W. Armitage Memorial Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the W. Armitage Memorial Dark Sky Site clear sky chart from 2005-11-06 to 2024-05-13.

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% 13% 5% 10% 6% 5% 4 3 5% 6% 26%	83556
	Night	17% 14% 5% 10% 6% 5% 4 3 5% 5% 25%	85919
Transparency	Day	60% 3 9% 18% 9% 1	83261
	Night	62% 3 9% 17% 8% 1	85748
Seeing	Day	27% 13% 31% 19% 7% 2	13830
	Night	27% 11% 28% 24% 9% 1	22833

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% 19% 8% 17% 9% 4 5% 3 3 5% 12%	2517
	11th to 20th	Day	13% 15% 6% 20% 9% 6% 3 3 4 4 16%	2520
	21st to end	Day	12% 20% 10% 15% 5% 5% 5% 3 5% 4 15%	2815
February	1st to 10th	Day	9% 19% 7% 14% 9% 7% 4 4 4 6% 17%	1710
	11th to 20th	Day	12% 18% 6% 15% 8% 5% 4 3 4 5% 20%	1663
	21st to end	Day	16% 20% 6% 10% 5% 5% 5% 3 5% 4 21%	1522
March	1st to 10th	Day	19% 18% 4 9% 5% 4 4 3 3 5% 26%	3138
	11th to 20th	Day	12% 17% 7% 15% 6% 5% 3 3 3 4 25%	3480
	21st to end	Day	12% 10% 4 9% 6% 6% 5% 4 5% 5% 33%	4002
April	1st to 10th	Day	22% 17% 3 8% 4 3 3 3 3 5% 28%	2280
	11th to 20th	Day	23% 12% 4 8% 4 4 3 3 4 6% 30%	2254
	21st to end	Day	21% 11% 4 9% 5% 4 4 3 4 4 32%	2517
May	1st to 10th	Day	21% 12% 3 7% 5% 4 4 4 5% 6% 30%	2660
	11th to 20th	Day	22% 9% 3 7% 5% 5% 4 3 5% 6% 31%	2509
	21st to end	Day	19% 8% 4 8% 5% 5% 4 4 5% 6% 33%	2725
June	1st to 10th	Day	22% 9% 3 9% 5% 4 5% 4 6% 7% 26%	2520
	11th to 20th	Day	15% 8% 3 7% 5% 5% 4 5% 7% 7% 34%	2520
	21st to end	Day	19% 7% 3 9% 6% 5% 5% 5% 6% 8% 27%	2693
July	1st to 10th	Day	16% 7% 3 6% 5% 5% 5% 5% 7% 9% 32%	2552
	11th to 20th	Day	15% 7% 3 7% 7% 7% 5% 5% 6% 8% 30%	2520
	21st to end	Day	15% 9% 3 8% 5% 4 6% 4 7% 8% 31%	2744
August	1st to 10th	Day	16% 9% 4 10% 6% 6% 5% 4 6% 7% 27%	2506
	11th to 20th	Day	17% 8% 3 7% 5% 6% 6% 4 6% 7% 32%	2305
	21st to end	Day	17% 8% 3 8% 5% 6% 5% 4 6% 7% 32%	2404
September	1st to 10th	Day	18% 8% 3 8% 4 4 3 3 5% 7% 37%	2160
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 4 4 6% 36%	2160
	21st to end	Day	21% 11% 3 7% 3 4 3 3 5% 6% 33%	1909
October	1st to 10th	Day	21% 13% 4 8% 4 4 3 3 4 6% 29%	1800
	11th to 20th	Day	29% 17% 5% 9% 5% 4 3 2 4 4 18%	1800
	21st to end	Day	29% 17% 4 9% 5% 4 3 3 4 5% 17%	1822
November	1st to 10th	Day	18% 20% 6% 9% 5% 3 3 2 4 5% 26%	1591
	11th to 20th	Day	19% 18% 9% 13% 7% 4 4 2 4 4 16%	1520
	21st to end	Day	23% 22% 8% 14% 6% 5% 3 3 3 3 10%	1520
December	1st to 10th	Day	22% 21% 7% 12% 7% 5% 4 2 4 5% 11%	1520
	11th to 20th	Day	20% 17% 6% 15% 9% 5% 4 5% 4 3 14%	1906
	21st to end	Day	21% 21% 8% 14% 6% 3 3 2 4 4 14%	2772
January	1st to 10th	Night	15% 19% 7% 16% 8% 5% 4 3 3 4 14%	4429
	11th to 20th	Night	13% 16% 6% 15% 10% 6% 5% 4 6% 5% 15%	4440
	21st to end	Night	9% 18% 10% 16% 7% 8% 6% 3 5% 5% 13%	4838
February	1st to 10th	Night	10% 16% 5% 11% 11% 8% 5% 4 5% 5% 20%	2470
