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 2005-11-05 to 2024-10-14.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 35%	101017
	Night	15% 11% 4 8% 5% 5% 4 3 5% 6% 34%	97591
Transparency	Day	48% 7% 12% 20% 12% 1	100812
	Night	50% 6% 11% 20% 12% 1	97473
Seeing	Day	17% 11% 28% 24% 14% 6%	15713
	Night	19% 9% 30% 30% 12% 1	25536

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% 18% 6% 11% 7% 6% 5% 4 4 4 19%	2527
	11th to 20th	Day	15% 16% 7% 14% 10% 12% 6% 3 4 4 9%	2530
	21st to end	Day	24% 23% 7% 12% 5% 4 3 2 2 4 13%	2827
February	1st to 10th	Day	12% 16% 5% 12% 8% 6% 6% 4 5% 5% 20%	1736
	11th to 20th	Day	13% 13% 6% 13% 8% 5% 5% 3 3 6% 25%	1848
	21st to end	Day	14% 16% 4 11% 6% 6% 5% 4 5% 6% 24%	1570
March	1st to 10th	Day	15% 14% 4 8% 5% 4 4 4 5% 7% 31%	3120
	11th to 20th	Day	11% 9% 5% 10% 7% 6% 5% 4 6% 7% 31%	3438
	21st to end	Day	19% 13% 5% 8% 5% 4 4 3 5% 5% 28%	3872
April	1st to 10th	Day	17% 13% 3 9% 5% 4 4 3 5% 5% 30%	2280
	11th to 20th	Day	17% 9% 3 7% 4 5% 4 4 5% 7% 36%	2340
	21st to end	Day	17% 8% 3 7% 6% 4 5% 3 5% 7% 35%	2470
May	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 5% 7% 36%	4082
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 4 6% 8% 38%	4081
	21st to end	Day	11% 7% 2 6% 4 4 4 4 5% 8% 44%	4718
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 4 4 6% 9% 43%	2660
	11th to 20th	Day	11% 5% 2 6% 5% 5% 4 3 6% 9% 44%	2660
	21st to end	Day	9% 5% 3 7% 5% 5% 5% 4 7% 10% 40%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	4440
	11th to 20th	Day	7% 4 2 4 4 4 5% 4 6% 10% 50%	4440
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 9% 45%	4760
August	1st to 10th	Day	13% 4 2 6% 5% 4 5% 4 6% 9% 42%	4104
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 5% 8% 46%	4044
	21st to end	Day	8% 5% 2 7% 4 5% 5% 4 7% 8% 43%	4313
September	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 4 5% 7% 44%	2218
	11th to 20th	Day	12% 6% 2 5% 3 4 4 3 5% 8% 49%	2090
	21st to end	Day	13% 6% 3 7% 5% 5% 4 4 6% 9% 38%	2090
October	1st to 10th	Day	14% 6% 2 8% 4 3 4 3 4 7% 43%	2090
	11th to 20th	Day	19% 9% 3 8% 6% 4 5% 4 6% 6% 28%	1800
	21st to end	Day	22% 11% 4 8% 5% 5% 4 4 4 6% 27%	1782
November	1st to 10th	Day	11% 12% 3 6% 5% 5% 5% 3 5% 6% 39%	1672
	11th to 20th	Day	14% 15% 5% 9% 6% 5% 5% 4 5% 7% 25%	1710
	21st to end	Day	18% 15% 5% 9% 5% 4 4 3 5% 5% 27%	1710
December	1st to 10th	Day	13% 21% 7% 13% 6% 6% 5% 3 4 5% 18%	1634
	11th to 20th	Day	13% 19% 6% 11% 6% 6% 5% 4 6% 5% 19%	1918
	21st to end	Day	21% 23% 8% 12% 5% 4 4 3 4 3 11%	2783
January	1st to 10th	Night	16% 18% 8% 13% 6% 5% 4 3 4 4 18%	4428
	11th to 20th	Night	14% 19% 9% 16% 7% 9% 4 3 4 4 12%	4440
	21st to end	Night	22% 18% 9% 12% 6% 5% 4 2 4 4 14%	4838
February	1st to 10th	Night	15% 15% 5% 11% 9% 5% 5% 4 5% 5% 21%	2442
	11th to 20th	Night	12% 14% 6% 11% 8% 6% 5% 4 5% 5% 25%	2218
	21st to end	Night	17% 12% 5% 9% 5% 5% 4 3 6% 6% 27%	1884
March	1st to 10th	Night	14% 10% 4 8% 4 4 3 3 5% 7% 38%	3733
	11th to 20th	Night	9% 9% 2 6% 4 5% 4 4 5% 8% 45%	3521
	21st to end	Night	17% 11% 6% 10% 5% 4 4 3 5% 6% 32%	3786
April	1st to 10th	Night	15% 11% 3 7% 5% 4 5% 4 4 6% 37%	1900
	11th to 20th	Night	20% 8% 2 6% 4 4 3 4 5% 5% 38%	1788
	21st to end	Night	18% 7% 3 7% 5% 4 4 3 5% 7% 38%	1710
