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 2026-09-29.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 39%	149771
	Night	11% 8% 3 8% 4 4 4 3 5% 7% 42%	142019
Transparency	Day	42% 5% 13% 24% 14% 1	153369
	Night	41% 5% 12% 24% 15% 2	146500
Seeing	Day	16% 10% 30% 27% 14% 3	18523
	Night	15% 9% 33% 31% 10% 1	28253

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	8% 12% 5% 20% 8% 7% 5% 3 4 5% 24%	4691
	11th to 20th	Day	5% 9% 4 12% 7% 7% 7% 5% 7% 6% 31%	5126
	21st to end	Day	8% 11% 5% 13% 6% 7% 6% 4 5% 5% 29%	5812
February	1st to 10th	Day	10% 13% 5% 9% 7% 5% 5% 4 6% 6% 29%	1890
	11th to 20th	Day	12% 11% 4 9% 5% 5% 5% 3 6% 6% 34%	1844
	21st to end	Day	12% 12% 3 10% 6% 5% 3 3 5% 6% 36%	1691
March	1st to 10th	Day	18% 9% 3 6% 3 3 3 2 3 5% 45%	4972
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 5% 8% 47%	5090
	21st to end	Day	19% 14% 5% 8% 5% 5% 4 4 5% 6% 25%	5646
April	1st to 10th	Day	18% 11% 4 7% 5% 4 5% 3 5% 7% 32%	2520
	11th to 20th	Day	18% 9% 3 8% 4 4 4 4 5% 6% 35%	2494
	21st to end	Day	22% 8% 3 7% 4 4 4 3 6% 6% 33%	2619
May	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	7670
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 5% 7% 45%	8222
	21st to end	Day	15% 8% 3 7% 5% 4 4 4 6% 8% 37%	9047
June	1st to 10th	Day	10% 5% 2 6% 4 5% 4 4 7% 9% 44%	2940
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	2940
	21st to end	Day	11% 7% 2 6% 5% 5% 5% 4 7% 8% 40%	2940
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 10% 39%	8280
	11th to 20th	Day	6% 5% 2 5% 3 4 3 3 5% 8% 55%	8280
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	8465
August	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 43%	6014
	11th to 20th	Day	13% 7% 2 7% 4 5% 5% 4 7% 9% 36%	5948
	21st to end	Day	6% 4 2 5% 4 4 5% 3 6% 9% 51%	6112
September	1st to 10th	Day	11% 4 2 5% 4 3 3 3 6% 7% 52%	2520
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 46%	2510
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 48%	2175
October	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 4 6% 49%	2000
	11th to 20th	Day	16% 8% 2 6% 4 3 3 3 4 5% 45%	2000
	21st to end	Day	15% 9% 3 6% 4 4 4 4 5% 6% 41%	2200
November	1st to 10th	Day	12% 6% 3 6% 4 4 5% 4 5% 7% 45%	1893
	11th to 20th	Day	13% 12% 3 6% 5% 3 4 3 5% 7% 38%	1692
	21st to end	Day	15% 10% 5% 8% 4 3 4 3 4 7% 37%	1680
December	1st to 10th	Day	7% 9% 3 8% 4 4 3 3 5% 8% 46%	2690
	11th to 20th	Day	13% 11% 5% 16% 7% 4 4 3 5% 6% 25%	3088
	21st to end	Day	22% 19% 9% 12% 5% 3 3 3 3 5% 16%	4070
January	1st to 10th	Night	10% 12% 5% 16% 6% 5% 4 3 4 6% 28%	8232
	11th to 20th	Night	7% 11% 4 10% 7% 6% 7% 4 7% 6% 31%	7860
	21st to end	Night	9% 11% 5% 11% 6% 5% 4 3 5% 6% 34%	8402
February	1st to 10th	Night	11% 12% 5% 10% 6% 4 4 3 5% 6% 33%	2730
	11th to 20th	Night	12% 10% 3 7% 4 4 4 4 5% 7% 40%	2663
	21st to end	Night	12% 10% 3 8% 5% 5% 4 3 4 5% 42%	2113
