Northfield Mountain Environmental Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northfield Mountain Environmental Center clear sky chart from 2005-11-06 to 2025-10-19.

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	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	126462
	Night	17% 8% 3 7% 4 4 4 3 5% 6% 37%	123254
Transparency	Day	46% 6% 12% 21% 14% 2	129982
	Night	45% 5% 12% 21% 14% 2	127821
Seeing	Day	23% 16% 29% 20% 9% 3	16165
	Night	21% 12% 29% 28% 8% 1	25081

By Month

Percentage Cloud Cover NE (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	12% 7% 5% 14% 6% 4 4 3 5% 6% 34%	3597
	11th to 20th	Day	13% 10% 3 8% 5% 4 4 3 5% 6% 39%	3570
	21st to end	Day	15% 9% 3 8% 4 4 3 3 4 6% 39%	3960
February	1st to 10th	Day	16% 9% 3 8% 5% 5% 5% 4 7% 6% 32%	1843
	11th to 20th	Day	14% 9% 2 10% 6% 5% 4 3 5% 6% 35%	1948
	21st to end	Day	18% 10% 3 8% 5% 4 4 3 4 4 36%	1650
March	1st to 10th	Day	20% 12% 5% 9% 5% 3 3 3 4 4 33%	4476
	11th to 20th	Day	15% 10% 3 7% 4 4 5% 4 5% 6% 38%	4651
	21st to end	Day	24% 8% 2 7% 4 4 4 2 5% 6% 34%	5960
April	1st to 10th	Day	19% 10% 3 7% 5% 4 4 3 4 7% 35%	2400
	11th to 20th	Day	17% 7% 3 6% 5% 5% 5% 4 5% 6% 38%	2374
	21st to end	Day	20% 7% 3 6% 5% 4 4 3 5% 5% 35%	2560
May	1st to 10th	Day	32% 8% 3 7% 4 4 4 4 5% 7% 23%	5886
	11th to 20th	Day	21% 10% 3 8% 5% 4 4 3 5% 6% 31%	6195
	21st to end	Day	19% 7% 3 6% 5% 4 4 4 6% 8% 34%	6949
June	1st to 10th	Day	18% 6% 2 6% 6% 5% 5% 4 7% 9% 33%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 4 5% 3 6% 9% 36%	2800
	21st to end	Day	16% 7% 2 7% 5% 4 5% 4 7% 8% 35%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 5% 8% 9% 34%	6360
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 36%	6360
	21st to end	Day	15% 7% 2 6% 4 4 4 3 5% 8% 42%	6926
August	1st to 10th	Day	15% 6% 3 8% 5% 5% 5% 5% 6% 9% 34%	4414
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 4 6% 9% 37%	3987
	21st to end	Day	8% 5% 2 6% 5% 6% 5% 4 7% 10% 42%	4312
September	1st to 10th	Day	13% 5% 2 5% 4 5% 5% 4 5% 8% 44%	2400
	11th to 20th	Day	14% 6% 2 6% 3 4 3 3 4 7% 49%	2249
	21st to end	Day	17% 7% 2 5% 4 4 4 3 6% 8% 41%	2200
October	1st to 10th	Day	21% 6% 3 6% 5% 4 3 3 4 5% 40%	2182
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 3 5% 7% 43%	1925
	21st to end	Day	22% 8% 2 6% 4 4 4 3 4 6% 36%	1881
November	1st to 10th	Day	13% 8% 3 6% 4 3 3 3 3 6% 48%	1761
	11th to 20th	Day	18% 7% 2 5% 3 4 4 4 4 6% 43%	1800
	21st to end	Day	19% 9% 3 5% 4 4 4 3 4 6% 39%	1740
December	1st to 10th	Day	16% 10% 3 7% 5% 4 4 4 5% 6% 36%	2600
	11th to 20th	Day	21% 9% 3 6% 4 3 4 3 4 5% 39%	2986
	21st to end	Day	19% 11% 4 7% 4 4 4 3 4 6% 35%	3960
January	1st to 10th	Night	14% 7% 4 10% 4 4 4 4 5% 7% 37%	6349
	11th to 20th	Night	12% 10% 3 11% 6% 5% 5% 4 5% 6% 33%	6315
	21st to end	Night	15% 10% 3 7% 4 5% 5% 3 5% 7% 35%	6996
February	1st to 10th	Night	14% 10% 4 7% 5% 4 5% 4 5% 6% 35%	2554
