Amherst
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Amherst clear sky chart from 2005-11-06 to 2025-09-04.

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	18% 11% 4 8% 5% 4 4 4 5% 6% 30%	126833
	Night	18% 12% 4 9% 5% 4 4 3 5% 6% 30%	122025
Transparency	Day	54% 6% 11% 20% 9% 1	130403
	Night	54% 6% 11% 19% 10% 1	126542
Seeing	Day	27% 22% 31% 15% 4 1	18205
	Night	26% 18% 31% 19% 5%	25571

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	13% 25% 5% 10% 5% 6% 4 3 4 4 22%	3157
	11th to 20th	Day	13% 16% 8% 12% 5% 4 4 2 4 4 26%	3626
	21st to end	Day	10% 11% 4 11% 5% 4 5% 4 5% 7% 34%	4466
February	1st to 10th	Day	17% 13% 5% 10% 5% 6% 3 4 4 6% 29%	1800
	11th to 20th	Day	16% 14% 4 10% 6% 4 4 4 5% 6% 27%	1753
	21st to end	Day	11% 14% 4 9% 7% 5% 4 4 5% 7% 31%	1490
March	1st to 10th	Day	19% 15% 5% 13% 5% 4 4 3 4 5% 25%	4920
	11th to 20th	Day	15% 16% 5% 10% 5% 4 5% 4 5% 6% 26%	4980
	21st to end	Day	26% 17% 4 8% 5% 4 4 3 5% 5% 20%	5466
April	1st to 10th	Day	21% 15% 4 8% 5% 4 4 4 5% 4 26%	2388
	11th to 20th	Day	19% 14% 3 7% 5% 4 4 4 5% 6% 29%	2482
	21st to end	Day	22% 11% 3 8% 4 5% 4 3 5% 7% 29%	2600
May	1st to 10th	Day	27% 15% 3 8% 4 4 4 3 5% 5% 22%	6085
	11th to 20th	Day	21% 11% 2 7% 5% 4 4 3 4 6% 31%	6318
	21st to end	Day	19% 11% 3 8% 5% 5% 5% 4 5% 8% 28%	6948
June	1st to 10th	Day	25% 10% 3 6% 4 5% 4 4 5% 7% 26%	2870
	11th to 20th	Day	20% 8% 3 6% 4 5% 5% 4 6% 7% 31%	3000
	21st to end	Day	27% 9% 3 7% 4 4 4 3 6% 6% 28%	3000
July	1st to 10th	Day	17% 10% 3 7% 5% 5% 5% 4 7% 9% 27%	6820
	11th to 20th	Day	14% 7% 3 8% 5% 4 4 3 5% 8% 39%	6529
	21st to end	Day	12% 6% 3 6% 5% 5% 5% 5% 7% 9% 38%	6799
August	1st to 10th	Day	17% 9% 3 9% 5% 5% 4 4 6% 7% 31%	4235
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 37%	4175
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 44%	4525
September	1st to 10th	Day	17% 5% 2 6% 3 4 4 3 5% 8% 41%	2316
	11th to 20th	Day	16% 9% 3 6% 4 4 5% 3 4 7% 39%	2199
	21st to end	Day	18% 7% 2 6% 5% 4 4 3 4 6% 42%	2035
October	1st to 10th	Day	17% 8% 2 7% 5% 4 4 4 6% 7% 37%	1900
	11th to 20th	Day	18% 9% 3 8% 6% 4 4 3 4 7% 35%	1900
	21st to end	Day	17% 11% 4 8% 4 5% 5% 3 4 6% 32%	2090
November	1st to 10th	Day	19% 12% 3 9% 6% 5% 4 4 4 5% 31%	1569
	11th to 20th	Day	19% 15% 4 7% 5% 4 4 4 4 6% 28%	1600
	21st to end	Day	19% 15% 4 10% 5% 4 3 4 4 5% 28%	1600
December	1st to 10th	Day	17% 14% 5% 9% 6% 3 3 2 4 7% 30%	2610
	11th to 20th	Day	18% 15% 4 9% 4 3 3 2 3 3 33%	2996
	21st to end	Day	16% 13% 6% 13% 6% 5% 5% 4 4 4 25%	3586
January	1st to 10th	Night	14% 19% 5% 10% 5% 4 4 3 4 5% 28%	6808
	11th to 20th	Night	15% 17% 7% 12% 6% 4 4 3 4 4 24%	6269
	21st to end	Night	11% 12% 5% 10% 6% 4 3 3 4 6% 36%	6490
February	1st to 10th	Night	17% 14% 4 10% 5% 5% 4 4 5% 5% 27%	2600
	11th to 20th	Night	17% 15% 4 9% 5% 5% 4 4 4 5% 28%	2533
