Foymount
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foymount clear sky chart from 2005-11-06 to 2025-04-11.

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	15% 12% 4 10% 6% 5% 4 3 5% 6% 30%	113620
	Night	14% 12% 5% 10% 6% 5% 4 3 5% 6% 29%	114652
Transparency	Day	53% 6% 11% 19% 10% 1	117185
	Night	54% 6% 11% 18% 9% 1	119251
Seeing	Day	23% 17% 31% 18% 8% 2	14841
	Night	21% 15% 32% 23% 8% 1	24183

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% 15% 5% 15% 10% 7% 6% 4 4 4 17%	3597
	11th to 20th	Day	10% 15% 8% 19% 8% 6% 5% 3 4 6% 17%	3570
	21st to end	Day	12% 18% 8% 16% 7% 5% 4 2 3 4 19%	4326
February	1st to 10th	Day	11% 17% 6% 15% 8% 6% 4 4 5% 4 20%	1800
	11th to 20th	Day	11% 16% 6% 14% 7% 5% 5% 4 4 5% 25%	1753
	21st to end	Day	16% 16% 5% 10% 7% 5% 5% 5% 4 4 23%	1582
March	1st to 10th	Day	16% 14% 5% 12% 6% 4 4 3 4 7% 25%	4692
	11th to 20th	Day	9% 14% 6% 12% 7% 5% 5% 4 6% 7% 26%	4980
	21st to end	Day	14% 14% 4 10% 4 4 3 3 4 4 37%	5635
April	1st to 10th	Day	19% 15% 3 7% 5% 4 5% 3 4 6% 29%	2400
	11th to 20th	Day	22% 11% 3 6% 4 5% 3 3 3 4 36%	2254
	21st to end	Day	22% 11% 4 8% 4 3 3 3 4 5% 32%	2454
May	1st to 10th	Day	19% 9% 3 8% 5% 4 5% 4 5% 6% 32%	4389
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 3 6% 8% 33%	4398
	21st to end	Day	16% 7% 2 7% 5% 4 4 4 4 7% 38%	4837
June	1st to 10th	Day	22% 9% 2 7% 6% 4 5% 4 5% 6% 30%	2660
	11th to 20th	Day	16% 6% 2 6% 5% 4 5% 5% 6% 8% 37%	2660
	21st to end	Day	17% 8% 3 6% 6% 5% 5% 5% 6% 8% 33%	2660
July	1st to 10th	Day	13% 8% 3 7% 5% 6% 5% 4 6% 10% 33%	4440
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 4 6% 8% 40%	4440
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 7% 45%	4814
August	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	4200
	11th to 20th	Day	18% 8% 2 7% 4 5% 5% 4 6% 8% 34%	4044
	21st to end	Day	10% 5% 2 7% 4 5% 5% 5% 8% 9% 39%	4247
September	1st to 10th	Day	16% 6% 3 6% 3 4 5% 4 5% 7% 40%	2280
	11th to 20th	Day	14% 6% 2 6% 5% 4 3 2 5% 7% 45%	2280
	21st to end	Day	18% 9% 2 8% 4 4 4 4 6% 7% 35%	1994
October	1st to 10th	Day	21% 10% 3 8% 4 4 4 2 4 6% 33%	1900
	11th to 20th	Day	21% 14% 4 8% 5% 4 5% 3 4 5% 27%	1900
	21st to end	Day	25% 12% 4 10% 5% 4 5% 4 4 6% 22%	2007
November	1st to 10th	Day	14% 12% 4 8% 5% 4 5% 4 4 6% 33%	1681
	11th to 20th	Day	15% 15% 7% 11% 6% 4 3 3 5% 7% 26%	1600
	21st to end	Day	19% 19% 6% 12% 6% 5% 4 4 4 5% 17%	1600
December	1st to 10th	Day	16% 25% 7% 14% 8% 6% 4 3 3 5% 10%	2600
	11th to 20th	Day	15% 17% 8% 14% 6% 4 4 2 4 3 22%	2986
	21st to end	Day	19% 18% 7% 14% 7% 3 4 2 3 4 19%	3960
January	1st to 10th	Night	11% 15% 10% 18% 10% 5% 4 3 3 4 17%	6349
	11th to 20th	Night	9% 13% 6% 16% 9% 8% 6% 3 5% 6% 18%	6315
	21st to end	Night	11% 16% 8% 15% 7% 8% 5% 3 4 4 18%	6628
February	1st to 10th	Night	11% 16% 6% 13% 8% 6% 5% 4 5% 5% 21%	2600