	11th to 20th	Night	12% 17% 6% 12% 7% 6% 5% 4 5% 6% 20%	2403
	21st to end	Night	12% 17% 6% 11% 7% 5% 6% 4 5% 6% 22%	1931
March	1st to 10th	Night	17% 18% 4 7% 5% 4 5% 3 4 5% 26%	3697
	11th to 20th	Night	16% 19% 5% 13% 4 4 4 3 4 5% 22%	3479
	21st to end	Night	13% 10% 4 11% 6% 6% 5% 4 6% 5% 30%	3629
April	1st to 10th	Night	20% 13% 5% 10% 6% 4 3 2 4 4 29%	1900
	11th to 20th	Night	17% 11% 3 7% 5% 4 4 3 5% 5% 35%	1875
	21st to end	Night	22% 10% 4 8% 4 3 4 3 4 6% 32%	1678
May	1st to 10th	Night	19% 10% 4 8% 5% 4 4 3 5% 5% 34%	1520
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 3 4 5% 34%	1439
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 8% 36%	1550
June	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 5% 7% 7% 30%	1440
	11th to 20th	Night	17% 8% 3 7% 4 4 4 5% 5% 6% 37%	1435
	21st to end	Night	18% 7% 2 5% 4 5% 5% 4 5% 8% 35%	1260
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 4 4 6% 8% 36%	1397
	11th to 20th	Night	13% 6% 2 6% 5% 3 6% 5% 8% 8% 37%	1440
	21st to end	Night	13% 6% 2 7% 6% 5% 6% 4 7% 10% 37%	1574
August	1st to 10th	Night	14% 6% 2 7% 5% 4 5% 4 7% 8% 37%	1432
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 6% 6% 41%	1593
	21st to end	Night	15% 8% 3 8% 4 4 4 4 5% 7% 39%	1933
September	1st to 10th	Night	17% 8% 2 6% 4 4 3 3 5% 8% 41%	1800
	11th to 20th	Night	19% 7% 3 5% 4 4 4 3 5% 6% 40%	1800
	21st to end	Night	21% 8% 3 7% 4 3 3 3 4 6% 37%	2048
October	1st to 10th	Night	23% 10% 3 7% 4 4 4 3 5% 5% 32%	2160
	11th to 20th	Night	27% 16% 4 7% 4 3 4 2 4 5% 24%	2160
	21st to end	Night	26% 17% 4 9% 4 4 4 3 4 5% 20%	2533
November	1st to 10th	Night	19% 17% 5% 9% 6% 4 3 2 4 5% 25%	2484
	11th to 20th	Night	19% 18% 6% 11% 8% 5% 4 2 4 5% 17%	2660
	21st to end	Night	24% 20% 6% 13% 7% 4 4 2 3 3 13%	2660
December	1st to 10th	Night	19% 22% 7% 14% 7% 4 4 3 3 4 13%	2660
	11th to 20th	Night	21% 20% 5% 11% 6% 5% 5% 3 4 5% 15%	3288
	21st to end	Night	20% 21% 7% 16% 7% 4 3 2 3 4 14%	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	78% 2 8% 8% 4	2518
	11th to 20th	Day	72% 3 5% 15% 6%	2520
	21st to end	Day	72% 2 7% 12% 7%	2815
February	1st to 10th	Day	69% 2 6% 14% 8% 1	1710
	11th to 20th	Day	68% 2 8% 12% 10% 1	1672
	21st to end	Day	67% 2 6% 15% 9% 1	1522
March	1st to 10th	Day	64% 2 6% 15% 12% 1	3141
	11th to 20th	Day	65% 3 7% 13% 10% 2	3480
	21st to end	Day	52% 4 7% 19% 16% 3	4002
April	1st to 10th	Day	60% 2 8% 15% 12% 2	2280
	11th to 20th	Day	57% 3 6% 22% 11% 2	2254
	21st to end	Day	57% 2 7% 17% 14% 2	2517
May	1st to 10th	Day	55% 3 8% 21% 11% 3	2660
	11th to 20th	Day	53% 3 7% 20% 15% 1	2513
	21st to end	Day	51% 3 10% 19% 15% 2	2725
June	1st to 10th	Day	56% 4 9% 19% 10% 1	2520
	11th to 20th	Day	47% 5% 12% 22% 12% 1	2520
	21st to end	Day	54% 5% 12% 21% 9%	2693
July	1st to 10th	Day	45% 5% 13% 26% 9%	2482
	11th to 20th	Day	49% 5% 14% 23% 9%	2445
