Dean Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dean Memorial Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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% 8% 3 8% 5% 4 4 4 5% 7% 39%	114300
	Night	12% 9% 3 8% 4 4 4 3 5% 6% 41%	115187
Transparency	Day	43% 5% 12% 24% 14% 1	117939
	Night	42% 5% 12% 24% 15% 2	119690
Seeing	Day	16% 10% 30% 27% 14% 3	16554
	Night	15% 9% 33% 31% 10% 1	26517

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	7% 12% 5% 21% 8% 7% 5% 3 4 5% 23%	3617
	11th to 20th	Day	6% 10% 4 11% 7% 7% 7% 6% 7% 5% 30%	3938
	21st to end	Day	11% 12% 6% 14% 5% 5% 5% 4 5% 6% 29%	4488
February	1st to 10th	Day	11% 13% 5% 9% 7% 5% 5% 4 6% 6% 29%	1800
	11th to 20th	Day	12% 11% 4 8% 5% 5% 5% 3 6% 6% 34%	1754
	21st to end	Day	12% 12% 3 10% 6% 4 3 3 5% 6% 36%	1613
March	1st to 10th	Day	18% 9% 3 6% 3 3 3 2 3 5% 45%	4864
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 5% 8% 47%	4980
	21st to end	Day	19% 14% 5% 8% 5% 5% 4 4 5% 6% 25%	5524
April	1st to 10th	Day	18% 11% 4 7% 5% 4 5% 3 5% 7% 33%	2400
	11th to 20th	Day	18% 9% 3 7% 4 5% 4 4 5% 6% 34%	2374
	21st to end	Day	22% 8% 3 6% 4 4 4 3 6% 6% 33%	2494
May	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 3 5% 8% 34%	4239
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 5% 7% 41%	4539
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 8% 42%	4895
June	1st to 10th	Day	10% 5% 2 6% 4 5% 4 4 7% 8% 44%	2660
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 6% 9% 46%	2660
	21st to end	Day	10% 7% 2 6% 5% 5% 5% 4 7% 8% 40%	2660
July	1st to 10th	Day	10% 7% 3 7% 6% 5% 6% 4 7% 9% 35%	4440
	11th to 20th	Day	6% 4 2 5% 3 4 3 3 6% 8% 56%	4440
	21st to end	Day	5% 5% 2 6% 5% 6% 6% 6% 8% 10% 41%	4538
August	1st to 10th	Day	8% 5% 2 6% 4 5% 5% 5% 7% 9% 44%	4104
	11th to 20th	Day	11% 7% 2 8% 4 4 5% 4 7% 9% 40%	4041
	21st to end	Day	8% 4 2 6% 5% 4 5% 3 6% 8% 50%	4198
September	1st to 10th	Day	12% 4 2 5% 3 3 3 3 5% 7% 53%	2280
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 5% 8% 49%	2271
	21st to end	Day	11% 5% 2 5% 4 5% 4 3 5% 7% 50%	1986
October	1st to 10th	Day	15% 6% 2 5% 4 3 3 3 5% 6% 48%	1900
	11th to 20th	Day	16% 8% 2 6% 4 3 3 3 4 5% 45%	1900
	21st to end	Day	16% 9% 3 6% 4 4 4 4 5% 6% 42%	2090
November	1st to 10th	Day	12% 6% 3 6% 4 4 5% 4 5% 7% 45%	1801
	11th to 20th	Day	14% 12% 3 6% 5% 3 4 3 5% 8% 37%	1612
	21st to end	Day	15% 10% 5% 7% 4 3 4 3 4 7% 38%	1600
December	1st to 10th	Day	7% 8% 3 8% 4 4 3 3 5% 8% 46%	2610
	11th to 20th	Day	13% 11% 5% 16% 7% 4 4 4 5% 6% 25%	3008
	21st to end	Day	22% 20% 9% 11% 5% 3 3 3 3 5% 16%	3982
January	1st to 10th	Night	9% 12% 5% 18% 6% 5% 4 3 4 6% 28%	6348
	11th to 20th	Night	9% 13% 4 11% 6% 5% 6% 4 7% 6% 29%	6038
	21st to end	Night	12% 12% 6% 12% 5% 4 5% 3 5% 5% 31%	6490
February	1st to 10th	Night	11% 12% 5% 10% 6% 4 4 3 5% 6% 33%	2600