	11th to 20th	Night	11% 14% 6% 12% 7% 5% 5% 3 5% 5% 27%	2533
	21st to end	Night	12% 14% 4 12% 6% 5% 5% 3 5% 6% 27%	2046
March	1st to 10th	Night	17% 15% 4 11% 6% 5% 4 3 3 4 28%	5144
	11th to 20th	Night	9% 13% 5% 10% 6% 6% 6% 4 7% 8% 26%	4979
	21st to end	Night	14% 12% 4 11% 6% 4 4 3 5% 6% 30%	5296
April	1st to 10th	Night	19% 13% 3 9% 5% 4 3 3 4 6% 31%	2000
	11th to 20th	Night	19% 10% 3 7% 4 5% 4 3 3 6% 36%	1882
	21st to end	Night	20% 11% 3 8% 5% 4 3 4 5% 6% 31%	1743
May	1st to 10th	Night	19% 10% 2 7% 4 4 4 4 5% 6% 34%	2549
	11th to 20th	Night	18% 7% 3 7% 4 4 4 4 5% 8% 36%	2495
	21st to end	Night	13% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	2738
June	1st to 10th	Night	16% 7% 3 8% 4 4 5% 5% 6% 8% 33%	1520
	11th to 20th	Night	14% 6% 3 5% 4 4 4 4 5% 8% 43%	1520
	21st to end	Night	18% 7% 3 7% 5% 5% 4 4 6% 7% 36%	1520
July	1st to 10th	Night	11% 7% 3 8% 6% 6% 6% 5% 5% 7% 38%	2520
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 6% 7% 43%	2520
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 8% 46%	2739
August	1st to 10th	Night	12% 6% 2 7% 4 4 5% 4 6% 8% 43%	2726
	11th to 20th	Night	15% 7% 3 5% 4 3 4 3 6% 7% 44%	2785
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 6% 8% 44%	3376
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 6% 47%	1900
	11th to 20th	Night	14% 5% 2 5% 3 4 3 3 5% 5% 51%	1900
	21st to end	Night	16% 8% 2 5% 5% 4 3 4 5% 6% 41%	2180
October	1st to 10th	Night	19% 8% 2 7% 3 4 3 3 4 6% 39%	2280
	11th to 20th	Night	19% 12% 3 8% 5% 4 4 4 5% 5% 30%	2280
	21st to end	Night	21% 15% 4 8% 4 4 4 3 5% 6% 25%	2590
November	1st to 10th	Night	15% 13% 4 8% 5% 4 4 3 5% 5% 32%	2615
	11th to 20th	Night	16% 14% 5% 10% 6% 5% 5% 3 5% 5% 27%	2800
	21st to end	Night	17% 17% 4 12% 6% 5% 4 4 5% 5% 20%	2800
December	1st to 10th	Night	16% 19% 7% 14% 8% 5% 5% 3 5% 5% 13%	4580
	11th to 20th	Night	17% 18% 6% 14% 6% 4 4 3 3 4 22%	5208
	21st to end	Night	20% 18% 5% 15% 7% 4 3 2 3 4 18%	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	69% 8% 7% 9% 7%	3598
	11th to 20th	Day	68% 6% 9% 9% 8% 1	3600
	21st to end	Day	69% 4 7% 11% 8% 1	4326
February	1st to 10th	Day	65% 7% 8% 14% 6% 1	1800
	11th to 20th	Day	59% 6% 12% 13% 8% 1	1762
	21st to end	Day	62% 6% 9% 12% 9% 2	1582
March	1st to 10th	Day	60% 5% 9% 11% 14% 2	4695
	11th to 20th	Day	54% 6% 14% 15% 9% 2	4980
	21st to end	Day	50% 5% 7% 19% 16% 3	5635
April	1st to 10th	Day	56% 5% 8% 18% 11% 3	2400
	11th to 20th	Day	52% 4 7% 21% 14% 2	2254
	21st to end	Day	53% 5% 8% 15% 15% 3	2454
May	1st to 10th	Day	48% 9% 10% 19% 11% 2	4403
	11th to 20th	Day	44% 10% 15% 20% 10% 2	4402
	21st to end	Day	43% 8% 12% 18% 16% 2	4837
June	1st to 10th	Day	52% 7% 11% 18% 11% 1	2660
	11th to 20th	Day	43% 8% 11% 22% 13% 2	2660
	21st to end	Day	47% 8% 13% 19% 12% 1	2660
July	1st to 10th	Day	45% 5% 14% 25% 10% 1	4370
	11th to 20th	Day	39% 6% 16% 29% 10%	4365
