Northfield
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northfield clear sky chart from 2005-11-06 to 2026-08-15.

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	12% 8% 3 8% 5% 4 5% 4 6% 7% 38%	144527
	Night	11% 9% 4 9% 5% 4 4 4 5% 7% 38%	137741
Transparency	Day	44% 5% 12% 26% 12% 1	148104
	Night	45% 5% 12% 24% 13% 1	142243
Seeing	Day	16% 10% 31% 27% 13% 4	18497
	Night	15% 9% 33% 31% 10% 1	28297

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	7% 14% 7% 19% 8% 6% 5% 3 4 6% 19%	4691
	11th to 20th	Day	5% 12% 7% 13% 7% 6% 7% 8% 9% 5% 20%	4809
	21st to end	Day	5% 13% 6% 11% 6% 6% 7% 4 7% 6% 30%	5812
February	1st to 10th	Day	9% 11% 5% 11% 8% 7% 5% 6% 6% 6% 27%	1890
	11th to 20th	Day	9% 10% 5% 11% 6% 5% 5% 5% 5% 6% 33%	1844
	21st to end	Day	12% 11% 3 10% 5% 6% 5% 3 6% 6% 33%	1633
March	1st to 10th	Day	12% 8% 3 5% 4 3 4 3 5% 7% 45%	4944
	11th to 20th	Day	11% 9% 3 6% 4 4 4 3 6% 8% 41%	5090
	21st to end	Day	23% 11% 4 10% 6% 4 4 3 4 6% 26%	5647
April	1st to 10th	Day	17% 11% 4 8% 4 4 4 3 6% 6% 33%	2520
	11th to 20th	Day	19% 11% 4 7% 5% 5% 4 4 4 5% 32%	2494
	21st to end	Day	20% 12% 4 7% 5% 4 5% 3 5% 5% 30%	2720
May	1st to 10th	Day	11% 5% 2 5% 4 4 5% 4 6% 7% 48%	7910
	11th to 20th	Day	15% 7% 2 6% 3 3 4 3 5% 7% 45%	8222
	21st to end	Day	16% 8% 2 7% 5% 4 4 3 6% 7% 38%	9047
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 45%	2940
	11th to 20th	Day	13% 7% 2 7% 5% 4 4 4 7% 10% 36%	2940
	21st to end	Day	11% 6% 2 6% 5% 6% 5% 4 6% 9% 39%	2940
July	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 5% 7% 10% 38%	8280
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 9% 59%	8280
	21st to end	Day	7% 4 2 6% 4 4 5% 4 6% 9% 48%	8822
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 8% 41%	4364
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 7% 8% 43%	4224
	21st to end	Day	8% 6% 3 7% 4 4 5% 4 6% 7% 47%	4448
September	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 6% 9% 46%	2400
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 5% 7% 47%	2375
	21st to end	Day	14% 7% 2 5% 3 4 5% 3 5% 7% 45%	2100
October	1st to 10th	Day	14% 7% 3 5% 4 3 5% 3 6% 6% 45%	2000
	11th to 20th	Day	14% 9% 2 7% 4 5% 4 4 4 7% 41%	2000
	21st to end	Day	17% 12% 4 7% 3 3 4 3 5% 6% 35%	2184
November	1st to 10th	Day	11% 10% 4 7% 5% 5% 4 3 6% 7% 39%	1749
	11th to 20th	Day	10% 14% 6% 10% 6% 4 5% 4 5% 6% 31%	1680
	21st to end	Day	12% 12% 4 11% 5% 4 4 3 4 7% 35%	1680
December	1st to 10th	Day	7% 10% 4 13% 12% 7% 6% 4 5% 7% 26%	2690
	11th to 20th	Day	11% 14% 5% 11% 9% 6% 5% 4 5% 6% 24%	3088
	21st to end	Day	19% 18% 8% 19% 5% 4 3 3 3 4 14%	4070
January	1st to 10th	Night	9% 12% 5% 15% 8% 6% 5% 4 5% 6% 25%	8232
	11th to 20th	Night	5% 11% 5% 14% 8% 7% 9% 9% 7% 4 20%	8178
	21st to end	Night	6% 12% 5% 10% 6% 5% 6% 3 5% 6% 35%	8402
February	1st to 10th	Night	7% 13% 5% 10% 7% 6% 5% 4 5% 6% 33%	2730
