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 2025-02-25.

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% 11% 4 10% 6% 6% 5% 4 7% 9% 27%	111226
	Night	8% 12% 5% 12% 7% 6% 6% 5% 8% 10% 19%	116174
Transparency	Day	51% 6% 15% 22% 6%	110907
	Night	56% 6% 13% 18% 6%	116142
Seeing	Day	16% 2 10% 26% 31% 16%	17148
	Night	14% 3 13% 30% 29% 11%	24470

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	3 8% 3 8% 5% 5% 5% 5% 10% 15% 33%	1820
	11th to 20th	Day	2 12% 5% 18% 8% 8% 8% 4 9% 8% 18%	1996
	21st to end	Day	1 4 2 9% 8% 6% 6% 7% 10% 20% 27%	2730
February	1st to 10th	Day	1 5% 3 7% 8% 8% 8% 8% 14% 20% 17%	1325
	11th to 20th	Day	1 6% 3 7% 8% 9% 8% 7% 11% 19% 20%	1472
	21st to end	Day	1 7% 3 10% 8% 7% 9% 7% 11% 20% 17%	1394
March	1st to 10th	Day	4 8% 3 11% 8% 8% 10% 6% 10% 14% 18%	2992
	11th to 20th	Day	7% 11% 7% 20% 9% 7% 5% 5% 7% 10% 12%	3335
	21st to end	Day	1 8% 6% 15% 8% 8% 6% 5% 9% 10% 24%	4115
April	1st to 10th	Day	5% 10% 4 10% 7% 8% 7% 5% 8% 8% 26%	2387
	11th to 20th	Day	7% 9% 4 8% 6% 8% 9% 6% 8% 9% 26%	2630
	21st to end	Day	8% 7% 4 11% 9% 8% 7% 5% 7% 10% 25%	2794
May	1st to 10th	Day	14% 17% 6% 13% 8% 5% 5% 3 4 5% 20%	5070
	11th to 20th	Day	10% 12% 4 10% 7% 5% 5% 4 6% 7% 30%	5273
	21st to end	Day	11% 11% 5% 11% 9% 5% 5% 4 5% 6% 28%	6205
June	1st to 10th	Day	14% 12% 4 10% 5% 5% 5% 3 6% 8% 30%	3420
	11th to 20th	Day	10% 10% 3 7% 5% 5% 5% 4 6% 8% 39%	3591
	21st to end	Day	12% 7% 3 8% 5% 5% 5% 4 7% 9% 36%	3610
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 9% 43%	5810
	11th to 20th	Day	8% 5% 3 8% 6% 6% 5% 5% 7% 9% 38%	5700
	21st to end	Day	9% 5% 4 6% 5% 5% 5% 5% 7% 9% 42%	5860
August	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 6% 9% 42%	5050
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 4 6% 9% 40%	4645
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 5% 5% 31%	4884
September	1st to 10th	Day	19% 13% 5% 8% 5% 5% 5% 3 5% 5% 27%	2415
	11th to 20th	Day	21% 16% 6% 10% 5% 5% 4 4 5% 4 20%	2280
	21st to end	Day	23% 18% 5% 9% 4 3 4 3 5% 5% 22%	2000
October	1st to 10th	Day	21% 27% 14% 11% 4 3 3 2 3 3 9%	3102
	11th to 20th	Day	21% 29% 8% 16% 5% 4 2 1 2 2 10%	2681
	21st to end	Day	15% 22% 9% 28% 12% 5% 2 2 2 2 3	2595
November	1st to 10th	Day	8% 24% 10% 14% 9% 9% 13% 6% 3 2 2	1257
	11th to 20th	Day	7% 17% 6% 12% 5% 10% 10% 8% 6% 10% 9%	1100
	21st to end	Day	8% 12% 7% 13% 9% 6% 8% 9% 9% 6% 11%	868
December	1st to 10th	Day	2 7% 3 12% 8% 8% 9% 7% 18% 17% 8%	1310
	11th to 20th	Day	3 7% 4 7% 8% 8% 7% 7% 18% 20% 12%	1508
	21st to end	Day	4 14% 10% 19% 8% 7% 6% 5% 6% 8% 12%	2002
January	1st to 10th	Night	2 8% 2 7% 6% 6% 7% 5% 10% 16% 31%	8145
	11th to 20th	Night	3 14% 6% 16% 7% 6% 8% 6% 9% 10% 15%	7910
	21st to end	Night	2 6% 3 7% 5% 5% 6% 7% 11% 17% 31%	8237
February	1st to 10th	Night	1 4 2 9% 9% 8% 10% 11% 13% 15% 16%	3074