	11th to 20th	Night	12% 10% 3 7% 4 4 4 4 5% 7% 40%	2533
	21st to end	Night	13% 10% 3 8% 5% 4 3 3 4 5% 42%	2015
March	1st to 10th	Night	16% 8% 2 5% 2 3 3 2 4 6% 48%	5017
	11th to 20th	Night	10% 7% 2 5% 4 3 3 3 5% 8% 50%	4980
	21st to end	Night	18% 11% 5% 11% 6% 4 4 3 5% 5% 27%	5404
April	1st to 10th	Night	15% 9% 3 7% 3 3 4 3 5% 6% 42%	2000
	11th to 20th	Night	13% 10% 3 6% 5% 4 5% 3 4 5% 41%	1975
	21st to end	Night	18% 7% 2 7% 4 4 4 3 4 6% 42%	1903
May	1st to 10th	Night	19% 6% 2 6% 4 4 4 3 5% 6% 39%	2885
	11th to 20th	Night	11% 8% 2 6% 4 4 3 3 5% 7% 46%	2586
	21st to end	Night	11% 7% 2 5% 5% 4 4 4 6% 8% 46%	2788
June	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 5% 8% 50%	1520
	11th to 20th	Night	7% 5% 1 4 4 3 4 4 7% 11% 49%	1520
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 5% 8% 48%	1520
July	1st to 10th	Night	9% 4 3 5% 4 5% 5% 5% 7% 9% 44%	2530
	11th to 20th	Night	3 3 1 4 4 3 3 3 5% 8% 61%	2530
	21st to end	Night	6% 4 2 6% 4 4 5% 4 7% 8% 50%	3022
August	1st to 10th	Night	5% 4 1 5% 4 3 4 4 7% 8% 54%	2836
	11th to 20th	Night	8% 5% 1 5% 4 3 4 3 4 7% 56%	2798
	21st to end	Night	8% 4 2 6% 3 4 4 3 5% 8% 54%	3445
September	1st to 10th	Night	9% 5% 2 5% 3 3 4 2 4 6% 57%	1900
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 5% 7% 55%	1905
	21st to end	Night	10% 5% 2 4 3 3 3 2 5% 7% 57%	2175
October	1st to 10th	Night	13% 6% 2 5% 3 3 4 2 3 7% 54%	2280
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 4 6% 49%	2280
	21st to end	Night	16% 8% 2 6% 4 4 3 3 5% 5% 43%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	2496
	11th to 20th	Night	13% 9% 3 7% 5% 4 3 3 5% 6% 42%	2785
	21st to end	Night	15% 10% 3 7% 4 4 4 3 4 7% 40%	2800
December	1st to 10th	Night	6% 8% 4 7% 5% 4 5% 4 6% 7% 45%	4580
	11th to 20th	Night	13% 10% 4 13% 5% 4 4 3 5% 5% 34%	5199
	21st to end	Night	20% 18% 7% 13% 5% 4 3 3 4 4 18%	6996

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	64% 5% 10% 15% 6%	3618
	11th to 20th	Day	52% 4 11% 17% 14% 1	3938
	21st to end	Day	56% 5% 13% 19% 7%	4488
February	1st to 10th	Day	54% 5% 13% 19% 9% 1	1800
	11th to 20th	Day	50% 5% 11% 21% 11% 2	1763
	21st to end	Day	50% 3 13% 21% 10% 3	1613
March	1st to 10th	Day	44% 3 9% 26% 16% 2	4864
	11th to 20th	Day	36% 5% 8% 32% 17% 2	4980
	21st to end	Day	59% 5% 15% 15% 5% 1	5526
April	1st to 10th	Day	50% 5% 11% 24% 10%	2400
	11th to 20th	Day	49% 5% 11% 22% 12% 1	2374
	21st to end	Day	51% 5% 12% 19% 12% 1	2494
May	1st to 10th	Day	48% 7% 13% 18% 13% 1	4277
	11th to 20th	Day	41% 7% 11% 28% 12% 1	4543
	21st to end	Day	40% 7% 10% 25% 16% 1	4908
June	1st to 10th	Day	35% 7% 17% 24% 15% 2	2660
	11th to 20th	Day	34% 7% 14% 23% 20% 3	2660
	21st to end	Day	39% 8% 14% 22% 14% 2	2660
July	1st to 10th	Day	42% 8% 14% 26% 8% 2	4371
	11th to 20th	Day	26% 6% 15% 32% 19% 2	4365
	21st to end	Day	32% 10% 19% 21% 17% 1	4395
August	1st to 10th	Day	33% 7% 16% 27% 15% 1	4090