	21st to end	Day	51% 4 14% 25% 6%	2591
August	1st to 10th	Day	56% 4 12% 23% 4	2492
	11th to 20th	Day	51% 4 17% 20% 8%	2305
	21st to end	Day	51% 4 12% 24% 9%	2404
September	1st to 10th	Day	47% 3 12% 24% 13% 1	2160
	11th to 20th	Day	49% 4 15% 23% 9% 1	2160
	21st to end	Day	52% 3 14% 24% 7% 1	1909
October	1st to 10th	Day	57% 3 11% 19% 9% 1	1800
	11th to 20th	Day	71% 2 10% 13% 3 1	1800
	21st to end	Day	71% 3 9% 12% 5% 1	1822
November	1st to 10th	Day	63% 2 11% 17% 7%	1591
	11th to 20th	Day	73% 3 8% 10% 5%	1520
	21st to end	Day	81% 3 6% 6% 4	1520
December	1st to 10th	Day	79% 3 6% 10% 3	1520
	11th to 20th	Day	75% 4 4 12% 5% 1	1906
	21st to end	Day	75% 2 6% 13% 3	2772
January	1st to 10th	Night	76% 2 11% 9% 2	4429
	11th to 20th	Night	71% 3 8% 13% 4 1	4440
	21st to end	Night	74% 2 6% 9% 7%	4838
February	1st to 10th	Night	66% 3 6% 16% 8% 1	2470
	11th to 20th	Night	66% 3 7% 15% 7% 1	2405
	21st to end	Night	63% 2 8% 15% 10% 1	1931
March	1st to 10th	Night	62% 3 9% 12% 11% 2	3707
	11th to 20th	Night	66% 3 8% 15% 7% 1	3480
	21st to end	Night	52% 6% 7% 20% 12% 2	3629
April	1st to 10th	Night	60% 2 6% 16% 14% 2	1900
	11th to 20th	Night	50% 3 8% 23% 13% 2	1875
	21st to end	Night	54% 3 8% 19% 14% 2	1678
May	1st to 10th	Night	52% 3 9% 19% 13% 3	1520
	11th to 20th	Night	53% 4 7% 20% 15% 2	1440
	21st to end	Night	48% 4 10% 22% 14% 3	1551
June	1st to 10th	Night	50% 5% 10% 23% 10% 1	1440
	11th to 20th	Night	47% 3 12% 21% 14% 3	1435
	21st to end	Night	47% 3 13% 25% 10% 1	1260
July	1st to 10th	Night	44% 5% 15% 26% 9% 1	1357
	11th to 20th	Night	41% 6% 17% 28% 9%	1392
	21st to end	Night	44% 5% 17% 28% 7%	1486
August	1st to 10th	Night	43% 6% 17% 29% 5%	1422
	11th to 20th	Night	43% 5% 18% 25% 9%	1593
	21st to end	Night	45% 4 15% 28% 8%	1933
September	1st to 10th	Night	43% 3 17% 26% 11%	1800
	11th to 20th	Night	46% 3 17% 25% 8%	1800
	21st to end	Night	50% 4 12% 28% 6%	2048
October	1st to 10th	Night	54% 4 11% 19% 11% 1	2160
	11th to 20th	Night	65% 3 10% 18% 3 1	2160
	21st to end	Night	68% 3 8% 15% 5% 1	2533
November	1st to 10th	Night	64% 3 11% 16% 6%	2484
	11th to 20th	Night	71% 3 9% 12% 5% 1	2660
	21st to end	Night	78% 3 7% 9% 3	2660
December	1st to 10th	Night	78% 3 7% 9% 4	2660
	11th to 20th	Night	72% 3 5% 13% 6%	3288
	21st to end	Night	77% 2 4 12% 5%	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	26% 19% 39% 13% 2 1	180
	11th to 20th	Day	24% 14% 38% 16% 7% 1	180
	21st to end	Day	25% 18% 33% 20% 4	213
February	1st to 10th	Day	26% 24% 39% 8% 3 1	270
	11th to 20th	Day	27% 26% 36% 11% 1	270
	21st to end	Day	33% 21% 33% 10% 2 1	334
March	1st to 10th	Day	26% 18% 33% 17% 5%	460
	11th to 20th	Day	28% 18% 33% 16% 5% 1	460
	21st to end	Day	29% 17% 33% 16% 4 1	506
April	1st to 10th	Day	35% 15% 26% 16% 7% 1	460
	11th to 20th	Day	34% 15% 26% 16% 7% 2	452
	21st to end	Day	28% 17% 33% 16% 4 1	460
