 42%	3733
	11th to 20th	Night	9% 7% 2 6% 4 5% 4 3 5% 7% 47%	3682
	21st to end	Night	25% 11% 4 9% 5% 4 3 3 4 5% 28%	3474
April	1st to 10th	Night	17% 10% 3 6% 4 4 5% 4 4 6% 38%	1900
	11th to 20th	Night	18% 10% 3 6% 5% 4 3 3 4 6% 39%	1875
	21st to end	Night	18% 7% 3 6% 4 4 4 3 6% 7% 38%	1824
May	1st to 10th	Night	17% 6% 2 6% 5% 5% 4 5% 5% 7% 37%	2786
	11th to 20th	Night	14% 6% 2 6% 4 5% 5% 5% 7% 7% 40%	2506
	21st to end	Night	11% 8% 2 6% 5% 4 3 3 7% 7% 43%	2743
June	1st to 10th	Night	8% 5% 2 5% 5% 5% 5% 4 7% 7% 47%	1520
	11th to 20th	Night	10% 5% 2 4 4 5% 5% 4 5% 10% 47%	1520
	21st to end	Night	9% 7% 3 6% 6% 4 5% 4 5% 9% 44%	1520
July	1st to 10th	Night	15% 6% 3 6% 4 4 5% 4 6% 7% 39%	2530
	11th to 20th	Night	5% 4 2 4 4 4 4 5% 6% 8% 54%	2530
	21st to end	Night	6% 6% 2 6% 5% 4 5% 5% 7% 8% 45%	2750
August	1st to 10th	Night	7% 4 2 5% 4 4 4 4 6% 9% 51%	2563
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 50%	3092
	21st to end	Night	8% 4 1 5% 4 4 4 4 5% 9% 52%	3476
September	1st to 10th	Night	9% 5% 1 6% 4 3 3 3 5% 5% 55%	1900
	11th to 20th	Night	12% 4 1 5% 3 3 3 2 5% 7% 54%	1967
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 6% 49%	2090
October	1st to 10th	Night	11% 6% 2 6% 4 4 3 3 4 5% 51%	2193
	11th to 20th	Night	17% 8% 2 6% 4 3 3 3 4 6% 45%	2280
	21st to end	Night	20% 9% 2 6% 4 3 5% 3 5% 6% 38%	2699
November	1st to 10th	Night	13% 8% 2 6% 4 3 3 2 5% 6% 48%	2536
	11th to 20th	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 36%	2600
	21st to end	Night	16% 12% 4 8% 5% 4 3 3 4 5% 36%	2740
December	1st to 10th	Night	15% 12% 4 10% 5% 5% 4 4 4 7% 31%	2687
	11th to 20th	Night	15% 13% 5% 9% 4 4 5% 3 5% 5% 31%	3279
	21st to end	Night	22% 17% 6% 10% 4 5% 5% 4 5% 4 17%	4884

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% 4 11% 16% 6%	2528
	11th to 20th	Day	62% 5% 13% 12% 8%	2530
	21st to end	Day	68% 4 8% 14% 5%	2910
February	1st to 10th	Day	57% 5% 12% 15% 9% 1	1710
	11th to 20th	Day	48% 5% 10% 24% 10% 2	1840
	21st to end	Day	54% 6% 8% 21% 10% 2	1570
March	1st to 10th	Day	45% 7% 11% 24% 11% 3	3120
	11th to 20th	Day	40% 6% 11% 31% 11% 1	3306
	21st to end	Day	63% 7% 12% 14% 4 1	4155
April	1st to 10th	Day	55% 4 10% 21% 9%	2280
	11th to 20th	Day	49% 6% 9% 23% 11% 2	2254
	21st to end	Day	52% 6% 11% 17% 13% 1	2356
May	1st to 10th	Day	47% 7% 9% 22% 15% 1	4145
	11th to 20th	Day	40% 6% 11% 28% 15%	4403
	21st to end	Day	39% 6% 13% 21% 16% 4	4837
June	1st to 10th	Day	36% 6% 14% 27% 16% 2	2660
	11th to 20th	Day	34% 6% 14% 25% 19% 2	2660
	21st to end	Day	40% 8% 13% 23% 14% 2	2660
July	1st to 10th	Day	44% 6% 13% 25% 10% 2	4371
	11th to 20th	Day	28% 6% 17% 29% 18% 1	4365
	21st to end	Day	39% 7% 14% 17% 19% 3	4662
August	1st to 10th	Day	36% 6% 16% 28% 12% 1	4359
	11th to 20th	Day	34% 6% 12% 33% 14% 1	3771
	21st to end	Day	34% 6% 13% 28% 17% 2	4179
