Deer Run Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deer Run Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	11% 6% 2 6% 5% 5% 5% 4 7% 9% 40%	148999
	Night	11% 6% 2 6% 4 4 5% 4 6% 8% 43%	143086
Transparency	Day	37% 8% 13% 18% 17% 7%	152811
	Night	37% 7% 12% 18% 18% 8%	147346
Seeing	Day	14% 9% 23% 25% 19% 9%	17568
	Night	14% 8% 23% 30% 21% 4	27510

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	13% 7% 3 7% 4 3 3 3 5% 6% 46%	5258
	11th to 20th	Day	13% 6% 2 4 2 2 3 3 5% 6% 54%	5280
	21st to end	Day	17% 7% 3 7% 5% 5% 4 3 4 6% 38%	6293
February	1st to 10th	Day	14% 8% 2 7% 4 4 5% 3 5% 7% 41%	2100
	11th to 20th	Day	18% 7% 2 6% 4 3 3 3 5% 6% 42%	2048
	21st to end	Day	15% 7% 3 6% 5% 4 4 4 5% 7% 42%	1730
March	1st to 10th	Day	19% 8% 2 7% 4 4 4 4 6% 6% 37%	4718
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 7% 53%	5090
	21st to end	Day	15% 7% 2 6% 4 4 3 3 5% 6% 45%	5586
April	1st to 10th	Day	11% 6% 2 5% 5% 4 5% 4 6% 6% 46%	2354
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 5% 7% 52%	2494
	21st to end	Day	7% 4 1 5% 4 4 4 4 5% 7% 54%	2625
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 4 3 6% 8% 38%	7616
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 38%	7626
	21st to end	Day	8% 7% 3 8% 6% 6% 7% 4 7% 10% 35%	8391
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 7% 6% 8% 9% 31%	2730
	11th to 20th	Day	6% 5% 2 6% 5% 5% 7% 6% 8% 12% 38%	2730
	21st to end	Day	5% 5% 2 7% 7% 8% 7% 6% 9% 9% 34%	2730
July	1st to 10th	Day	7% 5% 2 7% 6% 7% 8% 7% 10% 12% 28%	7680
	11th to 20th	Day	5% 4 2 8% 7% 7% 7% 6% 9% 12% 33%	7680
	21st to end	Day	5% 6% 3 8% 7% 7% 8% 7% 10% 14% 25%	8340
August	1st to 10th	Day	9% 6% 3 8% 6% 7% 7% 6% 9% 13% 28%	5854
	11th to 20th	Day	5% 3 1 4 4 5% 5% 5% 8% 12% 48%	5486
	21st to end	Day	7% 5% 2 7% 5% 6% 7% 5% 9% 11% 35%	6063
September	1st to 10th	Day	9% 7% 3 7% 5% 6% 6% 5% 7% 9% 36%	2310
	11th to 20th	Day	13% 5% 2 6% 5% 5% 4 4 6% 8% 40%	2310
	21st to end	Day	12% 6% 2 7% 4 4 4 3 5% 6% 45%	2310
October	1st to 10th	Day	12% 6% 2 6% 4 3 3 3 4 6% 50%	2310
	11th to 20th	Day	10% 4 1 4 3 3 3 2 5% 6% 60%	2237
	21st to end	Day	10% 5% 1 5% 3 4 3 3 4 6% 56%	2280
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 4 5% 55%	1912
	11th to 20th	Day	12% 5% 2 4 2 3 3 3 4 6% 56%	1890
	21st to end	Day	14% 6% 3 6% 3 4 4 3 5% 7% 47%	1890
December	1st to 10th	Day	15% 7% 2 4 4 3 4 3 4 6% 48%	3020
	11th to 20th	Day	14% 7% 2 6% 4 3 4 4 5% 6% 47%	3463
	21st to end	Day	19% 8% 3 6% 3 3 3 2 3 5% 44%	4565
January	1st to 10th	Night	12% 6% 3 5% 4 4 4 4 5% 7% 46%	7635
	11th to 20th	Night	13% 7% 2 7% 4 4 4 3 4 6% 47%	7680
	21st to end	Night	17% 9% 3 6% 4 3 3 3 4 6% 39%	7885
February	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 45%	2520
	11th to 20th	Night	19% 7% 2 5% 4 3 3 2 5% 6% 45%	2458
	21st to end	Night	15% 7% 2 6% 3 3 4 4 5% 7% 44%	2076
