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	17% 7% 3 7% 4 4 4 4 6% 7% 37%	101257
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 38%	97762
Transparency	Day	45% 6% 12% 22% 14% 2	104795
	Night	45% 6% 12% 22% 14% 2	102313
Seeing	Day	24% 15% 29% 20% 9% 3	14950
	Night	22% 12% 30% 27% 8% 1	23919

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% 7% 3 7% 4 4 4 3 6% 7% 41%	2517
	11th to 20th	Day	16% 10% 4 9% 5% 4 4 3 5% 5% 33%	2520
	21st to end	Day	20% 11% 4 9% 5% 4 4 3 5% 5% 30%	2772
February	1st to 10th	Day	17% 8% 3 8% 5% 5% 4 4 6% 7% 32%	1763
	11th to 20th	Day	13% 8% 3 8% 5% 4 4 3 5% 6% 39%	1848
	21st to end	Day	20% 10% 3 7% 5% 4 4 4 4 5% 34%	1570
March	1st to 10th	Day	23% 13% 4 8% 4 3 3 2 4 5% 31%	3116
	11th to 20th	Day	12% 7% 3 8% 4 5% 5% 4 6% 8% 37%	3246
	21st to end	Day	21% 8% 3 6% 4 3 4 3 5% 6% 37%	4161
April	1st to 10th	Day	19% 9% 3 6% 5% 4 4 3 5% 7% 36%	2280
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 6% 6% 39%	2254
	21st to end	Day	20% 8% 3 6% 4 4 5% 4 5% 6% 36%	2422
May	1st to 10th	Day	25% 8% 3 7% 5% 4 5% 4 5% 7% 28%	4106
	11th to 20th	Day	22% 8% 3 7% 4 3 4 3 5% 7% 35%	4297
	21st to end	Day	13% 5% 2 5% 4 4 5% 5% 6% 8% 43%	4837
June	1st to 10th	Day	16% 6% 2 6% 5% 5% 5% 4 7% 9% 35%	2660
	11th to 20th	Day	17% 6% 2 5% 5% 5% 5% 4 7% 9% 37%	2660
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 8% 35%	2660
July	1st to 10th	Day	12% 6% 2 7% 6% 5% 5% 4 7% 9% 37%	4440
	11th to 20th	Day	12% 5% 2 6% 4 5% 6% 5% 7% 11% 37%	4440
	21st to end	Day	17% 7% 3 7% 4 5% 5% 4 6% 8% 35%	4814
August	1st to 10th	Day	15% 7% 3 7% 5% 6% 6% 4 7% 8% 32%	4275
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 7% 8% 38%	3838
	21st to end	Day	9% 4 2 6% 4 5% 6% 5% 7% 10% 42%	4180
September	1st to 10th	Day	13% 5% 3 6% 5% 4 4 4 6% 6% 45%	2280
	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	13% 7% 4 4 4 4 3 3 4 6% 48%	1698
	11th to 20th	Day	19% 7% 2 4 3 4 3 4 5% 6% 42%	1710
	21st to end	Day	19% 7% 2 6% 5% 4 4 4 5% 6% 40%	1661
December	1st to 10th	Day	20% 9% 3 6% 5% 4 4 4 5% 5% 34%	1520
	11th to 20th	Day	22% 7% 3 6% 3 3 4 3 3 6% 41%	1906
	21st to end	Day	21% 12% 3 8% 4 4 4 3 4 6% 30%	2772
January	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 5% 6% 43%	4429
	11th to 20th	Night	15% 9% 4 10% 6% 5% 4 3 5% 5% 34%	4440
	21st to end	Night	21% 13% 4 7% 4 4 4 4 5% 5% 29%	4884
February	1st to 10th	Night	15% 10% 3 6% 5% 4 5% 4 5% 5% 37%	2410
	11th to 20th	Night	13% 8% 3 7% 6% 5% 5% 3 5% 6% 40%	2218
	21st to end	Night	18% 9% 3 8% 5% 3 4 3 5% 5% 37%	1884
March	1st to 10th	Night	27% 9% 3 6% 4 4 4 3 5% 6% 30%	3724
	11th to 20th	Night	13% 8% 2 7% 5% 4 5% 4 5% 7% 39%	3712
	21st to end	Night	21% 9% 3 6% 3 4 4 3 6% 5% 36%	3469
April	1st to 10th	Night	19% 9% 3 6% 5% 4 4 3 5% 6% 37%	1900
	11th to 20th	Night	21% 6% 2 6% 4 4 4 3 4 6% 39%	1875
	21st to end	Night	23% 6% 2 6% 3 3 3 3 4 6% 40%	1756
May	1st to 10th	Night	25% 10% 3 6% 4 4 4 4 6% 6% 29%	2834
	11th to 20th	Night	19% 8% 3 9% 6% 5% 5% 4 5% 7% 28%	2625
	21st to end	Night	16% 6% 2 5% 4 4 4 3 6% 9% 42%	2738
June	1st to 10th	Night	17% 7% 3 6% 6% 4 4 3 4 6% 40%	1520
	11th to 20th	Night	18% 7% 2 5% 4 4 4 4 6% 7% 39%	1520
	21st to end	Night	16% 5% 2 7% 4 4 4 4 6% 8% 40%	1520
July	1st to 10th	Night	12% 5% 2 6% 5% 5% 5% 4 7% 9% 38%	2520
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 3 7% 8% 39%	2520
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 39%	2739
August	1st to 10th	Night	22% 6% 3 7% 5% 4 4 4 5% 7% 33%	2624
	11th to 20th	Night	13% 7% 2 8% 5% 5% 5% 4 6% 7% 38%	2973
	21st to end	Night	9% 4 1 4 3 4 4 3 5% 8% 54%	3476
September	1st to 10th	Night	15% 6% 2 5% 4 4 4 4 5% 6% 46%	1900
	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 5% 4 3 4 3 5% 5% 49%	2453
	11th to 20th	Night	20% 6% 2 5% 4 3 4 3 4 5% 44%	2470
	21st to end	Night	16% 8% 2 4 5% 3 4 4 5% 6% 42%	2517
December	1st to 10th	Night	21% 8% 3 6% 5% 4 4 3 5% 7% 35%	2660
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 6% 5% 38%	3288
	21st to end	Night	22% 11% 4 7% 4 4 4 3 4 5% 31%	4884

