Dunham Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dunham Hill Observatory clear sky chart from 2005-11-06 to 2024-02-10.

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	14% 10% 4 7% 6% 4 5% 5% 5% 7% 33%	78236
	Night	15% 11% 5% 8% 6% 3 4 4 5% 6% 32%	80780
Transparency	Day	50% 7% 12% 19% 11% 1	77940
	Night	52% 6% 11% 18% 11% 1	80597
Seeing	Day	18% 11% 28% 24% 14% 6%	14689
	Night	19% 9% 29% 30% 12% 1	24709

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	16% 17% 7% 9% 8% 5% 5% 6% 3 4 19%	2527
	11th to 20th	Day	15% 15% 8% 12% 12% 9% 7% 5% 4 4 9%	2530
	21st to end	Day	24% 21% 9% 11% 6% 3 3 3 3 4 13%	2827
February	1st to 10th	Day	12% 15% 6% 10% 10% 4 7% 5% 5% 5% 21%	1726
	11th to 20th	Day	14% 11% 6% 12% 10% 5% 5% 4 3 6% 25%	1748
	21st to end	Day	15% 15% 6% 10% 6% 4 5% 5% 4 7% 23%	1480
March	1st to 10th	Day	12% 11% 5% 8% 7% 4 5% 6% 6% 7% 29%	1800
	11th to 20th	Day	13% 10% 5% 7% 7% 4 4 3 6% 7% 34%	1952
	21st to end	Day	18% 12% 5% 7% 6% 3 4 3 5% 6% 31%	2209
April	1st to 10th	Day	18% 11% 4 8% 6% 3 4 4 5% 5% 30%	2160
	11th to 20th	Day	17% 9% 3 6% 5% 3 5% 4 5% 7% 35%	2215
	21st to end	Day	17% 8% 3 6% 7% 3 5% 4 5% 7% 35%	2340
May	1st to 10th	Day	17% 8% 3 6% 6% 4 5% 5% 5% 7% 34%	2340
	11th to 20th	Day	19% 6% 3 5% 6% 3 4 4 5% 7% 38%	2301
	21st to end	Day	13% 6% 2 6% 5% 3 5% 5% 5% 8% 41%	2661
June	1st to 10th	Day	10% 5% 2 5% 6% 4 5% 5% 6% 9% 43%	2520
	11th to 20th	Day	11% 5% 2 5% 6% 3 4 4 5% 9% 44%	2520
	21st to end	Day	9% 4 3 6% 6% 3 5% 5% 7% 10% 41%	2520
July	1st to 10th	Day	7% 4 2 4 4 3 5% 6% 8% 10% 48%	2520
	11th to 20th	Day	10% 5% 3 4 4 3 5% 6% 6% 8% 45%	2520
	21st to end	Day	7% 4 2 5% 4 4 6% 6% 7% 10% 46%	2716
August	1st to 10th	Day	10% 4 2 5% 6% 3 5% 6% 7% 10% 42%	2327
	11th to 20th	Day	10% 4 2 5% 6% 4 6% 5% 6% 9% 43%	2303
	21st to end	Day	10% 5% 3 5% 5% 3 5% 5% 7% 8% 43%	2453
September	1st to 10th	Day	12% 7% 3 5% 6% 3 5% 5% 5% 7% 42%	2102
	11th to 20th	Day	12% 6% 2 5% 4 2 4 4 5% 8% 47%	1980
	21st to end	Day	13% 6% 2 6% 6% 4 5% 5% 6% 9% 39%	1980
October	1st to 10th	Day	15% 6% 3 7% 6% 3 4 4 4 7% 41%	1980
	11th to 20th	Day	20% 9% 3 7% 8% 3 4 5% 6% 6% 29%	1770
	21st to end	Day	22% 10% 4 7% 6% 4 4 5% 4 6% 27%	1782
November	1st to 10th	Day	12% 11% 3 6% 5% 4 5% 4 4 7% 39%	1672
	11th to 20th	Day	15% 13% 5% 9% 7% 4 5% 5% 5% 7% 25%	1710
	21st to end	Day	19% 14% 5% 8% 6% 3 4 4 5% 5% 28%	1710
December	1st to 10th	Day	13% 18% 9% 12% 7% 4 5% 4 4 5% 18%	1634
	11th to 20th	Day	13% 17% 7% 10% 7% 4 6% 5% 6% 5% 20%	1918
	21st to end	Day	22% 22% 8% 11% 7% 3 5% 4 4 3 11%	2783
January	1st to 10th	Night	16% 17% 9% 11% 7% 4 4 4 4 4 18%	4428
	11th to 20th	Night	14% 18% 9% 15% 8% 8% 5% 4 4 4 12%	4440
	21st to end	Night	22% 17% 10% 11% 7% 3 4 3 4 4 14%	4838
February	1st to 10th	Night	15% 14% 5% 10% 10% 5% 5% 5% 5% 5% 21%	2437
	11th to 20th	Night	13% 13% 6% 9% 9% 4 6% 5% 5% 5% 25%	2098
