Chesterfield
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chesterfield clear sky chart from 2005-11-06 to 2026-07-01.

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	10% 10% 5% 11% 7% 6% 6% 5% 7% 8% 26%	143408
	Night	7% 12% 5% 13% 8% 7% 7% 5% 8% 10% 18%	137758
Transparency	Day	53% 6% 14% 21% 6%	143039
	Night	57% 6% 13% 19% 6%	137713
Seeing	Day	16% 2 10% 27% 31% 15%	19133
	Night	14% 3 13% 31% 29% 11%	26053

By Month

Percentage Cloud Cover NW (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	2 7% 3 11% 8% 6% 6% 6% 8% 16% 27%	2362
	11th to 20th	Day	2 10% 5% 17% 8% 8% 9% 5% 11% 11% 15%	2598
	21st to end	Day	1 4 3 12% 9% 7% 7% 7% 11% 17% 23%	3532
February	1st to 10th	Day	1 5% 3 7% 8% 8% 8% 8% 14% 20% 16%	1391
	11th to 20th	Day	1 6% 3 7% 8% 9% 8% 7% 11% 19% 20%	1548
	21st to end	Day	1 7% 3 10% 8% 7% 8% 8% 11% 20% 17%	1499
March	1st to 10th	Day	2 6% 2 12% 10% 9% 12% 8% 11% 12% 14%	4396
	11th to 20th	Day	5% 10% 6% 19% 12% 11% 9% 4 6% 8% 9%	4870
	21st to end	Day	1 6% 5% 12% 7% 9% 10% 8% 14% 12% 17%	6021
April	1st to 10th	Day	5% 10% 4 10% 7% 8% 7% 7% 9% 8% 25%	2639
	11th to 20th	Day	7% 8% 4 9% 7% 7% 9% 6% 9% 9% 25%	2910
	21st to end	Day	8% 8% 4 12% 10% 8% 6% 5% 6% 10% 24%	3089
May	1st to 10th	Day	12% 19% 7% 14% 9% 5% 5% 3 4 5% 17%	9420
	11th to 20th	Day	7% 11% 6% 17% 9% 6% 5% 4 5% 6% 25%	9822
	21st to end	Day	9% 9% 4 12% 8% 6% 5% 4 6% 7% 29%	11613
June	1st to 10th	Day	14% 11% 4 10% 5% 5% 5% 3 6% 7% 29%	3780
	11th to 20th	Day	12% 10% 3 7% 5% 4 5% 4 6% 8% 37%	3969
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	3990
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 43%	8327
	11th to 20th	Day	8% 6% 3 8% 6% 6% 6% 5% 7% 9% 37%	8160
	21st to end	Day	13% 7% 4 7% 5% 5% 5% 4 6% 8% 37%	8408
August	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 6% 9% 43%	5210
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 4 6% 9% 40%	4794
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 5% 6% 31%	5038
September	1st to 10th	Day	18% 13% 5% 8% 5% 5% 5% 3 5% 5% 26%	2542
	11th to 20th	Day	21% 16% 6% 10% 5% 5% 5% 4 5% 4 19%	2400
	21st to end	Day	23% 18% 5% 9% 4 3 4 3 4 5% 22%	2105
October	1st to 10th	Day	21% 27% 13% 11% 4 3 3 2 3 3 9%	3200
	11th to 20th	Day	21% 29% 9% 16% 5% 4 2 1 2 2 10%	2766
	21st to end	Day	14% 21% 9% 28% 12% 5% 2 2 2 2 3	2678
November	1st to 10th	Day	8% 24% 10% 14% 9% 10% 12% 5% 3 2 2	1321
	11th to 20th	Day	9% 17% 5% 13% 5% 10% 10% 8% 6% 9% 9%	1155
