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 2024-11-30.

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	101165
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	98378
Transparency	Day	39% 7% 12% 22% 17% 4	104802
	Night	39% 6% 11% 22% 17% 4	102880
Seeing	Day	15% 9% 25% 26% 18% 6%	15437
	Night	16% 7% 26% 33% 16% 2	25056

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	16% 6% 2 5% 4 2 3 3 4 5% 51%	2711
	11th to 20th	Day	12% 6% 3 6% 4 4 3 3 5% 6% 48%	2840
	21st to end	Day	14% 11% 3 7% 4 4 4 3 4 6% 40%	3124
February	1st to 10th	Day	13% 8% 3 6% 5% 5% 3 4 6% 7% 42%	1710
	11th to 20th	Day	18% 8% 2 5% 4 3 3 3 5% 4 44%	1835
	21st to end	Day	17% 7% 3 5% 4 4 4 4 5% 7% 40%	1570
March	1st to 10th	Day	24% 9% 2 5% 3 4 3 3 5% 6% 37%	3311
	11th to 20th	Day	12% 5% 2 5% 3 3 3 2 4 6% 55%	3480
	21st to end	Day	17% 6% 2 5% 4 4 4 3 4 6% 46%	3937
April	1st to 10th	Day	12% 6% 2 4 3 3 3 3 5% 7% 52%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 5% 7% 50%	2254
	21st to end	Day	11% 6% 2 6% 5% 5% 4 4 5% 6% 47%	2280
May	1st to 10th	Day	12% 7% 3 7% 6% 5% 5% 4 6% 9% 35%	3765
	11th to 20th	Day	18% 8% 2 8% 5% 5% 5% 4 6% 8% 31%	4119
	21st to end	Day	9% 6% 2 6% 5% 4 6% 4 7% 9% 43%	4823
June	1st to 10th	Day	10% 5% 2 5% 6% 5% 6% 6% 8% 9% 37%	2660
	11th to 20th	Day	10% 6% 2 5% 5% 5% 7% 5% 7% 10% 39%	2660
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 12% 36%	2660
July	1st to 10th	Day	8% 5% 2 7% 4 6% 6% 6% 9% 13% 35%	4440
	11th to 20th	Day	10% 8% 3 8% 6% 6% 6% 5% 8% 10% 30%	4154
	21st to end	Day	10% 7% 4 10% 6% 6% 6% 6% 8% 9% 28%	4466
August	1st to 10th	Day	17% 10% 4 9% 7% 6% 6% 5% 7% 9% 20%	4023
	11th to 20th	Day	9% 7% 3 7% 6% 6% 6% 5% 8% 11% 33%	3730
	21st to end	Day	8% 4 2 6% 5% 6% 7% 5% 7% 11% 39%	4180
September	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 6% 6% 8% 37%	2280
	11th to 20th	Day	13% 7% 3 7% 6% 5% 5% 4 5% 7% 37%	2280
	21st to end	Day	20% 8% 3 7% 5% 5% 4 3 6% 7% 32%	2041
October	1st to 10th	Day	15% 7% 2 5% 4 3 4 3 6% 6% 46%	1900
	11th to 20th	Day	12% 6% 1 6% 4 3 3 3 5% 5% 50%	1900
	21st to end	Day	11% 4 2 6% 4 4 4 3 4 5% 52%	2090
November	1st to 10th	Day	15% 6% 2 6% 4 3 3 3 5% 5% 48%	1877
	11th to 20th	Day	16% 6% 2 4 3 3 3 2 4 7% 49%	1800
	21st to end	Day	13% 5% 2 4 4 4 4 3 5% 6% 49%	1783
December	1st to 10th	Day	16% 8% 2 6% 5% 4 4 3 5% 7% 41%	1520
	11th to 20th	Day	16% 5% 2 4 3 2 3 3 3 5% 53%	1910
	21st to end	Day	16% 6% 2 5% 4 3 4 3 5% 7% 45%	2772
January	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 47%	4232
	11th to 20th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 42%	4120
	21st to end	Night	16% 8% 2 6% 4 5% 4 3 5% 4 43%	4532
February	1st to 10th	Night	18% 7% 2 5% 4 3 4 3 6% 7% 42%	2470
	11th to 20th	Night	19% 9% 3 6% 4 4 3 3 5% 5% 41%	2228
	21st to end	Night	15% 7% 2 6% 4 3 4 2 4 6% 45%	1884