March	1st to 10th	Night	19% 8% 2 7% 4 4 3 3 4 6% 39%	5352
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 51%	5090
	21st to end	Night	13% 6% 2 5% 4 4 5% 4 5% 5% 47%	5588
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 7% 7% 47%	2282
	11th to 20th	Night	11% 5% 1 4 4 3 4 3 5% 8% 52%	2075
	21st to end	Night	7% 4 1 6% 4 3 4 3 5% 7% 55%	1993
May	1st to 10th	Night	13% 6% 2 6% 5% 4 4 4 5% 9% 40%	5237
	11th to 20th	Night	13% 5% 2 6% 4 4 5% 4 8% 9% 40%	5234
	21st to end	Night	9% 7% 3 7% 6% 5% 6% 4 7% 11% 36%	5761
June	1st to 10th	Night	9% 7% 3 7% 6% 6% 6% 5% 7% 11% 33%	1890
	11th to 20th	Night	4 4 2 5% 5% 5% 6% 6% 8% 14% 40%	1890
	21st to end	Night	4 5% 2 7% 6% 6% 6% 6% 8% 12% 38%	1890
July	1st to 10th	Night	7% 6% 3 8% 6% 7% 7% 6% 10% 14% 27%	5278
	11th to 20th	Night	4 5% 2 8% 6% 7% 7% 6% 10% 14% 30%	5277
	21st to end	Night	6% 6% 3 9% 6% 6% 7% 6% 9% 13% 28%	5736
August	1st to 10th	Night	9% 6% 3 8% 6% 7% 7% 5% 9% 13% 28%	4289
	11th to 20th	Night	4 3 1 5% 3 4 6% 5% 9% 15% 44%	4560
	21st to end	Night	7% 7% 2 7% 6% 6% 5% 5% 8% 10% 39%	5046
September	1st to 10th	Night	10% 6% 2 6% 5% 6% 5% 4 5% 12% 38%	2290
	11th to 20th	Night	12% 7% 3 6% 4 4 4 4 5% 8% 42%	2310
	21st to end	Night	9% 5% 2 7% 4 5% 5% 4 6% 8% 45%	2310
October	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 49%	2310
	11th to 20th	Night	9% 3 2 4 3 3 3 3 5% 5% 62%	2381
	21st to end	Night	9% 6% 1 5% 3 4 3 3 4 5% 58%	2737
November	1st to 10th	Night	10% 6% 1 4 3 3 4 3 5% 4 56%	2601
	11th to 20th	Night	14% 4 2 4 3 3 3 2 4 5% 57%	2730
	21st to end	Night	13% 6% 2 5% 4 3 4 4 5% 6% 48%	2730
December	1st to 10th	Night	14% 6% 2 4 3 3 2 3 4 6% 53%	4380
	11th to 20th	Night	14% 5% 2 5% 3 4 3 3 4 6% 50%	4952
	21st to end	Night	18% 8% 3 6% 4 3 3 2 4 5% 43%	6633

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	39% 4 6% 19% 26% 7%	5259
	11th to 20th	Day	31% 3 4 15% 32% 15%	5280
	21st to end	Day	47% 4 7% 13% 18% 12%	6293
February	1st to 10th	Day	43% 4 7% 16% 21% 9%	2100
	11th to 20th	Day	42% 5% 5% 15% 24% 9%	2058
	21st to end	Day	42% 4 6% 17% 19% 11%	1730
March	1st to 10th	Day	46% 5% 8% 14% 19% 9%	4718
	11th to 20th	Day	30% 5% 8% 13% 21% 22%	5090
	21st to end	Day	39% 4 9% 18% 18% 11%	5586
April	1st to 10th	Day	37% 6% 9% 20% 17% 10%	2354
	11th to 20th	Day	32% 4 8% 20% 24% 11%	2494
	21st to end	Day	29% 5% 9% 25% 24% 7%	2625
May	1st to 10th	Day	43% 6% 13% 20% 17% 3	7651
	11th to 20th	Day	38% 9% 17% 22% 12% 2	7630
	21st to end	Day	42% 10% 14% 18% 15% 1	8405
June	1st to 10th	Day	44% 9% 15% 19% 11% 2	2730
	11th to 20th	Day	33% 11% 18% 22% 15% 1	2730
	21st to end	Day	39% 13% 16% 19% 11% 2	2730
July	1st to 10th	Day	38% 16% 26% 16% 2	7615
	11th to 20th	Day	35% 17% 30% 16% 2	7611
	21st to end	Day	41% 16% 24% 16% 4	8156
August	1st to 10th	Day	41% 13% 22% 19% 5%	5841
	11th to 20th	Day	25% 10% 23% 30% 11% 1	5522