March	1st to 10th	Night	16% 8% 3 5% 2 3 3 2 4 6% 48%	5130
	11th to 20th	Night	10% 7% 2 5% 4 3 3 3 5% 8% 49%	5090
	21st to end	Night	17% 11% 5% 11% 6% 4 4 3 5% 5% 28%	5524
April	1st to 10th	Night	15% 10% 3 7% 3 3 4 3 5% 6% 41%	2100
	11th to 20th	Night	13% 10% 3 6% 5% 4 5% 4 4 5% 42%	2075
	21st to end	Night	17% 7% 2 7% 4 4 3 3 4 6% 42%	1998
May	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 47%	5212
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	4670
	21st to end	Night	16% 8% 2 6% 4 4 4 3 5% 6% 40%	5139
June	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 6% 8% 49%	1680
	11th to 20th	Night	7% 5% 1 4 4 3 4 4 7% 11% 48%	1680
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 5% 8% 49%	1680
July	1st to 10th	Night	7% 4 2 6% 4 5% 5% 5% 7% 10% 45%	4708
	11th to 20th	Night	5% 4 1 4 3 3 3 3 5% 7% 61%	4707
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 7% 8% 46%	5626
August	1st to 10th	Night	9% 5% 1 5% 4 3 4 3 6% 8% 53%	4156
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 47%	4118
	21st to end	Night	6% 3 2 5% 3 3 4 4 6% 9% 55%	5002
September	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 4 6% 56%	2100
	11th to 20th	Night	11% 5% 2 5% 4 3 3 3 5% 7% 51%	2106
	21st to end	Night	11% 5% 2 4 3 3 3 2 5% 7% 55%	2381
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 3 7% 53%	2400
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 6% 49%	2400
	21st to end	Night	16% 8% 2 6% 4 3 3 3 5% 5% 44%	2640
November	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 52%	2624
	11th to 20th	Night	13% 9% 3 7% 5% 4 3 3 5% 6% 42%	2924
	21st to end	Night	15% 10% 3 7% 4 4 3 3 4 7% 40%	2940
December	1st to 10th	Night	6% 8% 4 8% 5% 4 5% 4 6% 7% 45%	4720
	11th to 20th	Night	13% 10% 4 12% 5% 4 4 3 5% 5% 34%	5339
	21st to end	Night	20% 18% 7% 13% 5% 4 3 3 4 4 18%	7150

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% 5% 12% 13% 8% 1	4692
	11th to 20th	Day	51% 4 9% 21% 14% 1	5126
	21st to end	Day	56% 5% 12% 20% 6%	5812
February	1st to 10th	Day	53% 5% 15% 19% 8% 1	1890
	11th to 20th	Day	50% 5% 11% 22% 10% 2	1853
	21st to end	Day	50% 3 13% 21% 10% 3	1691
March	1st to 10th	Day	45% 3 9% 26% 16% 2	4972
	11th to 20th	Day	37% 5% 9% 32% 17% 2	5090
	21st to end	Day	58% 5% 15% 15% 5% 1	5648
April	1st to 10th	Day	50% 5% 11% 24% 9%	2520
	11th to 20th	Day	49% 5% 11% 22% 12% 1	2494
	21st to end	Day	50% 5% 12% 20% 12% 1	2619
May	1st to 10th	Day	41% 6% 11% 24% 17% 1	7708
	11th to 20th	Day	38% 5% 11% 28% 16% 2	8226
	21st to end	Day	44% 7% 10% 24% 15% 1	9061
June	1st to 10th	Day	36% 7% 16% 24% 15% 2	2940
	11th to 20th	Day	34% 6% 13% 23% 21% 2	2940
	21st to end	Day	39% 8% 14% 23% 14% 2	2940
July	1st to 10th	Day	39% 7% 15% 28% 10% 1	8211
	11th to 20th	Day	27% 6% 14% 32% 20% 2	8205
	21st to end	Day	37% 8% 20% 24% 10% 1	8280
August	1st to 10th	Day	35% 7% 15% 27% 14% 1	6000
