Chamberlain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chamberlain 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	9% 7% 3 7% 5% 4 4 3 5% 7% 46%	112049
	Night	8% 7% 3 7% 4 4 4 4 6% 7% 46%	115711
Transparency	Day	36% 7% 14% 25% 15% 2	111795
	Night	36% 6% 15% 25% 16% 2	115556
Seeing	Day	13% 9% 31% 31% 14% 2	16323
	Night	12% 8% 34% 36% 10% 1	26448

By Month

Percentage Cloud Cover SW (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	7% 11% 5% 19% 9% 6% 4 3 4 6% 28%	3638
	11th to 20th	Day	4 9% 5% 10% 7% 6% 7% 4 11% 6% 31%	3640
	21st to end	Day	3 8% 4 13% 7% 5% 5% 3 4 5% 44%	4070
February	1st to 10th	Day	9% 13% 3 10% 6% 5% 5% 3 5% 7% 35%	1800
	11th to 20th	Day	7% 8% 4 10% 6% 6% 4 3 5% 6% 40%	1914
	21st to end	Day	8% 11% 3 8% 5% 4 3 3 5% 7% 42%	1650
March	1st to 10th	Day	7% 10% 3 6% 3 3 4 3 5% 6% 49%	3230
	11th to 20th	Day	8% 7% 2 6% 4 4 4 3 5% 8% 49%	3486
	21st to end	Day	21% 12% 3 10% 6% 4 3 3 5% 6% 26%	3814
April	1st to 10th	Day	14% 8% 3 6% 5% 3 4 4 5% 7% 42%	2266
	11th to 20th	Day	18% 8% 3 6% 5% 4 4 3 5% 6% 38%	2382
	21st to end	Day	15% 8% 3 6% 4 4 4 4 5% 7% 40%	2470
May	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 3 6% 5% 40%	4120
	11th to 20th	Day	11% 6% 2 4 4 3 4 4 5% 7% 51%	4082
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 6% 44%	4751
June	1st to 10th	Day	9% 4 2 4 3 4 5% 4 6% 9% 50%	2755
	11th to 20th	Day	10% 4 1 4 4 4 4 4 7% 8% 48%	2850
	21st to end	Day	6% 4 2 5% 3 4 4 4 7% 8% 53%	2729
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 48%	4430
	11th to 20th	Day	4 3 2 4 3 4 4 4 6% 8% 60%	4430
	21st to end	Day	4 3 1 4 4 4 5% 4 7% 9% 56%	4756
August	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 6% 7% 52%	4103
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 42%	4041
	21st to end	Day	5% 4 1 4 3 4 4 4 6% 9% 56%	4418
September	1st to 10th	Day	8% 4 2 4 3 4 4 2 5% 6% 58%	2218
	11th to 20th	Day	8% 5% 2 4 3 3 3 2 5% 6% 59%	2090
	21st to end	Day	11% 5% 2 4 2 2 2 2 5% 6% 58%	2090
October	1st to 10th	Day	8% 3 1 3 2 3 2 3 3 5% 66%	3430
	11th to 20th	Day	8% 5% 2 6% 4 3 3 2 4 7% 55%	3087
	21st to end	Day	11% 7% 3 7% 4 4 4 3 5% 7% 44%	3091
November	1st to 10th	Day	10% 7% 2 6% 4 3 5% 3 6% 6% 48%	1763
	11th to 20th	Day	6% 8% 3 8% 6% 3 4 3 5% 6% 49%	1800
	21st to end	Day	7% 9% 3 7% 5% 4 4 3 5% 7% 46%	1690
December	1st to 10th	Day	6% 10% 4 10% 7% 6% 5% 4 5% 6% 37%	2436
	11th to 20th	Day	6% 10% 3 10% 7% 7% 7% 4 5% 7% 35%	2643
	21st to end	Day	13% 10% 3 12% 5% 4 4 3 4 6% 37%	3886
January	1st to 10th	Night	6% 12% 5% 13% 8% 6% 5% 5% 6% 7% 27%	6327
	11th to 20th	Night	3 9% 4 10% 7% 6% 7% 4 8% 7% 35%	6340
	21st to end	Night	3 6% 4 12% 5% 4 5% 4 5% 6% 46%	6906
February	1st to 10th	Night	10% 11% 4 8% 5% 5% 5% 4 5% 8% 35%	2600
	11th to 20th	Night	6% 10% 3 8% 5% 5% 5% 4 6% 7% 41%	2393
	21st to end	Night	9% 7% 2 6% 5% 4 4 4 6% 8% 44%	1980