May	1st to 10th	Night	12% 8% 3 8% 4 5% 5% 4 6% 8% 37%	2813
	11th to 20th	Night	18% 7% 3 6% 3 4 3 3 4 6% 43%	2812
	21st to end	Night	11% 6% 3 6% 4 4 4 4 6% 7% 44%	2843
June	1st to 10th	Night	11% 5% 2 5% 4 3 5% 4 6% 9% 46%	1520
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 8% 8% 44%	1520
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 6% 10% 45%	1520
July	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 5% 7% 10% 45%	2530
	11th to 20th	Night	8% 4 1 5% 3 4 3 3 5% 7% 57%	2530
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 7% 9% 47%	2800
August	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 48%	2825
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 6% 8% 46%	2785
	21st to end	Night	9% 6% 3 8% 4 5% 3 4 6% 7% 46%	3339
September	1st to 10th	Night	9% 5% 3 5% 5% 5% 4 3 6% 8% 49%	1943
	11th to 20th	Night	10% 4 1 4 3 4 3 3 5% 7% 55%	2090
	21st to end	Night	14% 7% 3 6% 5% 4 4 3 4 6% 43%	2090
October	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 6% 6% 47%	2090
	11th to 20th	Night	20% 10% 4 7% 5% 3 4 3 5% 6% 33%	2229
	21st to end	Night	23% 10% 3 8% 4 4 4 3 5% 6% 31%	2574
November	1st to 10th	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	2406
	11th to 20th	Night	14% 15% 3 8% 5% 5% 6% 5% 6% 6% 27%	2470
	21st to end	Night	18% 14% 4 9% 4 4 5% 4 4 7% 28%	2470
December	1st to 10th	Night	16% 18% 7% 12% 7% 5% 4 4 5% 5% 17%	2541
	11th to 20th	Night	15% 16% 7% 13% 7% 5% 5% 3 5% 5% 20%	3279
	21st to end	Night	26% 20% 6% 10% 5% 4 4 3 4 4 12%	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%	1736
	11th to 20th	Day	63% 4 9% 15% 9% 1	1858
	21st to end	Day	61% 6% 11% 15% 7% 1	1570
March	1st to 10th	Day	52% 7% 13% 20% 7% 1	3120
	11th to 20th	Day	49% 8% 13% 20% 9% 1	3438
	21st to end	Day	57% 6% 10% 17% 8% 2	3872
April	1st to 10th	Day	55% 5% 10% 19% 11%	2280
	11th to 20th	Day	48% 6% 11% 20% 14% 2	2340
	21st to end	Day	48% 7% 11% 18% 14% 2	2470
May	1st to 10th	Day	45% 7% 12% 21% 13% 1	4120
	11th to 20th	Day	41% 9% 14% 24% 11% 1	4085
	21st to end	Day	36% 7% 11% 20% 23% 2	4731
June	1st to 10th	Day	37% 7% 13% 27% 15% 2	2660
	11th to 20th	Day	36% 8% 13% 23% 19% 2	2660
	21st to end	Day	37% 9% 13% 22% 17% 2	2660
July	1st to 10th	Day	41% 7% 14% 20% 15% 3	4371
	11th to 20th	Day	28% 7% 14% 32% 18% 1	4365
	21st to end	Day	33% 8% 15% 20% 22% 2	4610
August	1st to 10th	Day	37% 8% 12% 26% 16% 1	4091
	11th to 20th	Day	36% 7% 11% 32% 14% 1	4083
	21st to end	Day	36% 7% 12% 29% 13% 2	4312
September	1st to 10th	Day	38% 8% 13% 24% 16% 2	2218
	11th to 20th	Day	33% 6% 13% 30% 18% 1	2090
	21st to end	Day	42% 6% 13% 23% 15%	2090
October	1st to 10th	Day	42% 7% 9% 26% 15% 1	2090
	11th to 20th	Day	54% 6% 13% 19% 8%	1801
	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% 9% 14% 5% 1	2442
	11th to 20th	Night	59% 6% 9% 16% 9% 1	2219
	21st to end	Night	57% 6% 12% 16% 8% 2	1884
March	1st to 10th	Night	46% 6% 13% 27% 7% 1	3733
	11th to 20th	Night	36% 7% 15% 22% 19% 2	3521
	21st to end	Night	52% 6% 12% 20% 9% 1	3786
April	1st to 10th	Night	47% 6% 11% 20% 14% 1	1900
	11th to 20th	Night	47% 5% 10% 19% 16% 2	1788
	21st to end	Night	45% 6% 13% 20% 14% 3	1710
May	1st to 10th	Night	43% 7% 13% 21% 14% 1	2840
	11th to 20th	Night	41% 6% 10% 23% 17% 2	2813