March	1st to 10th	Night	26% 9% 2 6% 5% 3 4 3 4 5% 33%	3526
	11th to 20th	Night	14% 6% 2 6% 4 4 3 3 4 6% 50%	3480
	21st to end	Night	17% 9% 2 5% 3 3 3 3 4 4 47%	3695
April	1st to 10th	Night	13% 5% 2 6% 4 4 5% 4 6% 7% 46%	1900
	11th to 20th	Night	11% 6% 2 6% 4 4 3 4 5% 7% 48%	1875
	21st to end	Night	13% 7% 1 5% 4 4 4 3 4 7% 48%	1900
May	1st to 10th	Night	11% 7% 2 8% 6% 6% 4 4 6% 8% 38%	3130
	11th to 20th	Night	19% 8% 2 7% 4 4 5% 4 5% 7% 35%	2799
	21st to end	Night	8% 6% 3 6% 5% 4 6% 4 7% 11% 40%	2732
June	1st to 10th	Night	10% 7% 2 5% 4 4 6% 4 8% 9% 41%	1520
	11th to 20th	Night	7% 6% 2 6% 5% 4 4 6% 8% 10% 41%	1520
	21st to end	Night	9% 4 1 6% 5% 5% 5% 6% 7% 10% 43%	1520
July	1st to 10th	Night	6% 5% 3 7% 5% 6% 6% 6% 9% 12% 35%	2520
	11th to 20th	Night	9% 7% 3 8% 5% 5% 6% 5% 7% 11% 34%	2800
	21st to end	Night	14% 9% 4 9% 6% 6% 5% 4 7% 8% 29%	3087
August	1st to 10th	Night	17% 8% 3 7% 5% 5% 5% 4 7% 9% 29%	2875
	11th to 20th	Night	9% 8% 4 8% 5% 5% 5% 4 6% 8% 38%	3097
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 7% 10% 43%	3476
September	1st to 10th	Night	10% 5% 2 5% 4 6% 5% 5% 5% 10% 44%	1900
	11th to 20th	Night	13% 7% 3 6% 4 4 4 4 5% 7% 43%	1900
	21st to end	Night	19% 6% 3 7% 5% 4 4 3 5% 7% 35%	2133
October	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Night	13% 7% 2 5% 3 2 3 2 5% 6% 52%	2280
	21st to end	Night	11% 6% 2 4 4 4 3 2 4 7% 54%	2508
November	1st to 10th	Night	15% 6% 2 6% 3 3 3 3 5% 6% 48%	2419
	11th to 20th	Night	16% 6% 2 5% 4 3 4 4 4 6% 47%	2600
	21st to end	Night	12% 5% 2 5% 4 4 4 3 6% 6% 50%	2611
December	1st to 10th	Night	15% 8% 3 7% 5% 4 4 3 5% 6% 40%	2660
	11th to 20th	Night	17% 6% 2 5% 3 3 4 3 5% 6% 45%	3285
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 7% 46%	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	36% 4 7% 27% 23% 4	2712
	11th to 20th	Day	37% 4 7% 19% 28% 5%	2840
	21st to end	Day	46% 5% 8% 17% 18% 5%	3124
February	1st to 10th	Day	40% 5% 11% 25% 15% 4	1710
	11th to 20th	Day	43% 4 8% 18% 21% 6%	1844
	21st to end	Day	42% 6% 12% 17% 18% 6%	1570
March	1st to 10th	Day	49% 5% 9% 18% 14% 6%	3311
	11th to 20th	Day	31% 5% 7% 20% 26% 11%	3480
	21st to end	Day	41% 4 14% 18% 19% 4	3937
April	1st to 10th	Day	33% 4 12% 23% 23% 6%	2280
	11th to 20th	Day	33% 6% 10% 24% 20% 7%	2254
	21st to end	Day	35% 7% 12% 23% 18% 5%	2280
May	1st to 10th	Day	42% 10% 17% 20% 10% 2	3800
	11th to 20th	Day	49% 8% 12% 21% 9% 1	4123
	21st to end	Day	32% 10% 14% 27% 14% 2	4837
June	1st to 10th	Day	37% 11% 14% 23% 15% 2	2660
	11th to 20th	Day	37% 9% 15% 24% 13% 1	2660
	21st to end	Day	37% 11% 15% 23% 12% 2	2660
July	1st to 10th	Day	36% 10% 20% 22% 10% 1	4370
	11th to 20th	Day	47% 10% 22% 16% 5% 1	4083
	21st to end	Day	47% 10% 13% 18% 11% 1	4324
August	1st to 10th	Day	57% 9% 17% 13% 5%	4010
	11th to 20th	Day	40% 9% 16% 23% 11%	3766
