Ferry Point observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ferry Point observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	130553
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	130276
Transparency	Day	39% 6% 12% 22% 17% 4	134146
	Night	39% 5% 11% 21% 18% 5%	134753
Seeing	Day	15% 10% 26% 26% 17% 6%	16967
	Night	16% 7% 27% 33% 16% 2	26681

By Month

Percentage Cloud Cover SE (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% 5% 2 5% 3 3 3 3 4 5% 56%	5037
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 4 55%	5280
	21st to end	Day	13% 10% 3 7% 4 4 4 3 4 5% 42%	5780
February	1st to 10th	Day	13% 8% 3 6% 4 4 3 4 6% 6% 42%	1890
	11th to 20th	Day	19% 8% 2 5% 4 3 3 3 5% 4 43%	1985
	21st to end	Day	16% 7% 3 5% 3 4 4 4 5% 7% 42%	1650
March	1st to 10th	Day	20% 7% 2 6% 3 4 4 3 5% 6% 40%	4755
	11th to 20th	Day	11% 6% 2 6% 4 3 3 2 4 6% 53%	4980
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 6% 49%	5643
April	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 5% 7% 51%	2400
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 5% 7% 50%	2374
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 7% 46%	2400
May	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 5% 7% 9% 35%	5405
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 9% 31%	5913
	21st to end	Day	13% 7% 2 6% 5% 4 5% 4 6% 9% 40%	6934
June	1st to 10th	Day	10% 5% 2 6% 6% 5% 6% 6% 8% 9% 38%	2800
	11th to 20th	Day	9% 6% 2 5% 5% 6% 7% 5% 7% 10% 39%	2800
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 12% 37%	2800
July	1st to 10th	Day	7% 5% 2 7% 5% 6% 7% 6% 9% 14% 31%	6360
	11th to 20th	Day	9% 7% 3 9% 6% 7% 7% 6% 9% 12% 26%	5948
	21st to end	Day	8% 6% 3 9% 6% 6% 6% 6% 9% 11% 30%	6424
August	1st to 10th	Day	17% 10% 4 9% 7% 6% 6% 5% 7% 9% 20%	4150
	11th to 20th	Day	9% 7% 3 8% 6% 6% 6% 5% 8% 11% 32%	3850
	21st to end	Day	8% 4 3 6% 5% 6% 7% 5% 7% 11% 39%	4312
September	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 6% 6% 8% 38%	2400
	11th to 20th	Day	14% 7% 3 7% 6% 5% 5% 4 6% 7% 36%	2400
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 5% 7% 32%	2148
October	1st to 10th	Day	15% 6% 3 5% 4 3 5% 3 6% 6% 45%	2000
	11th to 20th	Day	13% 6% 1 6% 4 3 3 3 5% 6% 50%	2000
	21st to end	Day	12% 5% 2 6% 4 4 4 3 4 5% 52%	2200
November	1st to 10th	Day	15% 5% 2 6% 3 3 3 3 5% 5% 49%	1973
	11th to 20th	Day	15% 6% 2 4 3 3 3 2 4 7% 50%	1890
	21st to end	Day	12% 5% 2 4 4 4 4 3 5% 6% 50%	1874
December	1st to 10th	Day	12% 6% 2 5% 3 3 4 3 4 6% 51%	2680
	11th to 20th	Day	14% 7% 2 7% 4 3 4 3 4 5% 45%	3070
	21st to end	Day	14% 5% 2 5% 4 3 4 3 5% 7% 49%	4048
January	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 5% 6% 51%	7840
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	7680
	21st to end	Night	16% 8% 2 7% 4 4 4 3 4 5% 44%	8401
February	1st to 10th	Night	18% 7% 2 5% 4 3 4 3 5% 6% 42%	2730
	11th to 20th	Night	19% 8% 2 5% 4 3 3 3 5% 5% 42%	2409