	11th to 20th	Day	44% 6% 16% 23% 10% 1	5987
	21st to end	Day	30% 6% 17% 27% 18% 2	6111
September	1st to 10th	Day	32% 5% 14% 24% 21% 3	2520
	11th to 20th	Day	35% 6% 13% 29% 16% 1	2510
	21st to end	Day	35% 6% 10% 30% 18% 1	2175
October	1st to 10th	Day	23% 3 6% 28% 38% 2	3260
	11th to 20th	Day	30% 3 9% 30% 24% 4	3240
	21st to end	Day	39% 3 10% 29% 15% 3	3538
November	1st to 10th	Day	37% 6% 14% 24% 16% 2	1893
	11th to 20th	Day	45% 4 13% 22% 14% 1	1692
	21st to end	Day	48% 4 15% 23% 10% 1	1680
December	1st to 10th	Day	38% 3 14% 37% 7% 1	2690
	11th to 20th	Day	60% 4 12% 14% 9%	3085
	21st to end	Day	72% 4 8% 12% 3	4070
January	1st to 10th	Night	57% 4 15% 16% 7% 1	8233
	11th to 20th	Night	50% 4 13% 17% 15% 1	7860
	21st to end	Night	50% 4 13% 20% 11% 1	8402
February	1st to 10th	Night	51% 4 12% 21% 11% 1	2730
	11th to 20th	Night	44% 5% 11% 26% 12% 2	2664
	21st to end	Night	45% 3 11% 23% 16% 3	2113
March	1st to 10th	Night	38% 3 8% 30% 19% 2	5130
	11th to 20th	Night	34% 4 9% 32% 17% 4	5090
	21st to end	Night	57% 6% 15% 14% 6% 2	5524
April	1st to 10th	Night	44% 5% 10% 26% 14% 2	2100
	11th to 20th	Night	44% 5% 11% 21% 16% 3	2075
	21st to end	Night	44% 4 13% 23% 14% 2	1998
May	1st to 10th	Night	37% 4 9% 24% 25% 2	5239
	11th to 20th	Night	37% 5% 10% 25% 18% 5%	4671
	21st to end	Night	44% 6% 9% 22% 16% 2	5149
June	1st to 10th	Night	31% 7% 15% 28% 15% 4	1680
	11th to 20th	Night	29% 6% 14% 22% 24% 5%	1680
	21st to end	Night	33% 6% 14% 26% 18% 4	1680
July	1st to 10th	Night	31% 7% 17% 30% 14% 1	4670
	11th to 20th	Night	23% 5% 15% 31% 23% 4	4662
	21st to end	Night	33% 8% 17% 30% 11% 1	5500
August	1st to 10th	Night	29% 6% 14% 32% 17% 2	4146
	11th to 20th	Night	36% 7% 17% 29% 11% 1	4145
	21st to end	Night	26% 6% 19% 27% 18% 4	5002
September	1st to 10th	Night	29% 6% 9% 28% 25% 3	2100
	11th to 20th	Night	33% 5% 10% 31% 20% 1	2106
	21st to end	Night	30% 4 10% 29% 24% 3	2381
October	1st to 10th	Night	23% 2 6% 25% 40% 3	3920
	11th to 20th	Night	27% 3 9% 26% 27% 8%	3899
	21st to end	Night	42% 4 10% 24% 17% 3	4252
November	1st to 10th	Night	33% 5% 14% 27% 18% 3	2624
	11th to 20th	Night	44% 4 12% 26% 13% 2	2924
	21st to end	Night	46% 4 12% 26% 12% 1	2940
December	1st to 10th	Night	38% 5% 13% 36% 9%	4720
	11th to 20th	Night	52% 4 13% 24% 6% 1	5341
	21st to end	Night	69% 5% 9% 12% 5%	7150

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% 37% 22% 8% 2	330
	11th to 20th	Day	19% 15% 44% 18% 3	330
	21st to end	Day	17% 15% 40% 25% 2 1	363
February	1st to 10th	Day	18% 15% 43% 18% 5% 1	330
	11th to 20th	Day	19% 17% 37% 20% 6% 1	330
	21st to end	Day	21% 16% 44% 17% 2	273
March	1st to 10th	Day	18% 20% 42% 15% 5%	330
	11th to 20th	Day	23% 18% 29% 23% 6%	330
	21st to end	Day	30% 19% 34% 14% 2 1	388
April	1st to 10th	Day	27% 15% 35% 19% 4 1	540
	11th to 20th	Day	23% 17% 31% 22% 6% 1	532