	21st to end	Day	36% 8% 14% 27% 12% 2	4180
September	1st to 10th	Day	41% 11% 14% 15% 16% 3	2280
	11th to 20th	Day	44% 8% 10% 19% 17% 2	2280
	21st to end	Day	50% 6% 10% 18% 14% 2	2041
October	1st to 10th	Day	32% 5% 8% 26% 27% 3	3160
	11th to 20th	Day	26% 3 5% 22% 32% 12%	3140
	21st to end	Day	23% 3 7% 29% 30% 8%	3428
November	1st to 10th	Day	38% 3 11% 22% 21% 4	1877
	11th to 20th	Day	37% 3 11% 22% 22% 6%	1800
	21st to end	Day	35% 5% 8% 25% 22% 5%	1783
December	1st to 10th	Day	43% 5% 12% 21% 16% 4	1520
	11th to 20th	Day	35% 3 8% 28% 18% 8%	1906
	21st to end	Day	39% 4 8% 27% 19% 2	2772
January	1st to 10th	Night	38% 4 6% 25% 21% 5%	4232
	11th to 20th	Night	43% 5% 6% 21% 21% 4	4120
	21st to end	Night	43% 5% 7% 18% 19% 7%	4532
February	1st to 10th	Night	41% 5% 9% 24% 17% 4	2470
	11th to 20th	Night	45% 5% 7% 19% 18% 6%	2230
	21st to end	Night	40% 5% 8% 20% 19% 7%	1884
March	1st to 10th	Night	54% 5% 9% 16% 13% 3	3526
	11th to 20th	Night	36% 5% 7% 17% 23% 12%	3480
	21st to end	Night	40% 4 9% 27% 15% 5%	3695
April	1st to 10th	Night	37% 6% 12% 17% 23% 5%	1900
	11th to 20th	Night	36% 5% 11% 24% 18% 6%	1875
	21st to end	Night	36% 6% 11% 26% 18% 4	1900
May	1st to 10th	Night	43% 9% 16% 22% 8% 1	3160
	11th to 20th	Night	48% 8% 12% 23% 8% 1	2800
	21st to end	Night	34% 11% 18% 24% 11% 1	2739
June	1st to 10th	Night	37% 9% 15% 26% 13% 1	1520
	11th to 20th	Night	34% 10% 18% 24% 13% 1	1520
	21st to end	Night	34% 11% 16% 25% 13% 2	1520
July	1st to 10th	Night	38% 13% 19% 19% 10% 1	2480
	11th to 20th	Night	42% 10% 26% 17% 5%	2748
	21st to end	Night	51% 10% 13% 16% 11%	2988
August	1st to 10th	Night	51% 9% 19% 17% 4	2864
	11th to 20th	Night	43% 7% 17% 22% 12%	3127
	21st to end	Night	35% 7% 14% 27% 16% 2	3476
September	1st to 10th	Night	35% 10% 18% 18% 15% 3	1900
	11th to 20th	Night	40% 7% 11% 21% 18% 3	1900
	21st to end	Night	48% 6% 11% 20% 14% 2	2133
October	1st to 10th	Night	38% 4 7% 23% 25% 3	3800
	11th to 20th	Night	26% 4 5% 19% 31% 15%	3779
	21st to end	Night	21% 3 7% 30% 26% 13%	4120
November	1st to 10th	Night	38% 4 9% 22% 23% 4	2419
	11th to 20th	Night	39% 5% 10% 18% 21% 8%	2600
	21st to end	Night	35% 4 8% 26% 22% 5%	2611
December	1st to 10th	Night	45% 5% 10% 19% 17% 4	2660
	11th to 20th	Night	40% 4 9% 20% 21% 6%	3288
	21st to end	Night	37% 5% 12% 24% 20% 2	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	19% 16% 44% 15% 6%	236
	11th to 20th	Day	17% 24% 39% 17% 4	270
	21st to end	Day	17% 21% 42% 15% 4 1	297
February	1st to 10th	Day	16% 19% 39% 20% 5%	270
	11th to 20th	Day	28% 16% 36% 17% 3	443
	21st to end	Day	20% 18% 31% 26% 4 1	382
March	1st to 10th	Day	18% 17% 36% 23% 5% 1	460
	11th to 20th	Day	18% 17% 37% 22% 5% 2	460
	21st to end	Day	18% 14% 35% 24% 8% 2	506
April	1st to 10th	Day	18% 17% 33% 22% 7% 2	460
	11th to 20th	Day	12% 15% 33% 24% 13% 4	452