	11th to 20th	Night	1 5% 2 8% 9% 10% 11% 9% 10% 15% 19%	2815
	21st to end	Night	1 6% 3 11% 8% 7% 9% 6% 13% 15% 20%	2154
March	1st to 10th	Night	3 7% 3 10% 8% 9% 8% 7% 9% 14% 22%	3863
	11th to 20th	Night	5% 12% 7% 19% 10% 8% 6% 5% 7% 8% 14%	3654
	21st to end	Night	2 7% 4 16% 13% 9% 5% 5% 7% 8% 26%	3516
April	1st to 10th	Night	5% 10% 3 11% 5% 8% 8% 6% 7% 9% 26%	1791
	11th to 20th	Night	7% 9% 3 7% 8% 9% 9% 6% 6% 8% 29%	1501
	21st to end	Night	7% 8% 4 11% 7% 7% 8% 5% 5% 7% 30%	1385
May	1st to 10th	Night	15% 17% 5% 14% 5% 4 4 4 4 4 23%	1900
	11th to 20th	Night	8% 10% 5% 14% 8% 4 4 4 4 6% 32%	1631
	21st to end	Night	8% 13% 7% 10% 8% 6% 8% 3 5% 5% 26%	1381
June	1st to 10th	Night	15% 12% 4 6% 4 5% 4 4 4 6% 36%	760
	11th to 20th	Night	12% 9% 2 7% 5% 4 4 6% 3 7% 39%	587
	21st to end	Night	11% 11% 3 8% 4 5% 5% 5% 6% 8% 34%	570
July	1st to 10th	Night	13% 8% 3 10% 6% 6% 5% 5% 6% 7% 31%	1159
	11th to 20th	Night	8% 6% 3 9% 4 5% 6% 5% 6% 11% 37%	1270
	21st to end	Night	10% 7% 3 7% 4 4 4 4 7% 8% 43%	1695
August	1st to 10th	Night	12% 9% 4 9% 5% 5% 6% 4 8% 7% 32%	1890
	11th to 20th	Night	12% 8% 3 6% 5% 5% 3 4 6% 7% 41%	2193
	21st to end	Night	20% 9% 4 8% 5% 4 4 3 5% 7% 31%	2783
September	1st to 10th	Night	23% 13% 4 8% 5% 3 4 3 4 6% 28%	1765
	11th to 20th	Night	21% 17% 4 8% 5% 5% 5% 3 5% 5% 22%	1900
	21st to end	Night	21% 22% 7% 9% 4 4 4 3 4 4 17%	2159
October	1st to 10th	Night	23% 29% 14% 11% 4 4 3 1 3 3 7%	3858
	11th to 20th	Night	23% 27% 8% 16% 5% 3 3 2 2 2 9%	4241
	21st to end	Night	15% 23% 9% 30% 10% 5% 2 1 1 1 3	4963
November	1st to 10th	Night	11% 22% 8% 18% 8% 9% 11% 5% 3 2 3	3039
	11th to 20th	Night	10% 16% 5% 12% 9% 10% 11% 5% 7% 9% 6%	3300
	21st to end	Night	7% 15% 6% 14% 9% 8% 8% 7% 8% 9% 10%	3530
December	1st to 10th	Night	2 6% 3 15% 10% 8% 8% 6% 13% 20% 9%	5880
	11th to 20th	Night	3 10% 4 10% 7% 7% 7% 6% 14% 19% 11%	6699
	21st to end	Night	6% 16% 9% 17% 8% 6% 6% 4 7% 9% 13%	8976

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	35% 6% 22% 23% 14%	1820
	11th to 20th	Day	60% 6% 19% 12% 4	2009
	21st to end	Day	35% 7% 17% 25% 15% 1	2730
February	1st to 10th	Day	40% 6% 21% 27% 5%	1325
	11th to 20th	Day	43% 6% 19% 23% 10%	1479
	21st to end	Day	43% 6% 14% 24% 13%	1394
March	1st to 10th	Day	51% 4 17% 19% 9%	2992
	11th to 20th	Day	64% 5% 10% 16% 5%	3337
	21st to end	Day	51% 6% 9% 18% 17% 1	4115
April	1st to 10th	Day	50% 6% 12% 22% 9%	2387
	11th to 20th	Day	48% 7% 11% 24% 9%	2630
	21st to end	Day	51% 8% 13% 20% 9%	2794
May	1st to 10th	Day	65% 5% 9% 15% 7%	5070
	11th to 20th	Day	50% 7% 11% 24% 8%	5277
	21st to end	Day	53% 7% 12% 22% 5%	6206
June	1st to 10th	Day	52% 6% 15% 21% 6%	3420
	11th to 20th	Day	43% 7% 14% 32% 5%	3591
	21st to end	Day	42% 7% 17% 31% 3	3610
July	1st to 10th	Day	33% 6% 16% 40% 5%	5721
	11th to 20th	Day	38% 7% 17% 35% 3	5601