	21st to end	Night	12% 15% 4 9% 6% 4 5% 3 5% 4 33%	2140
March	1st to 10th	Night	24% 14% 5% 12% 5% 5% 3 2 3 3 24%	4915
	11th to 20th	Night	20% 18% 5% 8% 5% 4 3 3 4 4 26%	4979
	21st to end	Night	28% 14% 4 8% 4 3 4 3 4 3 25%	5466
April	1st to 10th	Night	22% 15% 4 9% 5% 5% 4 3 4 5% 25%	2011
	11th to 20th	Night	22% 12% 3 6% 4 3 4 3 4 6% 32%	1864
	21st to end	Night	23% 13% 4 7% 5% 4 4 3 5% 6% 28%	1800
May	1st to 10th	Night	27% 14% 3 7% 5% 5% 4 4 5% 6% 20%	3862
	11th to 20th	Night	23% 9% 2 7% 4 4 4 3 4 6% 34%	3596
	21st to end	Night	18% 11% 3 8% 5% 5% 5% 4 6% 6% 29%	3948
June	1st to 10th	Night	25% 11% 3 7% 5% 4 4 4 5% 8% 26%	1544
	11th to 20th	Night	21% 8% 3 6% 5% 4 5% 3 4 8% 34%	1400
	21st to end	Night	22% 9% 3 9% 5% 4 5% 4 5% 8% 26%	1400
July	1st to 10th	Night	16% 10% 4 9% 5% 5% 6% 4 7% 8% 25%	3160
	11th to 20th	Night	12% 9% 3 8% 5% 5% 5% 4 6% 9% 34%	3390
	21st to end	Night	16% 7% 2 6% 4 5% 4 4 7% 8% 36%	4067
August	1st to 10th	Night	16% 9% 3 9% 6% 5% 5% 4 6% 7% 29%	2915
	11th to 20th	Night	11% 9% 3 9% 5% 5% 5% 4 7% 9% 34%	2872
	21st to end	Night	14% 6% 2 5% 3 4 4 4 6% 8% 45%	3371
September	1st to 10th	Night	16% 6% 2 6% 4 4 4 3 5% 8% 41%	1931
	11th to 20th	Night	15% 7% 3 6% 4 3 4 3 4 6% 45%	1979
	21st to end	Night	17% 8% 2 6% 4 4 3 3 6% 6% 41%	2143
October	1st to 10th	Night	19% 7% 2 7% 5% 4 3 3 5% 6% 39%	2280
	11th to 20th	Night	18% 9% 3 6% 5% 4 3 3 5% 7% 37%	2280
	21st to end	Night	24% 10% 3 7% 4 4 3 3 4 6% 32%	2508
November	1st to 10th	Night	21% 11% 3 8% 5% 4 4 4 4 6% 31%	2724
	11th to 20th	Night	20% 13% 4 7% 5% 5% 4 4 5% 5% 29%	2800
	21st to end	Night	23% 13% 4 8% 4 4 4 3 5% 5% 29%	2800
December	1st to 10th	Night	16% 11% 4 9% 4 4 3 4 5% 6% 34%	4580
	11th to 20th	Night	18% 16% 4 9% 5% 3 3 2 3 4 32%	5213
	21st to end	Night	20% 15% 5% 12% 4 3 3 3 4 4 27%	7387

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% 5% 11% 10% 6% 1	3158
	11th to 20th	Day	63% 6% 9% 14% 8% 1	3655
	21st to end	Day	50% 5% 10% 15% 16% 4	4466
February	1st to 10th	Day	59% 5% 10% 14% 11% 1	1800
	11th to 20th	Day	60% 5% 9% 15% 10% 1	1761
	21st to end	Day	53% 4 12% 18% 11% 2	1490
March	1st to 10th	Day	64% 4 8% 13% 10% 2	4924
	11th to 20th	Day	59% 6% 13% 16% 6% 1	4980
	21st to end	Day	67% 3 6% 16% 7% 1	5466
April	1st to 10th	Day	62% 5% 7% 14% 10% 2	2388
	11th to 20th	Day	56% 5% 8% 18% 11% 1	2482
	21st to end	Day	57% 5% 9% 17% 10% 2	2600
May	1st to 10th	Day	64% 6% 8% 15% 7%	6096
	11th to 20th	Day	53% 8% 11% 22% 5%	6322
	21st to end	Day	53% 9% 13% 18% 8%	6948
June	1st to 10th	Day	58% 6% 11% 20% 6% 1	2870
	11th to 20th	Day	51% 7% 11% 21% 9% 1	3000
	21st to end	Day	57% 6% 11% 17% 9% 1	3000
July	1st to 10th	Day	53% 6% 15% 17% 8%	6745
	11th to 20th	Day	43% 6% 15% 28% 8% 1	6449
	21st to end	Day	42% 8% 17% 27% 6%	6604
August	1st to 10th	Day	52% 7% 11% 24% 6%	4222