	21st to end	Night	36% 8% 11% 22% 20% 3	2851
June	1st to 10th	Night	34% 7% 11% 29% 17% 3	1520
	11th to 20th	Night	35% 7% 13% 26% 17% 2	1520
	21st to end	Night	33% 7% 13% 27% 17% 2	1520
July	1st to 10th	Night	34% 6% 13% 27% 13% 7%	2490
	11th to 20th	Night	27% 6% 12% 32% 21% 1	2482
	21st to end	Night	33% 8% 16% 17% 23% 3	2711
August	1st to 10th	Night	33% 6% 13% 29% 19% 1	2814
	11th to 20th	Night	35% 7% 10% 32% 15% 1	2812
	21st to end	Night	37% 6% 11% 30% 15% 2	3339
September	1st to 10th	Night	33% 7% 11% 28% 19% 2	1943
	11th to 20th	Night	28% 6% 11% 32% 22% 2	2090
	21st to end	Night	42% 6% 11% 25% 15% 1	2090
October	1st to 10th	Night	38% 6% 11% 27% 17% 1	2090
	11th to 20th	Night	52% 6% 12% 19% 11% 1	2233
	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% 20% 33% 20% 2 1	270
	11th to 20th	Day	21% 19% 39% 19% 2	270
	21st to end	Day	26% 23% 36% 13% 2	225
March	1st to 10th	Day	21% 19% 42% 13% 5%	270
	11th to 20th	Day	21% 16% 33% 22% 8%	431
	21st to end	Day	28% 14% 28% 22% 6% 1	506
April	1st to 10th	Day	22% 13% 35% 23% 6% 1	460
	11th to 20th	Day	24% 19% 30% 16% 6% 5%	452
	21st to end	Day	20% 12% 34% 21% 11% 2	460
May	1st to 10th	Day	25% 13% 26% 24% 10% 2	463
	11th to 20th	Day	19% 11% 24% 26% 14% 6%	487
	21st to end	Day	15% 8% 25% 27% 18% 8%	667
June	1st to 10th	Day	11% 10% 27% 31% 16% 5%	680
	11th to 20th	Day	14% 6% 30% 29% 17% 5%	680
	21st to end	Day	12% 8% 29% 31% 15% 6%	680
July	1st to 10th	Day	9% 4 20% 31% 22% 14%	671
	11th to 20th	Day	11% 7% 24% 28% 21% 10%	668
	21st to end	Day	8% 4 23% 33% 23% 10%	710
August	1st to 10th	Day	10% 4 19% 28% 21% 19%	486
	11th to 20th	Day	12% 4 16% 26% 31% 11%	479
	21st to end	Day	10% 4 21% 28% 24% 13%	539
September	1st to 10th	Day	16% 5% 21% 26% 21% 11%	490
	11th to 20th	Day	10% 5% 18% 23% 29% 15%	490
	21st to end	Day	16% 11% 22% 24% 17% 10%	490
October	1st to 10th	Day	17% 7% 28% 25% 16% 7%	490
	11th to 20th	Day	22% 18% 25% 28% 6% 1	418
	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	23% 13% 37% 24% 3	760
	11th to 20th	Night	20% 14% 36% 26% 4	743
	21st to end	Night	22% 14% 39% 21% 4	628
March	1st to 10th	Night	20% 11% 36% 25% 7% 1	760
	11th to 20th	Night	17% 14% 36% 26% 6% 1	760
	21st to end	Night	23% 9% 32% 28% 7%	817
April	1st to 10th	Night	18% 11% 36% 27% 6% 1	570
	11th to 20th	Night	24% 10% 34% 24% 7% 1	563
	21st to end	Night	22% 6% 35% 29% 7% 1	570
May	1st to 10th	Night	19% 7% 33% 32% 8% 1	570
	11th to 20th	Night	21% 5% 30% 32% 11% 1	561
	21st to end	Night	13% 3 26% 34% 21% 2	617
June	1st to 10th	Night	11% 5% 24% 44% 15% 1	570
	11th to 20th	Night	13% 3 27% 39% 17% 1	570
	21st to end	Night	9% 4 23% 42% 22% 1	570
July	1st to 10th	Night	7% 3 19% 40% 27% 5%	555
	11th to 20th	Night	11% 2 18% 42% 24% 3	552
	21st to end	Night	7% 2 18% 45% 25% 2	624
August	1st to 10th	Night	10% 1 15% 45% 27% 3	567
	11th to 20th	Night	10% 2 15% 40% 28% 5%	564
	21st to end	Night	11% 3 17% 39% 28% 3	627
September	1st to 10th	Night	11% 3 18% 38% 26% 4	613
	11th to 20th	Night	13% 4 16% 36% 28% 4	760
	21st to end	Night	17% 5% 23% 30% 21% 4	760
October	1st to 10th	Night	18% 6% 25% 32% 18% 1	760
	11th to 20th	Night	25% 13% 28% 26% 7% 1	735
	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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