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 2025-04-11.

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% 9% 3 8% 5% 4 5% 4 6% 7% 37%	113739
	Night	11% 9% 4 9% 5% 4 4 4 5% 6% 38%	114790
Transparency	Day	45% 6% 13% 24% 11% 1	117360
	Night	45% 5% 12% 23% 13% 1	119311
Seeing	Day	16% 10% 31% 27% 13% 4	16482
	Night	15% 9% 33% 31% 10% 1	26557

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% 6% 18% 9% 7% 6% 3 4 6% 20%	3617
	11th to 20th	Day	5% 12% 7% 12% 7% 6% 7% 9% 10% 6% 19%	3691
	21st to end	Day	6% 16% 7% 13% 6% 5% 5% 4 5% 5% 29%	4488
February	1st to 10th	Day	9% 11% 4 11% 8% 6% 5% 6% 6% 6% 27%	1800
	11th to 20th	Day	9% 10% 4 10% 6% 5% 5% 5% 5% 6% 32%	1754
	21st to end	Day	13% 11% 3 10% 5% 5% 5% 3 5% 6% 33%	1557
March	1st to 10th	Day	12% 8% 3 5% 3 3 4 3 5% 7% 46%	4837
	11th to 20th	Day	11% 9% 3 6% 4 4 4 4 6% 8% 41%	4980
	21st to end	Day	23% 12% 4 10% 6% 4 4 3 4 6% 26%	5525
April	1st to 10th	Day	17% 12% 4 7% 4 4 4 4 6% 6% 33%	2400
	11th to 20th	Day	20% 11% 4 7% 4 5% 4 3 4 5% 33%	2254
	21st to end	Day	20% 12% 4 7% 5% 4 5% 3 5% 5% 30%	2460
May	1st to 10th	Day	17% 6% 2 6% 4 4 4 4 6% 8% 38%	4237
	11th to 20th	Day	12% 7% 2 7% 3 4 4 4 6% 7% 44%	4399
	21st to end	Day	17% 7% 2 7% 5% 4 4 4 6% 7% 37%	4824
June	1st to 10th	Day	11% 5% 2 5% 4 3 4 4 6% 8% 46%	2660
	11th to 20th	Day	13% 6% 2 7% 5% 4 4 4 7% 10% 37%	2660
	21st to end	Day	10% 5% 2 6% 5% 6% 5% 4 7% 10% 39%	2660
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 4 8% 10% 37%	4440
	11th to 20th	Day	5% 4 1 4 3 3 4 4 6% 8% 58%	4440
	21st to end	Day	6% 4 1 6% 4 5% 6% 4 7% 9% 47%	4729
August	1st to 10th	Day	11% 7% 2 6% 5% 5% 5% 4 7% 8% 41%	4104
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 7% 8% 43%	4041
	21st to end	Day	8% 6% 3 7% 4 4 4 4 5% 7% 48%	4312
September	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 5% 8% 47%	2280
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 5% 7% 48%	2256
	21st to end	Day	14% 7% 2 5% 4 4 5% 3 5% 7% 44%	1995
October	1st to 10th	Day	15% 7% 3 5% 4 3 4 3 5% 5% 44%	1900
	11th to 20th	Day	14% 9% 2 7% 4 5% 4 4 4 6% 42%	1900
	21st to end	Day	17% 12% 3 7% 3 4 4 3 5% 7% 35%	2075
November	1st to 10th	Day	11% 10% 4 7% 5% 5% 4 3 6% 7% 39%	1664
	11th to 20th	Day	11% 13% 6% 10% 7% 4 5% 4 5% 6% 30%	1600
	21st to end	Day	12% 12% 4 10% 5% 4 3 3 4 7% 36%	1600
December	1st to 10th	Day	7% 9% 4 13% 12% 7% 6% 4 5% 7% 26%	2610
	11th to 20th	Day	12% 14% 6% 11% 9% 6% 5% 4 5% 6% 23%	3008
	21st to end	Day	19% 18% 8% 19% 5% 3 3 3 3 4 14%	3982
January	1st to 10th	Night	7% 12% 5% 16% 8% 7% 5% 4 5% 6% 25%	6348
	11th to 20th	Night	6% 11% 5% 13% 8% 7% 9% 8% 8% 5% 20%	6286
	21st to end	Night	8% 15% 6% 11% 6% 5% 5% 2 5% 5% 33%	6490
February	1st to 10th	Night	7% 13% 5% 10% 6% 6% 5% 4 4 6% 34%	2600
	11th to 20th	Night	7% 10% 4 9% 6% 6% 6% 3 4 6% 39%	2533
