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 2024-11-04.

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% 31%	101854
	Night	19% 12% 4 8% 5% 4 4 3 5% 6% 30%	97219
Transparency	Day	53% 6% 11% 19% 9% 1	105442
	Night	54% 6% 11% 19% 9% 1	101720
Seeing	Day	27% 22% 31% 15% 4 1	17042
	Night	26% 18% 31% 19% 5%	24475

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	14% 15% 4 9% 5% 6% 4 3 4 5% 31%	2207
	11th to 20th	Day	15% 14% 5% 10% 5% 4 4 2 4 4 32%	2539
	21st to end	Day	13% 11% 5% 13% 5% 4 5% 3 4 7% 28%	3124
February	1st to 10th	Day	18% 13% 5% 10% 5% 5% 3 4 4 6% 29%	1710
	11th to 20th	Day	16% 14% 4 10% 6% 4 4 4 5% 6% 27%	1663
	21st to end	Day	11% 14% 4 8% 7% 5% 4 4 5% 7% 31%	1418
March	1st to 10th	Day	22% 16% 4 7% 4 4 4 3 5% 5% 26%	3420
	11th to 20th	Day	13% 22% 6% 10% 5% 4 4 3 5% 4 23%	3480
	21st to end	Day	29% 15% 4 8% 5% 4 4 3 4 4 20%	3816
April	1st to 10th	Day	21% 15% 4 8% 5% 4 4 3 5% 4 26%	2268
	11th to 20th	Day	19% 14% 3 7% 4 4 4 3 5% 6% 30%	2356
	21st to end	Day	22% 11% 3 8% 4 5% 4 3 5% 7% 29%	2470
May	1st to 10th	Day	24% 17% 3 7% 4 4 4 3 4 5% 24%	4249
	11th to 20th	Day	17% 9% 2 7% 5% 5% 5% 3 5% 7% 35%	4398
	21st to end	Day	17% 10% 3 8% 5% 5% 5% 5% 6% 8% 27%	4836
June	1st to 10th	Day	26% 10% 3 6% 4 5% 4 4 5% 7% 26%	2726
	11th to 20th	Day	21% 8% 3 6% 4 5% 5% 4 5% 6% 32%	2850
	21st to end	Day	27% 9% 3 7% 4 4 4 3 6% 6% 28%	2850
July	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 7% 8% 31%	4760
	11th to 20th	Day	16% 7% 3 9% 5% 4 4 4 6% 8% 34%	4553
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 5% 7% 8% 35%	4729
August	1st to 10th	Day	17% 9% 3 9% 6% 5% 4 4 6% 7% 29%	4105
	11th to 20th	Day	14% 6% 3 8% 5% 5% 6% 4 6% 8% 36%	4045
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 44%	4386
September	1st to 10th	Day	17% 5% 2 6% 3 4 4 3 5% 8% 41%	2280
	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% 5% 5% 4 3 4 5% 30%	1513
	11th to 20th	Day	18% 14% 4 7% 6% 4 4 4 4 6% 30%	1520
	21st to end	Day	18% 14% 4 10% 5% 4 3 4 4 5% 29%	1520
December	1st to 10th	Day	23% 13% 5% 7% 5% 4 3 2 4 7% 27%	1520
	11th to 20th	Day	18% 19% 5% 11% 5% 4 4 3 4 4 25%	1906
	21st to end	Day	19% 13% 7% 14% 5% 4 4 3 3 4 24%	2513
January	1st to 10th	Night	14% 12% 4 8% 5% 4 4 3 5% 5% 35%	4748
	11th to 20th	Night	17% 14% 4 10% 5% 4 4 3 4 5% 29%	4423
	21st to end	Night	15% 12% 4 12% 7% 4 3 3 4 6% 29%	4532
February	1st to 10th	Night	17% 14% 4 10% 5% 5% 4 4 5% 5% 26%	2470
	11th to 20th	Night	17% 16% 4 9% 5% 5% 4 4 4 5% 27%	2403
	21st to end	Night	12% 15% 4 9% 6% 4 5% 3 5% 4 32%	2036
March	1st to 10th	Night	24% 13% 4 11% 5% 5% 4 3 4 3 23%	3415
	11th to 20th	Night	20% 21% 6% 8% 6% 5% 3 3 5% 4 20%	3479
	21st to end	Night	27% 14% 4 9% 4 4 4 3 3 3 25%	3816
April	1st to 10th	Night	22% 15% 4 9% 5% 5% 4 3 4 5% 25%	1911
	11th to 20th	Night	22% 12% 3 6% 4 4 4 3 4 6% 32%	1770
	21st to end	Night	22% 13% 4 7% 4 4 4 3 5% 6% 29%	1710
May	1st to 10th	Night	24% 15% 3 6% 5% 5% 4 4 5% 7% 22%	2694
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 7% 38%	2506
	21st to end	Night	17% 10% 4 8% 6% 5% 5% 4 6% 7% 28%	2749
June	1st to 10th	Night	25% 12% 3 7% 5% 4 4 3 4 8% 25%	1467
	11th to 20th	Night	21% 8% 3 6% 5% 4 5% 3 4 8% 34%	1330
	21st to end	Night	22% 9% 3 9% 5% 4 5% 4 5% 8% 25%	1330
July	1st to 10th	Night	16% 9% 4 9% 5% 5% 6% 4 7% 7% 27%	2210
	11th to 20th	Night	14% 10% 4 7% 6% 5% 6% 4 6% 9% 29%	2370
	21st to end	Night	14% 7% 2 7% 4 5% 5% 4 7% 8% 38%	2830
August	1st to 10th	Night	16% 9% 3 9% 6% 5% 5% 5% 6% 7% 28%	2825
	11th to 20th	Night	11% 9% 3 9% 5% 5% 5% 4 7% 9% 34%	2782
	21st to end	Night	14% 6% 2 5% 3 4 4 4 6% 8% 45%	3268
September	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 5% 7% 41%	1900
	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% 10% 3 8% 5% 4 4 4 4 6% 31%	2633
	11th to 20th	Night	19% 11% 4 7% 5% 5% 4 4 5% 5% 30%	2660
	21st to end	Night	21% 13% 4 8% 4 4 4 3 5% 5% 30%	2660
December	1st to 10th	Night	24% 13% 5% 8% 4 4 4 3 5% 6% 25%	2660
	11th to 20th	Night	22% 17% 5% 9% 5% 3 3 2 3 4 26%	3293
	21st to end	Night	24% 15% 6% 11% 3 3 3 3 3 4 26%	5149