	21st to end	Day	8% 13% 7% 13% 9% 6% 9% 9% 9% 6% 11%	911
December	1st to 10th	Day	2 6% 3 11% 8% 9% 9% 7% 19% 17% 8%	1350
	11th to 20th	Day	3 7% 3 7% 7% 8% 7% 7% 18% 20% 12%	1548
	21st to end	Day	4 14% 9% 18% 8% 7% 7% 5% 6% 8% 12%	2046
January	1st to 10th	Night	2 8% 3 9% 7% 7% 8% 5% 10% 16% 25%	10561
	11th to 20th	Night	2 11% 6% 15% 8% 7% 8% 5% 10% 13% 16%	10314
	21st to end	Night	2 7% 4 8% 7% 7% 8% 7% 10% 15% 25%	10664
February	1st to 10th	Night	1 4 2 9% 10% 8% 10% 11% 13% 15% 15%	3227
	11th to 20th	Night	1 6% 3 9% 9% 10% 10% 8% 10% 14% 19%	2959
	21st to end	Night	1 6% 3 10% 8% 8% 9% 7% 13% 15% 20%	2305
March	1st to 10th	Night	2 5% 3 11% 10% 9% 10% 8% 11% 13% 18%	5683
	11th to 20th	Night	4 11% 6% 19% 13% 10% 9% 5% 6% 7% 11%	5354
	21st to end	Night	1 5% 4 13% 11% 8% 8% 7% 12% 10% 21%	5152
April	1st to 10th	Night	5% 10% 3 11% 5% 7% 8% 7% 8% 10% 27%	1979
	11th to 20th	Night	7% 8% 3 7% 8% 9% 9% 6% 6% 8% 30%	1661
	21st to end	Night	6% 9% 5% 11% 7% 7% 8% 5% 5% 7% 30%	1529
May	1st to 10th	Night	13% 18% 6% 15% 9% 5% 4 3 4 4 19%	3528
	11th to 20th	Night	7% 9% 7% 21% 10% 6% 5% 4 5% 5% 21%	3057
	21st to end	Night	8% 11% 5% 11% 7% 6% 7% 4 6% 7% 28%	2591
June	1st to 10th	Night	15% 12% 5% 6% 5% 6% 3 3 5% 5% 35%	840
	11th to 20th	Night	16% 9% 3 7% 5% 4 4 5% 3 7% 38%	649
	21st to end	Night	10% 10% 2 8% 4 4 5% 4 7% 8% 37%	630
July	1st to 10th	Night	10% 7% 3 11% 5% 7% 5% 5% 6% 6% 33%	1653
	11th to 20th	Night	11% 7% 3 7% 4 4 5% 5% 7% 10% 37%	1820
	21st to end	Night	12% 7% 3 8% 4 4 4 4 7% 8% 40%	2429
August	1st to 10th	Night	12% 9% 4 9% 5% 5% 6% 4 8% 7% 33%	1950
	11th to 20th	Night	12% 8% 3 6% 5% 5% 3 4 5% 7% 41%	2264
	21st to end	Night	20% 9% 4 8% 5% 4 4 3 5% 7% 31%	2871
September	1st to 10th	Night	22% 13% 4 8% 5% 3 4 3 4 6% 28%	1858
	11th to 20th	Night	21% 17% 5% 8% 5% 5% 5% 3 5% 5% 21%	2000
	21st to end	Night	21% 22% 7% 9% 4 4 4 3 4 4 17%	2273
October	1st to 10th	Night	23% 29% 14% 11% 4 3 3 1 3 3 7%	3980
	11th to 20th	Night	24% 27% 9% 15% 5% 3 3 2 2 2 9%	4375
	21st to end	Night	15% 23% 9% 29% 10% 5% 2 1 1 1 3	5122
November	1st to 10th	Night	11% 22% 8% 18% 9% 10% 10% 5% 3 2 3	3195
	11th to 20th	Night	10% 17% 6% 12% 9% 10% 10% 5% 7% 9% 6%	3465
	21st to end	Night	7% 16% 6% 13% 9% 9% 8% 7% 8% 9% 9%	3707
December	1st to 10th	Night	2 6% 3 15% 10% 8% 8% 6% 14% 20% 9%	6060
	11th to 20th	Night	3 10% 4 10% 7% 7% 7% 6% 15% 20% 11%	6879
	21st to end	Night	6% 16% 9% 17% 8% 6% 6% 4 7% 9% 12%	9174