	21st to end	Day	34% 8% 21% 29% 7%	5674
August	1st to 10th	Day	38% 6% 24% 24% 7% 1	5032
	11th to 20th	Day	40% 6% 21% 25% 6% 2	4692
	21st to end	Day	55% 6% 16% 21% 1	4883
September	1st to 10th	Day	58% 7% 17% 15% 3	2415
	11th to 20th	Day	65% 7% 18% 9% 1	2280
	21st to end	Day	64% 5% 15% 14% 2	2000
October	1st to 10th	Day	81% 4 7% 8% 1	3102
	11th to 20th	Day	83% 4 5% 7% 2	2681
	21st to end	Day	87% 5% 4 2 1	2595
November	1st to 10th	Day	78% 12% 6% 3	1257
	11th to 20th	Day	62% 10% 16% 10% 2	1100
	21st to end	Day	60% 13% 13% 11% 2 1	868
December	1st to 10th	Day	47% 7% 20% 24% 2	1310
	11th to 20th	Day	43% 6% 15% 33% 2	1508
	21st to end	Day	67% 6% 8% 9% 7% 3	2002
January	1st to 10th	Night	37% 5% 18% 31% 10%	8145
	11th to 20th	Night	60% 6% 19% 12% 2	7959
	21st to end	Night	34% 6% 16% 26% 18%	8237
February	1st to 10th	Night	43% 9% 20% 22% 5%	3074
	11th to 20th	Night	46% 9% 17% 20% 8%	2818
	21st to end	Night	43% 6% 14% 25% 12%	2154
March	1st to 10th	Night	47% 5% 12% 25% 10%	3863
	11th to 20th	Night	64% 6% 8% 16% 5%	3654
	21st to end	Night	54% 5% 8% 16% 15% 2	3516
April	1st to 10th	Night	48% 8% 12% 22% 9% 1	1791
	11th to 20th	Night	47% 8% 13% 25% 7% 1	1501
	21st to end	Night	50% 5% 11% 24% 10%	1385
May	1st to 10th	Night	63% 4 9% 17% 8%	1900
	11th to 20th	Night	46% 10% 10% 22% 12%	1631
	21st to end	Night	54% 9% 10% 21% 6%	1381
June	1st to 10th	Night	48% 4 16% 24% 8%	760
	11th to 20th	Night	44% 7% 11% 31% 8%	587
	21st to end	Night	43% 8% 15% 29% 4	570
July	1st to 10th	Night	49% 7% 11% 26% 6%	1139
	11th to 20th	Night	38% 6% 14% 33% 9%	1246
	21st to end	Night	35% 7% 18% 33% 7%	1642
August	1st to 10th	Night	48% 6% 17% 22% 6% 1	1884
	11th to 20th	Night	42% 5% 17% 25% 8% 3	2212
	21st to end	Night	53% 6% 18% 21% 3	2783
September	1st to 10th	Night	57% 6% 19% 15% 2	1765
	11th to 20th	Night	63% 7% 17% 12%	1900
	21st to end	Night	70% 6% 10% 13% 1	2159
October	1st to 10th	Night	84% 4 7% 5%	3858
	11th to 20th	Night	84% 3 6% 6% 1	4241
	21st to end	Night	91% 4 3 2 1	4963
November	1st to 10th	Night	80% 10% 7% 3	3039
	11th to 20th	Night	67% 10% 12% 11% 1	3300
	21st to end	Night	63% 8% 14% 12% 2	3530
December	1st to 10th	Night	50% 6% 20% 21% 2	5880
	11th to 20th	Night	49% 5% 15% 29% 3	6699
	21st to end	Night	67% 4 10% 10% 9%	8976

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 9% 36% 34% 19%	100
	11th to 20th	Day	6% 2 17% 44% 24% 7%	100
	21st to end	Day	2 2 14% 33% 39% 11%	161
February	1st to 10th	Day	4 1 10% 28% 33% 23%	263
	11th to 20th	Day	2 10% 32% 38% 19%	300
	21st to end	Day	1 8% 27% 36% 28%	237
March	1st to 10th	Day	5% 1 16% 27% 34% 16%	270
	11th to 20th	Day	8% 1 9% 27% 34% 20%	270
	21st to end	Day	5% 3 19% 26% 29% 18%	469
April	1st to 10th	Day	11% 1 9% 24% 32% 22%	539
	11th to 20th	Day	10% 1 12% 29% 29% 20%	640
	21st to end	Day	8% 2 12% 30% 26% 22%	650
