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 2024-04-03.

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 6% 5% 3 5% 5% 5% 7% 38%	78682
	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 39%	81440
Transparency	Day	45% 6% 11% 22% 14% 2	78387
	Night	46% 6% 11% 22% 13% 2	81269
Seeing	Day	24% 15% 29% 20% 9% 3	14107
	Night	22% 12% 30% 27% 8% 1	23255

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	15% 7% 4 6% 5% 3 3 4 6% 7% 41%	2517
	11th to 20th	Day	16% 10% 4 8% 6% 3 4 4 5% 6% 33%	2520
	21st to end	Day	20% 10% 4 8% 6% 3 5% 4 4 6% 31%	2772
February	1st to 10th	Day	17% 8% 3 7% 6% 3 5% 5% 6% 8% 33%	1763
	11th to 20th	Day	13% 8% 4 8% 6% 3 4 4 5% 7% 39%	1848
	21st to end	Day	20% 9% 3 7% 5% 4 4 5% 4 5% 35%	1570
March	1st to 10th	Day	16% 8% 3 7% 5% 3 4 4 5% 7% 38%	1897
	11th to 20th	Day	16% 8% 4 6% 5% 3 5% 5% 5% 7% 36%	1947
	21st to end	Day	19% 7% 4 6% 5% 3 4 3 4 5% 40%	2503
April	1st to 10th	Day	19% 7% 4 6% 5% 3 5% 3 5% 7% 37%	2184
	11th to 20th	Day	17% 6% 3 5% 5% 3 5% 5% 5% 7% 39%	2134
	21st to end	Day	21% 7% 3 5% 4 3 5% 4 5% 6% 36%	2294
May	1st to 10th	Day	24% 8% 3 6% 4 3 5% 5% 5% 6% 32%	2340
	11th to 20th	Day	18% 6% 3 5% 4 2 4 4 5% 7% 41%	2419
	21st to end	Day	15% 5% 2 5% 6% 3 4 5% 5% 8% 41%	2725
June	1st to 10th	Day	17% 6% 3 6% 6% 4 5% 5% 6% 9% 34%	2520
	11th to 20th	Day	18% 5% 3 5% 5% 4 5% 5% 6% 9% 37%	2520
	21st to end	Day	16% 5% 2 6% 6% 3 6% 5% 7% 8% 36%	2520
July	1st to 10th	Day	13% 5% 3 6% 6% 3 5% 5% 6% 8% 40%	2520
	11th to 20th	Day	13% 5% 2 5% 5% 3 6% 6% 7% 9% 40%	2520
	21st to end	Day	13% 5% 2 6% 5% 4 6% 5% 7% 9% 38%	2744
August	1st to 10th	Day	10% 5% 3 5% 6% 5% 7% 6% 7% 8% 39%	2427
	11th to 20th	Day	12% 5% 2 5% 6% 4 5% 6% 6% 9% 39%	2195
	21st to end	Day	13% 4 2 5% 4 3 5% 6% 6% 9% 44%	2376
September	1st to 10th	Day	14% 5% 3 5% 5% 3 4 5% 6% 6% 44%	2160
	11th to 20th	Day	16% 5% 2 4 4 2 3 4 3 7% 50%	2026
	21st to end	Day	17% 5% 2 5% 4 3 3 5% 6% 7% 43%	1980
October	1st to 10th	Day	23% 6% 2 5% 5% 3 5% 4 3 6% 38%	1963
	11th to 20th	Day	14% 6% 3 4 5% 3 4 4 6% 8% 42%	1756
	21st to end	Day	23% 7% 2 5% 6% 3 4 4 4 5% 36%	1782
November	1st to 10th	Day	13% 7% 4 4 4 3 3 4 4 6% 48%	1671
	11th to 20th	Day	19% 6% 3 4 3 2 4 5% 5% 6% 42%	1710
	21st to end	Day	19% 6% 2 5% 5% 3 4 5% 4 7% 40%	1661
December	1st to 10th	Day	20% 8% 4 5% 5% 3 4 6% 5% 5% 34%	1520
	11th to 20th	Day	23% 6% 3 5% 3 3 4 3 3 6% 41%	1906
	21st to end	Day	21% 11% 4 7% 5% 3 4 4 4 6% 30%	2772
January	1st to 10th	Night	17% 7% 3 4 4 3 4 4 5% 6% 43%	4429
	11th to 20th	Night	15% 9% 4 9% 7% 4 4 4 5% 6% 34%	4440
	21st to end	Night	21% 13% 4 6% 5% 3 4 4 5% 5% 29%	4884
February	1st to 10th	Night	15% 9% 3 6% 5% 3 5% 5% 5% 6% 37%	2410
	11th to 20th	Night	13% 7% 3 7% 6% 3 5% 5% 5% 6% 40%	2218
	21st to end	Night	18% 8% 3 7% 6% 3 4 4 4 5% 38%	1884
March	1st to 10th	Night	16% 8% 4 6% 5% 3 4 5% 5% 7% 39%	2270
	11th to 20th	Night	16% 7% 2 5% 6% 3 6% 5% 4 6% 40%	2231
	21st to end	Night	17% 8% 4 5% 4 3 4 4 5% 6% 40%	2093
April	1st to 10th	Night	18% 8% 3 6% 5% 3 4 3 5% 7% 37%	1821
