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	17% 7% 3 7% 4 4 4 4 6% 7% 37%	125940
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	122788
Transparency	Day	45% 6% 11% 22% 14% 2	129460
	Night	45% 6% 12% 21% 14% 2	127355
Seeing	Day	24% 15% 29% 20% 9% 3	16006
	Night	22% 12% 30% 27% 8% 1	24935

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% 4 10% 5% 5% 5% 4 6% 7% 35%	3597
	11th to 20th	Day	13% 10% 3 8% 4 4 4 4 5% 6% 40%	3570
	21st to end	Day	16% 9% 3 7% 4 4 4 3 5% 6% 39%	3960
February	1st to 10th	Day	16% 8% 3 7% 5% 5% 4 4 7% 7% 32%	1857
	11th to 20th	Day	14% 8% 4 9% 5% 4 4 3 5% 7% 37%	1948
	21st to end	Day	19% 10% 3 7% 4 4 4 4 4 5% 35%	1650
March	1st to 10th	Day	21% 11% 4 8% 4 3 4 3 4 5% 34%	4476
	11th to 20th	Day	16% 8% 2 7% 4 4 5% 4 5% 7% 38%	4648
	21st to end	Day	24% 8% 3 5% 4 3 4 3 5% 6% 35%	5965
April	1st to 10th	Day	20% 9% 3 7% 5% 4 4 3 5% 7% 36%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 3 6% 7% 38%	2374
	21st to end	Day	19% 8% 3 6% 4 4 5% 4 5% 6% 37%	2550
May	1st to 10th	Day	31% 8% 3 7% 4 4 4 4 5% 7% 23%	5886
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 5% 7% 32%	6175
	21st to end	Day	18% 7% 3 7% 5% 4 5% 4 6% 8% 34%	6949
June	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 4 7% 9% 34%	2800
	11th to 20th	Day	18% 6% 2 5% 5% 5% 5% 4 6% 9% 36%	2800
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 8% 36%	2800
July	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 7% 9% 34%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 11% 35%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 8% 42%	6926
August	1st to 10th	Day	15% 7% 3 7% 5% 6% 6% 4 7% 8% 33%	4410
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	3962
	21st to end	Day	9% 4 2 6% 4 5% 6% 5% 7% 10% 43%	4312
September	1st to 10th	Day	13% 5% 3 6% 5% 4 4 4 6% 6% 45%	2316
	11th to 20th	Day	15% 5% 2 5% 4 3 3 3 3 7% 50%	2138
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 41%	2090
October	1st to 10th	Day	22% 6% 2 6% 4 4 4 3 4 6% 39%	2072
	11th to 20th	Day	14% 6% 2 5% 4 3 4 3 6% 8% 44%	1853
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 5% 37%	1881
November	1st to 10th	Day	12% 7% 3 4 4 4 3 3 4 6% 48%	1761
	11th to 20th	Day	18% 7% 2 4 3 3 3 4 5% 6% 44%	1800
	21st to end	Day	19% 7% 2 5% 4 4 4 4 5% 6% 40%	1748
December	1st to 10th	Day	17% 8% 3 6% 4 4 4 4 5% 6% 40%	2600
	11th to 20th	Day	22% 7% 3 6% 2 3 3 3 3 6% 42%	2986
	21st to end	Day	20% 11% 3 7% 4 3 3 3 4 6% 36%	3960
January	1st to 10th	Night	15% 7% 3 8% 4 4 4 3 5% 6% 40%	6349
	11th to 20th	Night	13% 9% 4 10% 5% 5% 4 4 5% 6% 35%	6315
	21st to end	Night	15% 10% 3 6% 4 4 5% 4 6% 6% 37%	6996
February	1st to 10th	Night	15% 10% 3 6% 5% 4 5% 4 5% 5% 37%	2536
	11th to 20th	Night	13% 8% 3 8% 6% 5% 5% 3 5% 6% 38%	2338
	21st to end	Night	17% 9% 3 8% 5% 3 4 3 4 5% 37%	1980