March	1st to 10th	Night	9% 9% 3 6% 4 4 4 3 4 6% 47%	3854
	11th to 20th	Night	8% 7% 3 5% 3 4 5% 4 6% 9% 47%	3502
	21st to end	Night	18% 12% 3 8% 4 4 4 3 5% 6% 32%	3817
April	1st to 10th	Night	14% 6% 3 6% 4 3 5% 3 6% 8% 42%	1910
	11th to 20th	Night	15% 7% 3 7% 3 4 3 3 5% 7% 42%	1746
	21st to end	Night	15% 9% 2 6% 4 4 4 3 6% 7% 42%	1710
May	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 6% 8% 44%	2850
	11th to 20th	Night	10% 4 2 4 3 4 4 3 5% 8% 53%	2823
	21st to end	Night	13% 4 2 5% 4 4 5% 4 6% 8% 46%	2832
June	1st to 10th	Night	7% 5% 2 4 4 3 3 4 6% 9% 54%	1417
	11th to 20th	Night	9% 5% 1 6% 4 4 5% 4 6% 8% 49%	1330
	21st to end	Night	5% 3 1 3 4 2 4 3 7% 9% 58%	1442
July	1st to 10th	Night	7% 4 1 5% 4 3 4 4 7% 11% 51%	2540
	11th to 20th	Night	3 2 1 4 3 3 4 4 5% 8% 64%	2540
	21st to end	Night	3 3 1 4 3 4 4 3 6% 9% 60%	2806
August	1st to 10th	Night	6% 4 2 5% 4 4 5% 4 8% 11% 47%	2836
	11th to 20th	Night	11% 6% 2 6% 3 3 4 4 6% 8% 47%	2798
	21st to end	Night	5% 4 2 5% 3 3 4 3 6% 8% 57%	3246
September	1st to 10th	Night	9% 4 1 5% 3 3 4 2 4 6% 60%	1962
	11th to 20th	Night	6% 5% 1 4 2 3 3 3 5% 7% 60%	2090
	21st to end	Night	9% 5% 2 3 2 3 3 3 4 5% 60%	2090
October	1st to 10th	Night	8% 4 1 2 2 2 2 2 3 5% 68%	3498
	11th to 20th	Night	9% 4 1 3 2 2 3 3 5% 9% 58%	3831
	21st to end	Night	8% 6% 2 7% 4 4 5% 4 5% 7% 48%	4467
November	1st to 10th	Night	7% 6% 2 5% 4 4 5% 3 6% 7% 50%	2535
	11th to 20th	Night	7% 7% 2 8% 5% 4 4 4 4 8% 48%	2600
	21st to end	Night	6% 8% 3 6% 4 4 4 4 5% 7% 49%	2705
December	1st to 10th	Night	4 7% 4 8% 6% 4 4 4 4 6% 49%	4740
	11th to 20th	Night	5% 9% 3 8% 5% 5% 5% 4 7% 8% 41%	5564
	21st to end	Night	13% 12% 3 10% 4 4 4 3 5% 6% 38%	7084

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	59% 5% 15% 15% 6%	3639
	11th to 20th	Day	47% 6% 16% 16% 15% 1	3640
	21st to end	Day	42% 6% 17% 21% 13% 1	4070
February	1st to 10th	Day	50% 7% 15% 17% 12%	1800
	11th to 20th	Day	45% 6% 14% 21% 12% 2	1924
	21st to end	Day	42% 6% 17% 21% 12% 2	1650
March	1st to 10th	Day	36% 6% 10% 28% 18% 2	3230
	11th to 20th	Day	32% 6% 15% 28% 18% 1	3486
	21st to end	Day	58% 8% 16% 14% 4 1	3814
April	1st to 10th	Day	41% 6% 14% 26% 11% 2	2266
	11th to 20th	Day	45% 7% 12% 21% 12% 2	2385
	21st to end	Day	43% 7% 14% 27% 10%	2470
May	1st to 10th	Day	40% 9% 12% 22% 15% 2	4120
	11th to 20th	Day	31% 8% 14% 27% 18% 3	4086
	21st to end	Day	38% 8% 13% 27% 14% 1	4751
June	1st to 10th	Day	29% 8% 17% 31% 11% 3	2755
	11th to 20th	Day	31% 9% 16% 26% 17% 2	2850
	21st to end	Day	27% 7% 15% 29% 18% 3	2729
July	1st to 10th	Day	31% 8% 17% 26% 15% 2	4361
	11th to 20th	Day	22% 7% 12% 32% 22% 4	4355
	21st to end	Day	21% 9% 16% 29% 21% 5%	4606
August	1st to 10th	Day	29% 8% 14% 26% 19% 4	4090
	11th to 20th	Day	39% 8% 15% 24% 12% 2	4080
	21st to end	Day	24% 7% 19% 27% 20% 3	4417