	21st to end	Day	37% 6% 12% 33% 12%	4661
August	1st to 10th	Day	49% 5% 13% 23% 9%	4186
	11th to 20th	Day	47% 6% 15% 26% 6%	4083
	21st to end	Day	37% 7% 14% 32% 10% 1	4247
September	1st to 10th	Day	41% 8% 14% 26% 11% 1	2280
	11th to 20th	Day	38% 6% 12% 29% 14%	2280
	21st to end	Day	47% 7% 13% 23% 10%	1994
October	1st to 10th	Day	52% 4 8% 28% 7%	3160
	11th to 20th	Day	52% 5% 11% 21% 11%	3140
	21st to end	Day	58% 5% 15% 16% 5%	3284
November	1st to 10th	Day	50% 8% 14% 21% 7%	1681
	11th to 20th	Day	58% 7% 12% 13% 8% 1	1600
	21st to end	Day	69% 8% 10% 8% 5% 1	1600
December	1st to 10th	Day	79% 4 9% 7% 2	2600
	11th to 20th	Day	66% 6% 7% 13% 8%	2986
	21st to end	Day	71% 4 6% 14% 4	3960
January	1st to 10th	Night	71% 6% 8% 10% 4	6349
	11th to 20th	Night	64% 9% 10% 11% 6%	6360
	21st to end	Night	68% 5% 6% 9% 10% 1	6628
February	1st to 10th	Night	63% 6% 10% 14% 6% 1	2600
	11th to 20th	Night	59% 7% 10% 17% 7% 1	2535
	21st to end	Night	56% 7% 10% 18% 9% 1	2046
March	1st to 10th	Night	60% 5% 8% 14% 9% 4	5154
	11th to 20th	Night	52% 8% 16% 15% 7% 1	4980
	21st to end	Night	51% 6% 10% 19% 11% 2	5296
April	1st to 10th	Night	55% 6% 7% 19% 11% 2	2000
	11th to 20th	Night	49% 6% 8% 22% 14% 1	1882
	21st to end	Night	53% 6% 8% 16% 15% 2	1743
May	1st to 10th	Night	45% 10% 12% 20% 10% 2	2555
	11th to 20th	Night	42% 13% 14% 21% 9% 1	2496
	21st to end	Night	37% 11% 13% 21% 16% 2	2739
June	1st to 10th	Night	44% 9% 15% 20% 11% 1	1520
	11th to 20th	Night	38% 8% 12% 25% 15% 2	1520
	21st to end	Night	45% 8% 14% 20% 12% 1	1520
July	1st to 10th	Night	44% 6% 15% 19% 14% 1	2480
	11th to 20th	Night	39% 4 14% 32% 10%	2472
	21st to end	Night	35% 7% 15% 29% 14%	2651
August	1st to 10th	Night	38% 6% 15% 27% 12% 2	2716
	11th to 20th	Night	40% 5% 16% 29% 9%	2812
	21st to end	Night	37% 5% 15% 30% 10% 2	3376
September	1st to 10th	Night	36% 8% 14% 28% 13% 1	1900
	11th to 20th	Night	35% 6% 15% 32% 11%	1900
	21st to end	Night	42% 7% 13% 28% 10%	2180
October	1st to 10th	Night	40% 4 12% 30% 14%	3800
	11th to 20th	Night	50% 5% 15% 19% 11%	3779
	21st to end	Night	53% 5% 17% 18% 6%	4263
November	1st to 10th	Night	51% 7% 16% 18% 8%	2615
	11th to 20th	Night	58% 8% 11% 15% 7% 1	2800
	21st to end	Night	65% 6% 12% 13% 3	2800
December	1st to 10th	Night	73% 5% 12% 9% 2	4580
	11th to 20th	Night	66% 5% 7% 12% 10% 1	5208
	21st to end	Night	71% 4 6% 11% 8%	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	20% 23% 39% 16% 3 1	200
	11th to 20th	Day	28% 16% 39% 13% 4 1	200
	21st to end	Day	22% 26% 32% 17% 3	300
February	1st to 10th	Day	28% 28% 33% 7% 3 1	300
	11th to 20th	Day	27% 30% 38% 5%	300
	21st to end	Day	34% 21% 33% 12%	346
March	1st to 10th	Day	25% 25% 32% 15% 3	500
	11th to 20th	Day	28% 20% 33% 16% 4	500
	21st to end	Day	30% 23% 30% 12% 4 1	550
April	1st to 10th	Day	31% 22% 29% 13% 6%	500
	11th to 20th	Day	33% 21% 22% 14% 8% 2	452