	11th to 20th	Night	7% 10% 4 8% 6% 6% 5% 3 4 6% 39%	2663
	21st to end	Night	11% 9% 4 8% 6% 5% 4 3 5% 6% 37%	2172
March	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 5% 6% 48%	5172
	11th to 20th	Night	10% 7% 2 7% 5% 5% 5% 4 6% 9% 41%	5090
	21st to end	Night	23% 12% 4 10% 4 3 3 3 4 5% 29%	5524
April	1st to 10th	Night	14% 12% 3 7% 4 4 4 3 5% 5% 37%	2100
	11th to 20th	Night	19% 9% 4 7% 5% 4 4 3 5% 6% 35%	2075
	21st to end	Night	19% 10% 3 7% 4 4 4 3 4 7% 35%	1895
May	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 51%	4973
	11th to 20th	Night	15% 5% 1 5% 3 3 4 3 5% 7% 49%	4670
	21st to end	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 38%	5139
June	1st to 10th	Night	10% 4 2 6% 4 3 4 4 6% 8% 49%	1680
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 3 6% 8% 43%	1680
	21st to end	Night	8% 5% 2 5% 4 3 4 4 5% 9% 51%	1680
July	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 5% 8% 12% 41%	4708
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 8% 60%	4707
	21st to end	Night	8% 5% 2 5% 3 4 4 4 6% 8% 51%	5268
August	1st to 10th	Night	11% 7% 2 6% 4 3 5% 3 5% 8% 47%	3016
	11th to 20th	Night	12% 5% 1 4 3 3 3 4 5% 8% 51%	2924
	21st to end	Night	8% 6% 2 6% 4 4 4 3 5% 7% 50%	3453
September	1st to 10th	Night	9% 4 2 5% 3 4 4 3 5% 7% 55%	2000
	11th to 20th	Night	11% 5% 1 5% 3 3 3 3 6% 7% 53%	2020
	21st to end	Night	11% 7% 2 5% 3 3 3 2 4 6% 54%	2280
October	1st to 10th	Night	14% 6% 2 4 4 3 3 3 4 6% 52%	2400
	11th to 20th	Night	15% 8% 3 5% 4 4 4 3 5% 6% 43%	2400
	21st to end	Night	17% 12% 4 6% 4 3 3 3 5% 6% 38%	2655
November	1st to 10th	Night	12% 9% 3 6% 4 4 4 4 5% 6% 42%	2766
	11th to 20th	Night	12% 12% 5% 10% 6% 4 3 3 4 5% 36%	2940
	21st to end	Night	14% 11% 3 10% 6% 4 4 2 4 6% 37%	2940
December	1st to 10th	Night	6% 9% 5% 14% 7% 8% 5% 4 5% 7% 30%	4720
	11th to 20th	Night	10% 15% 7% 14% 7% 5% 4 4 5% 5% 24%	5339
	21st to end	Night	21% 15% 8% 15% 5% 4 4 3 4 4 16%	7150

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	67% 4 11% 13% 5%	4692
	11th to 20th	Day	60% 4 8% 17% 11%	4809
	21st to end	Day	52% 5% 12% 22% 8%	5812
February	1st to 10th	Day	55% 6% 13% 16% 8% 1	1890
	11th to 20th	Day	51% 6% 11% 24% 7% 1	1853
	21st to end	Day	51% 5% 14% 19% 11%	1633
March	1st to 10th	Day	38% 4 17% 27% 12% 2	4944
	11th to 20th	Day	41% 4 15% 28% 10% 1	5090
	21st to end	Day	60% 5% 10% 16% 7%	5649
April	1st to 10th	Day	51% 5% 11% 21% 11% 1	2520
	11th to 20th	Day	53% 5% 11% 18% 11% 2	2494
	21st to end	Day	55% 6% 11% 19% 9% 1	2720
May	1st to 10th	Day	34% 4 11% 31% 19% 1	7948
	11th to 20th	Day	39% 4 8% 27% 18% 4	8226
	21st to end	Day	46% 5% 9% 29% 10% 2	9061
June	1st to 10th	Day	34% 8% 13% 30% 14% 1	2940
	11th to 20th	Day	42% 7% 13% 22% 14% 1	2940
	21st to end	Day	39% 9% 16% 24% 10% 2	2940
July	1st to 10th	Day	39% 7% 16% 30% 7%	8211
	11th to 20th	Day	23% 5% 13% 39% 19% 1	8205
	21st to end	Day	30% 7% 14% 33% 15% 1	8628