	11th to 20th	Night	20% 5% 3 6% 4 3 5% 4 4 6% 40%	1775
	21st to end	Night	23% 6% 3 5% 4 2 3 4 4 6% 39%	1664
May	1st to 10th	Night	23% 9% 3 6% 5% 3 4 5% 5% 6% 32%	1620
	11th to 20th	Night	19% 5% 2 5% 5% 4 4 5% 6% 8% 38%	1489
	21st to end	Night	17% 5% 2 6% 4 3 3 4 6% 9% 40%	1550
June	1st to 10th	Night	18% 6% 3 6% 6% 3 3 4 4 6% 40%	1440
	11th to 20th	Night	19% 6% 3 5% 5% 3 5% 4 5% 7% 38%	1440
	21st to end	Night	16% 4 3 5% 5% 2 5% 4 7% 8% 42%	1440
July	1st to 10th	Night	14% 4 2 4 5% 3 6% 5% 6% 8% 44%	1440
	11th to 20th	Night	15% 5% 2 5% 5% 3 5% 5% 8% 9% 38%	1440
	21st to end	Night	11% 4 2 5% 4 4 5% 5% 8% 9% 45%	1574
August	1st to 10th	Night	10% 5% 2 5% 5% 4 4 5% 5% 9% 46%	1494
	11th to 20th	Night	13% 5% 2 6% 6% 4 5% 5% 7% 8% 39%	1703
	21st to end	Night	14% 4 2 4 4 3 4 4 5% 8% 47%	1980
September	1st to 10th	Night	15% 6% 2 4 5% 3 4 5% 5% 6% 45%	1800
	11th to 20th	Night	13% 5% 2 5% 3 3 4 4 4 6% 52%	1934
	21st to end	Night	17% 6% 2 5% 4 3 4 4 4 7% 45%	1980
October	1st to 10th	Night	23% 6% 2 5% 5% 3 4 3 4 5% 41%	1993
	11th to 20th	Night	17% 6% 2 5% 4 3 4 4 6% 6% 44%	2204
	21st to end	Night	23% 8% 3 5% 5% 2 3 4 4 5% 37%	2574
November	1st to 10th	Night	15% 6% 2 5% 4 2 4 4 5% 5% 48%	2407
	11th to 20th	Night	20% 5% 3 4 4 2 4 4 4 5% 44%	2470
	21st to end	Night	17% 7% 2 4 5% 3 4 5% 5% 6% 42%	2517
December	1st to 10th	Night	21% 7% 3 6% 5% 3 4 4 5% 7% 35%	2660
	11th to 20th	Night	21% 8% 2 6% 4 3 3 4 6% 6% 38%	3288
	21st to end	Night	22% 11% 4 6% 5% 3 5% 4 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	47% 4 13% 19% 13% 3	1900
	11th to 20th	Day	47% 5% 11% 20% 14% 3	1947
	21st to end	Day	49% 4 7% 17% 18% 5%	2503
April	1st to 10th	Day	48% 4 9% 21% 14% 4	2184
	11th to 20th	Day	44% 5% 11% 18% 18% 3	2134
	21st to end	Day	49% 5% 7% 19% 16% 3	2294
May	1st to 10th	Day	54% 5% 7% 19% 13% 2	2340
	11th to 20th	Day	44% 5% 9% 23% 18% 2	2423
	21st to end	Day	42% 6% 14% 22% 14% 2	2725
June	1st to 10th	Day	47% 7% 12% 23% 11% 1	2520
	11th to 20th	Day	45% 6% 13% 20% 14% 1	2520
	21st to end	Day	46% 7% 12% 22% 13% 1	2520
July	1st to 10th	Day	42% 7% 10% 23% 15% 2	2450
	11th to 20th	Day	38% 8% 15% 25% 12% 1	2445
	21st to end	Day	40% 9% 13% 21% 14% 3	2591
August	1st to 10th	Day	40% 8% 15% 24% 12% 1	2413
	11th to 20th	Day	41% 7% 13% 25% 12% 1	2195
	21st to end	Day	36% 8% 11% 26% 17% 2	2376
September	1st to 10th	Day	40% 7% 12% 24% 16% 1	2160
	11th to 20th	Day	37% 4 9% 24% 23% 2	2026
	21st to end	Day	40% 6% 10% 23% 19% 3	1980
October	1st to 10th	Day	50% 5% 8% 21% 13% 4	1963
	11th to 20th	Day	39% 6% 12% 28% 13% 2	1756
	21st to end	Day	50% 4 12% 18% 13% 3	1782
November	1st to 10th	Day	39% 5% 13% 25% 15% 2	1671
	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	46% 5% 9% 23% 14% 3	2280
	11th to 20th	Night	46% 6% 11% 20% 14% 3	2232
	21st to end	Night	46% 5% 9% 17% 19% 5%	2093
April	1st to 10th	Night	50% 4 9% 20% 15% 2	1821
	11th to 20th	Night	48% 5% 9% 20% 18% 1	1775
	21st to end	Night	48% 5% 9% 20% 13% 5%	1664
May	1st to 10th	Night	54% 6% 8% 18% 11% 3	1620
