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 2024-09-29.

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% 4 10% 7% 6% 6% 4 7% 9% 28%	103600
	Night	8% 12% 5% 12% 7% 7% 7% 5% 8% 10% 20%	94755
Transparency	Day	50% 7% 15% 22% 6%	103281
	Night	55% 7% 13% 18% 7%	94723
Seeing	Day	16% 2 10% 26% 31% 16%	16864
	Night	14% 3 12% 30% 30% 11%	23682

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 5% 2 9% 7% 8% 7% 7% 11% 17% 27%	1270
	11th to 20th	Day	3 14% 6% 14% 8% 9% 8% 5% 10% 11% 13%	1381
	21st to end	Day	1 6% 3 10% 9% 7% 6% 8% 10% 19% 21%	1898
February	1st to 10th	Day	1 6% 3 7% 7% 8% 8% 7% 14% 20% 18%	1258
	11th to 20th	Day	1 5% 3 7% 8% 9% 8% 8% 11% 20% 20%	1396
	21st to end	Day	1 8% 3 10% 8% 7% 9% 7% 11% 20% 17%	1360
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	22% 18% 6% 9% 4 3 4 3 4 5% 22%	1980
October	1st to 10th	Day	20% 22% 11% 14% 6% 4 4 2 3 3 11%	1770
	11th to 20th	Day	23% 21% 7% 20% 7% 5% 2 2 3 3 7%	1542
	21st to end	Day	19% 23% 7% 20% 13% 6% 3 2 2 2 3	1501
November	1st to 10th	Day	8% 24% 10% 15% 9% 8% 13% 6% 3 2 2	1193
	11th to 20th	Day	7% 16% 5% 12% 5% 9% 10% 8% 7% 10% 10%	1045
	21st to end	Day	7% 12% 8% 13% 10% 6% 8% 9% 9% 7% 11%	825
December	1st to 10th	Day	4 9% 4 11% 10% 11% 8% 5% 13% 11% 14%	760
	11th to 20th	Day	5% 10% 4 8% 8% 8% 7% 7% 10% 16% 18%	958
	21st to end	Day	4 12% 6% 13% 9% 7% 7% 6% 8% 11% 17%	1397
January	1st to 10th	Night	1 5% 2 8% 7% 8% 9% 6% 10% 17% 26%	5685
	11th to 20th	Night	4 16% 6% 13% 6% 6% 9% 6% 10% 13% 11%	5520
	21st to end	Night	3 8% 4 8% 6% 5% 6% 6% 11% 19% 24%	5763
February	1st to 10th	Night	1 5% 3 9% 10% 8% 9% 10% 13% 15% 16%	2921
	11th to 20th	Night	1 5% 3 9% 9% 10% 11% 9% 9% 15% 19%	2671
	21st to end	Night	1 6% 3 11% 8% 7% 9% 7% 12% 15% 20%	2093
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% 23% 7% 9% 4 4 4 3 4 4 16%	2147
October	1st to 10th	Night	19% 26% 13% 13% 4 4 3 2 3 4 9%	2190
	11th to 20th	Night	23% 22% 7% 20% 6% 3 3 2 2 2 9%	2416
	21st to end	Night	21% 23% 6% 21% 12% 6% 3 1 1 1 4	2855
November	1st to 10th	Night	11% 23% 8% 18% 9% 9% 11% 5% 3 2 3	2883
	11th to 20th	Night	9% 15% 5% 13% 9% 10% 11% 5% 7% 9% 7%	3135
	21st to end	Night	6% 15% 6% 14% 9% 8% 8% 7% 8% 10% 10%	3353
December	1st to 10th	Night	4 9% 4 13% 9% 9% 9% 7% 12% 12% 13%	3420
	11th to 20th	Night	4 10% 4 11% 9% 9% 8% 7% 11% 15% 12%	4239
	21st to end	Night	6% 14% 6% 13% 7% 7% 7% 5% 7% 12% 18%	6270