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	42% 6% 18% 23% 11%	2362
	11th to 20th	Day	58% 5% 15% 18% 4	2611
	21st to end	Day	42% 6% 19% 20% 11% 1	3532
February	1st to 10th	Day	40% 6% 21% 27% 5%	1391
	11th to 20th	Day	43% 6% 18% 22% 11%	1555
	21st to end	Day	42% 6% 13% 25% 13%	1499
March	1st to 10th	Day	53% 5% 12% 17% 12%	4396
	11th to 20th	Day	72% 4 7% 13% 4	4872
	21st to end	Day	48% 7% 7% 21% 17%	6021
April	1st to 10th	Day	50% 6% 11% 24% 9%	2639
	11th to 20th	Day	49% 7% 11% 24% 9%	2910
	21st to end	Day	52% 7% 12% 20% 9%	3089
May	1st to 10th	Day	68% 5% 7% 13% 6%	9417
	11th to 20th	Day	57% 6% 10% 21% 5%	9826
	21st to end	Day	52% 7% 14% 23% 5%	11618
June	1st to 10th	Day	53% 6% 15% 20% 6%	3780
	11th to 20th	Day	44% 6% 14% 31% 5%	3969
	21st to end	Day	40% 7% 17% 33% 3	3990
July	1st to 10th	Day	35% 6% 16% 37% 6% 1	8238
	11th to 20th	Day	40% 7% 19% 29% 5%	8061
	21st to end	Day	43% 7% 18% 27% 6%	8171
August	1st to 10th	Day	38% 6% 24% 25% 7% 1	5192
	11th to 20th	Day	40% 6% 21% 25% 6% 2	4841
	21st to end	Day	54% 6% 17% 21% 1	5037
September	1st to 10th	Day	58% 7% 17% 15% 2	2542
	11th to 20th	Day	66% 7% 17% 9% 1	2400
	21st to end	Day	64% 5% 15% 13% 2	2105
October	1st to 10th	Day	80% 4 7% 8% 1	3200
	11th to 20th	Day	83% 4 5% 7% 2	2766
	21st to end	Day	87% 5% 4 3 1	2678
November	1st to 10th	Day	79% 12% 6% 3	1321
	11th to 20th	Day	64% 10% 15% 9% 2	1155
	21st to end	Day	61% 12% 13% 11% 2 1	911
December	1st to 10th	Day	47% 7% 19% 25% 2	1350
	11th to 20th	Day	43% 6% 15% 34% 2	1548
	21st to end	Day	67% 6% 8% 9% 7% 2	2046
January	1st to 10th	Night	43% 5% 16% 28% 8%	10562
	11th to 20th	Night	57% 5% 16% 18% 4	10363
	21st to end	Night	43% 5% 17% 21% 14%	10665
February	1st to 10th	Night	44% 9% 20% 23% 5%	3227
	11th to 20th	Night	47% 8% 16% 20% 9%	2962
	21st to end	Night	43% 6% 13% 26% 12%	2305
March	1st to 10th	Night	49% 6% 10% 24% 11%	5683
	11th to 20th	Night	71% 4 6% 13% 5%	5354
	21st to end	Night	49% 6% 7% 22% 15% 1	5152
April	1st to 10th	Night	46% 8% 12% 25% 9% 1	1979
	11th to 20th	Night	47% 8% 12% 26% 7% 1	1661
	21st to end	Night	51% 5% 10% 23% 10%	1529
May	1st to 10th	Night	67% 3 7% 14% 8%	3528
	11th to 20th	Night	59% 8% 9% 17% 6%	3057
	21st to end	Night	52% 7% 13% 21% 6%	2591
June	1st to 10th	Night	50% 4 16% 23% 8%	840
	11th to 20th	Night	47% 6% 10% 29% 8%	649
	21st to end	Night	41% 7% 16% 33% 4	630
July	1st to 10th	Night	48% 7% 11% 26% 8%	1633
	11th to 20th	Night	40% 6% 15% 28% 11%	1796
	21st to end	Night	39% 7% 16% 32% 6%	2361
August	1st to 10th	Night	47% 6% 17% 23% 6% 1	1944
	11th to 20th	Night	42% 5% 17% 25% 8% 3	2283
	21st to end	Night	52% 6% 18% 21% 3	2871
September	1st to 10th	Night	57% 6% 19% 15% 2	1858
	11th to 20th	Night	64% 7% 17% 12% 1	2000
	21st to end	Night	69% 6% 10% 13% 2	2273
October	1st to 10th	Night	84% 4 7% 5%	3980
	11th to 20th	Night	84% 3 6% 6% 1	4375
	21st to end	Night	90% 4 3 2 1	5122
November	1st to 10th	Night	80% 10% 7% 3	3195
	11th to 20th	Night	68% 9% 11% 10% 1	3465
	21st to end	Night	65% 8% 14% 12% 2	3707
December	1st to 10th	Night	50% 6% 20% 22% 2	6060
	11th to 20th	Night	48% 5% 14% 29% 3	6879
	21st to end	Night	66% 5% 10% 11% 9%	9174

