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 2025-03-12.

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% 4 9% 5% 5% 5% 4 5% 7% 33%	108697
	Night	14% 11% 4 9% 5% 5% 4 3 5% 6% 32%	110479
Transparency	Day	48% 6% 12% 20% 12% 1	112284
	Night	51% 6% 11% 20% 12% 1	115033
Seeing	Day	17% 11% 28% 24% 14% 6%	16164
	Night	19% 9% 30% 30% 11% 1	26186

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	12% 16% 7% 15% 7% 6% 5% 4 4 5% 19%	3617
	11th to 20th	Day	12% 14% 7% 18% 10% 12% 7% 3 4 3 8%	3620
	21st to end	Day	19% 19% 6% 14% 6% 4 6% 3 3 5% 16%	4052
February	1st to 10th	Day	12% 16% 5% 12% 8% 6% 6% 4 5% 5% 21%	1828
	11th to 20th	Day	13% 13% 6% 14% 9% 5% 5% 3 3 5% 24%	1948
	21st to end	Day	14% 16% 4 11% 6% 5% 5% 4 5% 6% 24%	1650
March	1st to 10th	Day	15% 13% 4 8% 5% 4 4 4 5% 6% 31%	3220
	11th to 20th	Day	11% 9% 5% 10% 7% 6% 5% 4 6% 7% 31%	3448
	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	18% 9% 3 8% 6% 4 4 4 6% 6% 30%	1867
	21st to end	Day	21% 11% 4 8% 5% 5% 4 4 5% 6% 27%	1881
November	1st to 10th	Day	11% 12% 3 7% 5% 5% 5% 3 5% 6% 39%	1762
	11th to 20th	Day	14% 14% 5% 10% 6% 5% 5% 4 5% 7% 24%	1800
	21st to end	Day	18% 15% 5% 10% 5% 4 4 3 5% 5% 27%	1800
December	1st to 10th	Day	9% 18% 9% 14% 7% 6% 5% 3 5% 6% 17%	2802
	11th to 20th	Day	14% 23% 9% 11% 8% 5% 4 3 4 3 16%	3008
	21st to end	Day	24% 20% 7% 13% 5% 4 4 3 4 4 12%	3982
January	1st to 10th	Night	12% 17% 7% 17% 6% 5% 4 3 5% 5% 19%	6348
	11th to 20th	Night	12% 15% 8% 17% 9% 9% 5% 4 5% 5% 12%	6360
	21st to end	Night	16% 17% 8% 12% 6% 6% 5% 3 5% 5% 17%	6922
February	1st to 10th	Night	15% 16% 5% 11% 9% 5% 5% 3 5% 4 21%	2570
	11th to 20th	Night	12% 13% 6% 11% 8% 6% 5% 4 5% 5% 25%	2338
	21st to end	Night	17% 12% 5% 9% 5% 5% 5% 4 6% 6% 28%	1980
March	1st to 10th	Night	14% 10% 4 8% 4 4 4 3 5% 7% 37%	3853
	11th to 20th	Night	9% 9% 2 6% 4 5% 4 4 5% 8% 45%	3540
	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 4 6% 34%	2309
	21st to end	Night	22% 9% 3 7% 4 4 4 3 6% 6% 31%	2717
November	1st to 10th	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	2536
	11th to 20th	Night	14% 16% 3 8% 5% 5% 6% 5% 6% 6% 27%	2600
	21st to end	Night	17% 15% 4 9% 4 4 5% 3 4 6% 27%	2600
December	1st to 10th	Night	12% 18% 8% 14% 6% 5% 4 3 4 5% 20%	4377
	11th to 20th	Night	15% 20% 7% 15% 8% 5% 4 3 5% 5% 15%	5199
	21st to end	Night	24% 20% 6% 11% 5% 4 4 3 4 5% 14%	6996