	21st to end	Day	36% 12% 18% 23% 10% 1	6063
September	1st to 10th	Day	41% 10% 16% 16% 14% 3	2310
	11th to 20th	Day	40% 8% 9% 21% 18% 4	2310
	21st to end	Day	39% 5% 9% 20% 20% 6%	2310
October	1st to 10th	Day	34% 4 7% 12% 24% 19%	3700
	11th to 20th	Day	16% 3 4 17% 24% 36%	3555
	21st to end	Day	21% 2 4 18% 39% 16%	3618
November	1st to 10th	Day	32% 5% 7% 14% 27% 15%	1912
	11th to 20th	Day	30% 3 7% 17% 29% 14%	1890
	21st to end	Day	37% 4 8% 18% 23% 11%	1890
December	1st to 10th	Day	38% 4 7% 16% 27% 9%	3020
	11th to 20th	Day	38% 4 7% 20% 23% 8%	3460
	21st to end	Day	44% 3 4 20% 24% 4	4565
January	1st to 10th	Night	36% 5% 8% 17% 27% 8%	7636
	11th to 20th	Night	39% 4 4 16% 24% 13%	7680
	21st to end	Night	46% 3 5% 16% 21% 9%	7885
February	1st to 10th	Night	40% 3 7% 16% 22% 12%	2520
	11th to 20th	Night	42% 3 5% 15% 22% 12%	2459
	21st to end	Night	39% 5% 7% 16% 17% 15%	2076
March	1st to 10th	Night	46% 5% 8% 14% 17% 10%	5352
	11th to 20th	Night	32% 5% 8% 13% 23% 19%	5090
	21st to end	Night	38% 5% 7% 17% 22% 12%	5588
April	1st to 10th	Night	35% 5% 11% 17% 19% 13%	2282
	11th to 20th	Night	31% 4 7% 21% 24% 13%	2075
	21st to end	Night	29% 5% 11% 25% 23% 8%	1993
May	1st to 10th	Night	39% 8% 15% 17% 15% 6%	5264
	11th to 20th	Night	37% 8% 19% 22% 12% 2	5235
	21st to end	Night	40% 10% 15% 20% 13% 2	5771
June	1st to 10th	Night	42% 10% 15% 18% 14% 1	1890
	11th to 20th	Night	31% 11% 19% 25% 12% 2	1890
	21st to end	Night	35% 12% 20% 19% 12% 3	1890
July	1st to 10th	Night	41% 16% 24% 16% 3	5235
	11th to 20th	Night	38% 17% 29% 13% 2	5226
	21st to end	Night	42% 15% 23% 16% 4	5606
August	1st to 10th	Night	41% 14% 23% 18% 4	4278
	11th to 20th	Night	24% 11% 27% 27% 10% 1	4590
	21st to end	Night	36% 11% 19% 24% 9% 1	5046
September	1st to 10th	Night	38% 9% 18% 17% 13% 4	2290
	11th to 20th	Night	39% 8% 13% 17% 19% 4	2310
	21st to end	Night	35% 8% 12% 18% 21% 7%	2310
October	1st to 10th	Night	35% 5% 8% 13% 22% 17%	3700
	11th to 20th	Night	16% 2 4 12% 30% 35%	3802
	21st to end	Night	20% 3 3 18% 34% 21%	4349
November	1st to 10th	Night	31% 4 7% 15% 28% 15%	2601
	11th to 20th	Night	31% 3 5% 16% 29% 16%	2730
	21st to end	Night	37% 4 7% 18% 22% 12%	2730
December	1st to 10th	Night	35% 3 6% 21% 26% 10%	4380
	11th to 20th	Night	36% 4 8% 20% 24% 9%	4954
	21st to end	Night	44% 3 5% 20% 25% 3	6633

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	20% 23% 43% 10% 4	330
	11th to 20th	Day	19% 24% 36% 14% 5% 2	330
	21st to end	Day	24% 26% 38% 9% 2	363
February	1st to 10th	Day	18% 24% 41% 13% 4	330
	11th to 20th	Day	28% 19% 39% 13% 1	330
	21st to end	Day	23% 21% 43% 11% 2	273
March	1st to 10th	Day	19% 20% 31% 24% 6% 1	390
	11th to 20th	Day	17% 16% 39% 19% 9% 1	540
	21st to end	Day	19% 14% 36% 22% 8% 1	594
April	1st to 10th	Day	17% 16% 30% 24% 11% 1	540
	11th to 20th	Day	14% 12% 31% 27% 13% 3	532
	21st to end	Day	10% 10% 29% 31% 16% 4	540