	21st to end	Day	13% 11% 31% 30% 13% 2	460
May	1st to 10th	Day	14% 14% 32% 24% 14% 3	463
	11th to 20th	Day	16% 7% 26% 31% 15% 5%	487
	21st to end	Day	14% 5% 22% 29% 21% 10%	529
June	1st to 10th	Day	11% 5% 21% 32% 24% 7%	490
	11th to 20th	Day	13% 4 19% 28% 26% 11%	490
	21st to end	Day	8% 4 15% 37% 27% 9%	490
July	1st to 10th	Day	11% 2 10% 32% 34% 12%	485
	11th to 20th	Day	10% 1 9% 30% 36% 13%	484
	21st to end	Day	11% 1 14% 32% 33% 8%	532
August	1st to 10th	Day	14% 4 12% 30% 27% 13%	486
	11th to 20th	Day	9% 4 12% 29% 30% 17%	479
	21st to end	Day	11% 4 12% 30% 29% 15%	539
September	1st to 10th	Day	13% 3 18% 26% 23% 17%	490
	11th to 20th	Day	15% 4 16% 27% 26% 11%	490
	21st to end	Day	22% 7% 22% 22% 20% 8%	490
October	1st to 10th	Day	17% 5% 19% 28% 20% 10%	490
	11th to 20th	Day	12% 8% 25% 31% 20% 4	490
	21st to end	Day	12% 9% 27% 28% 18% 7%	539
November	1st to 10th	Day	16% 7% 32% 26% 14% 4	455
	11th to 20th	Day	17% 11% 30% 29% 12% 1	400
	21st to end	Day	14% 14% 28% 25% 18% 2	385
December	1st to 10th	Day	18% 16% 38% 20% 6% 2	180
	11th to 20th	Day	17% 13% 39% 19% 11% 1	180
	21st to end	Day	22% 10% 34% 19% 14% 1	198
January	1st to 10th	Night	18% 11% 38% 27% 5%	760
	11th to 20th	Night	19% 11% 33% 30% 6% 1	760
	21st to end	Night	18% 8% 43% 27% 3	836
February	1st to 10th	Night	20% 9% 38% 29% 4	760
	11th to 20th	Night	23% 13% 41% 21% 2	743
	21st to end	Night	18% 13% 36% 26% 6%	628
March	1st to 10th	Night	18% 9% 37% 29% 7%	760
	11th to 20th	Night	19% 11% 40% 24% 7%	760
	21st to end	Night	17% 10% 38% 29% 6%	836
April	1st to 10th	Night	16% 10% 39% 29% 5%	760
	11th to 20th	Night	13% 11% 35% 31% 11%	751
	21st to end	Night	15% 6% 34% 37% 8% 1	760
May	1st to 10th	Night	13% 9% 32% 36% 9% 1	760
	11th to 20th	Night	16% 6% 22% 40% 15% 1	561
	21st to end	Night	13% 1 15% 43% 25% 3	411
June	1st to 10th	Night	12% 3 16% 45% 23% 1	380
	11th to 20th	Night	9% 3 12% 44% 28% 3	380
	21st to end	Night	8% 3 13% 49% 27% 1	380
July	1st to 10th	Night	8% 1 11% 44% 35% 2	370
	11th to 20th	Night	9% 1 6% 39% 37% 7%	527
	21st to end	Night	11% 2 12% 41% 31% 3	624
August	1st to 10th	Night	13% 2 10% 38% 33% 4	591
	11th to 20th	Night	11% 1 9% 40% 34% 5%	750
	21st to end	Night	13% 2 9% 31% 38% 7%	836
September	1st to 10th	Night	11% 3 11% 35% 31% 9%	760
	11th to 20th	Night	16% 2 13% 33% 31% 4	760
	21st to end	Night	21% 5% 14% 32% 25% 2	760
October	1st to 10th	Night	14% 3 18% 31% 27% 6%	760
	11th to 20th	Night	15% 6% 23% 34% 20% 2	760
	21st to end	Night	14% 6% 24% 34% 20% 4	836
November	1st to 10th	Night	17% 4 24% 36% 15% 4	780
	11th to 20th	Night	19% 7% 29% 32% 11% 1	800
	21st to end	Night	13% 7% 32% 32% 15% 2	800
December	1st to 10th	Night	19% 9% 32% 30% 9%	760
	11th to 20th	Night	17% 8% 33% 33% 9%	760
	21st to end	Night	18% 8% 33% 33% 8%	836

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