September	1st to 10th	Day	27% 6% 12% 29% 22% 5%	2218
	11th to 20th	Day	27% 5% 14% 31% 22% 2	2090
	21st to end	Day	28% 4 10% 33% 22% 2	2090
October	1st to 10th	Day	22% 4 9% 23% 36% 6%	3430
	11th to 20th	Day	30% 4 11% 34% 18% 2	3087
	21st to end	Day	40% 5% 12% 24% 18% 2	3091
November	1st to 10th	Day	35% 6% 18% 24% 15% 3	1763
	11th to 20th	Day	36% 5% 16% 26% 14% 3	1800
	21st to end	Day	38% 6% 17% 25% 14% 1	1690
December	1st to 10th	Day	47% 6% 17% 22% 8% 1	2436
	11th to 20th	Day	47% 7% 17% 19% 10% 1	2640
	21st to end	Day	49% 5% 12% 30% 4	3886
January	1st to 10th	Night	53% 6% 14% 18% 8%	6327
	11th to 20th	Night	44% 6% 16% 18% 14% 1	6340
	21st to end	Night	37% 7% 16% 25% 14% 1	6906
February	1st to 10th	Night	47% 6% 16% 19% 11% 1	2600
	11th to 20th	Night	41% 6% 14% 22% 13% 3	2393
	21st to end	Night	36% 7% 15% 26% 11% 4	1980
March	1st to 10th	Night	38% 6% 13% 23% 19% 2	3854
	11th to 20th	Night	31% 7% 13% 30% 17% 1	3502
	21st to end	Night	52% 8% 14% 19% 7% 1	3817
April	1st to 10th	Night	39% 6% 15% 25% 13% 1	1910
	11th to 20th	Night	41% 6% 12% 22% 17% 3	1746
	21st to end	Night	41% 7% 13% 26% 13% 2	1710
May	1st to 10th	Night	35% 8% 14% 22% 16% 4	2850
	11th to 20th	Night	27% 7% 12% 28% 23% 2	2824
	21st to end	Night	33% 10% 17% 25% 13% 1	2833
June	1st to 10th	Night	26% 7% 18% 29% 16% 4	1417
	11th to 20th	Night	32% 8% 15% 23% 19% 3	1330
	21st to end	Night	22% 6% 17% 32% 19% 3	1442
July	1st to 10th	Night	27% 7% 22% 24% 17% 3	2500
	11th to 20th	Night	19% 6% 14% 29% 27% 6%	2492
	21st to end	Night	21% 6% 18% 31% 19% 5%	2718
August	1st to 10th	Night	28% 9% 16% 28% 12% 6%	2826
	11th to 20th	Night	35% 8% 15% 26% 14% 2	2825
	21st to end	Night	25% 6% 15% 27% 22% 5%	3246
September	1st to 10th	Night	26% 5% 10% 29% 22% 7%	1962
	11th to 20th	Night	23% 6% 14% 33% 21% 3	2090
	21st to end	Night	27% 5% 8% 29% 27% 4	2090
October	1st to 10th	Night	21% 3 7% 17% 46% 7%	3498
	11th to 20th	Night	25% 4 11% 34% 21% 6%	3831
	21st to end	Night	34% 5% 15% 23% 20% 2	4467
November	1st to 10th	Night	31% 5% 14% 25% 19% 5%	2535
	11th to 20th	Night	35% 5% 13% 24% 18% 4	2600
	21st to end	Night	34% 5% 16% 28% 16% 2	2705
December	1st to 10th	Night	36% 5% 16% 30% 13% 1	4740
	11th to 20th	Night	38% 7% 19% 27% 8%	5566
	21st to end	Night	48% 4 14% 26% 8%	7084

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	11% 10% 27% 36% 13% 3	300
	11th to 20th	Day	11% 13% 37% 30% 8% 2	300
	21st to end	Day	14% 9% 40% 25% 10% 3	330
February	1st to 10th	Day	12% 16% 41% 26% 6%	300
	11th to 20th	Day	13% 11% 41% 24% 9% 2	300
	21st to end	Day	16% 11% 38% 28% 7%	249
March	1st to 10th	Day	16% 10% 38% 28% 6% 2	300
	11th to 20th	Day	12% 9% 32% 33% 12% 2	273
	21st to end	Day	22% 13% 30% 19% 12% 3	297
April	1st to 10th	Day	21% 15% 37% 20% 7%	446
	11th to 20th	Day	22% 16% 31% 24% 6%	583
	21st to end	Day	18% 15% 29% 26% 10% 2	650