	21st to end	Night	15% 7% 2 5% 4 3 4 2 4 6% 47%	1980
March	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 4 5% 41%	5082
	11th to 20th	Night	14% 5% 2 6% 4 4 4 3 4 5% 48%	4980
	21st to end	Night	14% 8% 2 5% 4 3 3 3 4 5% 48%	5289
April	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 7% 45%	2000
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 5% 7% 49%	1975
	21st to end	Night	12% 7% 1 5% 4 4 4 3 4 6% 49%	2000
May	1st to 10th	Night	10% 7% 3 8% 6% 6% 5% 4 7% 10% 34%	4490
	11th to 20th	Night	18% 9% 3 8% 4 5% 5% 3 5% 7% 32%	4005
	21st to end	Night	13% 8% 3 7% 5% 4 5% 4 6% 9% 37%	3920
June	1st to 10th	Night	10% 7% 2 5% 4 4 6% 4 8% 9% 41%	1600
	11th to 20th	Night	8% 6% 2 6% 5% 4 4 6% 8% 10% 40%	1600
	21st to end	Night	8% 4 1 6% 5% 5% 5% 6% 6% 10% 44%	1600
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 6% 6% 10% 14% 33%	3598
	11th to 20th	Night	8% 7% 3 9% 6% 6% 7% 5% 8% 12% 29%	4003
	21st to end	Night	11% 7% 3 7% 5% 6% 5% 4 7% 9% 35%	4427
August	1st to 10th	Night	17% 8% 3 8% 5% 5% 5% 4 7% 8% 28%	2966
	11th to 20th	Night	9% 8% 4 8% 5% 5% 5% 4 6% 8% 38%	3197
	21st to end	Night	8% 6% 2 6% 4 4 5% 4 7% 10% 43%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 6% 5% 4 5% 9% 44%	2000
	11th to 20th	Night	13% 8% 3 6% 4 4 4 4 5% 7% 42%	2000
	21st to end	Night	19% 6% 3 7% 5% 4 4 3 5% 7% 35%	2246
October	1st to 10th	Night	12% 7% 2 4 4 4 4 3 5% 7% 48%	2400
	11th to 20th	Night	14% 7% 2 5% 3 2 3 2 4 5% 51%	2400
	21st to end	Night	12% 5% 2 4 3 4 3 2 4 7% 53%	2640
November	1st to 10th	Night	14% 6% 2 6% 3 3 3 3 5% 6% 49%	2543
	11th to 20th	Night	15% 6% 2 5% 3 3 4 4 4 6% 47%	2730
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 6% 50%	2744
December	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 4 5% 53%	4720
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 4 5% 6% 38%	5345
	21st to end	Night	12% 6% 2 5% 4 3 4 3 6% 7% 50%	7150

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	32% 3 11% 24% 20% 11%	5038
	11th to 20th	Day	34% 3 4 15% 37% 8%	5280
	21st to end	Day	46% 4 6% 22% 17% 6%	5780
February	1st to 10th	Day	41% 5% 11% 24% 16% 4	1890
	11th to 20th	Day	44% 4 8% 17% 21% 6%	1994
	21st to end	Day	41% 5% 11% 18% 19% 5%	1650
March	1st to 10th	Day	46% 4 7% 19% 16% 8%	4755
	11th to 20th	Day	33% 4 6% 19% 27% 11%	4980
	21st to end	Day	37% 4 13% 20% 21% 5%	5643
April	1st to 10th	Day	34% 4 12% 23% 22% 6%	2400
	11th to 20th	Day	32% 6% 11% 25% 20% 6%	2374
	21st to end	Day	35% 7% 13% 23% 18% 4	2400
May	1st to 10th	Day	42% 8% 18% 20% 11% 1	5437
	11th to 20th	Day	48% 7% 16% 18% 10% 1	5917
	21st to end	Day	38% 8% 13% 27% 13% 2	6949
June	1st to 10th	Day	37% 10% 13% 23% 15% 1	2800
	11th to 20th	Day	37% 9% 16% 24% 13% 1	2800
	21st to end	Day	35% 10% 16% 25% 12% 2	2800
July	1st to 10th	Day	39% 9% 21% 22% 9% 1	6290
	11th to 20th	Day	47% 11% 25% 13% 4	5877
	21st to end	Day	43% 9% 14% 24% 9%	6240