	11th to 20th	Night	12% 9% 4 8% 5% 4 5% 4 6% 6% 37%	2338
	21st to end	Night	17% 8% 3 8% 5% 4 4 3 4 6% 37%	1980
March	1st to 10th	Night	21% 8% 2 7% 5% 4 4 3 5% 6% 34%	5364
	11th to 20th	Night	16% 9% 3 6% 5% 5% 5% 3 5% 7% 37%	5306
	21st to end	Night	24% 10% 3 7% 4 3 4 3 5% 5% 32%	4967
April	1st to 10th	Night	20% 7% 3 8% 5% 4 4 4 5% 6% 34%	2000
	11th to 20th	Night	20% 8% 2 5% 4 5% 3 4 4 6% 39%	1975
	21st to end	Night	21% 7% 3 6% 4 4 3 3 5% 5% 39%	1840
May	1st to 10th	Night	32% 9% 3 6% 4 4 4 3 5% 6% 24%	4054
	11th to 20th	Night	18% 10% 4 10% 5% 5% 5% 4 5% 7% 28%	3727
	21st to end	Night	22% 8% 3 7% 5% 4 4 3 5% 7% 32%	3926
June	1st to 10th	Night	18% 8% 3 8% 5% 3 4 3 5% 6% 38%	1600
	11th to 20th	Night	18% 7% 2 6% 4 3 4 4 6% 7% 38%	1600
	21st to end	Night	16% 7% 2 6% 3 3 4 4 6% 7% 40%	1600
July	1st to 10th	Night	12% 7% 3 7% 5% 4 5% 4 6% 9% 40%	3600
	11th to 20th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 9% 36%	3600
	21st to end	Night	12% 6% 2 5% 4 4 3 3 5% 6% 50%	3926
August	1st to 10th	Night	22% 5% 2 7% 4 5% 4 4 5% 7% 34%	2703
	11th to 20th	Night	13% 7% 3 8% 5% 5% 4 4 6% 7% 37%	3056
	21st to end	Night	9% 4 1 4 4 4 4 3 5% 7% 55%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 3 4 3 6% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 3 4 3 3 4 6% 53%	2150
	21st to end	Night	16% 6% 2 7% 3 3 4 3 5% 7% 43%	2200
October	1st to 10th	Night	20% 6% 2 6% 4 4 3 3 4 6% 41%	2215
	11th to 20th	Night	17% 5% 2 5% 4 3 4 3 6% 7% 43%	2431
	21st to end	Night	23% 7% 2 6% 3 4 4 3 4 6% 37%	2717
November	1st to 10th	Night	15% 6% 2 6% 4 3 3 3 4 7% 47%	2537
	11th to 20th	Night	18% 7% 2 5% 5% 4 4 3 4 6% 42%	2600
	21st to end	Night	15% 8% 3 6% 5% 4 4 3 5% 6% 41%	2658
December	1st to 10th	Night	18% 9% 3 7% 4 4 4 4 6% 7% 34%	4580
	11th to 20th	Night	24% 10% 3 6% 3 3 3 3 4 4 37%	5208
	21st to end	Night	20% 13% 4 8% 4 3 3 2 4 5% 33%	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	51% 4 9% 24% 11% 1	3598
	11th to 20th	Day	45% 5% 11% 20% 17% 3	3600
	21st to end	Day	46% 5% 8% 21% 18% 2	3960
February	1st to 10th	Day	50% 6% 12% 18% 12% 1	1843
	11th to 20th	Day	49% 5% 10% 20% 14% 3	1957
	21st to end	Day	51% 5% 9% 20% 13% 2	1650
March	1st to 10th	Day	55% 4 11% 13% 10% 7%	4479
	11th to 20th	Day	45% 6% 16% 21% 9% 3	4651
	21st to end	Day	51% 3 7% 15% 18% 6%	5960
April	1st to 10th	Day	49% 5% 9% 20% 13% 3	2400
	11th to 20th	Day	46% 7% 10% 18% 18% 2	2374
	21st to end	Day	48% 6% 8% 19% 16% 3	2560
May	1st to 10th	Day	60% 8% 11% 13% 8% 1	5897
	11th to 20th	Day	54% 6% 13% 17% 10% 1	6199
	21st to end	Day	46% 7% 11% 23% 13% 1	6949
June	1st to 10th	Day	45% 8% 13% 21% 12% 1	2800
	11th to 20th	Day	44% 7% 13% 19% 14% 2	2800
	21st to end	Day	45% 8% 11% 21% 13% 1	2800
July	1st to 10th	Day	45% 8% 12% 21% 13% 1	6290
	11th to 20th	Day	40% 8% 17% 23% 11% 1	6285
	21st to end	Day	40% 5% 12% 27% 15% 1	6728