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	42% 5% 12% 32% 8% 1	2518
	11th to 20th	Day	51% 5% 12% 22% 9% 1	2520
	21st to end	Day	55% 5% 9% 19% 9% 1	2772
February	1st to 10th	Day	50% 5% 13% 19% 12% 1	1763
	11th to 20th	Day	47% 5% 9% 21% 14% 3	1857
	21st to end	Day	52% 5% 9% 19% 13% 2	1570
March	1st to 10th	Day	57% 4 11% 14% 11% 2	3119
	11th to 20th	Day	43% 9% 13% 23% 11% 2	3246
	21st to end	Day	50% 4 5% 17% 19% 5%	4161
April	1st to 10th	Day	49% 4 9% 21% 14% 3	2280
	11th to 20th	Day	45% 5% 11% 18% 17% 3	2254
	21st to end	Day	49% 5% 7% 19% 16% 3	2422
May	1st to 10th	Day	55% 7% 11% 15% 11% 1	4120
	11th to 20th	Day	50% 6% 14% 19% 10% 1	4301
	21st to end	Day	36% 9% 11% 25% 18% 2	4837
June	1st to 10th	Day	46% 7% 12% 23% 11% 1	2660
	11th to 20th	Day	44% 7% 13% 21% 14% 1	2660
	21st to end	Day	46% 7% 12% 21% 13% 1	2660
July	1st to 10th	Day	43% 8% 11% 22% 15% 1	4370
	11th to 20th	Day	39% 9% 16% 23% 12% 1	4365
	21st to end	Day	46% 7% 12% 19% 14% 2	4661
August	1st to 10th	Day	47% 8% 15% 20% 8% 1	4261
	11th to 20th	Day	41% 7% 15% 27% 8% 1	3877
	21st to end	Day	33% 8% 15% 28% 14% 1	4180
September	1st to 10th	Day	39% 7% 11% 25% 17% 1	2280
	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% 15% 2	1698
	11th to 20th	Day	42% 5% 11% 21% 18% 2	1710
	21st to end	Day	45% 4 14% 25% 11% 2	1661
December	1st to 10th	Day	52% 5% 13% 16% 12% 2	1520
	11th to 20th	Day	47% 4 14% 18% 16% 1	1906
	21st to end	Day	56% 5% 11% 19% 10%	2772
January	1st to 10th	Night	42% 5% 13% 27% 11% 3	4429
	11th to 20th	Night	51% 5% 13% 19% 10% 2	4440
	21st to end	Night	56% 6% 9% 18% 10% 2	4884
February	1st to 10th	Night	47% 6% 13% 20% 11% 2	2410
	11th to 20th	Night	46% 5% 10% 20% 15% 3	2220
	21st to end	Night	50% 4 9% 22% 11% 4	1884
March	1st to 10th	Night	55% 5% 9% 19% 10% 2	3734
	11th to 20th	Night	43% 8% 13% 21% 13% 2	3713
	21st to end	Night	50% 5% 6% 13% 20% 6%	3469
April	1st to 10th	Night	50% 5% 9% 19% 15% 2	1900
	11th to 20th	Night	48% 5% 8% 19% 18% 1	1875
	21st to end	Night	47% 5% 8% 20% 14% 5%	1756
May	1st to 10th	Night	54% 8% 12% 15% 10% 2	2840
	11th to 20th	Night	54% 9% 12% 15% 9% 2	2626
	21st to end	Night	39% 8% 13% 24% 14% 1	2739
June	1st to 10th	Night	45% 6% 11% 24% 12% 1	1520
	11th to 20th	Night	45% 7% 13% 24% 12% 1	1520
	21st to end	Night	41% 9% 13% 23% 14% 1	1520
July	1st to 10th	Night	41% 8% 15% 21% 15%	2480
	11th to 20th	Night	42% 8% 13% 23% 13%	2472
	21st to end	Night	44% 7% 11% 19% 17% 1	2651
August	1st to 10th	Night	51% 7% 12% 20% 9% 1	2614
	11th to 20th	Night	43% 8% 14% 25% 9%	3000
	21st to end	Night	25% 7% 17% 33% 17% 1	3476
September	1st to 10th	Night	38% 7% 10% 25% 18% 1	1900
	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	38% 6% 11% 24% 19% 3	2453
	11th to 20th	Night	44% 5% 11% 20% 17% 3	2470
	21st to end	Night	43% 6% 14% 24% 11% 1	2517
December	1st to 10th	Night	49% 6% 12% 20% 12%	2660
	11th to 20th	Night	47% 6% 14% 17% 15% 2	3288
	21st to end	Night	54% 4 10% 22% 9% 1	4884

