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 2024-11-02.

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% 8% 3 8% 5% 4 4 4 5% 7% 38%	101448
	Night	12% 9% 3 8% 4 4 4 3 5% 6% 42%	97610
Transparency	Day	44% 6% 13% 23% 13% 1	101247
	Night	43% 5% 12% 24% 14% 2	97482
Seeing	Day	16% 10% 30% 27% 14% 4	15852
	Night	15% 9% 33% 31% 11% 1	25677

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% 11% 5% 21% 8% 8% 5% 4 4 6% 20%	2527
	11th to 20th	Day	9% 14% 5% 11% 8% 7% 7% 5% 7% 5% 24%	2750
	21st to end	Day	13% 15% 8% 17% 6% 5% 4 3 4 6% 18%	3135
February	1st to 10th	Day	11% 14% 5% 9% 7% 5% 5% 4 6% 6% 30%	1710
	11th to 20th	Day	12% 10% 4 8% 5% 5% 5% 4 6% 7% 35%	1664
	21st to end	Day	13% 12% 3 10% 6% 4 3 3 5% 6% 35%	1534
March	1st to 10th	Day	17% 10% 3 7% 4 3 3 3 4 5% 41%	3392
	11th to 20th	Day	10% 7% 3 8% 5% 4 4 3 5% 7% 45%	3480
	21st to end	Day	22% 16% 5% 9% 5% 4 4 4 5% 6% 22%	3862
April	1st to 10th	Day	18% 11% 3 7% 5% 4 4 3 5% 8% 33%	2280
	11th to 20th	Day	18% 9% 3 7% 4 4 4 4 5% 6% 35%	2254
	21st to end	Day	23% 8% 3 6% 4 4 4 3 6% 6% 34%	2369
May	1st to 10th	Day	19% 7% 3 7% 5% 5% 5% 4 5% 8% 33%	4108
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 3 5% 7% 40%	4399
	21st to end	Day	15% 7% 2 6% 4 4 4 3 5% 8% 43%	4824
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% 10% 3 6% 4 4 4 3 5% 6% 42%	2090
November	1st to 10th	Day	11% 6% 3 5% 4 4 5% 4 5% 8% 46%	1719
	11th to 20th	Day	13% 12% 4 6% 5% 3 4 3 5% 8% 38%	1532
	21st to end	Day	16% 10% 4 7% 4 3 3 3 4 7% 38%	1520
December	1st to 10th	Day	12% 13% 4 10% 5% 5% 4 4 4 7% 32%	1520
	11th to 20th	Day	12% 10% 5% 10% 6% 4 4 3 5% 7% 33%	1918
	21st to end	Day	21% 18% 9% 12% 5% 3 2 3 4 6% 17%	2783
January	1st to 10th	Night	11% 12% 5% 20% 6% 6% 4 3 4 6% 25%	4428
	11th to 20th	Night	11% 17% 5% 12% 5% 4 6% 3 8% 6% 24%	4216
	21st to end	Night	15% 15% 7% 14% 6% 5% 5% 3 4 6% 22%	4532
February	1st to 10th	Night	11% 12% 5% 10% 6% 4 4 3 5% 6% 33%	2470
	11th to 20th	Night	12% 10% 3 7% 4 4 4 4 5% 7% 40%	2403
	21st to end	Night	13% 10% 4 8% 5% 4 3 3 4 5% 40%	1918
March	1st to 10th	Night	15% 9% 3 6% 3 3 3 3 4 5% 46%	3489
	11th to 20th	Night	11% 7% 2 5% 4 3 3 3 5% 7% 51%	3480
	21st to end	Night	21% 12% 5% 13% 5% 4 4 3 4 4 23%	3768
April	1st to 10th	Night	15% 10% 3 7% 3 3 4 3 5% 5% 42%	1900
	11th to 20th	Night	13% 10% 3 6% 5% 4 4 3 4 5% 42%	1875
	21st to end	Night	18% 7% 2 7% 4 4 4 3 4 6% 43%	1808
May	1st to 10th	Night	19% 7% 2 6% 4 4 5% 3 5% 6% 39%	2796
	11th to 20th	Night	11% 8% 3 6% 4 4 3 3 5% 7% 46%	2506
	21st to end	Night	11% 7% 2 5% 4 4 4 3 6% 8% 46%	2743
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% 54%	2381
	11th to 20th	Night	13% 9% 3 7% 5% 4 3 3 5% 6% 42%	2645
	21st to end	Night	16% 9% 3 7% 4 3 4 3 4 7% 41%	2660
December	1st to 10th	Night	11% 11% 5% 9% 5% 4 4 3 5% 7% 35%	2660
	11th to 20th	Night	12% 10% 3 10% 5% 4 4 3 5% 5% 37%	3279
	21st to end	Night	19% 16% 6% 13% 6% 4 3 4 4 4 21%	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	66% 6% 10% 12% 6%	2528
	11th to 20th	Day	60% 4 11% 19% 6%	2750
	21st to end	Day	68% 5% 10% 12% 4	3135
February	1st to 10th	Day	54% 4 13% 19% 9% 1	1710
	11th to 20th	Day	49% 5% 11% 22% 11% 2	1673
	21st to end	Day	51% 4 12% 19% 10% 3	1534
March	1st to 10th	Day	46% 4 10% 24% 14% 2	3392
	11th to 20th	Day	40% 5% 11% 27% 15% 2	3480
	21st to end	Day	63% 6% 12% 14% 4 1	3862
April	1st to 10th	Day	50% 5% 11% 24% 10%	2280
	11th to 20th	Day	49% 5% 11% 22% 13% 1	2254
	21st to end	Day	51% 6% 11% 20% 12% 1	2369
May	1st to 10th	Day	48% 7% 13% 18% 12% 1	4146
	11th to 20th	Day	42% 7% 12% 27% 12% 1	4403
	21st to end	Day	39% 7% 11% 25% 16% 1	4837
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	38% 5% 10% 26% 19% 2	1900