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	36% 9% 26% 19% 9%	1270
	11th to 20th	Day	60% 8% 19% 9% 4	1394
	21st to end	Day	40% 8% 8% 22% 20% 2	1898
February	1st to 10th	Day	40% 6% 21% 28% 5%	1258
	11th to 20th	Day	42% 6% 18% 23% 11%	1403
	21st to end	Day	43% 6% 14% 24% 13%	1360
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% 16% 13% 2	1980
October	1st to 10th	Day	78% 5% 10% 6%	1770
	11th to 20th	Day	84% 5% 6% 4 1	1542
	21st to end	Day	87% 5% 5% 2 1	1501
November	1st to 10th	Day	78% 13% 7% 3	1193
	11th to 20th	Day	60% 10% 16% 10% 2	1045
	21st to end	Day	59% 13% 13% 12% 2 1	825
December	1st to 10th	Day	55% 7% 16% 19% 4	760
	11th to 20th	Day	49% 8% 13% 27% 3	958
	21st to end	Day	55% 8% 11% 12% 10% 4	1397
January	1st to 10th	Night	39% 6% 22% 24% 8%	5685
	11th to 20th	Night	59% 8% 20% 10% 3	5569
	21st to end	Night	39% 6% 10% 23% 22% 1	5763
February	1st to 10th	Night	43% 9% 20% 23% 5%	2921
	11th to 20th	Night	46% 9% 16% 21% 8%	2674
	21st to end	Night	44% 6% 13% 24% 12%	2093
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	71% 6% 10% 13% 1	2147
October	1st to 10th	Night	80% 4 10% 5%	2190
	11th to 20th	Night	83% 4 8% 5% 1	2416
	21st to end	Night	88% 4 4 2 1	2855
November	1st to 10th	Night	80% 10% 7% 3	2883
	11th to 20th	Night	65% 10% 12% 11% 1	3135
	21st to end	Night	63% 8% 14% 12% 2	3353
December	1st to 10th	Night	55% 8% 15% 18% 3	3420
	11th to 20th	Night	54% 6% 15% 21% 3	4239
	21st to end	Night	56% 6% 11% 14% 12% 1	6270

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	1 7% 33% 38% 21%	90
	11th to 20th	Day	6% 2 19% 42% 26% 6%	90
	21st to end	Day	2 2 15% 33% 38% 11%	144
February	1st to 10th	Day	5% 1 9% 29% 32% 24%	236
	11th to 20th	Day	1 10% 31% 37% 21%	270
	21st to end	Day	1 8% 25% 36% 30%	225
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%	380
October	1st to 10th	Day	25% 6% 17% 27% 19% 6%	270
	11th to 20th	Day	31% 3 12% 21% 24% 8%	270
	21st to end	Day	24% 5% 13% 29% 24% 5%	255
November	1st to 10th	Day	12% 3 17% 30% 28% 9%	180
	11th to 20th	Day	11% 4 14% 32% 29% 10%	180
	21st to end	Day	11% 4 18% 23% 34% 10%	120
December	1st to 10th	Day	4 1 17% 32% 38% 8%	90
	11th to 20th	Day	7% 2 12% 31% 28% 20%	90
	21st to end	Day	5% 2 11% 35% 27% 19%	99
January	1st to 10th	Night	4 2 11% 30% 39% 14%	1040
	11th to 20th	Night	6% 2 16% 37% 29% 9%	1002
	21st to end	Night	4 2 16% 35% 30% 14%	1045
February	1st to 10th	Night	3 2 10% 32% 34% 18%	950
	11th to 20th	Night	3 1 8% 36% 37% 16%	929
	21st to end	Night	3 1 9% 33% 41% 13%	785
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	33% 4 15% 27% 17% 4	723
October	1st to 10th	Night	28% 7% 16% 27% 18% 3	720
	11th to 20th	Night	32% 5% 12% 24% 22% 5%	796
	21st to end	Night	35% 3 13% 25% 20% 4	990
November	1st to 10th	Night	19% 4 13% 30% 26% 7%	925
	11th to 20th	Night	14% 4 15% 29% 29% 8%	995
	21st to end	Night	12% 4 13% 27% 32% 13%	1040
December	1st to 10th	Night	8% 2 11% 36% 31% 13%	1040
	11th to 20th	Night	10% 3 16% 28% 27% 16%	1040
	21st to end	Night	11% 3 15% 29% 28% 16%	1144

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