March	1st to 10th	Night	22% 8% 3 6% 4 4 4 3 5% 6% 35%	5364
	11th to 20th	Night	17% 8% 3 6% 5% 4 5% 4 4 7% 38%	5310
	21st to end	Night	24% 9% 3 6% 3 4 4 3 5% 5% 35%	4965
April	1st to 10th	Night	19% 8% 3 7% 5% 4 4 3 5% 6% 35%	2000
	11th to 20th	Night	21% 7% 2 6% 4 4 4 3 4 6% 38%	1975
	21st to end	Night	22% 7% 2 6% 3 3 3 3 4 6% 40%	1848
May	1st to 10th	Night	33% 9% 3 6% 3 3 3 4 5% 6% 25%	4054
	11th to 20th	Night	18% 9% 4 9% 6% 5% 5% 4 5% 6% 29%	3761
	21st to end	Night	21% 9% 3 8% 4 4 4 3 5% 7% 32%	3926
June	1st to 10th	Night	18% 8% 3 6% 5% 4 3 3 4 6% 40%	1600
	11th to 20th	Night	19% 7% 2 5% 4 4 4 4 6% 7% 39%	1600
	21st to end	Night	15% 5% 2 7% 4 4 4 4 7% 8% 41%	1600
July	1st to 10th	Night	13% 6% 2 6% 4 4 4 4 6% 9% 39%	3600
	11th to 20th	Night	14% 7% 2 7% 5% 5% 5% 4 7% 8% 36%	3600
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 6% 48%	3926
August	1st to 10th	Night	22% 6% 3 7% 5% 4 4 4 5% 7% 34%	2708
	11th to 20th	Night	13% 7% 2 8% 5% 5% 5% 4 6% 7% 38%	3069
	21st to end	Night	10% 4 1 4 3 4 3 3 5% 8% 55%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 5% 6% 47%	1931
	11th to 20th	Night	13% 5% 2 5% 3 4 4 3 4 6% 51%	2042
	21st to end	Night	17% 7% 2 6% 4 4 4 3 4 7% 43%	2090
October	1st to 10th	Night	21% 6% 2 6% 4 4 4 3 4 5% 41%	2104
	11th to 20th	Night	16% 6% 2 5% 3 4 4 3 6% 6% 46%	2327
	21st to end	Night	22% 8% 2 6% 4 3 3 3 4 5% 39%	2717
November	1st to 10th	Night	14% 6% 2 6% 4 3 4 3 5% 6% 49%	2537
	11th to 20th	Night	19% 6% 2 5% 4 3 4 3 4 5% 45%	2600
	21st to end	Night	16% 8% 2 4 5% 3 4 4 5% 6% 42%	2650
December	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 5% 6% 38%	4580
	11th to 20th	Night	26% 8% 2 5% 3 2 3 3 5% 5% 39%	5208
	21st to end	Night	20% 11% 3 7% 4 3 4 3 4 5% 37%	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	46% 7% 10% 26% 10% 1	3598
	11th to 20th	Day	44% 5% 11% 20% 16% 4	3600
	21st to end	Day	46% 5% 10% 21% 18% 1	3960
February	1st to 10th	Day	49% 5% 13% 19% 13% 1	1857
	11th to 20th	Day	48% 6% 10% 21% 14% 3	1957
	21st to end	Day	51% 4 9% 20% 14% 2	1650
March	1st to 10th	Day	53% 4 11% 13% 11% 7%	4479
	11th to 20th	Day	45% 7% 13% 23% 9% 2	4648
	21st to end	Day	51% 4 5% 16% 17% 6%	5965
April	1st to 10th	Day	50% 4 9% 21% 14% 4	2400
	11th to 20th	Day	45% 5% 11% 18% 18% 3	2374
	21st to end	Day	48% 5% 7% 20% 16% 3	2550
May	1st to 10th	Day	60% 7% 11% 13% 8% 1	5897
	11th to 20th	Day	52% 7% 13% 18% 10% 1	6179
	21st to end	Day	46% 8% 10% 22% 12% 1	6949
June	1st to 10th	Day	46% 7% 12% 23% 12% 1	2800
	11th to 20th	Day	44% 7% 13% 21% 14% 1	2800
	21st to end	Day	46% 7% 12% 21% 14% 1	2800
July	1st to 10th	Day	46% 8% 11% 21% 13% 1	6290
	11th to 20th	Day	40% 10% 17% 22% 11% 1	6285
	21st to end	Day	40% 6% 12% 26% 15% 1	6728
August	1st to 10th	Day	46% 8% 15% 21% 9% 1	4396
	11th to 20th	Day	41% 7% 15% 27% 8% 1	4001