	11th to 20th	Day	45% 7% 12% 24% 12%	4214
	21st to end	Day	36% 6% 15% 27% 16%	4525
September	1st to 10th	Day	41% 7% 14% 25% 13%	2316
	11th to 20th	Day	45% 6% 12% 24% 12% 1	2199
	21st to end	Day	44% 5% 9% 27% 14% 1	2035
October	1st to 10th	Day	46% 6% 11% 28% 9%	3160
	11th to 20th	Day	48% 3 10% 31% 8%	3140
	21st to end	Day	46% 4 13% 21% 16%	3428
November	1st to 10th	Day	57% 6% 9% 19% 9%	1569
	11th to 20th	Day	58% 6% 12% 14% 9% 1	1600
	21st to end	Day	60% 6% 11% 15% 8%	1600
December	1st to 10th	Day	56% 5% 12% 23% 4	2610
	11th to 20th	Day	58% 4 9% 17% 10% 1	2995
	21st to end	Day	64% 5% 10% 13% 7% 1	3586
January	1st to 10th	Night	59% 5% 11% 21% 5%	6808
	11th to 20th	Night	62% 6% 10% 16% 6% 1	6315
	21st to end	Night	51% 5% 7% 21% 14% 2	6490
February	1st to 10th	Night	59% 5% 9% 12% 13% 2	2600
	11th to 20th	Night	59% 6% 9% 14% 11% 1	2536
	21st to end	Night	54% 5% 8% 18% 13% 2	2140
March	1st to 10th	Night	67% 3 5% 11% 12% 2	4924
	11th to 20th	Night	61% 5% 11% 13% 8% 1	4980
	21st to end	Night	65% 3 6% 15% 10% 1	5466
April	1st to 10th	Night	61% 5% 7% 12% 11% 3	2011
	11th to 20th	Night	54% 5% 9% 20% 12% 1	1864
	21st to end	Night	57% 5% 9% 15% 12% 2	1800
May	1st to 10th	Night	64% 7% 9% 12% 7% 1	3868
	11th to 20th	Night	51% 7% 12% 22% 6% 1	3597
	21st to end	Night	52% 8% 18% 16% 6%	3949
June	1st to 10th	Night	57% 7% 11% 18% 8%	1544
	11th to 20th	Night	50% 5% 12% 24% 9% 1	1400
	21st to end	Night	56% 7% 13% 14% 9% 1	1400
July	1st to 10th	Night	55% 6% 13% 18% 7%	3125
	11th to 20th	Night	45% 7% 18% 22% 8%	3347
	21st to end	Night	43% 8% 18% 21% 10%	3948
August	1st to 10th	Night	52% 8% 13% 21% 5%	2903
	11th to 20th	Night	44% 9% 17% 18% 12%	2899
	21st to end	Night	37% 6% 18% 26% 14%	3370
September	1st to 10th	Night	40% 8% 11% 29% 11%	1931
	11th to 20th	Night	41% 5% 13% 27% 14% 1	1979
	21st to end	Night	43% 6% 11% 27% 12% 1	2143
October	1st to 10th	Night	50% 5% 12% 26% 8%	3800
	11th to 20th	Night	39% 5% 11% 30% 15%	3779
	21st to end	Night	46% 4 13% 22% 15%	4120
November	1st to 10th	Night	56% 7% 11% 17% 9% 1	2724
	11th to 20th	Night	57% 7% 10% 14% 10% 1	2800
	21st to end	Night	58% 6% 12% 16% 7%	2800
December	1st to 10th	Night	52% 5% 10% 24% 9%	4580
	11th to 20th	Night	58% 4 8% 17% 12% 2	5215
	21st to end	Night	61% 5% 11% 14% 8% 1	7387

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	35% 28% 28% 9% 1	200
	11th to 20th	Day	31% 24% 27% 14% 4	382
	21st to end	Day	20% 25% 38% 13% 3	440
February	1st to 10th	Day	29% 25% 30% 12% 4	400
	11th to 20th	Day	32% 29% 27% 10% 2 1	396
	21st to end	Day	24% 25% 34% 15% 3	331
March	1st to 10th	Day	31% 28% 28% 11% 2 1	496
	11th to 20th	Day	27% 27% 31% 12% 3 1	500
	21st to end	Day	33% 29% 24% 9% 3 1	550
April	1st to 10th	Day	40% 29% 21% 8% 2	500
	11th to 20th	Day	34% 28% 24% 11% 2	492