	21st to end	Night	18% 11% 6% 7% 6% 4 5% 4 5% 6% 28%	1776
March	1st to 10th	Night	15% 10% 4 8% 6% 3 4 4 5% 7% 36%	2160
	11th to 20th	Night	12% 10% 4 7% 5% 3 5% 4 5% 6% 40%	2007
	21st to end	Night	19% 9% 4 7% 5% 2 4 3 5% 7% 35%	2160
April	1st to 10th	Night	15% 10% 4 7% 6% 3 4 6% 4 7% 37%	1800
	11th to 20th	Night	19% 8% 3 6% 5% 3 3 4 5% 5% 38%	1693
	21st to end	Night	18% 6% 3 6% 5% 3 4 4 5% 7% 38%	1620
May	1st to 10th	Night	15% 8% 3 7% 6% 4 5% 6% 5% 7% 35%	1620
	11th to 20th	Night	20% 7% 2 4 4 3 4 4 4 7% 40%	1592
	21st to end	Night	11% 6% 3 5% 5% 3 6% 5% 6% 8% 43%	1612
June	1st to 10th	Night	12% 5% 2 5% 5% 2 4 5% 5% 9% 47%	1440
	11th to 20th	Night	11% 5% 2 4 4 3 5% 4 7% 9% 45%	1440
	21st to end	Night	9% 5% 2 5% 5% 3 5% 5% 6% 10% 45%	1440
July	1st to 10th	Night	6% 4 2 4 5% 2 4 4 6% 9% 54%	1440
	11th to 20th	Night	8% 4 2 4 4 3 5% 4 5% 9% 52%	1440
	21st to end	Night	6% 3 2 4 4 2 4 5% 6% 9% 53%	1601
August	1st to 10th	Night	8% 4 1 4 5% 4 4 6% 6% 9% 49%	1611
	11th to 20th	Night	9% 4 2 4 5% 3 5% 4 7% 9% 47%	1598
	21st to end	Night	9% 5% 3 5% 4 3 3 4 5% 8% 51%	1899
September	1st to 10th	Night	10% 5% 3 4 6% 4 4 4 5% 8% 47%	1840
	11th to 20th	Night	11% 5% 2 3 4 3 3 4 5% 7% 53%	1980
	21st to end	Night	14% 6% 3 4 6% 3 4 4 5% 6% 44%	1980
October	1st to 10th	Night	16% 6% 3 5% 4 3 4 3 5% 6% 45%	1980
	11th to 20th	Night	20% 10% 4 6% 6% 3 4 4 5% 6% 33%	2186
	21st to end	Night	23% 9% 3 7% 5% 3 5% 4 5% 6% 31%	2574
November	1st to 10th	Night	15% 8% 4 6% 5% 3 4 3 4 6% 41%	2406
	11th to 20th	Night	14% 14% 4 7% 6% 3 6% 6% 6% 6% 27%	2470
	21st to end	Night	18% 13% 4 7% 6% 3 4 5% 4 7% 28%	2470
December	1st to 10th	Night	16% 16% 8% 11% 8% 4 5% 5% 5% 5% 18%	2541
	11th to 20th	Night	15% 14% 8% 12% 8% 3 5% 4 5% 5% 20%	3279
	21st to end	Night	26% 20% 7% 9% 6% 3 4 4 4 5% 13%	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	67% 8% 10% 12% 2	2528
	11th to 20th	Day	79% 6% 8% 5% 2	2530
	21st to end	Day	77% 4 10% 6% 2	2827
February	1st to 10th	Day	65% 6% 9% 13% 6%	1726
	11th to 20th	Day	63% 4 9% 15% 8% 1	1758
	21st to end	Day	62% 6% 10% 14% 7% 1	1480
March	1st to 10th	Day	53% 7% 13% 19% 8% 1	1800
	11th to 20th	Day	50% 5% 11% 18% 14% 2	1952
	21st to end	Day	54% 5% 11% 19% 9% 1	2209
April	1st to 10th	Day	56% 5% 10% 19% 10%	2160
	11th to 20th	Day	49% 6% 11% 19% 14% 2	2215
	21st to end	Day	49% 7% 11% 17% 14% 3	2340
May	1st to 10th	Day	49% 7% 11% 21% 10% 2	2340
	11th to 20th	Day	46% 6% 12% 20% 15% 2	2305
	21st to end	Day	41% 7% 13% 24% 13% 1	2661
June	1st to 10th	Day	36% 7% 13% 27% 15% 2	2520
	11th to 20th	Day	36% 8% 13% 22% 19% 2	2520
	21st to end	Day	37% 9% 13% 22% 17% 2	2520
July	1st to 10th	Day	30% 8% 13% 27% 18% 4	2451
	11th to 20th	Day	33% 8% 13% 26% 18% 2	2445
	21st to end	Day	32% 9% 13% 25% 19% 3	2565
August	1st to 10th	Day	35% 9% 15% 27% 13% 2	2314