	21st to end	Day	24% 13% 35% 20% 6% 2	639
May	1st to 10th	Day	20% 14% 32% 23% 9% 2	753
	11th to 20th	Day	23% 13% 27% 23% 11% 3	775
	21st to end	Day	16% 9% 30% 27% 15% 4	844
June	1st to 10th	Day	8% 7% 28% 31% 21% 6%	780
	11th to 20th	Day	12% 7% 31% 33% 14% 3	780
	21st to end	Day	12% 8% 32% 30% 14% 4	780
July	1st to 10th	Day	8% 5% 18% 34% 24% 11%	771
	11th to 20th	Day	7% 5% 25% 32% 24% 7%	768
	21st to end	Day	6% 4 22% 39% 24% 5%	849
August	1st to 10th	Day	8% 3 23% 35% 25% 5%	776
	11th to 20th	Day	13% 4 20% 34% 24% 6%	765
	21st to end	Day	8% 2 22% 36% 25% 6%	647
September	1st to 10th	Day	12% 7% 21% 33% 22% 6%	570
	11th to 20th	Day	11% 5% 24% 32% 23% 5%	560
	21st to end	Day	14% 10% 26% 29% 16% 5%	354
October	1st to 10th	Day	17% 6% 33% 23% 16% 5%	330
	11th to 20th	Day	16% 22% 38% 17% 6% 1	330
	21st to end	Day	20% 17% 33% 21% 6% 2	363
November	1st to 10th	Day	16% 14% 38% 24% 6% 2	330
	11th to 20th	Day	19% 19% 35% 17% 8% 1	330
	21st to end	Day	22% 15% 38% 21% 5%	330
December	1st to 10th	Day	14% 14% 44% 19% 8% 2	330
	11th to 20th	Day	17% 15% 37% 22% 8% 2	330
	21st to end	Day	29% 13% 40% 15% 2	363
January	1st to 10th	Night	14% 13% 39% 27% 6%	1050
	11th to 20th	Night	17% 13% 38% 26% 5% 1	1050
	21st to end	Night	18% 12% 38% 27% 5%	1155
February	1st to 10th	Night	18% 12% 39% 26% 5%	1050
	11th to 20th	Night	17% 13% 41% 25% 4	1029
	21st to end	Night	17% 14% 41% 24% 3 1	729
March	1st to 10th	Night	19% 13% 39% 22% 6%	683
	11th to 20th	Night	18% 10% 39% 25% 7%	630
	21st to end	Night	23% 12% 39% 20% 4 1	693
April	1st to 10th	Night	20% 15% 40% 22% 3	630
	11th to 20th	Night	17% 14% 38% 24% 6%	623
	21st to end	Night	21% 11% 35% 25% 7% 1	630
May	1st to 10th	Night	17% 8% 35% 33% 7% 1	630
	11th to 20th	Night	17% 8% 36% 28% 11%	621
	21st to end	Night	16% 7% 28% 36% 14%	683
June	1st to 10th	Night	9% 6% 25% 42% 16% 1	630
	11th to 20th	Night	8% 5% 29% 46% 11% 1	630
	21st to end	Night	10% 5% 30% 40% 14%	630
July	1st to 10th	Night	7% 2 14% 45% 30% 2	615
	11th to 20th	Night	5% 2 21% 43% 26% 3	612
	21st to end	Night	8% 2 17% 50% 22% 2	690
August	1st to 10th	Night	7% 4 15% 46% 26% 2	627
	11th to 20th	Night	8% 2 18% 46% 24% 3	624
	21st to end	Night	9% 3 19% 42% 25% 2	693
September	1st to 10th	Night	10% 5% 21% 42% 19% 3	630
	11th to 20th	Night	14% 3 20% 40% 21% 2	630
	21st to end	Night	11% 6% 29% 35% 17% 1	717
October	1st to 10th	Night	14% 7% 31% 34% 13% 1	800
	11th to 20th	Night	16% 11% 36% 30% 8%	800
	21st to end	Night	20% 11% 34% 27% 7%	880
November	1st to 10th	Night	14% 10% 38% 30% 8%	820
	11th to 20th	Night	17% 14% 41% 22% 6%	1034
	21st to end	Night	19% 12% 38% 23% 8%	1050
December	1st to 10th	Night	15% 13% 41% 26% 4 1	1050
	11th to 20th	Night	21% 12% 34% 28% 5%	1050
	21st to end	Night	24% 12% 37% 22% 5%	1155

Page generated at 2026-09-26 03:18:42. Version ac07

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