	21st to end	Night	12% 10% 4 8% 6% 5% 4 3 5% 5% 37%	2072
March	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 5% 6% 48%	5059
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	4980
	21st to end	Night	24% 12% 4 10% 4 3 3 3 4 5% 29%	5404
April	1st to 10th	Night	14% 13% 3 7% 5% 4 4 4 5% 5% 37%	2000
	11th to 20th	Night	19% 10% 4 7% 5% 4 4 3 5% 6% 34%	1881
	21st to end	Night	18% 10% 2 8% 4 5% 4 2 4 7% 36%	1715
May	1st to 10th	Night	14% 7% 3 7% 5% 5% 4 3 5% 7% 42%	2668
	11th to 20th	Night	13% 4 1 5% 3 3 3 3 5% 8% 50%	2506
	21st to end	Night	14% 8% 3 6% 4 4 3 3 4 7% 44%	2743
June	1st to 10th	Night	10% 4 2 6% 4 3 4 4 6% 7% 49%	1520
	11th to 20th	Night	10% 5% 2 6% 5% 5% 5% 3 6% 9% 44%	1520
	21st to end	Night	8% 4 2 5% 4 3 3 3 5% 9% 53%	1520
July	1st to 10th	Night	10% 4 1 5% 3 4 4 5% 8% 11% 45%	2530
	11th to 20th	Night	3 2 1 4 3 3 4 4 6% 9% 61%	2530
	21st to end	Night	5% 4 1 4 3 4 5% 3 6% 8% 57%	2831
August	1st to 10th	Night	11% 7% 2 6% 4 3 5% 3 5% 8% 47%	2836
	11th to 20th	Night	12% 5% 1 5% 3 3 3 4 5% 8% 51%	2798
	21st to end	Night	8% 7% 2 6% 4 4 4 3 5% 7% 49%	3347
September	1st to 10th	Night	9% 4 2 5% 3 3 3 3 4 7% 56%	1900
	11th to 20th	Night	11% 5% 1 5% 3 3 3 3 6% 7% 54%	1919
	21st to end	Night	12% 7% 2 5% 3 3 3 2 4 6% 53%	2166
October	1st to 10th	Night	14% 6% 2 4 4 3 3 3 4 6% 51%	2280
	11th to 20th	Night	14% 8% 3 5% 4 4 4 3 5% 7% 43%	2280
	21st to end	Night	18% 12% 4 6% 4 3 3 3 5% 6% 38%	2522
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 4 5% 6% 42%	2631
	11th to 20th	Night	12% 12% 5% 10% 6% 4 3 3 4 5% 35%	2800
	21st to end	Night	14% 11% 3 9% 6% 4 4 3 4 6% 37%	2800
December	1st to 10th	Night	6% 8% 5% 14% 7% 8% 5% 4 5% 7% 30%	4580
	11th to 20th	Night	10% 15% 7% 14% 7% 5% 4 4 5% 5% 25%	5199
	21st to end	Night	21% 15% 8% 16% 5% 4 4 3 4 4 16%	6996

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	66% 5% 9% 16% 5%	3618
	11th to 20th	Day	59% 5% 8% 16% 11%	3691
	21st to end	Day	57% 6% 14% 15% 7%	4488
February	1st to 10th	Day	55% 6% 13% 15% 9% 1	1800
	11th to 20th	Day	51% 6% 11% 23% 8% 1	1763
	21st to end	Day	52% 6% 13% 18% 11%	1557
March	1st to 10th	Day	38% 4 17% 27% 12% 2	4837
	11th to 20th	Day	41% 4 15% 28% 10% 1	4980
	21st to end	Day	61% 5% 10% 16% 7%	5527
April	1st to 10th	Day	50% 5% 11% 21% 11% 1	2400
	11th to 20th	Day	53% 5% 10% 19% 11% 2	2254
	21st to end	Day	56% 6% 11% 18% 8% 1	2460
May	1st to 10th	Day	42% 6% 13% 25% 12% 1	4275
	11th to 20th	Day	38% 7% 11% 29% 14% 2	4403
	21st to end	Day	46% 6% 11% 26% 10% 1	4837
June	1st to 10th	Day	34% 9% 13% 29% 14% 1	2660
	11th to 20th	Day	41% 7% 13% 22% 15% 1	2660
	21st to end	Day	38% 9% 17% 24% 10% 2	2660
July	1st to 10th	Day	38% 9% 19% 24% 10% 1	4371
	11th to 20th	Day	23% 6% 16% 34% 19% 2	4365
	21st to end	Day	30% 9% 14% 29% 17% 2	4580
August	1st to 10th	Day	39% 7% 12% 28% 13% 1	4090
	11th to 20th	Day	37% 7% 13% 33% 11%	4080