August	1st to 10th	Day	58% 8% 16% 13% 4	4137
	11th to 20th	Day	41% 9% 16% 23% 11%	3886
	21st to end	Day	37% 8% 14% 27% 12% 2	4312
September	1st to 10th	Day	41% 11% 14% 17% 15% 3	2400
	11th to 20th	Day	44% 8% 9% 20% 17% 2	2400
	21st to end	Day	49% 6% 11% 18% 14% 2	2148
October	1st to 10th	Day	32% 5% 8% 25% 27% 3	3260
	11th to 20th	Day	27% 3 5% 22% 32% 12%	3240
	21st to end	Day	24% 3 7% 28% 30% 8%	3538
November	1st to 10th	Day	37% 3 11% 22% 22% 4	1973
	11th to 20th	Day	35% 3 11% 23% 22% 6%	1890
	21st to end	Day	35% 4 9% 26% 22% 5%	1874
December	1st to 10th	Day	35% 4 8% 27% 22% 4	2680
	11th to 20th	Day	42% 3 12% 21% 16% 6%	3066
	21st to end	Day	36% 3 11% 29% 19% 1	4048
January	1st to 10th	Night	35% 3 10% 22% 22% 8%	7841
	11th to 20th	Night	34% 3 4 17% 32% 9%	7680
	21st to end	Night	44% 3 6% 21% 19% 6%	8401
February	1st to 10th	Night	41% 4 10% 22% 18% 4	2730
	11th to 20th	Night	44% 5% 8% 19% 19% 6%	2411
	21st to end	Night	39% 5% 8% 21% 20% 8%	1980
March	1st to 10th	Night	46% 4 9% 17% 16% 9%	5082
	11th to 20th	Night	38% 4 8% 14% 23% 12%	4980
	21st to end	Night	39% 4 11% 24% 16% 6%	5289
April	1st to 10th	Night	37% 6% 12% 17% 23% 5%	2000
	11th to 20th	Night	35% 5% 11% 26% 18% 6%	1975
	21st to end	Night	36% 6% 11% 25% 18% 4	2000
May	1st to 10th	Night	44% 8% 18% 21% 8% 1	4520
	11th to 20th	Night	51% 6% 12% 21% 9% 1	4006
	21st to end	Night	42% 9% 14% 23% 12% 1	3927
June	1st to 10th	Night	37% 9% 15% 25% 13% 1	1600
	11th to 20th	Night	36% 10% 18% 24% 12% 1	1600
	21st to end	Night	33% 10% 16% 27% 13% 1	1600
July	1st to 10th	Night	36% 10% 22% 22% 8% 1	3560
	11th to 20th	Night	44% 12% 26% 14% 4	3954
	21st to end	Night	44% 8% 15% 20% 12%	4297
August	1st to 10th	Night	51% 9% 19% 17% 4	2955
	11th to 20th	Night	44% 7% 16% 21% 12%	3227
	21st to end	Night	35% 6% 14% 27% 16% 2	3586
September	1st to 10th	Night	35% 10% 18% 19% 15% 3	2000
	11th to 20th	Night	41% 7% 11% 20% 18% 2	2000
	21st to end	Night	48% 6% 12% 19% 13% 1	2246
October	1st to 10th	Night	38% 4 7% 23% 26% 3	3920
	11th to 20th	Night	27% 4 5% 19% 30% 14%	3899
	21st to end	Night	21% 3 7% 29% 26% 13%	4252
November	1st to 10th	Night	38% 3 9% 22% 24% 4	2543
	11th to 20th	Night	38% 4 10% 18% 21% 8%	2730
	21st to end	Night	34% 4 9% 26% 22% 5%	2744
December	1st to 10th	Night	34% 4 7% 21% 25% 8%	4720
	11th to 20th	Night	48% 4 11% 15% 16% 6%	5348
	21st to end	Night	33% 5% 15% 28% 18% 2	7150

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	19% 17% 45% 14% 4	289
	11th to 20th	Day	18% 24% 40% 15% 3	330
	21st to end	Day	17% 23% 42% 14% 3 1	363
February	1st to 10th	Day	18% 21% 37% 19% 5%	330
	11th to 20th	Day	30% 15% 35% 17% 3	503
	21st to end	Day	19% 19% 31% 27% 4 1	414
March	1st to 10th	Day	18% 18% 36% 23% 5% 1	500
	11th to 20th	Day	18% 17% 36% 22% 5% 2	500
	21st to end	Day	17% 15% 37% 23% 7% 1	550
April	1st to 10th	Day	18% 18% 34% 22% 7% 2	500
	11th to 20th	Day	12% 15% 34% 24% 12% 3	492