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	2 12% 35% 33% 18%	110
	11th to 20th	Day	5% 2 18% 45% 24% 6%	110
	21st to end	Day	2 2 16% 33% 38% 10%	177
February	1st to 10th	Day	4 1 13% 28% 33% 21%	289
	11th to 20th	Day	2 1 10% 33% 36% 17%	330
	21st to end	Day	1 1 10% 30% 32% 27%	273
March	1st to 10th	Day	4 1 14% 26% 36% 19%	330
	11th to 20th	Day	7% 1 10% 29% 35% 18%	330
	21st to end	Day	4 3 16% 25% 31% 21%	553
April	1st to 10th	Day	10% 1 11% 25% 32% 21%	627
	11th to 20th	Day	9% 1 12% 31% 30% 18%	740
	21st to end	Day	8% 2 14% 30% 26% 20%	750
May	1st to 10th	Day	16% 2 11% 25% 29% 17%	753
	11th to 20th	Day	15% 2 16% 28% 27% 12%	861
	21st to end	Day	18% 1 11% 28% 29% 13%	1072
June	1st to 10th	Day	19% 2 9% 26% 29% 15%	990
	11th to 20th	Day	17% 1 8% 29% 31% 13%	1179
	21st to end	Day	14% 2 6% 27% 35% 17%	1200
July	1st to 10th	Day	17% 1 5% 23% 36% 18%	992
	11th to 20th	Day	15% 1 6% 28% 36% 14%	912
	21st to end	Day	16% 1 5% 21% 37% 20%	846
August	1st to 10th	Day	18% 2 7% 21% 32% 19%	726
	11th to 20th	Day	20% 1 5% 22% 34% 18%	715
	21st to end	Day	22% 2 9% 27% 26% 14%	803
September	1st to 10th	Day	29% 3 9% 20% 28% 12%	617
	11th to 20th	Day	30% 2 9% 27% 25% 6%	530
	21st to end	Day	32% 2 13% 26% 21% 6%	420
October	1st to 10th	Day	27% 6% 18% 26% 18% 5%	330
	11th to 20th	Day	33% 5% 14% 21% 21% 6%	330
	21st to end	Day	23% 5% 17% 28% 23% 5%	311
November	1st to 10th	Day	14% 3 18% 31% 26% 8%	220
	11th to 20th	Day	15% 4 16% 30% 25% 9%	220
	21st to end	Day	14% 4 15% 25% 33% 9%	146
December	1st to 10th	Day	4 1 15% 34% 38% 8%	110
	11th to 20th	Day	5% 3 16% 34% 25% 16%	110
	21st to end	Day	7% 2 12% 38% 25% 17%	121
January	1st to 10th	Night	4 2 14% 30% 36% 13%	1160
	11th to 20th	Night	6% 2 17% 37% 29% 9%	1113
	21st to end	Night	4 2 16% 36% 30% 13%	1155
February	1st to 10th	Night	3 2 10% 34% 34% 17%	1050
	11th to 20th	Night	3 1 9% 36% 36% 15%	1029
	21st to end	Night	2 1 12% 33% 39% 12%	865
March	1st to 10th	Night	6% 2 14% 34% 31% 12%	931
	11th to 20th	Night	6% 1 10% 33% 38% 11%	751
	21st to end	Night	6% 1 14% 32% 32% 14%	693
April	1st to 10th	Night	9% 2 13% 27% 35% 15%	630
	11th to 20th	Night	10% 2 12% 34% 32% 10%	623
	21st to end	Night	9% 2 15% 32% 29% 14%	554
May	1st to 10th	Night	17% 4 12% 31% 28% 9%	420
	11th to 20th	Night	14% 4 15% 34% 25% 8%	345
	21st to end	Night	19% 3 11% 33% 24% 11%	227
June	1st to 10th	Night	20% 1 10% 31% 28% 10%	210
	11th to 20th	Night	18% 8% 35% 35% 4	210
	21st to end	Night	15% 3 8% 27% 37% 11%	210
July	1st to 10th	Night	17% 1 6% 27% 38% 11%	195
	11th to 20th	Night	15% 2 9% 31% 34% 9%	194
	21st to end	Night	17% 2 5% 23% 39% 14%	249
August	1st to 10th	Night	21% 2 10% 20% 36% 10%	398
	11th to 20th	Night	23% 2 3 23% 35% 13%	545
	21st to end	Night	24% 2 8% 30% 26% 9%	660
September	1st to 10th	Night	30% 3 9% 24% 28% 7%	600
	11th to 20th	Night	30% 2 11% 28% 24% 5%	600
	21st to end	Night	32% 4 15% 28% 17% 4	765
October	1st to 10th	Night	30% 7% 17% 26% 17% 3	800
	11th to 20th	Night	33% 5% 13% 24% 20% 5%	885
	21st to end	Night	34% 2 14% 27% 19% 4	1100
November	1st to 10th	Night	20% 4 14% 31% 25% 7%	1025
	11th to 20th	Night	16% 4 15% 29% 28% 8%	1105
	21st to end	Night	13% 4 13% 28% 31% 12%	1160
December	1st to 10th	Night	7% 2 12% 36% 31% 12%	1160
	11th to 20th	Night	10% 3 17% 30% 26% 14%	1160
	21st to end	Night	11% 3 16% 29% 26% 15%	1276

Page generated at 2026-06-28 01:10:50. Version ac07

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