August	1st to 10th	Day	46% 7% 16% 21% 10% 1	4400
	11th to 20th	Day	42% 7% 15% 26% 9% 1	4026
	21st to end	Day	34% 8% 14% 28% 15% 2	4312
September	1st to 10th	Day	37% 7% 12% 25% 18% 1	2400
	11th to 20th	Day	35% 5% 9% 26% 22% 2	2249
	21st to end	Day	42% 6% 10% 23% 18% 3	2200
October	1st to 10th	Day	43% 5% 8% 22% 20% 2	3559
	11th to 20th	Day	35% 4 10% 28% 21% 2	3128
	21st to end	Day	36% 5% 11% 30% 15% 3	3081
November	1st to 10th	Day	39% 5% 14% 25% 15% 2	1761
	11th to 20th	Day	42% 6% 10% 20% 20% 2	1800
	21st to end	Day	46% 5% 16% 23% 10% 2	1740
December	1st to 10th	Day	48% 6% 14% 24% 8% 1	2600
	11th to 20th	Day	48% 3 12% 18% 17% 1	2986
	21st to end	Day	51% 5% 7% 18% 14% 4	3960
January	1st to 10th	Night	46% 5% 12% 24% 11% 2	6349
	11th to 20th	Night	49% 5% 15% 16% 12% 3	6360
	21st to end	Night	48% 5% 10% 16% 17% 4	6996
February	1st to 10th	Night	48% 7% 13% 21% 10% 2	2554
	11th to 20th	Night	45% 7% 12% 20% 14% 3	2340
	21st to end	Night	48% 4 11% 22% 12% 4	1980
March	1st to 10th	Night	49% 4 12% 16% 15% 4	5374
	11th to 20th	Night	45% 7% 17% 19% 10% 3	5307
	21st to end	Night	52% 3 7% 13% 21% 4	4967
April	1st to 10th	Night	50% 5% 9% 20% 14% 2	2000
	11th to 20th	Night	47% 5% 11% 19% 18% 1	1975
	21st to end	Night	46% 5% 9% 19% 16% 5%	1840
May	1st to 10th	Night	59% 7% 12% 13% 8% 1	4060
	11th to 20th	Night	55% 7% 11% 16% 10% 2	3728
	21st to end	Night	51% 7% 13% 18% 11% 1	3927
June	1st to 10th	Night	46% 6% 11% 25% 11% 1	1600
	11th to 20th	Night	44% 8% 13% 22% 12% 1	1600
	21st to end	Night	40% 9% 13% 23% 15% 1	1600
July	1st to 10th	Night	40% 7% 14% 23% 15%	3560
	11th to 20th	Night	43% 8% 15% 19% 14%	3552
	21st to end	Night	35% 5% 13% 31% 16% 1	3809
August	1st to 10th	Night	49% 7% 13% 20% 10% 1	2693
	11th to 20th	Night	43% 7% 15% 24% 10%	3083
	21st to end	Night	26% 7% 15% 34% 16% 1	3586
September	1st to 10th	Night	37% 5% 11% 27% 18% 1	2000
	11th to 20th	Night	33% 5% 11% 26% 23% 2	2150
	21st to end	Night	41% 5% 9% 25% 17% 3	2200
October	1st to 10th	Night	40% 5% 13% 24% 16% 2	3618
	11th to 20th	Night	31% 5% 11% 26% 25% 3	3967
	21st to end	Night	36% 4 13% 33% 14% 1	4467
November	1st to 10th	Night	38% 5% 11% 24% 17% 3	2537
	11th to 20th	Night	43% 5% 12% 20% 17% 3	2600
	21st to end	Night	44% 5% 14% 24% 12% 1	2658
December	1st to 10th	Night	48% 5% 12% 26% 9%	4580
	11th to 20th	Night	50% 4 14% 14% 17% 2	5208
	21st to end	Night	53% 4 9% 19% 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	29% 26% 25% 17% 4 1	200
	11th to 20th	Day	32% 22% 33% 9% 4 1	200
	21st to end	Day	25% 25% 30% 15% 5%	220
February	1st to 10th	Day	31% 28% 30% 10% 1	351
	11th to 20th	Day	32% 24% 30% 13% 1	496
	21st to end	Day	30% 19% 30% 18% 2	414
March	1st to 10th	Day	31% 19% 30% 17% 2	500
	11th to 20th	Day	30% 21% 29% 13% 5% 1	500
	21st to end	Day	30% 23% 29% 11% 4 3	550
April	1st to 10th	Day	33% 23% 25% 12% 7% 1	500
	11th to 20th	Day	32% 21% 26% 15% 5% 1	492