	11th to 20th	Day	37% 8% 15% 26% 13% 2	2303
	21st to end	Day	36% 8% 13% 25% 16% 3	2453
September	1st to 10th	Day	40% 8% 12% 23% 15% 2	2102
	11th to 20th	Day	34% 6% 13% 30% 15% 1	1980
	21st to end	Day	41% 7% 13% 23% 15%	1980
October	1st to 10th	Day	43% 7% 9% 25% 14% 2	1980
	11th to 20th	Day	54% 6% 13% 18% 9%	1770
	21st to end	Day	59% 6% 12% 15% 7% 1	1782
November	1st to 10th	Day	45% 6% 13% 22% 13% 1	1672
	11th to 20th	Day	58% 6% 12% 18% 6% 1	1710
	21st to end	Day	59% 5% 15% 16% 5%	1710
December	1st to 10th	Day	69% 6% 10% 12% 3 1	1634
	11th to 20th	Day	65% 7% 14% 12% 3	1915
	21st to end	Day	77% 5% 8% 6% 3	2783
January	1st to 10th	Night	67% 6% 11% 11% 5% 1	4428
	11th to 20th	Night	76% 5% 9% 6% 4	4440
	21st to end	Night	75% 4 9% 9% 3	4838
February	1st to 10th	Night	64% 6% 10% 14% 6% 1	2437
	11th to 20th	Night	60% 6% 9% 15% 8% 1	2099
	21st to end	Night	57% 6% 12% 15% 8% 2	1776
March	1st to 10th	Night	49% 5% 13% 23% 10% 1	2160
	11th to 20th	Night	45% 6% 11% 21% 15% 2	2007
	21st to end	Night	51% 5% 10% 23% 10% 2	2160
April	1st to 10th	Night	48% 7% 11% 20% 14% 1	1800
	11th to 20th	Night	47% 5% 11% 19% 17% 3	1693
	21st to end	Night	44% 6% 13% 20% 14% 3	1620
May	1st to 10th	Night	49% 5% 12% 20% 11% 2	1620
	11th to 20th	Night	44% 6% 10% 20% 19% 3	1593
	21st to end	Night	37% 8% 13% 25% 15% 2	1613
June	1st to 10th	Night	34% 7% 10% 30% 16% 3	1440
	11th to 20th	Night	34% 7% 13% 26% 18% 2	1440
	21st to end	Night	33% 7% 13% 27% 17% 2	1440
July	1st to 10th	Night	26% 7% 13% 30% 19% 4	1400
	11th to 20th	Night	29% 7% 12% 30% 20% 2	1392
	21st to end	Night	27% 8% 16% 24% 21% 4	1512
August	1st to 10th	Night	30% 7% 16% 29% 17% 1	1600
	11th to 20th	Night	31% 8% 13% 29% 16% 2	1598
	21st to end	Night	32% 6% 12% 26% 21% 3	1899
September	1st to 10th	Night	34% 7% 11% 27% 18% 2	1840
	11th to 20th	Night	30% 6% 12% 32% 19% 2	1980
	21st to end	Night	40% 6% 12% 26% 15% 1	1980
October	1st to 10th	Night	40% 6% 11% 25% 17% 1	1980
	11th to 20th	Night	52% 6% 12% 19% 11% 1	2186
	21st to end	Night	54% 5% 11% 20% 9% 1	2574
November	1st to 10th	Night	45% 5% 10% 24% 15% 1	2406
	11th to 20th	Night	54% 7% 13% 17% 7% 1	2470
	21st to end	Night	56% 6% 13% 19% 5%	2470
December	1st to 10th	Night	68% 6% 10% 10% 5%	2541
	11th to 20th	Night	65% 6% 12% 12% 5%	3281
	21st to end	Night	75% 6% 10% 7% 3	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	23% 15% 34% 20% 7% 1	270
	11th to 20th	Day	19% 14% 42% 19% 5% 1	270
	21st to end	Day	27% 14% 31% 23% 6%	297
February	1st to 10th	Day	23% 19% 34% 20% 2 1	267
	11th to 20th	Day	22% 19% 41% 16% 2	240
	21st to end	Day	26% 24% 36% 12% 2	198
March	1st to 10th	Day	23% 20% 40% 13% 5%	240
	11th to 20th	Day	20% 15% 33% 23% 8% 1	392
	21st to end	Day	29% 13% 28% 23% 6% 2	462
April	1st to 10th	Day	24% 11% 34% 24% 7% 1	420
	11th to 20th	Day	23% 19% 29% 16% 7% 6%	412