	11th to 20th	Day	41% 6% 14% 25% 14% 1	4080
	21st to end	Day	32% 6% 15% 28% 16% 2	4197
September	1st to 10th	Day	32% 5% 13% 24% 23% 4	2280
	11th to 20th	Day	32% 6% 13% 31% 17% 1	2271
	21st to end	Day	33% 6% 11% 32% 18% 1	1986
October	1st to 10th	Day	23% 3 6% 28% 37% 2	3160
	11th to 20th	Day	30% 2 9% 30% 24% 4	3140
	21st to end	Day	39% 3 10% 29% 15% 3	3428
November	1st to 10th	Day	38% 6% 15% 24% 16% 2	1801
	11th to 20th	Day	46% 4 13% 21% 15% 1	1612
	21st to end	Day	47% 4 15% 24% 9% 1	1600
December	1st to 10th	Day	37% 3 14% 38% 7% 1	2610
	11th to 20th	Day	60% 4 12% 14% 9%	3005
	21st to end	Day	72% 3 8% 12% 3	3982
January	1st to 10th	Night	57% 4 12% 17% 8% 1	6348
	11th to 20th	Night	54% 4 13% 13% 15% 2	6038
	21st to end	Night	54% 4 10% 18% 12% 1	6490
February	1st to 10th	Night	52% 4 11% 21% 11% 1	2600
	11th to 20th	Night	44% 5% 11% 26% 12% 2	2534
	21st to end	Night	45% 3 11% 22% 16% 3	2015
March	1st to 10th	Night	38% 3 7% 30% 20% 2	5017
	11th to 20th	Night	33% 4 9% 33% 18% 4	4980
	21st to end	Night	58% 6% 15% 14% 6% 2	5404
April	1st to 10th	Night	42% 5% 10% 26% 14% 2	2000
	11th to 20th	Night	44% 5% 10% 21% 17% 3	1975
	21st to end	Night	43% 4 14% 23% 13% 2	1903
May	1st to 10th	Night	44% 5% 11% 20% 19% 1	2912
	11th to 20th	Night	38% 6% 9% 28% 15% 3	2587
	21st to end	Night	36% 8% 11% 25% 18% 3	2796
June	1st to 10th	Night	31% 7% 15% 27% 16% 4	1520
	11th to 20th	Night	29% 6% 14% 22% 23% 5%	1520
	21st to end	Night	33% 6% 14% 26% 17% 4	1520
July	1st to 10th	Night	34% 6% 16% 31% 12% 2	2490
	11th to 20th	Night	21% 6% 19% 33% 19% 2	2482
	21st to end	Night	28% 9% 18% 25% 18% 2	2926
August	1st to 10th	Night	25% 7% 17% 34% 15% 2	2826
	11th to 20th	Night	29% 6% 15% 33% 15% 1	2825
	21st to end	Night	29% 6% 14% 28% 18% 5%	3445
September	1st to 10th	Night	29% 6% 9% 27% 27% 3	1900
	11th to 20th	Night	30% 5% 10% 32% 21% 1	1905
	21st to end	Night	28% 4 10% 31% 24% 3	2175
October	1st to 10th	Night	23% 2 6% 25% 41% 3	3800
	11th to 20th	Night	26% 3 9% 26% 28% 8%	3779
	21st to end	Night	42% 4 10% 24% 17% 3	4120
November	1st to 10th	Night	33% 5% 14% 26% 18% 4	2496
	11th to 20th	Night	43% 4 12% 25% 13% 2	2785
	21st to end	Night	46% 4 12% 26% 12% 1	2800
December	1st to 10th	Night	37% 5% 13% 36% 9%	4580
	11th to 20th	Night	51% 4 13% 24% 6% 1	5201
	21st to end	Night	70% 5% 9% 12% 5%	6996

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% 13% 36% 21% 9% 2	300
	11th to 20th	Day	20% 14% 44% 18% 3	300
	21st to end	Day	18% 15% 39% 24% 3 1	330
February	1st to 10th	Day	20% 15% 43% 18% 4 1	300
	11th to 20th	Day	19% 17% 37% 21% 6% 1	300
	21st to end	Day	21% 15% 44% 18% 2	249
March	1st to 10th	Day	17% 21% 42% 14% 6%	300
	11th to 20th	Day	22% 19% 29% 23% 7%	300
	21st to end	Day	32% 18% 32% 14% 3 1	354
April	1st to 10th	Day	26% 15% 34% 19% 5% 1	500
	11th to 20th	Day	23% 17% 30% 23% 6% 1	492