May	1st to 10th	Day	10% 11% 27% 29% 17% 5%	543
	11th to 20th	Day	11% 4 25% 31% 20% 9%	567
	21st to end	Day	12% 5% 19% 31% 24% 10%	617
June	1st to 10th	Day	10% 3 18% 32% 25% 12%	570
	11th to 20th	Day	8% 3 14% 28% 27% 19%	570
	21st to end	Day	5% 2 14% 31% 31% 17%	570
July	1st to 10th	Day	7% 2 7% 25% 32% 26%	565
	11th to 20th	Day	7% 2 5% 20% 38% 28%	564
	21st to end	Day	5% 2 8% 26% 37% 22%	620
August	1st to 10th	Day	11% 3 9% 27% 31% 19%	566
	11th to 20th	Day	8% 1 7% 25% 36% 24%	559
	21st to end	Day	12% 3 11% 28% 30% 16%	627
September	1st to 10th	Day	8% 4 14% 28% 31% 14%	570
	11th to 20th	Day	14% 3 13% 35% 25% 10%	570
	21st to end	Day	14% 5% 14% 27% 28% 12%	570
October	1st to 10th	Day	16% 6% 28% 27% 18% 5%	570
	11th to 20th	Day	11% 8% 22% 33% 20% 6%	570
	21st to end	Day	14% 9% 26% 30% 19% 2	615
November	1st to 10th	Day	17% 9% 24% 33% 13% 4	390
	11th to 20th	Day	13% 14% 39% 24% 8% 2	330
	21st to end	Day	14% 14% 40% 23% 8% 1	330
December	1st to 10th	Day	25% 17% 33% 20% 3 1	330
	11th to 20th	Day	23% 18% 34% 16% 7% 1	330
	21st to end	Day	25% 14% 32% 25% 4 1	363
January	1st to 10th	Night	19% 16% 35% 25% 5%	840
	11th to 20th	Night	19% 15% 28% 29% 9%	840
	21st to end	Night	21% 16% 37% 22% 4	924
February	1st to 10th	Night	18% 12% 33% 30% 8%	840
	11th to 20th	Night	23% 13% 34% 26% 5%	823
	21st to end	Night	17% 15% 38% 26% 3	692
March	1st to 10th	Night	15% 13% 31% 33% 8%	840
	11th to 20th	Night	15% 13% 33% 28% 10% 1	840
	21st to end	Night	13% 14% 33% 30% 10%	924
April	1st to 10th	Night	13% 10% 32% 32% 12%	812
	11th to 20th	Night	14% 9% 27% 32% 17% 1	623
	21st to end	Night	8% 7% 28% 37% 21%	630
May	1st to 10th	Night	10% 7% 26% 36% 19% 2	630
	11th to 20th	Night	10% 3 24% 36% 22% 5%	621
	21st to end	Night	10% 2 16% 35% 33% 4	683
June	1st to 10th	Night	9% 3 15% 37% 32% 4	630
	11th to 20th	Night	5% 2 10% 33% 38% 12%	630
	21st to end	Night	6% 2 9% 34% 40% 10%	630
July	1st to 10th	Night	8% 2 5% 27% 42% 16%	615
	11th to 20th	Night	8% 1 3 23% 48% 17%	612
	21st to end	Night	4 2 6% 29% 48% 12%	690
August	1st to 10th	Night	9% 2 8% 26% 41% 14%	627
	11th to 20th	Night	10% 1 5% 24% 48% 13%	624
	21st to end	Night	14% 2 11% 25% 35% 13%	693
September	1st to 10th	Night	11% 2 11% 30% 37% 8%	820
	11th to 20th	Night	13% 2 13% 34% 32% 5%	840
	21st to end	Night	11% 2 15% 32% 36% 4	840
October	1st to 10th	Night	14% 3 24% 34% 21% 4	840
	11th to 20th	Night	11% 4 24% 31% 27% 3	840
	21st to end	Night	13% 8% 22% 33% 22% 1	913
November	1st to 10th	Night	14% 8% 23% 34% 19% 2	820
	11th to 20th	Night	15% 9% 30% 32% 12% 1	840
	21st to end	Night	15% 11% 30% 30% 13% 1	840
December	1st to 10th	Night	18% 10% 32% 28% 10% 1	840
	11th to 20th	Night	18% 11% 31% 30% 9% 1	840
	21st to end	Night	24% 12% 26% 28% 9%	924

Page generated at 2026-10-26 04:37:49. Version ac07

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