May	1st to 10th	Day	15% 1 9% 25% 31% 19%	653
	11th to 20th	Day	16% 2 16% 28% 27% 11%	753
	21st to end	Day	19% 1 11% 27% 27% 14%	940
June	1st to 10th	Day	18% 2 9% 26% 30% 16%	870
	11th to 20th	Day	15% 1 7% 30% 33% 13%	1041
	21st to end	Day	14% 2 6% 29% 34% 15%	1060
July	1st to 10th	Day	18% 1 5% 22% 35% 19%	927
	11th to 20th	Day	15% 1 6% 27% 37% 14%	852
	21st to end	Day	15% 1 5% 21% 37% 21%	789
August	1st to 10th	Day	19% 2 6% 20% 32% 20%	676
	11th to 20th	Day	20% 1 5% 22% 33% 19%	665
	21st to end	Day	23% 2 9% 27% 25% 13%	748
September	1st to 10th	Day	31% 3 8% 19% 28% 12%	573
	11th to 20th	Day	30% 2 10% 26% 26% 7%	490
	21st to end	Day	32% 3 13% 25% 21% 6%	386
October	1st to 10th	Day	27% 6% 17% 26% 19% 6%	300
	11th to 20th	Day	33% 5% 12% 20% 23% 7%	300
	21st to end	Day	24% 5% 14% 29% 23% 5%	283
November	1st to 10th	Day	14% 3 16% 31% 28% 9%	200
	11th to 20th	Day	12% 5% 17% 31% 27% 9%	200
	21st to end	Day	14% 4 17% 23% 33% 9%	133
December	1st to 10th	Day	4 1 16% 35% 37% 7%	100
	11th to 20th	Day	6% 3 14% 32% 27% 18%	100
	21st to end	Day	6% 2 12% 35% 26% 18%	110
January	1st to 10th	Night	4 2 13% 30% 37% 14%	1100
	11th to 20th	Night	6% 2 16% 36% 30% 9%	1057
	21st to end	Night	4 2 16% 35% 30% 13%	1100
February	1st to 10th	Night	3 2 10% 33% 33% 18%	1000
	11th to 20th	Night	3 1 8% 35% 37% 16%	979
	21st to end	Night	2 1 11% 33% 40% 13%	808
March	1st to 10th	Night	7% 3 15% 33% 30% 12%	843
	11th to 20th	Night	6% 1 11% 33% 37% 13%	679
	21st to end	Night	6% 1 15% 33% 30% 14%	627
April	1st to 10th	Night	10% 2 13% 26% 33% 16%	570
	11th to 20th	Night	10% 2 11% 33% 33% 11%	563
	21st to end	Night	9% 2 13% 32% 29% 15%	502
May	1st to 10th	Night	16% 3 11% 31% 30% 9%	380
	11th to 20th	Night	15% 4 15% 34% 25% 7%	312
	21st to end	Night	20% 3 11% 30% 23% 12%	205
June	1st to 10th	Night	20% 1 9% 31% 28% 10%	190
	11th to 20th	Night	15% 1 8% 35% 37% 4	190
	21st to end	Night	16% 3 8% 27% 36% 9%	190
July	1st to 10th	Night	18% 1 5% 25% 39% 11%	185
	11th to 20th	Night	14% 2 9% 32% 34% 9%	184
	21st to end	Night	17% 2 6% 22% 39% 14%	236
August	1st to 10th	Night	22% 2 9% 20% 36% 11%	378
	11th to 20th	Night	24% 2 3 23% 35% 12%	517
	21st to end	Night	25% 2 8% 30% 26% 9%	627
September	1st to 10th	Night	31% 3 9% 22% 28% 7%	570
	11th to 20th	Night	31% 2 10% 28% 24% 5%	570
	21st to end	Night	32% 4 15% 28% 17% 4	727
October	1st to 10th	Night	29% 7% 16% 27% 18% 3	760
	11th to 20th	Night	33% 5% 13% 24% 21% 5%	841
	21st to end	Night	34% 2 13% 27% 20% 4	1045
November	1st to 10th	Night	19% 4 13% 31% 26% 7%	975
	11th to 20th	Night	15% 4 16% 30% 28% 8%	1050
	21st to end	Night	13% 4 13% 27% 31% 12%	1100
December	1st to 10th	Night	7% 2 11% 37% 30% 12%	1100
	11th to 20th	Night	10% 3 16% 29% 26% 15%	1100
	21st to end	Night	11% 3 15% 29% 27% 16%	1210

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