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	56% 7% 14% 14% 9% 1	2208
	11th to 20th	Day	57% 7% 7% 17% 11% 1	2539
	21st to end	Day	56% 6% 12% 12% 14%	3124
February	1st to 10th	Day	59% 5% 10% 14% 11% 1	1710
	11th to 20th	Day	60% 6% 9% 15% 10% 1	1671
	21st to end	Day	53% 4 11% 18% 11% 2	1418
March	1st to 10th	Day	61% 4 9% 17% 6% 2	3424
	11th to 20th	Day	64% 6% 9% 14% 7% 1	3480
	21st to end	Day	69% 3 7% 13% 7% 2	3816
April	1st to 10th	Day	62% 5% 7% 15% 10% 2	2268
	11th to 20th	Day	55% 5% 8% 19% 11% 1	2356
	21st to end	Day	57% 5% 9% 17% 11% 2	2470
May	1st to 10th	Day	62% 7% 8% 17% 6% 1	4263
	11th to 20th	Day	47% 9% 13% 23% 7%	4402
	21st to end	Day	52% 11% 15% 16% 7%	4836
June	1st to 10th	Day	59% 6% 11% 19% 5%	2726
	11th to 20th	Day	52% 7% 11% 21% 9% 1	2850
	21st to end	Day	57% 6% 11% 16% 9% 1	2850
July	1st to 10th	Day	50% 7% 15% 18% 10% 1	4685
	11th to 20th	Day	48% 7% 14% 24% 7%	4473
	21st to end	Day	45% 9% 16% 24% 5%	4578
August	1st to 10th	Day	53% 8% 11% 24% 5%	4092
	11th to 20th	Day	45% 8% 12% 24% 11%	4084
	21st to end	Day	36% 6% 15% 26% 16%	4386
September	1st to 10th	Day	41% 7% 14% 25% 13%	2280
	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% 18% 9%	1513
	11th to 20th	Day	56% 7% 12% 15% 10% 1	1520
	21st to end	Day	58% 6% 12% 16% 8%	1520
December	1st to 10th	Day	60% 8% 10% 16% 6%	1520
	11th to 20th	Day	64% 5% 8% 15% 6% 1	1905
	21st to end	Day	66% 7% 11% 11% 6% 1	2513
January	1st to 10th	Night	50% 6% 13% 24% 7%	4748
	11th to 20th	Night	56% 7% 9% 18% 8% 1	4423
	21st to end	Night	56% 7% 9% 20% 8% 1	4532
February	1st to 10th	Night	59% 5% 9% 12% 12% 2	2470
	11th to 20th	Night	60% 6% 9% 13% 11% 1	2406
	21st to end	Night	54% 5% 8% 18% 13% 2	2036
March	1st to 10th	Night	66% 4 6% 15% 8% 2	3424
	11th to 20th	Night	67% 6% 7% 12% 7% 2	3480
	21st to end	Night	65% 4 6% 14% 9% 1	3816
April	1st to 10th	Night	61% 5% 8% 12% 10% 3	1911
	11th to 20th	Night	54% 5% 9% 21% 12% 1	1770
	21st to end	Night	57% 6% 9% 15% 12% 2	1710
May	1st to 10th	Night	60% 9% 12% 13% 6% 1	2700
	11th to 20th	Night	44% 9% 16% 24% 7% 1	2507
	21st to end	Night	52% 10% 18% 15% 5%	2750
June	1st to 10th	Night	58% 7% 11% 16% 8%	1467
	11th to 20th	Night	50% 5% 12% 23% 9% 1	1330
	21st to end	Night	56% 8% 13% 14% 8% 1	1330
July	1st to 10th	Night	53% 7% 13% 17% 9% 1	2175
	11th to 20th	Night	50% 8% 16% 18% 7%	2327
	21st to end	Night	42% 9% 16% 22% 11%	2741
August	1st to 10th	Night	53% 9% 14% 20% 5%	2813
	11th to 20th	Night	45% 9% 16% 18% 12%	2809
	21st to end	Night	37% 6% 18% 25% 14%	3267
September	1st to 10th	Night	41% 8% 11% 29% 11%	1900
	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	2633
	11th to 20th	Night	56% 8% 11% 15% 10% 1	2660
	21st to end	Night	57% 6% 13% 16% 8%	2660
December	1st to 10th	Night	61% 7% 9% 16% 7%	2660
	11th to 20th	Night	64% 5% 7% 15% 9%	3295
	21st to end	Night	64% 6% 12% 12% 5% 1	5149

