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 2024-03-06.

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% 4 7% 6% 3 5% 5% 5% 7% 37%	78565
	Night	12% 9% 4 7% 5% 3 4 4 5% 6% 40%	81007
Transparency	Day	46% 6% 13% 22% 12% 2	78274
	Night	44% 5% 12% 23% 14% 2	80818
Seeing	Day	16% 10% 30% 27% 14% 4	14838
	Night	16% 9% 33% 31% 11% 1	24918

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% 10% 6% 18% 11% 6% 5% 5% 4 6% 20%	2527
	11th to 20th	Day	9% 13% 5% 9% 10% 5% 7% 6% 7% 5% 24%	2750
	21st to end	Day	14% 14% 8% 16% 7% 4 5% 4 4 6% 18%	3135
February	1st to 10th	Day	11% 12% 6% 8% 7% 4 5% 5% 6% 6% 30%	1710
	11th to 20th	Day	13% 10% 4 7% 6% 3 6% 4 6% 7% 35%	1664
	21st to end	Day	13% 11% 4 9% 6% 3 4 4 5% 6% 35%	1534
March	1st to 10th	Day	14% 10% 4 8% 6% 3 4 4 5% 7% 35%	2002
	11th to 20th	Day	14% 9% 4 8% 6% 3 5% 4 4 8% 34%	1980
	21st to end	Day	22% 13% 4 6% 6% 3 4 5% 5% 6% 27%	2199
April	1st to 10th	Day	19% 10% 4 6% 5% 3 5% 4 5% 8% 32%	2160
	11th to 20th	Day	18% 8% 4 7% 5% 3 4 5% 5% 7% 34%	2134
	21st to end	Day	23% 8% 3 5% 5% 2 4 4 6% 6% 33%	2244
May	1st to 10th	Day	19% 7% 3 6% 6% 4 5% 5% 5% 7% 32%	2352
	11th to 20th	Day	19% 8% 3 6% 5% 3 4 5% 5% 6% 37%	2479
	21st to end	Day	16% 7% 3 6% 5% 3 5% 5% 6% 8% 36%	2725
June	1st to 10th	Day	10% 4 2 5% 5% 3 5% 5% 7% 9% 45%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 3 5% 5% 5% 8% 47%	2520
	21st to end	Day	11% 6% 2 6% 5% 3 6% 5% 6% 9% 40%	2520
July	1st to 10th	Day	8% 4 2 4 4 3 5% 5% 6% 10% 49%	2520
	11th to 20th	Day	7% 5% 2 5% 5% 3 4 5% 6% 8% 49%	2520
	21st to end	Day	6% 4 2 5% 5% 3 5% 7% 7% 11% 46%	2591
August	1st to 10th	Day	8% 5% 2 5% 5% 4 6% 7% 8% 9% 42%	2328
	11th to 20th	Day	10% 6% 2 6% 5% 3 4 6% 7% 8% 45%	2301
	21st to end	Day	10% 4 2 5% 5% 3 3 4 5% 7% 52%	2394
September	1st to 10th	Day	13% 4 2 4 4 2 3 4 5% 7% 51%	2160
	11th to 20th	Day	10% 4 2 5% 4 3 4 5% 5% 8% 50%	2152
	21st to end	Day	11% 5% 2 4 4 3 4 4 5% 8% 50%	1882
October	1st to 10th	Day	16% 6% 3 5% 5% 3 4 4 5% 6% 45%	1800
	11th to 20th	Day	17% 8% 3 6% 5% 2 3 4 4 5% 43%	1800
	21st to end	Day	16% 9% 3 5% 5% 3 4 4 4 6% 41%	1980
November	1st to 10th	Day	11% 5% 3 5% 4 3 5% 5% 5% 7% 46%	1709
	11th to 20th	Day	13% 11% 4 6% 5% 2 4 4 5% 8% 38%	1532
	21st to end	Day	17% 9% 5% 5% 6% 2 3 4 4 8% 38%	1520
December	1st to 10th	Day	13% 12% 4 9% 6% 3 5% 5% 4 7% 33%	1520