	21st to end	Day	32% 8% 15% 28% 15% 1	4312
September	1st to 10th	Day	39% 6% 11% 26% 17% 1	2316
	11th to 20th	Day	37% 4 9% 26% 23% 3	2138
	21st to end	Day	41% 6% 10% 22% 18% 3	2090
October	1st to 10th	Day	41% 6% 9% 22% 19% 2	3449
	11th to 20th	Day	33% 5% 9% 27% 23% 3	3056
	21st to end	Day	35% 4 13% 29% 16% 3	3081
November	1st to 10th	Day	39% 5% 13% 25% 16% 2	1761
	11th to 20th	Day	41% 5% 11% 21% 20% 3	1800
	21st to end	Day	46% 4 13% 25% 11% 2	1748
December	1st to 10th	Day	45% 6% 13% 27% 8% 1	2600
	11th to 20th	Day	45% 4 13% 18% 18% 1	2986
	21st to end	Day	51% 4 8% 19% 14% 5%	3960
January	1st to 10th	Night	43% 6% 11% 26% 12% 2	6349
	11th to 20th	Night	48% 5% 14% 18% 12% 3	6360
	21st to end	Night	47% 5% 11% 16% 17% 4	6996
February	1st to 10th	Night	47% 7% 13% 21% 11% 2	2536
	11th to 20th	Night	46% 5% 11% 20% 14% 3	2340
	21st to end	Night	49% 5% 9% 21% 12% 4	1980
March	1st to 10th	Night	48% 6% 12% 16% 15% 4	5374
	11th to 20th	Night	45% 7% 16% 19% 10% 3	5311
	21st to end	Night	51% 5% 6% 13% 21% 4	4965
April	1st to 10th	Night	51% 5% 8% 19% 15% 2	2000
	11th to 20th	Night	48% 5% 9% 19% 18% 1	1975
	21st to end	Night	47% 5% 8% 20% 15% 5%	1848
May	1st to 10th	Night	59% 7% 13% 11% 9% 1	4060
	11th to 20th	Night	54% 9% 10% 16% 9% 2	3762
	21st to end	Night	51% 8% 12% 18% 11% 1	3927
June	1st to 10th	Night	46% 6% 11% 24% 12% 1	1600
	11th to 20th	Night	45% 7% 12% 24% 11% 1	1600
	21st to end	Night	41% 9% 12% 24% 14% 1	1600
July	1st to 10th	Night	41% 7% 14% 22% 15%	3560
	11th to 20th	Night	44% 8% 16% 19% 13% 1	3552
	21st to end	Night	36% 7% 12% 28% 17% 1	3809
August	1st to 10th	Night	50% 7% 12% 20% 9% 1	2698
	11th to 20th	Night	43% 8% 14% 25% 10%	3096
	21st to end	Night	25% 7% 16% 34% 17% 1	3586
September	1st to 10th	Night	38% 7% 10% 26% 18% 1	1931
	11th to 20th	Night	34% 4 10% 25% 24% 2	2042
	21st to end	Night	41% 6% 8% 24% 18% 3	2090
October	1st to 10th	Night	39% 6% 14% 24% 15% 2	3507
	11th to 20th	Night	29% 5% 11% 26% 25% 4	3863
	21st to end	Night	35% 4 13% 32% 15% 1	4467
November	1st to 10th	Night	37% 5% 11% 24% 19% 3	2537
	11th to 20th	Night	43% 5% 11% 20% 19% 3	2600
	21st to end	Night	43% 6% 14% 24% 11% 1	2650
December	1st to 10th	Night	46% 6% 12% 27% 8%	4580
	11th to 20th	Night	49% 4 13% 14% 18% 2	5208
	21st to end	Night	50% 4 9% 19% 14% 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	26% 28% 26% 17% 4	200
	11th to 20th	Day	32% 23% 32% 9% 4 1	200
	21st to end	Day	25% 23% 33% 15% 4	220
February	1st to 10th	Day	32% 28% 30% 10% 1	359
	11th to 20th	Day	32% 24% 31% 12% 1	496
	21st to end	Day	32% 19% 31% 16% 2	414
March	1st to 10th	Day	31% 21% 32% 13% 3	500
	11th to 20th	Day	31% 22% 30% 13% 4	500
	21st to end	Day	31% 23% 29% 10% 4 2	550
April	1st to 10th	Day	34% 20% 26% 12% 6% 1	500
	11th to 20th	Day	31% 22% 27% 13% 5% 1	492
	21st to end	Day	29% 18% 30% 16% 5% 2	500