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	32% 28% 29% 9% 1	180
	11th to 20th	Day	32% 26% 27% 12% 4	352
	21st to end	Day	21% 25% 37% 14% 3	407
February	1st to 10th	Day	29% 26% 29% 12% 4	370
	11th to 20th	Day	31% 30% 26% 10% 2 1	366
	21st to end	Day	23% 26% 35% 13% 3	307
March	1st to 10th	Day	30% 29% 28% 11% 2 1	456
	11th to 20th	Day	27% 27% 31% 12% 3 1	460
	21st to end	Day	33% 29% 25% 9% 3 1	506
April	1st to 10th	Day	38% 30% 21% 8% 2	460
	11th to 20th	Day	32% 29% 25% 12% 3	452
	21st to end	Day	31% 25% 28% 11% 5%	460
May	1st to 10th	Day	34% 27% 23% 13% 3 1	541
	11th to 20th	Day	28% 20% 28% 17% 5% 2	674
	21st to end	Day	28% 21% 34% 14% 2	735
June	1st to 10th	Day	34% 19% 29% 14% 4	680
	11th to 20th	Day	25% 20% 35% 17% 4	680
	21st to end	Day	29% 14% 31% 19% 5% 1	680
July	1st to 10th	Day	23% 18% 37% 17% 5% 1	670
	11th to 20th	Day	18% 12% 41% 22% 6% 1	668
	21st to end	Day	22% 14% 38% 19% 5%	690
August	1st to 10th	Day	16% 15% 40% 22% 7%	486
	11th to 20th	Day	20% 16% 33% 23% 7% 1	479
	21st to end	Day	17% 17% 34% 23% 6% 2	539
September	1st to 10th	Day	19% 17% 39% 19% 6% 1	490
	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	374
	11th to 20th	Day	30% 26% 25% 13% 4 2	370
	21st to end	Day	33% 26% 26% 11% 3	370
December	1st to 10th	Day	34% 25% 27% 11% 2 1	370
	11th to 20th	Day	37% 28% 23% 11% 1	370
	21st to end	Day	29% 27% 30% 11% 2 1	241
January	1st to 10th	Night	27% 24% 29% 16% 4	850
	11th to 20th	Night	30% 23% 29% 15% 3	835
	21st to end	Night	21% 24% 32% 18% 5%	836
February	1st to 10th	Night	28% 21% 31% 17% 3	760
	11th to 20th	Night	29% 27% 29% 13% 2 1	743
	21st to end	Night	22% 21% 35% 18% 4 1	628
March	1st to 10th	Night	26% 24% 32% 16% 3	760
	11th to 20th	Night	28% 20% 29% 18% 5% 1	760
	21st to end	Night	28% 20% 31% 16% 5%	836
April	1st to 10th	Night	31% 21% 29% 16% 3	581
	11th to 20th	Night	29% 19% 31% 15% 5%	564
	21st to end	Night	31% 17% 30% 16% 4 1	570
May	1st to 10th	Night	31% 17% 27% 21% 4 1	570
	11th to 20th	Night	26% 17% 27% 24% 5%	562
	21st to end	Night	23% 17% 41% 16% 2	616
June	1st to 10th	Night	32% 13% 32% 20% 4	517
	11th to 20th	Night	25% 12% 36% 24% 4	380
	21st to end	Night	25% 10% 35% 25% 5% 1	380
July	1st to 10th	Night	20% 7% 47% 19% 6%	370
	11th to 20th	Night	18% 11% 42% 25% 4	462
	21st to end	Night	19% 9% 39% 28% 5%	623
August	1st to 10th	Night	17% 10% 41% 24% 7%	568
	11th to 20th	Night	16% 10% 39% 27% 7% 1	562
	21st to end	Night	18% 11% 32% 30% 8%	627
September	1st to 10th	Night	19% 10% 41% 24% 5%	570
	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	29% 21% 27% 17% 5% 1	845
	11th to 20th	Night	28% 23% 25% 17% 5% 1	850
	21st to end	Night	31% 22% 27% 15% 3	850
December	1st to 10th	Night	32% 20% 29% 15% 3	850
	11th to 20th	Night	32% 26% 28% 12% 3	850
	21st to end	Night	32% 23% 25% 16% 3 1	935

Page generated at 2024-11-01 23:47:52. Version ac07

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