	21st to end	Day	25% 13% 35% 19% 7% 2	594
May	1st to 10th	Day	21% 13% 31% 23% 9% 2	703
	11th to 20th	Day	22% 13% 27% 24% 12% 3	725
	21st to end	Day	16% 10% 29% 27% 15% 3	760
June	1st to 10th	Day	8% 6% 28% 30% 21% 6%	680
	11th to 20th	Day	11% 6% 31% 33% 15% 4	680
	21st to end	Day	12% 8% 31% 30% 14% 4	680
July	1st to 10th	Day	8% 5% 19% 33% 24% 11%	671
	11th to 20th	Day	7% 4 27% 33% 23% 6%	668
	21st to end	Day	6% 3 22% 39% 24% 5%	739
August	1st to 10th	Day	8% 3 23% 36% 25% 5%	676
	11th to 20th	Day	12% 4 20% 33% 24% 6%	665
	21st to end	Day	9% 3 21% 36% 25% 7%	557
September	1st to 10th	Day	13% 7% 19% 32% 22% 6%	490
	11th to 20th	Day	9% 4 25% 33% 24% 5%	481
	21st to end	Day	11% 12% 29% 30% 15% 3	300
October	1st to 10th	Day	18% 6% 33% 23% 16% 4	300
	11th to 20th	Day	17% 21% 36% 18% 7% 1	300
	21st to end	Day	21% 18% 31% 21% 6% 3	330
November	1st to 10th	Day	17% 13% 36% 26% 7% 2	300
	11th to 20th	Day	20% 19% 34% 17% 9% 1	300
	21st to end	Day	21% 14% 38% 21% 4	300
December	1st to 10th	Day	13% 13% 45% 20% 8% 1	300
	11th to 20th	Day	16% 15% 38% 22% 7% 2	300
	21st to end	Day	29% 14% 38% 16% 2	330
January	1st to 10th	Night	14% 13% 39% 28% 6%	1000
	11th to 20th	Night	17% 13% 37% 27% 6% 1	1000
	21st to end	Night	18% 12% 36% 27% 6%	1100
February	1st to 10th	Night	19% 12% 38% 26% 5%	1000
	11th to 20th	Night	17% 12% 41% 25% 4	979
	21st to end	Night	17% 14% 40% 24% 4 1	695
March	1st to 10th	Night	19% 14% 38% 22% 6%	650
	11th to 20th	Night	17% 9% 39% 26% 8%	600
	21st to end	Night	24% 12% 39% 20% 5% 1	660
April	1st to 10th	Night	18% 14% 41% 23% 4	600
	11th to 20th	Night	18% 14% 38% 24% 6%	593
	21st to end	Night	21% 11% 35% 25% 7% 1	600
May	1st to 10th	Night	17% 8% 35% 32% 7% 1	600
	11th to 20th	Night	16% 8% 35% 29% 11%	591
	21st to end	Night	15% 6% 28% 37% 14%	634
June	1st to 10th	Night	10% 5% 24% 43% 16% 1	570
	11th to 20th	Night	9% 3 27% 48% 12% 1	570
	21st to end	Night	10% 5% 31% 40% 15% 1	570
July	1st to 10th	Night	6% 2 14% 46% 30% 2	555
	11th to 20th	Night	5% 2 22% 43% 25% 3	552
	21st to end	Night	5% 1 17% 51% 23% 2	624
August	1st to 10th	Night	6% 4 15% 48% 26% 2	567
	11th to 20th	Night	7% 2 19% 44% 25% 3	564
	21st to end	Night	10% 3 19% 41% 26% 2	627
September	1st to 10th	Night	11% 5% 20% 41% 19% 3	570
	11th to 20th	Night	14% 3 21% 39% 22% 2	570
	21st to end	Night	10% 6% 31% 35% 16% 1	655
October	1st to 10th	Night	14% 7% 31% 33% 13% 1	760
	11th to 20th	Night	16% 11% 35% 30% 8%	760
	21st to end	Night	20% 11% 34% 27% 8%	836
November	1st to 10th	Night	14% 10% 37% 30% 8%	780
	11th to 20th	Night	17% 13% 42% 22% 6% 1	985
	21st to end	Night	19% 12% 38% 24% 8%	1000
December	1st to 10th	Night	15% 13% 41% 27% 4 1	1000
	11th to 20th	Night	21% 11% 35% 29% 5% 1	1000
	21st to end	Night	24% 12% 36% 23% 5%	1100

Page generated at 2025-05-23 08:21:02. Version ac07

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