	21st to end	Day	13% 11% 32% 30% 13% 2	500
May	1st to 10th	Day	15% 13% 33% 23% 13% 2	503
	11th to 20th	Day	16% 8% 27% 30% 14% 5%	527
	21st to end	Day	15% 6% 23% 27% 20% 9%	573
June	1st to 10th	Day	11% 5% 22% 32% 23% 7%	530
	11th to 20th	Day	12% 4 20% 29% 25% 10%	530
	21st to end	Day	8% 3 14% 36% 28% 11%	530
July	1st to 10th	Day	10% 2 11% 33% 33% 11%	525
	11th to 20th	Day	10% 1 9% 30% 37% 14%	524
	21st to end	Day	10% 1 13% 32% 34% 10%	576
August	1st to 10th	Day	16% 5% 13% 29% 25% 12%	526
	11th to 20th	Day	9% 4 12% 29% 30% 16%	519
	21st to end	Day	11% 4 14% 30% 27% 14%	583
September	1st to 10th	Day	14% 3 19% 25% 23% 16%	530
	11th to 20th	Day	16% 5% 16% 26% 25% 11%	530
	21st to end	Day	23% 7% 21% 22% 20% 7%	530
October	1st to 10th	Day	17% 6% 18% 28% 20% 11%	530
	11th to 20th	Day	14% 8% 25% 30% 19% 4	530
	21st to end	Day	12% 10% 28% 27% 17% 7%	583
November	1st to 10th	Day	16% 8% 32% 26% 13% 4	491
	11th to 20th	Day	17% 11% 30% 29% 11% 1	430
	21st to end	Day	14% 14% 29% 26% 16% 2	414
December	1st to 10th	Day	19% 14% 40% 20% 5% 1	220
	11th to 20th	Day	16% 12% 40% 22% 9%	220
	21st to end	Day	20% 12% 37% 19% 11% 1	242
January	1st to 10th	Night	17% 13% 38% 26% 5%	840
	11th to 20th	Night	19% 12% 35% 28% 6%	840
	21st to end	Night	18% 9% 44% 26% 3	924
February	1st to 10th	Night	21% 9% 39% 28% 4	840
	11th to 20th	Night	23% 13% 41% 21% 2	804
	21st to end	Night	17% 13% 37% 27% 6%	660
March	1st to 10th	Night	17% 10% 38% 29% 7%	800
	11th to 20th	Night	19% 10% 39% 24% 7%	800
	21st to end	Night	17% 10% 39% 28% 6%	880
April	1st to 10th	Night	17% 11% 39% 29% 5%	800
	11th to 20th	Night	13% 11% 35% 31% 10%	791
	21st to end	Night	14% 6% 34% 37% 8% 1	800
May	1st to 10th	Night	13% 8% 32% 36% 10% 1	800
	11th to 20th	Night	16% 7% 22% 40% 15% 1	590
	21st to end	Night	14% 2 16% 41% 24% 3	433
June	1st to 10th	Night	12% 4 16% 44% 23% 1	400
	11th to 20th	Night	11% 4 12% 43% 28% 3	400
	21st to end	Night	8% 3 12% 48% 29% 1	400
July	1st to 10th	Night	7% 1 11% 45% 34% 2	390
	11th to 20th	Night	9% 1 6% 39% 37% 8%	556
	21st to end	Night	10% 2 12% 40% 33% 3	657
August	1st to 10th	Night	14% 2 10% 37% 32% 4	622
	11th to 20th	Night	11% 2 9% 39% 34% 6%	790
	21st to end	Night	13% 2 9% 32% 37% 7%	880
September	1st to 10th	Night	11% 3 12% 35% 31% 9%	800
	11th to 20th	Night	17% 2 13% 33% 31% 4	800
	21st to end	Night	21% 6% 14% 32% 25% 2	800
October	1st to 10th	Night	14% 3 19% 31% 27% 7%	800
	11th to 20th	Night	17% 6% 23% 33% 19% 2	800
	21st to end	Night	14% 6% 24% 33% 19% 4	880
November	1st to 10th	Night	17% 5% 25% 35% 15% 4	820
	11th to 20th	Night	18% 8% 30% 32% 11% 1	840
	21st to end	Night	13% 7% 32% 33% 14% 1	840
December	1st to 10th	Night	19% 9% 33% 30% 9%	840
	11th to 20th	Night	17% 9% 33% 33% 9%	840
	21st to end	Night	17% 8% 34% 34% 8%	924

Page generated at 2026-02-13 01:42:16. Version ac07

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