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	24% 25% 27% 19% 4	180
	11th to 20th	Day	32% 23% 31% 9% 4 1	180
	21st to end	Day	23% 21% 35% 17% 4 1	198
February	1st to 10th	Day	31% 27% 30% 11% 1	325
	11th to 20th	Day	30% 23% 33% 13% 1	456
	21st to end	Day	33% 17% 31% 17% 3	382
March	1st to 10th	Day	29% 21% 33% 14% 3	460
	11th to 20th	Day	30% 23% 29% 13% 4	460
	21st to end	Day	31% 23% 30% 10% 5% 2	506
April	1st to 10th	Day	32% 20% 27% 13% 6% 1	460
	11th to 20th	Day	29% 22% 27% 14% 6% 2	452
	21st to end	Day	29% 18% 30% 16% 5% 2	460
May	1st to 10th	Day	34% 18% 25% 15% 6% 2	463
	11th to 20th	Day	23% 16% 36% 17% 7% 1	487
	21st to end	Day	20% 11% 31% 23% 12% 2	529
June	1st to 10th	Day	17% 15% 33% 22% 10% 2	490
	11th to 20th	Day	17% 15% 32% 23% 10% 2	490
	21st to end	Day	18% 15% 32% 21% 10% 4	490
July	1st to 10th	Day	19% 7% 30% 26% 15% 3	485
	11th to 20th	Day	14% 8% 28% 30% 17% 3	484
	21st to end	Day	16% 7% 33% 27% 14% 3	532
August	1st to 10th	Day	13% 7% 26% 32% 18% 4	486
	11th to 20th	Day	15% 7% 30% 26% 16% 6%	479
	21st to end	Day	12% 10% 27% 27% 17% 6%	539
September	1st to 10th	Day	21% 7% 25% 24% 16% 7%	490
	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% 25% 9% 3	374
	11th to 20th	Day	30% 16% 28% 18% 6% 1	370
	21st to end	Day	29% 14% 31% 20% 6% 1	321
December	1st to 10th	Day	27% 24% 29% 16% 5%	180
	11th to 20th	Day	37% 17% 33% 12% 2	180
	21st to end	Day	29% 18% 33% 15% 5% 1	198
January	1st to 10th	Night	22% 15% 37% 23% 2	760
	11th to 20th	Night	28% 15% 32% 22% 3	760
	21st to end	Night	21% 15% 37% 24% 3	836
February	1st to 10th	Night	23% 16% 36% 23% 2	760
	11th to 20th	Night	23% 20% 38% 18% 1	743
	21st to end	Night	27% 17% 32% 22% 3	628
March	1st to 10th	Night	25% 18% 35% 20% 2	760
	11th to 20th	Night	23% 20% 34% 19% 3	760
	21st to end	Night	24% 17% 34% 20% 6%	836
April	1st to 10th	Night	26% 16% 33% 22% 4	760
	11th to 20th	Night	24% 17% 31% 23% 5%	751
	21st to end	Night	28% 15% 31% 22% 4	616
May	1st to 10th	Night	29% 13% 30% 23% 5%	570
	11th to 20th	Night	21% 12% 37% 25% 6%	452
	21st to end	Night	20% 9% 27% 33% 10% 1	411
June	1st to 10th	Night	22% 7% 26% 36% 10%	380
	11th to 20th	Night	20% 7% 29% 33% 10% 1	380
	21st to end	Night	17% 5% 28% 39% 10%	380
July	1st to 10th	Night	15% 4 26% 41% 14%	370
	11th to 20th	Night	16% 3 21% 42% 17% 1	368
	21st to end	Night	12% 5% 23% 46% 14%	416
August	1st to 10th	Night	13% 3 24% 45% 14% 1	444
	11th to 20th	Night	15% 4 22% 39% 17% 2	675
	21st to end	Night	15% 5% 21% 37% 21% 1	836
September	1st to 10th	Night	18% 4 27% 34% 15% 2	760
	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% 29% 8% 1	755
	11th to 20th	Night	23% 13% 31% 24% 7% 1	760
	21st to end	Night	23% 13% 32% 27% 5% 1	760
December	1st to 10th	Night	27% 13% 36% 19% 5%	760
	11th to 20th	Night	26% 15% 31% 25% 4	760
	21st to end	Night	27% 15% 34% 21% 2	836

Page generated at 2024-11-01 22:47:53. Version ac07

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