	11th to 20th	Day	12% 10% 6% 8% 7% 3 5% 4 5% 7% 33%	1918
	21st to end	Day	21% 17% 9% 11% 7% 2 3 4 4 6% 18%	2783
January	1st to 10th	Night	11% 11% 6% 18% 7% 5% 4 4 4 5% 25%	4428
	11th to 20th	Night	11% 16% 5% 10% 7% 3 6% 4 8% 6% 24%	4216
	21st to end	Night	15% 15% 7% 13% 7% 3 5% 3 4 6% 22%	4532
February	1st to 10th	Night	11% 11% 5% 9% 7% 3 5% 4 5% 6% 34%	2470
	11th to 20th	Night	12% 9% 3 6% 5% 3 4 5% 5% 7% 41%	2403
	21st to end	Night	14% 10% 4 7% 6% 3 4 3 4 5% 41%	1918
March	1st to 10th	Night	15% 9% 4 6% 4 2 3 3 4 7% 43%	2089
	11th to 20th	Night	14% 8% 4 6% 7% 3 3 4 4 7% 40%	1980
	21st to end	Night	17% 11% 5% 9% 7% 3 5% 4 5% 5% 28%	2156
April	1st to 10th	Night	15% 9% 4 6% 4 2 4 4 5% 6% 42%	1800
	11th to 20th	Night	14% 9% 4 5% 6% 3 5% 5% 4 6% 40%	1775
	21st to end	Night	18% 7% 2 6% 5% 3 4 4 4 7% 42%	1713
May	1st to 10th	Night	17% 6% 2 5% 5% 3 4 5% 6% 7% 40%	1607
	11th to 20th	Night	15% 7% 3 5% 4 3 4 4 6% 8% 40%	1416
	21st to end	Night	13% 6% 2 5% 5% 3 5% 4 5% 8% 43%	1551
June	1st to 10th	Night	7% 4 2 5% 5% 3 5% 5% 5% 8% 51%	1440
	11th to 20th	Night	7% 4 1 3 6% 2 4 5% 6% 12% 49%	1440
	21st to end	Night	9% 4 2 5% 5% 3 5% 5% 5% 8% 49%	1440
July	1st to 10th	Night	5% 2 2 3 4 2 5% 5% 6% 10% 55%	1440
	11th to 20th	Night	5% 3 2 3 3 3 3 4 5% 9% 61%	1440
	21st to end	Night	5% 3 2 5% 4 3 5% 5% 5% 8% 56%	1724
August	1st to 10th	Night	6% 5% 2 5% 5% 2 4 5% 6% 9% 52%	1611
	11th to 20th	Night	7% 4 2 4 4 2 4 4 4 7% 57%	1600
	21st to end	Night	9% 3 2 5% 4 3 4 4 4 7% 56%	1962
September	1st to 10th	Night	10% 5% 3 5% 4 2 4 3 4 6% 55%	1800
	11th to 20th	Night	11% 4 2 4 4 2 3 3 5% 7% 55%	1804
	21st to end	Night	10% 4 2 4 3 2 4 3 4 7% 57%	2060
October	1st to 10th	Night	14% 6% 2 4 3 2 4 3 3 6% 53%	2160
	11th to 20th	Night	15% 7% 2 5% 4 3 3 4 4 6% 47%	2160
	21st to end	Night	16% 8% 2 5% 4 3 3 4 5% 5% 44%	2376
November	1st to 10th	Night	11% 5% 2 4 4 2 3 4 4 6% 54%	2368
	11th to 20th	Night	13% 8% 4 7% 6% 3 4 4 5% 6% 42%	2645
	21st to end	Night	16% 8% 3 6% 5% 2 4 4 4 6% 42%	2660
December	1st to 10th	Night	11% 10% 5% 8% 5% 3 5% 4 5% 7% 36%	2660
	11th to 20th	Night	13% 10% 4 9% 6% 3 4 4 5% 6% 37%	3279
	21st to end	Night	19% 16% 7% 12% 7% 3 4 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	50% 5% 14% 21% 9% 1	2002
	11th to 20th	Day	51% 5% 11% 21% 10% 2	1980
	21st to end	Day	58% 5% 11% 17% 7% 2	2199