May	1st to 10th	Day	33% 13% 31% 16% 6% 2	463
	11th to 20th	Day	28% 14% 27% 19% 9% 2	456
	21st to end	Day	26% 18% 29% 21% 5% 1	485
June	1st to 10th	Day	28% 10% 36% 21% 5% 1	450
	11th to 20th	Day	23% 14% 33% 18% 10% 2	450
	21st to end	Day	23% 10% 31% 24% 10% 2	623
July	1st to 10th	Day	18% 7% 37% 29% 7% 2	477
	11th to 20th	Day	20% 7% 35% 26% 10% 2	444
	21st to end	Day	17% 9% 31% 29% 13% 2	488
August	1st to 10th	Day	19% 7% 34% 26% 11% 4	446
	11th to 20th	Day	20% 4 28% 28% 13% 7%	439
	21st to end	Day	22% 9% 32% 19% 13% 6%	495
September	1st to 10th	Day	23% 8% 29% 24% 12% 4	450
	11th to 20th	Day	22% 10% 29% 22% 12% 5%	450
	21st to end	Day	25% 14% 23% 20% 15% 3	450
October	1st to 10th	Day	31% 12% 25% 22% 8% 3	450
	11th to 20th	Day	36% 14% 29% 14% 5% 2	450
	21st to end	Day	41% 13% 25% 17% 5%	416
November	1st to 10th	Day	35% 15% 29% 16% 4 1	285
	11th to 20th	Day	30% 19% 33% 13% 4 1	180
	21st to end	Day	34% 22% 32% 8% 2 2	180
December	1st to 10th	Day	35% 14% 28% 19% 3 2	180
	11th to 20th	Day	31% 20% 33% 12% 3 1	180
	21st to end	Day	32% 13% 37% 13% 5% 1	198
January	1st to 10th	Night	31% 13% 33% 19% 4	760
	11th to 20th	Night	32% 11% 30% 20% 7%	760
	21st to end	Night	20% 9% 35% 26% 8% 1	836
February	1st to 10th	Night	19% 16% 35% 24% 6% 1	760
	11th to 20th	Night	24% 15% 34% 22% 4 1	743
	21st to end	Night	23% 18% 32% 22% 4	628
March	1st to 10th	Night	24% 16% 31% 23% 5% 1	760
	11th to 20th	Night	28% 15% 31% 20% 5% 1	760
	21st to end	Night	22% 15% 33% 24% 6%	836
April	1st to 10th	Night	30% 13% 27% 24% 6%	760
	11th to 20th	Night	26% 15% 28% 22% 9% 1	751
	21st to end	Night	28% 11% 29% 24% 7% 1	538
May	1st to 10th	Night	25% 7% 28% 31% 9%	380
	11th to 20th	Night	26% 8% 26% 27% 12% 1	360
	21st to end	Night	24% 7% 34% 28% 6%	389
June	1st to 10th	Night	23% 8% 32% 29% 8% 1	360
	11th to 20th	Night	21% 8% 27% 30% 13% 1	360
	21st to end	Night	23% 4 24% 34% 14% 1	360
July	1st to 10th	Night	16% 4 28% 40% 12%	350
	11th to 20th	Night	16% 4 24% 41% 13% 1	348
	21st to end	Night	15% 4 25% 39% 16% 1	394
August	1st to 10th	Night	19% 3 22% 33% 23% 1	358
	11th to 20th	Night	19% 3 18% 35% 22% 3	529
	21st to end	Night	20% 5% 26% 28% 19% 2	745
September	1st to 10th	Night	22% 5% 26% 32% 13% 2	720
	11th to 20th	Night	26% 6% 26% 26% 14% 2	720
	21st to end	Night	26% 7% 26% 28% 12% 2	720
October	1st to 10th	Night	29% 8% 28% 23% 12%	720
	11th to 20th	Night	37% 10% 25% 19% 8% 1	720
	21st to end	Night	38% 10% 27% 21% 4 1	792
November	1st to 10th	Night	31% 12% 31% 18% 7% 1	740
	11th to 20th	Night	31% 16% 29% 16% 7% 1	760
	21st to end	Night	38% 15% 22% 19% 5% 1	760
December	1st to 10th	Night	33% 13% 30% 18% 5% 1	760
	11th to 20th	Night	32% 15% 26% 21% 6% 1	760
	21st to end	Night	34% 14% 27% 19% 5% 1	836

Page generated at 2024-05-10 12:39:07. Version ac07

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