	21st to end	Day	34% 7% 15% 28% 14% 1	4311
September	1st to 10th	Day	35% 6% 13% 30% 16%	2280
	11th to 20th	Day	35% 7% 15% 32% 11% 1	2256
	21st to end	Day	39% 6% 11% 32% 12% 1	1995
October	1st to 10th	Day	30% 3 7% 36% 23%	3160
	11th to 20th	Day	38% 4 12% 25% 20% 1	3140
	21st to end	Day	43% 3 13% 30% 9%	3401
November	1st to 10th	Day	45% 5% 14% 19% 14% 3	1664
	11th to 20th	Day	55% 6% 11% 17% 10% 1	1600
	21st to end	Day	49% 6% 17% 21% 7% 1	1600
December	1st to 10th	Day	57% 6% 17% 13% 6% 1	2610
	11th to 20th	Day	61% 5% 15% 14% 5%	3005
	21st to end	Day	75% 4 8% 9% 3	3982
January	1st to 10th	Night	59% 4 10% 20% 6%	6348
	11th to 20th	Night	60% 5% 10% 14% 10%	6286
	21st to end	Night	54% 5% 11% 22% 8%	6490
February	1st to 10th	Night	52% 4 12% 21% 10% 1	2600
	11th to 20th	Night	47% 5% 13% 25% 9% 1	2534
	21st to end	Night	49% 5% 11% 22% 12% 2	2072
March	1st to 10th	Night	37% 4 12% 29% 16% 2	5059
	11th to 20th	Night	38% 4 14% 28% 15% 1	4980
	21st to end	Night	59% 3 9% 17% 10% 1	5404
April	1st to 10th	Night	48% 5% 10% 21% 15% 1	2000
	11th to 20th	Night	50% 6% 9% 17% 16% 2	1881
	21st to end	Night	48% 4 10% 21% 15% 2	1715
May	1st to 10th	Night	41% 6% 12% 18% 21% 1	2695
	11th to 20th	Night	32% 6% 11% 27% 20% 5%	2507
	21st to end	Night	40% 7% 11% 26% 16% 1	2751
June	1st to 10th	Night	31% 8% 12% 30% 17% 2	1520
	11th to 20th	Night	37% 7% 13% 22% 19% 3	1520
	21st to end	Night	28% 7% 16% 30% 17% 3	1520
July	1st to 10th	Night	31% 8% 21% 26% 13% 2	2490
	11th to 20th	Night	20% 6% 14% 39% 19% 2	2482
	21st to end	Night	24% 7% 17% 29% 21% 2	2741
August	1st to 10th	Night	36% 6% 10% 29% 14% 4	2826
	11th to 20th	Night	31% 6% 14% 32% 16% 1	2825
	21st to end	Night	34% 7% 16% 27% 15% 2	3347
September	1st to 10th	Night	28% 5% 11% 34% 20% 1	1900
	11th to 20th	Night	30% 6% 15% 31% 17% 1	1919
	21st to end	Night	33% 4 12% 31% 18% 2	2166
October	1st to 10th	Night	24% 3 6% 34% 28% 4	3800
	11th to 20th	Night	31% 5% 12% 26% 22% 4	3779
	21st to end	Night	43% 4 11% 28% 14% 1	4146
November	1st to 10th	Night	41% 6% 13% 24% 14% 2	2631
	11th to 20th	Night	51% 5% 11% 19% 12% 1	2800
	21st to end	Night	50% 5% 15% 22% 8% 1	2800
December	1st to 10th	Night	52% 6% 18% 19% 5%	4580
	11th to 20th	Night	61% 5% 11% 17% 5% 1	5201
	21st to end	Night	71% 4 9% 12% 4	6996

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% 12% 36% 27% 10% 4	300
	11th to 20th	Day	14% 16% 36% 28% 5% 1	300
	21st to end	Day	15% 12% 37% 24% 10% 2	330
February	1st to 10th	Day	14% 15% 41% 26% 4 1	300
	11th to 20th	Day	14% 13% 41% 24% 8% 1	300
	21st to end	Day	17% 13% 40% 28% 2	249
March	1st to 10th	Day	17% 11% 43% 22% 5% 1	300
	11th to 20th	Day	21% 11% 37% 23% 8% 1	300
	21st to end	Day	30% 12% 29% 20% 8% 2	355
April	1st to 10th	Day	24% 14% 35% 20% 6% 1	500
	11th to 20th	Day	27% 16% 23% 23% 10% 1	452