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% 7% 10% 14% 3	3618
	11th to 20th	Day	79% 5% 8% 5% 3	3620
	21st to end	Day	72% 4 12% 9% 3	4052
February	1st to 10th	Day	65% 6% 10% 14% 6%	1828
	11th to 20th	Day	64% 4 9% 15% 8% 1	1958
	21st to end	Day	61% 6% 11% 15% 7% 1	1650
March	1st to 10th	Day	52% 7% 13% 20% 8% 1	3220
	11th to 20th	Day	49% 8% 13% 20% 9% 1	3448
	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	30% 5% 8% 36% 20% 1	3480
	11th to 20th	Day	41% 5% 11% 27% 16%	3070
	21st to end	Day	49% 5% 12% 24% 10% 1	3081
November	1st to 10th	Day	46% 6% 13% 22% 12% 1	1762
	11th to 20th	Day	59% 5% 12% 18% 6% 1	1800
	21st to end	Day	60% 5% 14% 15% 5%	1800
December	1st to 10th	Day	67% 7% 15% 9% 2	2802
	11th to 20th	Day	73% 5% 9% 11% 2	3005
	21st to end	Day	76% 4 7% 9% 3	3982
January	1st to 10th	Night	66% 5% 10% 13% 5% 1	6348
	11th to 20th	Night	73% 5% 10% 7% 5%	6360
	21st to end	Night	69% 4 12% 10% 4	6922
February	1st to 10th	Night	64% 6% 10% 14% 5% 1	2570
	11th to 20th	Night	60% 6% 9% 16% 8% 1	2339
	21st to end	Night	56% 6% 12% 16% 8% 2	1980
March	1st to 10th	Night	46% 6% 13% 26% 7% 1	3853
	11th to 20th	Night	36% 7% 15% 22% 19% 2	3540
	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	26% 4 10% 36% 22% 1	3480
	11th to 20th	Night	45% 4 10% 20% 20%	3845
	21st to end	Night	41% 4 10% 30% 14% 1	4467
November	1st to 10th	Night	45% 5% 10% 24% 15% 1	2536
	11th to 20th	Night	55% 7% 13% 17% 8% 1	2600
	21st to end	Night	57% 6% 13% 19% 5%	2600
December	1st to 10th	Night	66% 6% 13% 12% 3	4377
	11th to 20th	Night	71% 5% 10% 10% 4	5201
	21st to end	Night	74% 5% 8% 8% 5%	6996

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% 15% 35% 20% 8% 1	300
	11th to 20th	Day	17% 16% 44% 17% 4 1	300
	21st to end	Day	25% 15% 31% 22% 6%	330
February	1st to 10th	Day	24% 21% 33% 19% 2 1	300
	11th to 20th	Day	22% 20% 38% 18% 2	300
	21st to end	Day	27% 22% 37% 13% 2	249
March	1st to 10th	Day	21% 19% 42% 14% 5%	300
	11th to 20th	Day	21% 16% 33% 22% 8%	434
	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	21% 17% 25% 28% 7% 2	443
	21st to end	Day	26% 15% 33% 20% 4 1	330
November	1st to 10th	Day	16% 17% 35% 24% 7% 1	300
	11th to 20th	Day	22% 20% 32% 20% 5% 1	300
	21st to end	Day	23% 16% 40% 15% 5%	300
December	1st to 10th	Day	25% 21% 28% 18% 7% 1	300
	11th to 20th	Day	23% 21% 39% 14% 4	300
	21st to end	Day	25% 20% 35% 16% 4 1	330
January	1st to 10th	Night	20% 15% 34% 25% 6% 1	1000
	11th to 20th	Night	22% 14% 36% 23% 5% 1	1000
	21st to end	Night	20% 11% 34% 29% 5%	1065
February	1st to 10th	Night	22% 13% 37% 24% 3	800
	11th to 20th	Night	20% 14% 36% 25% 4	783
	21st to end	Night	22% 14% 40% 20% 4	660
March	1st to 10th	Night	20% 11% 37% 24% 7% 1	800
	11th to 20th	Night	17% 14% 37% 26% 5% 1	767
	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	24% 13% 27% 27% 8% 1	760
	21st to end	Night	25% 10% 33% 25% 6%	836
November	1st to 10th	Night	21% 10% 31% 29% 9% 1	780
	11th to 20th	Night	21% 14% 36% 23% 5% 1	800
	21st to end	Night	25% 12% 34% 26% 4	800
December	1st to 10th	Night	23% 14% 34% 22% 6%	875
	11th to 20th	Night	24% 14% 36% 23% 3	1000
	21st to end	Night	31% 12% 35% 21% 2	1100

Page generated at 2025-03-09 08:08:42. Version ac07

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