	21st to end	Day	21% 12% 33% 20% 11% 2	420
May	1st to 10th	Day	26% 12% 27% 23% 10% 2	423
	11th to 20th	Day	20% 11% 25% 24% 13% 6%	447
	21st to end	Day	15% 7% 25% 26% 19% 8%	615
June	1st to 10th	Day	11% 8% 28% 31% 17% 5%	630
	11th to 20th	Day	14% 6% 28% 29% 17% 6%	630
	21st to end	Day	11% 7% 28% 33% 15% 7%	630
July	1st to 10th	Day	8% 4 19% 31% 22% 15%	621
	11th to 20th	Day	11% 7% 24% 28% 21% 10%	618
	21st to end	Day	8% 4 24% 33% 21% 9%	657
August	1st to 10th	Day	9% 3 20% 27% 21% 20%	446
	11th to 20th	Day	12% 4 16% 25% 32% 12%	439
	21st to end	Day	11% 4 21% 27% 24% 13%	495
September	1st to 10th	Day	17% 5% 19% 27% 21% 11%	450
	11th to 20th	Day	11% 6% 19% 24% 28% 13%	450
	21st to end	Day	15% 11% 22% 24% 18% 11%	450
October	1st to 10th	Day	18% 7% 28% 23% 16% 8%	450
	11th to 20th	Day	22% 18% 25% 27% 7% 1	406
	21st to end	Day	28% 15% 32% 20% 3 1	297
November	1st to 10th	Day	16% 16% 35% 24% 8% 1	270
	11th to 20th	Day	22% 21% 31% 20% 5% 1	270
	21st to end	Day	25% 16% 40% 14% 6%	270
December	1st to 10th	Day	27% 21% 28% 17% 7% 1	270
	11th to 20th	Day	24% 20% 38% 14% 4	270
	21st to end	Day	23% 21% 36% 16% 4	297
January	1st to 10th	Night	20% 15% 34% 24% 6% 1	950
	11th to 20th	Night	23% 14% 35% 23% 5% 1	950
	21st to end	Night	20% 10% 34% 30% 5%	1012
February	1st to 10th	Night	22% 14% 36% 24% 3	759
	11th to 20th	Night	20% 14% 36% 26% 4	703
	21st to end	Night	24% 14% 38% 21% 4	592
March	1st to 10th	Night	20% 11% 35% 25% 7% 1	720
	11th to 20th	Night	18% 13% 35% 27% 6% 1	720
	21st to end	Night	24% 9% 32% 28% 7% 1	774
April	1st to 10th	Night	18% 11% 36% 27% 6% 1	540
	11th to 20th	Night	24% 9% 34% 25% 7% 1	533
	21st to end	Night	22% 6% 34% 30% 7% 1	540
May	1st to 10th	Night	20% 7% 33% 32% 8% 1	540
	11th to 20th	Night	22% 5% 30% 31% 11% 1	531
	21st to end	Night	13% 2 26% 34% 22% 2	584
June	1st to 10th	Night	11% 4 24% 45% 15% 1	540
	11th to 20th	Night	13% 3 26% 39% 18% 1	540
	21st to end	Night	9% 4 22% 42% 23% 1	540
July	1st to 10th	Night	7% 2 18% 40% 29% 5%	525
	11th to 20th	Night	11% 2 17% 43% 24% 3	522
	21st to end	Night	7% 2 19% 45% 25% 2	591
August	1st to 10th	Night	10% 1 14% 45% 27% 3	537
	11th to 20th	Night	9% 2 16% 38% 29% 5%	534
	21st to end	Night	11% 3 17% 38% 28% 3	594
September	1st to 10th	Night	12% 3 18% 38% 25% 4	580
	11th to 20th	Night	14% 4 17% 35% 27% 4	720
	21st to end	Night	17% 5% 23% 29% 22% 5%	720
October	1st to 10th	Night	19% 5% 24% 32% 19% 1	720
	11th to 20th	Night	25% 13% 28% 26% 7% 1	720
	21st to end	Night	27% 11% 32% 24% 6%	792
November	1st to 10th	Night	20% 10% 31% 30% 9% 1	740
	11th to 20th	Night	20% 14% 37% 23% 5% 1	760
	21st to end	Night	24% 11% 34% 26% 4	760
December	1st to 10th	Night	24% 14% 34% 22% 6%	831
	11th to 20th	Night	23% 13% 37% 23% 3	950
	21st to end	Night	30% 12% 35% 21% 2	1045

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