August	1st to 10th	Day	39% 6% 12% 28% 13% 1	4350
	11th to 20th	Day	37% 7% 14% 32% 11%	4263
	21st to end	Day	34% 7% 15% 29% 14% 1	4447
September	1st to 10th	Day	35% 6% 14% 30% 15%	2400
	11th to 20th	Day	36% 7% 16% 31% 10% 1	2375
	21st to end	Day	39% 5% 11% 32% 12% 1	2100
October	1st to 10th	Day	30% 3 7% 37% 23%	3260
	11th to 20th	Day	39% 4 12% 24% 20% 1	3240
	21st to end	Day	44% 4 13% 30% 9%	3510
November	1st to 10th	Day	45% 5% 14% 19% 14% 2	1749
	11th to 20th	Day	54% 6% 11% 18% 10% 1	1680
	21st to end	Day	50% 6% 16% 21% 7% 1	1680
December	1st to 10th	Day	58% 6% 16% 13% 6% 1	2690
	11th to 20th	Day	61% 5% 15% 14% 5%	3085
	21st to end	Day	75% 4 8% 9% 3	4070
January	1st to 10th	Night	59% 4 13% 18% 5%	8233
	11th to 20th	Night	62% 5% 9% 15% 9%	8178
	21st to end	Night	49% 4 11% 25% 11%	8402
February	1st to 10th	Night	52% 4 13% 20% 10% 1	2730
	11th to 20th	Night	46% 5% 13% 26% 9% 1	2664
	21st to end	Night	48% 5% 11% 22% 12% 1	2172
March	1st to 10th	Night	37% 4 12% 29% 15% 2	5172
	11th to 20th	Night	39% 4 14% 27% 15% 1	5090
	21st to end	Night	59% 3 10% 17% 10% 1	5524
April	1st to 10th	Night	48% 5% 9% 22% 15% 1	2100
	11th to 20th	Night	49% 5% 9% 19% 16% 2	2075
	21st to end	Night	49% 4 9% 21% 15% 2	1895
May	1st to 10th	Night	33% 4 9% 31% 22% 2	5000
	11th to 20th	Night	35% 5% 9% 22% 24% 6%	4671
	21st to end	Night	46% 5% 8% 27% 12% 2	5149
June	1st to 10th	Night	31% 7% 13% 30% 17% 2	1680
	11th to 20th	Night	38% 7% 13% 21% 18% 3	1680
	21st to end	Night	29% 7% 16% 29% 17% 3	1680
July	1st to 10th	Night	33% 7% 20% 27% 11% 1	4670
	11th to 20th	Night	22% 5% 12% 39% 20% 1	4662
	21st to end	Night	31% 6% 15% 30% 17% 1	5151
August	1st to 10th	Night	36% 6% 11% 29% 14% 4	3006
	11th to 20th	Night	32% 6% 15% 32% 15% 1	2951
	21st to end	Night	33% 7% 16% 27% 15% 2	3453
September	1st to 10th	Night	29% 6% 12% 34% 19% 1	2000
	11th to 20th	Night	30% 6% 15% 31% 16% 1	2020
	21st to end	Night	33% 4 12% 31% 19% 2	2280
October	1st to 10th	Night	24% 3 6% 34% 28% 4	3920
	11th to 20th	Night	32% 5% 12% 26% 22% 4	3899
	21st to end	Night	43% 4 10% 28% 14% 1	4279
November	1st to 10th	Night	41% 6% 13% 24% 14% 2	2766
	11th to 20th	Night	51% 5% 11% 20% 12% 1	2940
	21st to end	Night	51% 5% 14% 22% 8% 1	2940
December	1st to 10th	Night	53% 6% 17% 19% 5%	4720
	11th to 20th	Night	61% 5% 11% 17% 5% 1	5341
	21st to end	Night	71% 4 9% 12% 4	7150

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	13% 11% 35% 28% 9% 3	330
	11th to 20th	Day	13% 16% 38% 27% 4 1	330
	21st to end	Day	14% 13% 37% 24% 9% 2	363
February	1st to 10th	Day	13% 16% 41% 25% 4 2	330
	11th to 20th	Day	14% 12% 41% 25% 8% 1	330
	21st to end	Day	18% 12% 39% 27% 3	273
March	1st to 10th	Day	18% 12% 42% 22% 5% 1	330
	11th to 20th	Day	22% 12% 35% 22% 7% 1	330
	21st to end	Day	29% 13% 30% 19% 7% 2	389
April	1st to 10th	Day	24% 15% 35% 20% 5% 1	540
	11th to 20th	Day	26% 17% 25% 22% 9% 1	532