	11th to 20th	Night	45% 7% 11% 19% 15% 3	1490
	21st to end	Night	43% 7% 13% 23% 12% 1	1551
June	1st to 10th	Night	46% 6% 12% 24% 12% 1	1440
	11th to 20th	Night	46% 7% 12% 23% 12% 1	1440
	21st to end	Night	40% 9% 13% 24% 14% 1	1440
July	1st to 10th	Night	38% 8% 14% 24% 15% 1	1400
	11th to 20th	Night	41% 10% 13% 27% 10%	1392
	21st to end	Night	34% 10% 14% 25% 14% 2	1486
August	1st to 10th	Night	36% 8% 13% 27% 14% 1	1484
	11th to 20th	Night	42% 8% 14% 24% 12%	1703
	21st to end	Night	35% 7% 11% 27% 19% 1	1980
September	1st to 10th	Night	39% 7% 11% 25% 17% 1	1800
	11th to 20th	Night	35% 5% 10% 24% 24% 2	1934
	21st to end	Night	40% 6% 8% 25% 18% 3	1980
October	1st to 10th	Night	46% 6% 11% 23% 11% 3	1993
	11th to 20th	Night	40% 6% 13% 26% 13% 1	2204
	21st to end	Night	49% 5% 12% 21% 12% 1	2574
November	1st to 10th	Night	38% 5% 11% 23% 19% 3	2407
	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% 14% 6% 1	428
	11th to 20th	Day	27% 21% 28% 15% 7% 2	412
	21st to end	Day	31% 17% 29% 17% 5% 2	420
May	1st to 10th	Day	35% 19% 24% 14% 7% 2	423
	11th to 20th	Day	23% 17% 37% 17% 6% 1	447
	21st to end	Day	20% 11% 32% 22% 12% 3	485
June	1st to 10th	Day	18% 14% 33% 21% 11% 3	450
	11th to 20th	Day	19% 14% 33% 23% 9% 2	450
	21st to end	Day	17% 15% 31% 22% 11% 4	450
July	1st to 10th	Day	19% 7% 30% 26% 15% 3	445
	11th to 20th	Day	14% 7% 27% 30% 17% 4	444
	21st to end	Day	15% 6% 34% 28% 15% 3	488
August	1st to 10th	Day	12% 8% 26% 32% 19% 4	446
	11th to 20th	Day	15% 7% 30% 26% 16% 6%	439
	21st to end	Day	13% 11% 27% 26% 17% 6%	495
September	1st to 10th	Day	22% 6% 24% 25% 16% 7%	450
	11th to 20th	Day	20% 10% 27% 19% 15% 10%	450
	21st to end	Day	20% 10% 29% 21% 12% 7%	450
October	1st to 10th	Day	29% 11% 23% 19% 12% 6%	450
	11th to 20th	Day	19% 15% 28% 24% 11% 3	428
	21st to end	Day	30% 14% 24% 22% 9% 1	396
November	1st to 10th	Day	23% 12% 27% 25% 9% 3	365
	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% 32% 22% 4	729
	11th to 20th	Night	24% 16% 31% 23% 6%	711
	21st to end	Night	29% 15% 31% 22% 3	584
May	1st to 10th	Night	30% 12% 30% 22% 5%	540
	11th to 20th	Night	21% 12% 36% 25% 6%	428
	21st to end	Night	20% 10% 25% 33% 11% 1	389
June	1st to 10th	Night	23% 6% 24% 36% 10%	360
	11th to 20th	Night	21% 6% 29% 34% 9%	360
	21st to end	Night	15% 5% 28% 41% 11%	360
July	1st to 10th	Night	15% 4 25% 41% 14%	350
	11th to 20th	Night	16% 2 20% 43% 17% 1	348
	21st to end	Night	10% 5% 23% 48% 14%	394
August	1st to 10th	Night	11% 2 25% 46% 15% 1	420
	11th to 20th	Night	15% 3 22% 40% 17% 2	639
	21st to end	Night	16% 5% 21% 37% 20% 2	792
September	1st to 10th	Night	18% 4 27% 35% 15% 2	720
	11th to 20th	Night	17% 7% 21% 33% 19% 3	720
	21st to end	Night	20% 6% 26% 33% 14% 2	720
October	1st to 10th	Night	26% 9% 20% 30% 12% 2	720
	11th to 20th	Night	20% 11% 30% 29% 10% 1	720
	21st to end	Night	29% 12% 29% 24% 6%	792
November	1st to 10th	Night	19% 13% 30% 29% 8% 1	740
	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 2024-03-31 06:08:52. Version ac07

Copyright © 2024 Attilla Danko.