	21st to end	Day	26% 12% 29% 20% 10% 3	640
May	1st to 10th	Day	19% 13% 33% 25% 9% 2	653
	11th to 20th	Day	18% 12% 29% 23% 13% 4	675
	21st to end	Day	19% 10% 26% 28% 15% 3	734
June	1st to 10th	Day	10% 9% 27% 30% 19% 4	680
	11th to 20th	Day	15% 7% 34% 32% 9% 3	680
	21st to end	Day	12% 9% 31% 28% 15% 5%	680
July	1st to 10th	Day	7% 6% 23% 30% 24% 10%	671
	11th to 20th	Day	9% 5% 28% 31% 20% 8%	668
	21st to end	Day	7% 3 23% 40% 20% 6%	739
August	1st to 10th	Day	10% 3 23% 35% 23% 5%	676
	11th to 20th	Day	14% 3 19% 33% 24% 6%	665
	21st to end	Day	11% 5% 27% 34% 17% 6%	619
September	1st to 10th	Day	14% 8% 29% 29% 14% 6%	490
	11th to 20th	Day	11% 6% 30% 32% 18% 4	466
	21st to end	Day	18% 10% 31% 22% 13% 7%	300
October	1st to 10th	Day	17% 9% 31% 26% 14% 4	300
	11th to 20th	Day	17% 18% 39% 20% 6% 1	300
	21st to end	Day	25% 16% 36% 14% 8% 1	330
November	1st to 10th	Day	18% 13% 39% 20% 10%	300
	11th to 20th	Day	17% 20% 40% 18% 5%	300
	21st to end	Day	22% 13% 41% 19% 4	300
December	1st to 10th	Day	15% 11% 40% 25% 7% 1	300
	11th to 20th	Day	20% 15% 35% 22% 8%	300
	21st to end	Day	26% 15% 34% 21% 3 1	330
January	1st to 10th	Night	11% 12% 38% 30% 8% 1	1000
	11th to 20th	Night	16% 12% 38% 27% 6% 1	1000
	21st to end	Night	17% 10% 38% 28% 6% 1	1100
February	1st to 10th	Night	13% 12% 41% 27% 6% 1	1000
	11th to 20th	Night	12% 12% 44% 25% 6% 1	979
	21st to end	Night	16% 11% 38% 26% 8% 1	752
March	1st to 10th	Night	17% 10% 38% 27% 8% 1	667
	11th to 20th	Night	16% 11% 38% 29% 6% 1	600
	21st to end	Night	22% 11% 31% 25% 10% 2	660
April	1st to 10th	Night	21% 12% 37% 25% 6%	600
	11th to 20th	Night	24% 10% 30% 28% 8%	565
	21st to end	Night	21% 10% 32% 26% 8% 2	570
May	1st to 10th	Night	19% 10% 35% 29% 6% 1	570
	11th to 20th	Night	17% 7% 31% 34% 10% 1	561
	21st to end	Night	17% 6% 27% 35% 14% 1	617
June	1st to 10th	Night	11% 5% 31% 38% 14% 1	570
	11th to 20th	Night	13% 4 32% 41% 10%	570
	21st to end	Night	10% 3 23% 47% 15% 1	570
July	1st to 10th	Night	7% 1 20% 42% 27% 3	555
	11th to 20th	Night	5% 3 25% 44% 20% 3	552
	21st to end	Night	5% 2 20% 51% 20% 2	624
August	1st to 10th	Night	7% 2 20% 46% 23% 2	567
	11th to 20th	Night	10% 3 20% 38% 25% 4	564
	21st to end	Night	10% 2 26% 38% 22% 2	627
September	1st to 10th	Night	10% 4 25% 40% 17% 4	570
	11th to 20th	Night	13% 5% 31% 31% 18% 2	570
	21st to end	Night	13% 9% 32% 29% 16%	646
October	1st to 10th	Night	18% 9% 29% 30% 14% 1	760
	11th to 20th	Night	18% 11% 37% 26% 7% 1	760
	21st to end	Night	25% 11% 29% 27% 7%	836
November	1st to 10th	Night	18% 12% 36% 27% 6% 1	875
	11th to 20th	Night	17% 13% 39% 25% 6%	1000
	21st to end	Night	16% 15% 38% 23% 7%	1000
December	1st to 10th	Night	15% 10% 41% 28% 6% 1	1000
	11th to 20th	Night	19% 11% 38% 25% 7% 1	1000
	21st to end	Night	24% 11% 34% 26% 4	1100

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