September	1st to 10th	Day	35% 6% 11% 27% 19% 3	2280
	11th to 20th	Day	31% 5% 11% 32% 19% 2	2193
	21st to end	Day	39% 5% 9% 29% 18% 1	2090
October	1st to 10th	Day	25% 3 7% 37% 27% 1	3297
	11th to 20th	Day	33% 4 11% 27% 24% 1	3140
	21st to end	Day	44% 4 8% 28% 13% 3	3097
November	1st to 10th	Day	41% 5% 14% 24% 15% 2	1762
	11th to 20th	Day	55% 3 11% 20% 9% 1	1800
	21st to end	Day	53% 4 13% 22% 8%	1659
December	1st to 10th	Day	56% 4 13% 18% 8% 1	1528
	11th to 20th	Day	56% 4 15% 16% 7% 2	1915
	21st to end	Day	76% 5% 7% 8% 4	2783
January	1st to 10th	Night	58% 5% 9% 18% 9% 1	4428
	11th to 20th	Night	66% 5% 9% 12% 8%	4440
	21st to end	Night	68% 4 9% 13% 6% 1	4756
February	1st to 10th	Night	54% 4 10% 18% 13% 1	2470
	11th to 20th	Night	48% 4 11% 23% 14% 1	2235
	21st to end	Night	51% 5% 10% 20% 11% 4	1884
March	1st to 10th	Night	46% 4 12% 23% 10% 5%	3733
	11th to 20th	Night	36% 6% 10% 29% 17% 2	3682
	21st to end	Night	60% 4 8% 18% 8% 1	3474
April	1st to 10th	Night	47% 5% 10% 22% 14% 2	1900
	11th to 20th	Night	48% 4 6% 21% 18% 4	1875
	21st to end	Night	45% 6% 11% 23% 12% 2	1824
May	1st to 10th	Night	44% 7% 11% 19% 16% 3	2813
	11th to 20th	Night	39% 9% 12% 27% 12% 1	2507
	21st to end	Night	37% 6% 13% 19% 21% 4	2751
June	1st to 10th	Night	32% 8% 14% 25% 19% 2	1520
	11th to 20th	Night	33% 7% 10% 24% 24% 3	1520
	21st to end	Night	37% 6% 14% 21% 19% 2	1520
July	1st to 10th	Night	44% 5% 14% 21% 12% 3	2490
	11th to 20th	Night	26% 6% 15% 33% 19% 1	2482
	21st to end	Night	33% 8% 14% 20% 21% 3	2662
August	1st to 10th	Night	29% 6% 14% 30% 19% 2	2554
	11th to 20th	Night	31% 6% 14% 31% 17% 1	3122
	21st to end	Night	30% 5% 12% 31% 18% 3	3476
September	1st to 10th	Night	32% 5% 10% 27% 24% 3	1900
	11th to 20th	Night	31% 4 9% 32% 21% 3	1967
	21st to end	Night	35% 5% 10% 30% 18% 2	2090
October	1st to 10th	Night	24% 3 7% 36% 28% 1	3661
	11th to 20th	Night	35% 3 9% 26% 25% 3	3779
	21st to end	Night	42% 4 9% 26% 16% 3	4449
November	1st to 10th	Night	38% 4 10% 27% 18% 3	2536
	11th to 20th	Night	49% 4 12% 22% 12% 1	2600
	21st to end	Night	51% 4 11% 22% 11% 1	2740
December	1st to 10th	Night	54% 5% 11% 21% 8% 1	2687
	11th to 20th	Night	54% 4 13% 19% 9% 1	3281
	21st to end	Night	69% 6% 9% 12% 4	4884

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	19% 12% 34% 26% 8%	270
	11th to 20th	Day	20% 14% 42% 19% 3 1	270
	21st to end	Day	22% 10% 36% 26% 5% 1	297
February	1st to 10th	Day	18% 17% 39% 20% 7%	270
	11th to 20th	Day	18% 21% 34% 21% 5%	270
	21st to end	Day	21% 19% 40% 17% 3	225
March	1st to 10th	Day	16% 21% 39% 19% 4	270
	11th to 20th	Day	23% 17% 37% 14% 7% 2	270
	21st to end	Day	28% 14% 30% 19% 7% 2	496
April	1st to 10th	Day	26% 13% 33% 23% 5%	460
	11th to 20th	Day	22% 20% 26% 19% 10% 3	452
	21st to end	Day	25% 13% 29% 22% 9% 2	460