	11th to 20th	Day	43% 3 8% 24% 20% 2	1900
	21st to end	Day	44% 4 12% 24% 14% 2	2090
November	1st to 10th	Day	37% 6% 14% 24% 17% 2	1719
	11th to 20th	Day	45% 4 13% 22% 14% 1	1532
	21st to end	Day	47% 4 16% 24% 10% 1	1520
December	1st to 10th	Day	52% 4 14% 20% 8% 2	1520
	11th to 20th	Day	51% 5% 16% 19% 8% 1	1915
	21st to end	Day	69% 4 9% 13% 4	2783
January	1st to 10th	Night	61% 5% 10% 14% 9% 1	4428
	11th to 20th	Night	59% 3 13% 14% 10% 1	4216
	21st to end	Night	64% 4 11% 14% 6% 1	4532
February	1st to 10th	Night	51% 4 11% 21% 11% 1	2470
	11th to 20th	Night	43% 5% 11% 27% 12% 2	2404
	21st to end	Night	46% 3 11% 22% 15% 3	1918
March	1st to 10th	Night	41% 4 9% 28% 17% 1	3489
	11th to 20th	Night	34% 4 11% 32% 15% 4	3480
	21st to end	Night	63% 6% 15% 10% 5% 1	3768
April	1st to 10th	Night	43% 5% 10% 27% 13% 2	1900
	11th to 20th	Night	45% 5% 10% 21% 17% 3	1875
	21st to end	Night	43% 4 13% 24% 14% 2	1808
May	1st to 10th	Night	44% 5% 11% 19% 19% 1	2823
	11th to 20th	Night	38% 7% 9% 28% 15% 3	2507
	21st to end	Night	36% 8% 11% 25% 18% 3	2751
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	34% 3 10% 26% 25% 2	2280
	11th to 20th	Night	37% 5% 10% 24% 19% 5%	2280
	21st to end	Night	42% 5% 12% 22% 17% 2	2508
November	1st to 10th	Night	32% 5% 14% 26% 19% 4	2381
	11th to 20th	Night	44% 4 12% 26% 13% 2	2645
	21st to end	Night	45% 4 12% 26% 12% 1	2660
December	1st to 10th	Night	48% 5% 12% 24% 9% 1	2660
	11th to 20th	Night	48% 5% 13% 26% 8% 1	3281
	21st to end	Night	66% 5% 8% 14% 6%	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	21% 12% 35% 21% 9% 2	270
	11th to 20th	Day	23% 13% 42% 19% 3	270
	21st to end	Day	19% 14% 36% 26% 3 1	297
February	1st to 10th	Day	21% 14% 43% 17% 4 1	270
	11th to 20th	Day	18% 16% 37% 21% 6% 1	270
	21st to end	Day	22% 16% 44% 16% 2	225
March	1st to 10th	Day	16% 22% 40% 14% 7%	270
	11th to 20th	Day	22% 18% 29% 24% 7%	270
	21st to end	Day	33% 19% 30% 14% 3 1	320
April	1st to 10th	Day	27% 14% 35% 19% 5% 1	460
	11th to 20th	Day	22% 17% 31% 23% 6% 1	452
	21st to end	Day	25% 12% 34% 19% 7% 2	549
May	1st to 10th	Day	23% 14% 32% 22% 8% 2	653
	11th to 20th	Day	22% 13% 28% 24% 11% 3	675
	21st to end	Day	16% 10% 29% 27% 16% 4	734
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	22% 17% 32% 21% 6% 3	330
November	1st to 10th	Day	15% 12% 38% 25% 7% 2	273
	11th to 20th	Day	19% 18% 35% 18% 10% 1	270
	21st to end	Day	22% 14% 37% 22% 4	270
December	1st to 10th	Day	14% 13% 43% 20% 9% 1	270
	11th to 20th	Day	15% 13% 37% 24% 8% 2	270
	21st to end	Day	28% 14% 40% 15% 3	297
January	1st to 10th	Night	14% 13% 39% 27% 6%	950
	11th to 20th	Night	18% 12% 37% 27% 6% 1	950
	21st to end	Night	19% 12% 35% 28% 6%	1045
February	1st to 10th	Night	20% 11% 38% 26% 5%	950
	11th to 20th	Night	17% 12% 41% 26% 4	929
	21st to end	Night	18% 14% 39% 23% 4 1	662
March	1st to 10th	Night	18% 14% 39% 22% 7%	618
	11th to 20th	Night	17% 9% 38% 27% 8%	570
	21st to end	Night	24% 11% 38% 21% 5% 1	627
April	1st to 10th	Night	18% 14% 41% 22% 4	570
	11th to 20th	Night	18% 13% 38% 24% 6%	563
	21st to end	Night	21% 10% 35% 25% 8% 1	570
May	1st to 10th	Night	18% 8% 35% 31% 7% 1	570
	11th to 20th	Night	16% 8% 35% 30% 10%	561
	21st to end	Night	15% 6% 28% 37% 15%	617
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% 38% 29% 8%	745
	11th to 20th	Night	17% 13% 42% 22% 6% 1	935
	21st to end	Night	19% 11% 38% 24% 8%	950
December	1st to 10th	Night	15% 12% 40% 27% 4 1	950
	11th to 20th	Night	21% 10% 34% 29% 5% 1	950
	21st to end	Night	23% 12% 37% 23% 5%	1045

Page generated at 2024-10-30 00:22:54. Version ac07

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