April	1st to 10th	Day	50% 5% 12% 23% 10%	2160
	11th to 20th	Day	50% 5% 10% 22% 12% 1	2134
	21st to end	Day	51% 5% 11% 20% 12% 1	2244
May	1st to 10th	Day	51% 7% 13% 19% 9% 1	2352
	11th to 20th	Day	48% 6% 12% 23% 11% 1	2483
	21st to end	Day	46% 8% 12% 20% 12% 2	2725
June	1st to 10th	Day	36% 7% 16% 24% 15% 2	2520
	11th to 20th	Day	34% 7% 14% 22% 21% 3	2520
	21st to end	Day	39% 8% 14% 22% 14% 2	2520
July	1st to 10th	Day	31% 8% 14% 31% 13% 3	2451
	11th to 20th	Day	32% 8% 15% 26% 16% 3	2445
	21st to end	Day	30% 8% 18% 25% 17% 2	2447
August	1st to 10th	Day	36% 9% 16% 26% 13% 1	2314
	11th to 20th	Day	35% 7% 14% 28% 15% 2	2301
	21st to end	Day	33% 6% 12% 26% 20% 3	2394
September	1st to 10th	Day	33% 5% 13% 23% 21% 4	2160
	11th to 20th	Day	32% 6% 13% 31% 17% 1	2152
	21st to end	Day	32% 6% 11% 32% 17% 1	1882
October	1st to 10th	Day	40% 5% 11% 25% 17% 2	1800
	11th to 20th	Day	44% 3 8% 24% 19% 2	1800
	21st to end	Day	44% 4 12% 23% 14% 2	1980
November	1st to 10th	Day	37% 6% 14% 24% 17% 2	1709
	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	42% 4 12% 29% 11% 1	2089
	11th to 20th	Night	45% 4 12% 22% 14% 2	1980
	21st to end	Night	57% 5% 13% 15% 8% 1	2156
April	1st to 10th	Night	43% 5% 10% 26% 14% 2	1800
	11th to 20th	Night	46% 5% 10% 21% 16% 3	1775
	21st to end	Night	43% 4 14% 24% 13% 2	1713
May	1st to 10th	Night	42% 6% 15% 21% 14% 1	1607
	11th to 20th	Night	42% 8% 12% 25% 13% 1	1417
	21st to end	Night	39% 8% 13% 22% 15% 4	1552
June	1st to 10th	Night	31% 7% 16% 28% 16% 3	1440
	11th to 20th	Night	29% 6% 15% 22% 24% 5%	1440
	21st to end	Night	32% 6% 14% 27% 17% 4	1440
July	1st to 10th	Night	23% 6% 18% 33% 16% 3	1400
	11th to 20th	Night	21% 6% 19% 33% 17% 4	1392
	21st to end	Night	24% 6% 16% 31% 20% 3	1628
August	1st to 10th	Night	28% 8% 16% 31% 13% 3	1601
	11th to 20th	Night	27% 7% 12% 34% 18% 2	1600
	21st to end	Night	28% 6% 11% 25% 24% 7%	1962
September	1st to 10th	Night	30% 6% 9% 25% 26% 3	1800
	11th to 20th	Night	30% 5% 10% 33% 21% 2	1804
	21st to end	Night	28% 4 11% 32% 23% 3	2060
October	1st to 10th	Night	35% 3 10% 26% 23% 3	2160
	11th to 20th	Night	38% 5% 11% 24% 18% 5%	2160
	21st to end	Night	42% 5% 12% 22% 17% 2	2376
November	1st to 10th	Night	32% 5% 15% 26% 19% 4	2368
	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% 41% 14% 6%	255