May	1st to 10th	Day	19% 16% 31% 23% 10% 2	475
	11th to 20th	Day	21% 11% 28% 25% 11% 3	675
	21st to end	Day	17% 10% 27% 23% 16% 7%	734
June	1st to 10th	Day	10% 7% 31% 29% 18% 5%	680
	11th to 20th	Day	13% 5% 33% 30% 15% 5%	680
	21st to end	Day	13% 7% 29% 31% 15% 5%	680
July	1st to 10th	Day	9% 3 19% 30% 25% 14%	671
	11th to 20th	Day	9% 4 25% 32% 21% 10%	668
	21st to end	Day	8% 2 24% 36% 21% 9%	739
August	1st to 10th	Day	10% 3 18% 32% 25% 12%	676
	11th to 20th	Day	10% 5% 19% 28% 21% 17%	485
	21st to end	Day	11% 5% 15% 31% 27% 11%	539
September	1st to 10th	Day	15% 7% 20% 28% 24% 7%	490
	11th to 20th	Day	11% 3 20% 29% 24% 13%	490
	21st to end	Day	15% 10% 28% 22% 17% 8%	490
October	1st to 10th	Day	17% 7% 29% 25% 17% 4	385
	11th to 20th	Day	22% 18% 31% 20% 9% 1	300
	21st to end	Day	31% 12% 29% 22% 5% 1	330
November	1st to 10th	Day	17% 13% 38% 20% 9% 3	300
	11th to 20th	Day	21% 15% 40% 16% 7% 1	300
	21st to end	Day	26% 10% 41% 18% 5%	300
December	1st to 10th	Day	19% 17% 41% 15% 7% 1	273
	11th to 20th	Day	17% 13% 37% 23% 9%	270
	21st to end	Day	26% 18% 38% 16% 1	297
January	1st to 10th	Night	19% 13% 37% 25% 6%	950
	11th to 20th	Night	22% 12% 38% 22% 5% 1	950
	21st to end	Night	20% 11% 35% 28% 6%	1045
February	1st to 10th	Night	22% 9% 40% 24% 5%	950
	11th to 20th	Night	18% 14% 37% 26% 5%	759
	21st to end	Night	24% 13% 38% 21% 4	628
March	1st to 10th	Night	22% 12% 37% 23% 6%	760
	11th to 20th	Night	15% 12% 36% 29% 8%	694
	21st to end	Night	24% 11% 32% 25% 7% 1	627
April	1st to 10th	Night	21% 14% 35% 24% 5%	570
	11th to 20th	Night	23% 13% 30% 27% 6%	563
	21st to end	Night	22% 8% 35% 26% 8% 1	570
May	1st to 10th	Night	19% 8% 36% 29% 8% 1	570
	11th to 20th	Night	18% 7% 31% 33% 12%	561
	21st to end	Night	15% 4 29% 34% 18%	617
June	1st to 10th	Night	9% 4 28% 43% 15% 1	570
	11th to 20th	Night	13% 3 32% 39% 11% 1	570
	21st to end	Night	9% 5% 29% 38% 19% 1	570
July	1st to 10th	Night	9% 1 16% 40% 30% 3	555
	11th to 20th	Night	8% 4 20% 41% 24% 2	552
	21st to end	Night	7% 3 18% 45% 25% 2	624
August	1st to 10th	Night	6% 2 14% 46% 27% 4	567
	11th to 20th	Night	9% 2 12% 41% 30% 5%	564
	21st to end	Night	10% 1 16% 41% 29% 4	627
September	1st to 10th	Night	10% 4 20% 37% 26% 4	570
	11th to 20th	Night	13% 3 19% 34% 27% 4	637
	21st to end	Night	13% 8% 25% 33% 19% 3	760
October	1st to 10th	Night	15% 5% 29% 34% 16% 2	760
	11th to 20th	Night	22% 10% 31% 26% 11%	760
	21st to end	Night	24% 11% 32% 25% 7%	836
November	1st to 10th	Night	17% 11% 37% 25% 10% 1	780
	11th to 20th	Night	21% 16% 34% 23% 6%	800
	21st to end	Night	21% 12% 36% 24% 6%	940
December	1st to 10th	Night	21% 12% 38% 23% 5%	960
	11th to 20th	Night	23% 12% 35% 24% 5% 1	950
	21st to end	Night	29% 11% 35% 21% 3	1045

Page generated at 2024-11-29 09:43:06. Version ac07

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