	21st to end	Day	30% 21% 30% 12% 5% 1	460
May	1st to 10th	Day	25% 19% 30% 16% 8% 2	463
	11th to 20th	Day	21% 17% 31% 21% 7% 3	487
	21st to end	Day	19% 18% 33% 19% 10% 2	529
June	1st to 10th	Day	24% 13% 35% 20% 7%	490
	11th to 20th	Day	17% 16% 32% 22% 9% 3	490
	21st to end	Day	19% 15% 33% 21% 10% 2	490
July	1st to 10th	Day	12% 8% 33% 34% 10% 2	485
	11th to 20th	Day	14% 11% 35% 25% 13% 2	484
	21st to end	Day	13% 10% 35% 27% 11% 3	532
August	1st to 10th	Day	16% 8% 33% 28% 12% 4	486
	11th to 20th	Day	15% 8% 28% 26% 17% 5%	479
	21st to end	Day	14% 12% 32% 26% 10% 6%	539
September	1st to 10th	Day	18% 13% 29% 21% 14% 6%	490
	11th to 20th	Day	14% 11% 23% 24% 18% 10%	490
	21st to end	Day	20% 14% 28% 21% 13% 5%	490
October	1st to 10th	Day	26% 16% 24% 18% 12% 4	490
	11th to 20th	Day	30% 22% 27% 15% 6% 1	490
	21st to end	Day	31% 19% 29% 16% 4	494
November	1st to 10th	Day	26% 19% 32% 16% 6% 1	315
	11th to 20th	Day	29% 22% 34% 10% 4 2	200
	21st to end	Day	30% 28% 31% 9% 2 2	200
December	1st to 10th	Day	29% 17% 41% 10% 4	200
	11th to 20th	Day	30% 31% 26% 10% 2 2	200
	21st to end	Day	30% 20% 30% 15% 3 1	220
January	1st to 10th	Night	23% 20% 37% 17% 4	800
	11th to 20th	Night	24% 20% 31% 20% 5%	800
	21st to end	Night	16% 16% 38% 23% 6%	880
February	1st to 10th	Night	19% 22% 38% 18% 3	800
	11th to 20th	Night	20% 20% 42% 15% 3	783
	21st to end	Night	22% 22% 33% 21% 3	660
March	1st to 10th	Night	24% 20% 34% 19% 3	800
	11th to 20th	Night	23% 21% 33% 19% 4 1	800
	21st to end	Night	20% 19% 34% 20% 6%	880
April	1st to 10th	Night	28% 16% 32% 21% 3	800
	11th to 20th	Night	25% 19% 31% 17% 8%	754
	21st to end	Night	25% 15% 27% 23% 10%	603
May	1st to 10th	Night	22% 10% 31% 28% 9%	402
	11th to 20th	Night	20% 10% 34% 27% 8%	374
	21st to end	Night	15% 8% 38% 27% 11%	411
June	1st to 10th	Night	19% 8% 37% 30% 5% 1	380
	11th to 20th	Night	18% 8% 34% 29% 10%	380
	21st to end	Night	21% 10% 31% 27% 12%	380
July	1st to 10th	Night	9% 7% 36% 35% 12% 1	370
	11th to 20th	Night	12% 8% 29% 38% 14%	368
	21st to end	Night	8% 6% 35% 39% 11% 1	416
August	1st to 10th	Night	10% 5% 27% 39% 19% 1	507
	11th to 20th	Night	12% 4 24% 39% 18% 3	564
	21st to end	Night	12% 8% 30% 31% 17% 2	775
September	1st to 10th	Night	15% 8% 31% 27% 17% 1	760
	11th to 20th	Night	19% 6% 23% 29% 19% 4	760
	21st to end	Night	20% 12% 22% 26% 18% 2	760
October	1st to 10th	Night	24% 11% 30% 23% 12% 1	760
	11th to 20th	Night	25% 17% 32% 18% 8% 1	760
	21st to end	Night	30% 18% 27% 20% 5%	836
November	1st to 10th	Night	26% 18% 29% 21% 5% 1	780
	11th to 20th	Night	22% 21% 31% 17% 9% 1	800
	21st to end	Night	31% 19% 26% 21% 4	800
December	1st to 10th	Night	27% 20% 34% 16% 3	800
	11th to 20th	Night	26% 21% 31% 19% 3 1	800
	21st to end	Night	30% 18% 30% 18% 3	880

Page generated at 2025-04-08 09:17:24. Version ac07

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