	11th to 20th	Day	23% 15% 29% 25% 8%	240
	21st to end	Day	34% 19% 29% 15% 3 1	285
April	1st to 10th	Day	26% 14% 35% 20% 5% 1	420
	11th to 20th	Day	22% 15% 31% 24% 7% 1	412
	21st to end	Day	25% 12% 34% 19% 8% 2	504
May	1st to 10th	Day	23% 14% 31% 23% 7% 2	603
	11th to 20th	Day	23% 12% 28% 23% 11% 2	625
	21st to end	Day	16% 9% 28% 27% 16% 4	679
June	1st to 10th	Day	8% 6% 26% 31% 22% 7%	630
	11th to 20th	Day	11% 5% 29% 35% 16% 4	630
	21st to end	Day	12% 8% 30% 30% 15% 5%	630
July	1st to 10th	Day	8% 4 18% 33% 25% 12%	621
	11th to 20th	Day	7% 4 27% 33% 23% 5%	618
	21st to end	Day	6% 3 22% 40% 24% 5%	684
August	1st to 10th	Day	8% 2 22% 37% 26% 6%	626
	11th to 20th	Day	11% 4 19% 33% 26% 6%	615
	21st to end	Day	10% 3 21% 35% 25% 7%	513
September	1st to 10th	Day	15% 7% 19% 32% 20% 7%	450
	11th to 20th	Day	9% 3 26% 33% 23% 5%	442
	21st to end	Day	11% 13% 30% 28% 16% 3	270
October	1st to 10th	Day	20% 6% 32% 24% 15% 4	270
	11th to 20th	Day	17% 23% 36% 16% 7%	270
	21st to end	Day	22% 18% 29% 21% 7% 3	297
November	1st to 10th	Day	16% 12% 39% 24% 7% 2	270
	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% 13% 40% 21% 7%	605
	11th to 20th	Night	18% 8% 37% 28% 8%	540
	21st to end	Night	24% 11% 38% 21% 5% 1	594
April	1st to 10th	Night	18% 14% 41% 23% 4	540
	11th to 20th	Night	18% 12% 38% 24% 6%	533
	21st to end	Night	21% 10% 36% 25% 8% 1	540
May	1st to 10th	Night	17% 8% 35% 31% 7% 1	540
	11th to 20th	Night	17% 8% 35% 30% 10%	531
	21st to end	Night	15% 5% 27% 36% 15%	584
June	1st to 10th	Night	9% 5% 23% 44% 17% 1	540
	11th to 20th	Night	9% 3 26% 49% 12% 1	540
	21st to end	Night	10% 4 30% 41% 15% 1	540
July	1st to 10th	Night	6% 2 13% 46% 31% 2	525
	11th to 20th	Night	5% 2 23% 42% 25% 3	522
	21st to end	Night	5% 1 17% 52% 23% 1	591
August	1st to 10th	Night	6% 4 14% 47% 26% 2	537
	11th to 20th	Night	7% 2 18% 44% 26% 3	534
	21st to end	Night	9% 3 20% 41% 25% 2	594
September	1st to 10th	Night	11% 5% 20% 40% 19% 4	540
	11th to 20th	Night	13% 3 21% 38% 22% 2	540
	21st to end	Night	11% 6% 32% 34% 16% 1	620
October	1st to 10th	Night	15% 7% 32% 32% 13% 2	720
	11th to 20th	Night	17% 11% 34% 30% 8%	720
	21st to end	Night	20% 11% 35% 27% 8%	792
November	1st to 10th	Night	14% 10% 38% 29% 8%	740
	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

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