May	1st to 10th	Day	19% 10% 33% 26% 11% 1	653
	11th to 20th	Day	19% 10% 29% 25% 15% 3	675
	21st to end	Day	16% 9% 34% 26% 12% 3	734
June	1st to 10th	Day	8% 7% 25% 34% 19% 6%	680
	11th to 20th	Day	11% 8% 36% 30% 13% 2	680
	21st to end	Day	8% 8% 31% 36% 15% 1	680
July	1st to 10th	Day	7% 4 23% 41% 21% 3	671
	11th to 20th	Day	6% 5% 29% 42% 17% 1	668
	21st to end	Day	4 3 28% 45% 17% 3	739
August	1st to 10th	Day	7% 4 24% 42% 21% 2	676
	11th to 20th	Day	11% 4 26% 39% 18% 3	665
	21st to end	Day	5% 5% 27% 37% 23% 3	686
September	1st to 10th	Day	10% 5% 29% 35% 19% 3	428
	11th to 20th	Day	11% 5% 29% 30% 20% 6%	300
	21st to end	Day	18% 16% 25% 24% 16% 2	300
October	1st to 10th	Day	16% 6% 30% 29% 14% 4	300
	11th to 20th	Day	16% 13% 38% 21% 10% 1	300
	21st to end	Day	17% 17% 32% 19% 13% 2	330
November	1st to 10th	Day	20% 10% 29% 27% 12% 3	300
	11th to 20th	Day	13% 15% 36% 26% 9%	300
	21st to end	Day	14% 15% 28% 29% 11% 2	300
December	1st to 10th	Day	16% 9% 37% 27% 9% 2	300
	11th to 20th	Day	13% 14% 35% 29% 7% 1	300
	21st to end	Day	19% 12% 32% 24% 11% 1	330
January	1st to 10th	Night	10% 10% 36% 34% 9% 1	1000
	11th to 20th	Night	9% 13% 35% 35% 8% 1	1000
	21st to end	Night	12% 8% 39% 32% 8%	1100
February	1st to 10th	Night	15% 10% 38% 31% 6%	1000
	11th to 20th	Night	10% 9% 37% 36% 8%	838
	21st to end	Night	12% 8% 36% 34% 9% 1	660
March	1st to 10th	Night	15% 9% 33% 37% 6%	800
	11th to 20th	Night	14% 9% 34% 36% 6%	580
	21st to end	Night	16% 10% 35% 30% 9% 1	627
April	1st to 10th	Night	16% 11% 38% 32% 3	570
	11th to 20th	Night	19% 10% 37% 30% 4	563
	21st to end	Night	16% 11% 31% 31% 9% 1	570
May	1st to 10th	Night	11% 8% 38% 33% 10%	570
	11th to 20th	Night	16% 8% 31% 33% 11% 1	561
	21st to end	Night	14% 8% 34% 34% 9% 1	617
June	1st to 10th	Night	9% 4 31% 37% 17% 1	570
	11th to 20th	Night	9% 6% 38% 39% 8%	570
	21st to end	Night	7% 4 29% 49% 11%	570
July	1st to 10th	Night	6% 3 26% 45% 19%	555
	11th to 20th	Night	5% 1 31% 48% 14%	552
	21st to end	Night	6% 1 27% 52% 13% 1	624
August	1st to 10th	Night	6% 3 27% 48% 16% 1	567
	11th to 20th	Night	8% 3 26% 46% 17%	564
	21st to end	Night	6% 4 29% 44% 17%	627
September	1st to 10th	Night	10% 5% 26% 46% 12% 1	570
	11th to 20th	Night	9% 4 32% 38% 15% 1	570
	21st to end	Night	11% 9% 33% 35% 12% 1	570
October	1st to 10th	Night	17% 7% 30% 35% 11%	575
	11th to 20th	Night	16% 10% 31% 33% 11%	788
	21st to end	Night	15% 11% 32% 33% 9%	1045
November	1st to 10th	Night	11% 8% 35% 36% 9% 1	975
	11th to 20th	Night	11% 10% 36% 33% 10% 1	1000
	21st to end	Night	9% 11% 34% 36% 9%	1000
December	1st to 10th	Night	11% 11% 38% 32% 8%	1000
	11th to 20th	Night	11% 9% 36% 36% 8%	1000
	21st to end	Night	18% 10% 31% 34% 7% 1	1100

Page generated at 2025-03-09 07:08:44. Version ac07

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