	21st to end	Day	32% 26% 27% 10% 4	500
May	1st to 10th	Day	35% 27% 23% 12% 3 1	585
	11th to 20th	Day	30% 20% 28% 16% 4 2	724
	21st to end	Day	28% 21% 33% 15% 3	790
June	1st to 10th	Day	34% 19% 29% 14% 4	730
	11th to 20th	Day	24% 21% 34% 16% 4	730
	21st to end	Day	30% 15% 31% 19% 5% 1	730
July	1st to 10th	Day	23% 18% 37% 17% 5% 1	720
	11th to 20th	Day	19% 12% 41% 22% 6% 1	718
	21st to end	Day	21% 15% 39% 19% 5%	742
August	1st to 10th	Day	15% 15% 41% 22% 7%	526
	11th to 20th	Day	19% 15% 33% 23% 8% 2	519
	21st to end	Day	17% 18% 34% 22% 7% 2	583
September	1st to 10th	Day	18% 16% 39% 20% 7% 1	502
	11th to 20th	Day	23% 18% 32% 19% 7% 1	490
	21st to end	Day	21% 21% 30% 19% 8% 2	460
October	1st to 10th	Day	24% 15% 33% 19% 8% 2	390
	11th to 20th	Day	24% 25% 28% 16% 6% 1	390
	21st to end	Day	26% 26% 29% 14% 5% 1	429
November	1st to 10th	Day	30% 24% 29% 10% 6% 1	395
	11th to 20th	Day	34% 26% 23% 13% 4 2	400
	21st to end	Day	35% 25% 26% 11% 3	400
December	1st to 10th	Day	34% 24% 28% 12% 2 1	400
	11th to 20th	Day	37% 26% 23% 12% 2	400
	21st to end	Day	29% 26% 31% 11% 3 1	265
January	1st to 10th	Night	27% 24% 29% 16% 4	900
	11th to 20th	Night	30% 22% 28% 16% 4	883
	21st to end	Night	21% 24% 33% 18% 4	880
February	1st to 10th	Night	28% 21% 31% 17% 3	800
	11th to 20th	Night	30% 26% 29% 13% 2 1	783
	21st to end	Night	23% 20% 35% 18% 3 1	660
March	1st to 10th	Night	27% 24% 31% 16% 3	800
	11th to 20th	Night	28% 19% 30% 18% 5% 1	800
	21st to end	Night	29% 20% 30% 16% 5%	880
April	1st to 10th	Night	32% 22% 29% 15% 2	611
	11th to 20th	Night	30% 19% 30% 15% 5%	594
	21st to end	Night	32% 17% 30% 17% 4 1	600
May	1st to 10th	Night	32% 17% 26% 21% 4 1	600
	11th to 20th	Night	27% 18% 27% 23% 5%	592
	21st to end	Night	24% 17% 41% 16% 2	649
June	1st to 10th	Night	32% 13% 32% 20% 4	544
	11th to 20th	Night	25% 13% 36% 23% 4	400
	21st to end	Night	25% 10% 35% 25% 5% 1	400
July	1st to 10th	Night	21% 7% 47% 19% 6%	390
	11th to 20th	Night	17% 11% 43% 25% 3	487
	21st to end	Night	19% 10% 38% 27% 5%	656
August	1st to 10th	Night	17% 10% 41% 25% 7%	598
	11th to 20th	Night	16% 10% 39% 28% 7% 1	592
	21st to end	Night	17% 11% 33% 30% 8%	660
September	1st to 10th	Night	19% 10% 41% 25% 6%	580
	11th to 20th	Night	19% 12% 30% 29% 9%	649
	21st to end	Night	22% 14% 30% 25% 9%	760
October	1st to 10th	Night	23% 12% 31% 25% 8% 1	760
	11th to 20th	Night	23% 16% 34% 20% 6% 1	760
	21st to end	Night	30% 24% 26% 17% 4	836
November	1st to 10th	Night	30% 21% 28% 17% 5% 1	877
	11th to 20th	Night	31% 23% 24% 16% 5% 1	900
	21st to end	Night	33% 21% 27% 15% 3	900
December	1st to 10th	Night	32% 20% 28% 17% 3	900
	11th to 20th	Night	32% 26% 27% 12% 3	900
	21st to end	Night	32% 22% 25% 17% 3 1	990

Page generated at 2025-09-01 02:51:04. Version ac07

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