May	1st to 10th	Day	36% 18% 24% 15% 6% 1	503
	11th to 20th	Day	23% 18% 36% 16% 6% 1	527
	21st to end	Day	21% 11% 32% 23% 11% 2	573
June	1st to 10th	Day	18% 14% 33% 22% 10% 2	530
	11th to 20th	Day	19% 15% 31% 23% 10% 2	530
	21st to end	Day	18% 14% 32% 22% 10% 4	530
July	1st to 10th	Day	19% 7% 30% 26% 14% 3	525
	11th to 20th	Day	14% 8% 28% 31% 16% 4	524
	21st to end	Day	16% 7% 33% 27% 14% 3	576
August	1st to 10th	Day	12% 7% 26% 31% 19% 4	526
	11th to 20th	Day	15% 7% 29% 26% 17% 6%	519
	21st to end	Day	12% 11% 29% 27% 15% 6%	583
September	1st to 10th	Day	20% 6% 25% 25% 16% 7%	502
	11th to 20th	Day	19% 9% 25% 19% 17% 11%	490
	21st to end	Day	20% 10% 30% 21% 12% 7%	490
October	1st to 10th	Day	28% 11% 23% 20% 13% 6%	490
	11th to 20th	Day	20% 16% 27% 23% 11% 3	465
	21st to end	Day	29% 14% 25% 22% 9% 1	429
November	1st to 10th	Day	23% 13% 28% 24% 9% 3	395
	11th to 20th	Day	30% 17% 30% 17% 6% 1	400
	21st to end	Day	31% 15% 30% 19% 5% 1	348
December	1st to 10th	Day	26% 25% 29% 16% 5%	200
	11th to 20th	Day	38% 18% 30% 12% 3	200
	21st to end	Day	28% 18% 34% 16% 5%	220
January	1st to 10th	Night	22% 16% 37% 22% 2	800
	11th to 20th	Night	28% 15% 33% 22% 3	800
	21st to end	Night	21% 15% 38% 23% 3	880
February	1st to 10th	Night	23% 17% 36% 23% 2	800
	11th to 20th	Night	24% 19% 38% 18% 1	783
	21st to end	Night	26% 17% 33% 22% 3	660
March	1st to 10th	Night	26% 18% 35% 20% 2	800
	11th to 20th	Night	24% 20% 34% 20% 3	800
	21st to end	Night	24% 17% 34% 19% 5%	880
April	1st to 10th	Night	26% 16% 33% 21% 3	800
	11th to 20th	Night	25% 17% 30% 22% 5%	791
	21st to end	Night	28% 15% 31% 22% 3	648
May	1st to 10th	Night	31% 12% 30% 22% 5%	600
	11th to 20th	Night	21% 12% 37% 25% 5%	476
	21st to end	Night	21% 9% 27% 33% 10% 1	433
June	1st to 10th	Night	22% 7% 26% 36% 10%	400
	11th to 20th	Night	21% 7% 29% 34% 9% 1	400
	21st to end	Night	16% 6% 28% 38% 11%	400
July	1st to 10th	Night	15% 4 27% 40% 14%	390
	11th to 20th	Night	16% 3 21% 41% 17% 1	388
	21st to end	Night	11% 5% 24% 47% 13%	438
August	1st to 10th	Night	13% 3 23% 46% 15% 1	468
	11th to 20th	Night	15% 4 23% 39% 18% 2	711
	21st to end	Night	15% 5% 23% 36% 20% 1	880
September	1st to 10th	Night	18% 4 26% 35% 15% 2	773
	11th to 20th	Night	16% 7% 20% 33% 20% 4	760
	21st to end	Night	19% 6% 26% 33% 13% 2	760
October	1st to 10th	Night	25% 9% 20% 31% 12% 2	760
	11th to 20th	Night	19% 11% 30% 29% 11% 1	760
	21st to end	Night	28% 11% 29% 25% 6%	836
November	1st to 10th	Night	19% 13% 31% 28% 8% 1	780
	11th to 20th	Night	23% 13% 32% 24% 7% 1	800
	21st to end	Night	23% 13% 32% 26% 5% 1	800
December	1st to 10th	Night	27% 13% 35% 19% 5% 1	800
	11th to 20th	Night	28% 14% 30% 25% 4	800
	21st to end	Night	26% 15% 34% 22% 3	880

Page generated at 2025-09-01 01:51:07. Version ac07

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