	21st to end	Day	30% 17% 33% 13% 5% 2	500
May	1st to 10th	Day	34% 20% 25% 14% 5% 2	503
	11th to 20th	Day	22% 18% 37% 15% 6% 2	527
	21st to end	Day	20% 14% 31% 23% 9% 2	573
June	1st to 10th	Day	19% 14% 32% 22% 11% 2	530
	11th to 20th	Day	18% 14% 34% 22% 10% 2	530
	21st to end	Day	17% 16% 34% 19% 11% 3	530
July	1st to 10th	Day	15% 10% 30% 28% 14% 3	525
	11th to 20th	Day	13% 7% 30% 29% 17% 3	524
	21st to end	Day	15% 9% 32% 30% 12% 2	576
August	1st to 10th	Day	10% 10% 26% 33% 14% 6%	526
	11th to 20th	Day	14% 8% 28% 27% 16% 7%	519
	21st to end	Day	12% 10% 30% 27% 16% 6%	583
September	1st to 10th	Day	18% 8% 24% 25% 17% 7%	530
	11th to 20th	Day	16% 9% 24% 22% 18% 11%	530
	21st to end	Day	19% 11% 28% 22% 13% 7%	530
October	1st to 10th	Day	27% 10% 24% 20% 14% 5%	530
	11th to 20th	Day	20% 15% 28% 23% 11% 3	492
	21st to end	Day	30% 14% 23% 25% 7% 1	429
November	1st to 10th	Day	22% 15% 29% 23% 9% 2	395
	11th to 20th	Day	28% 18% 27% 21% 6% 1	400
	21st to end	Day	31% 14% 27% 20% 6% 1	340
December	1st to 10th	Day	24% 23% 32% 17% 4 1	200
	11th to 20th	Day	37% 20% 32% 10% 3	200
	21st to end	Day	27% 20% 30% 18% 5%	220
January	1st to 10th	Night	22% 17% 35% 24% 3	800
	11th to 20th	Night	26% 15% 33% 23% 3	800
	21st to end	Night	19% 18% 35% 25% 4	880
February	1st to 10th	Night	22% 17% 36% 24% 1	800
	11th to 20th	Night	23% 18% 37% 21% 1	783
	21st to end	Night	25% 16% 34% 22% 3	660
March	1st to 10th	Night	25% 18% 33% 22% 3	800
	11th to 20th	Night	23% 19% 34% 21% 4	800
	21st to end	Night	25% 16% 31% 22% 6%	880
April	1st to 10th	Night	26% 15% 32% 23% 4	800
	11th to 20th	Night	25% 17% 31% 21% 6%	791
	21st to end	Night	26% 17% 29% 23% 5%	640
May	1st to 10th	Night	29% 13% 29% 24% 5%	600
	11th to 20th	Night	19% 15% 33% 28% 5%	468
	21st to end	Night	21% 7% 28% 33% 11%	433
June	1st to 10th	Night	18% 6% 28% 38% 9%	400
	11th to 20th	Night	19% 7% 30% 35% 9% 1	400
	21st to end	Night	15% 7% 28% 37% 14%	400
July	1st to 10th	Night	14% 4 24% 45% 13%	390
	11th to 20th	Night	15% 5% 22% 41% 16% 1	388
	21st to end	Night	11% 4 22% 48% 14%	438
August	1st to 10th	Night	13% 3 20% 46% 17% 1	463
	11th to 20th	Night	13% 6% 20% 40% 19% 2	698
	21st to end	Night	15% 5% 20% 36% 22% 2	880
September	1st to 10th	Night	17% 5% 24% 35% 18% 1	800
	11th to 20th	Night	15% 6% 21% 33% 21% 4	800
	21st to end	Night	18% 6% 26% 34% 16% 2	800
October	1st to 10th	Night	23% 9% 20% 32% 14% 2	800
	11th to 20th	Night	20% 10% 27% 31% 11% 1	793
	21st to end	Night	27% 10% 29% 27% 6%	836
November	1st to 10th	Night	18% 13% 31% 29% 8% 1	780
	11th to 20th	Night	22% 14% 32% 25% 7% 1	800
	21st to end	Night	21% 13% 33% 27% 5%	800
December	1st to 10th	Night	25% 15% 34% 20% 6%	800
	11th to 20th	Night	25% 14% 33% 25% 3	800
	21st to end	Night	27% 14% 33% 23% 3	880

Page generated at 2025-10-16 01:50:28. Version ac07

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