	21st to end	Day	25% 13% 30% 20% 9% 2	740
May	1st to 10th	Day	18% 13% 32% 25% 11% 2	753
	11th to 20th	Day	19% 12% 29% 24% 13% 3	775
	21st to end	Day	19% 9% 27% 28% 14% 3	844
June	1st to 10th	Day	9% 9% 27% 30% 20% 4	780
	11th to 20th	Day	16% 8% 34% 30% 9% 3	780
	21st to end	Day	13% 9% 32% 28% 14% 5%	780
July	1st to 10th	Day	8% 6% 21% 31% 24% 10%	771
	11th to 20th	Day	9% 5% 27% 31% 19% 8%	768
	21st to end	Day	7% 4 23% 41% 19% 6%	849
August	1st to 10th	Day	11% 3 22% 35% 23% 6%	776
	11th to 20th	Day	15% 3 19% 34% 24% 5%	736
	21st to end	Day	10% 5% 27% 34% 18% 6%	667
September	1st to 10th	Day	14% 9% 29% 28% 14% 6%	530
	11th to 20th	Day	11% 6% 29% 32% 18% 4	505
	21st to end	Day	16% 10% 29% 22% 15% 8%	330
October	1st to 10th	Day	16% 9% 33% 25% 14% 4	330
	11th to 20th	Day	18% 19% 38% 18% 5% 1	330
	21st to end	Day	25% 16% 36% 15% 9% 1	363
November	1st to 10th	Day	19% 14% 39% 18% 9% 1	330
	11th to 20th	Day	17% 19% 40% 19% 5%	330
	21st to end	Day	22% 14% 40% 20% 4	330
December	1st to 10th	Day	15% 11% 39% 25% 8% 1	330
	11th to 20th	Day	19% 16% 35% 22% 7%	330
	21st to end	Day	25% 15% 36% 21% 3 1	363
January	1st to 10th	Night	12% 12% 38% 30% 8% 1	1050
	11th to 20th	Night	16% 13% 38% 27% 6% 1	1050
	21st to end	Night	17% 11% 38% 29% 5% 1	1155
February	1st to 10th	Night	13% 12% 42% 27% 6% 1	1050
	11th to 20th	Night	12% 11% 44% 26% 6% 1	1029
	21st to end	Night	16% 12% 38% 26% 7% 1	788
March	1st to 10th	Night	17% 11% 37% 27% 8% 1	700
	11th to 20th	Night	17% 12% 37% 28% 6% 1	630
	21st to end	Night	21% 11% 32% 25% 9% 2	693
April	1st to 10th	Night	21% 12% 37% 24% 6%	630
	11th to 20th	Night	23% 10% 30% 28% 8%	623
	21st to end	Night	22% 10% 32% 27% 8% 1	630
May	1st to 10th	Night	18% 10% 34% 30% 7%	630
	11th to 20th	Night	17% 8% 31% 34% 9% 1	621
	21st to end	Night	17% 6% 27% 36% 13% 1	683
June	1st to 10th	Night	11% 5% 30% 39% 14%	630
	11th to 20th	Night	13% 5% 32% 40% 9%	630
	21st to end	Night	10% 3 25% 45% 14% 1	630
July	1st to 10th	Night	7% 1 20% 42% 27% 3	615
	11th to 20th	Night	5% 3 24% 45% 20% 2	612
	21st to end	Night	6% 3 21% 50% 19% 2	690
August	1st to 10th	Night	7% 2 20% 45% 24% 2	627
	11th to 20th	Night	11% 3 20% 38% 24% 4	606
	21st to end	Night	9% 2 26% 38% 23% 2	660
September	1st to 10th	Night	11% 5% 26% 38% 17% 4	600
	11th to 20th	Night	13% 5% 31% 32% 18% 2	600
	21st to end	Night	13% 9% 31% 30% 17% 1	680
October	1st to 10th	Night	17% 9% 29% 31% 14% 1	800
	11th to 20th	Night	19% 11% 37% 26% 7% 1	800
	21st to end	Night	25% 10% 30% 27% 7%	880
November	1st to 10th	Night	17% 13% 36% 27% 6% 1	920
	11th to 20th	Night	17% 14% 39% 24% 6%	1050
	21st to end	Night	17% 15% 38% 23% 7%	1050
December	1st to 10th	Night	15% 9% 41% 28% 6% 1	1050
	11th to 20th	Night	19% 12% 38% 25% 6%	1050
	21st to end	Night	24% 11% 33% 27% 4	1155

Page generated at 2026-08-12 02:37:17. Version ac07

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