PFO Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PFO Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	20% 9% 3 7% 4 4 4 3 5% 6% 33%	127390
	Night	21% 11% 3 7% 4 4 4 3 5% 6% 33%	124421
Transparency	Day	33% 39% 7% 13% 7% 1	130923
	Night	34% 38% 6% 13% 7% 1	128975
Seeing	Day	27% 18% 30% 18% 6% 1	18028
	Night	27% 15% 29% 22% 6%	25968

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	14% 13% 4 9% 5% 5% 4 4 5% 6% 32%	3227
	11th to 20th	Day	14% 14% 4 9% 5% 4 3 2 3 5% 37%	3552
	21st to end	Day	12% 9% 3 7% 6% 4 5% 4 6% 7% 39%	4462
February	1st to 10th	Day	16% 13% 3 6% 5% 4 5% 4 7% 6% 32%	1800
	11th to 20th	Day	15% 11% 3 8% 6% 4 4 4 6% 7% 32%	1753
	21st to end	Day	12% 11% 4 7% 5% 5% 4 4 4 7% 37%	1592
March	1st to 10th	Day	24% 11% 3 10% 5% 4 4 3 4 4 28%	4769
	11th to 20th	Day	20% 12% 3 7% 4 5% 4 3 6% 7% 28%	4980
	21st to end	Day	28% 11% 4 7% 4 4 4 4 5% 5% 25%	5466
April	1st to 10th	Day	21% 12% 4 7% 6% 4 4 3 5% 6% 29%	2319
	11th to 20th	Day	20% 10% 3 7% 5% 4 3 3 4 7% 35%	2572
	21st to end	Day	23% 7% 3 7% 5% 3 3 4 5% 7% 33%	2600
May	1st to 10th	Day	27% 12% 3 8% 6% 5% 4 4 4 6% 21%	5886
	11th to 20th	Day	23% 7% 3 7% 4 4 4 3 5% 6% 32%	6195
	21st to end	Day	21% 9% 3 8% 5% 4 5% 4 6% 7% 28%	6988
June	1st to 10th	Day	23% 9% 2 7% 5% 5% 4 4 5% 7% 29%	3000
	11th to 20th	Day	21% 8% 3 6% 5% 3 4 3 5% 7% 36%	3000
	21st to end	Day	29% 8% 2 6% 5% 4 4 3 6% 6% 28%	3000
July	1st to 10th	Day	19% 10% 3 8% 5% 5% 5% 4 5% 8% 29%	6820
	11th to 20th	Day	22% 11% 2 5% 4 3 4 3 5% 7% 35%	6802
	21st to end	Day	17% 8% 2 5% 4 4 4 4 6% 8% 37%	6452
August	1st to 10th	Day	23% 9% 3 7% 4 4 4 4 5% 7% 31%	4235
	11th to 20th	Day	18% 7% 2 5% 5% 5% 4 4 7% 7% 37%	4175
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 7% 45%	4652
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 8% 43%	2385
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 5% 6% 47%	2200
	21st to end	Day	20% 5% 2 5% 4 3 3 3 4 6% 45%	2200
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	2024
	11th to 20th	Day	19% 7% 2 7% 4 4 4 4 5% 5% 39%	2000
	21st to end	Day	20% 9% 3 7% 4 4 4 3 5% 6% 34%	2109
November	1st to 10th	Day	22% 8% 2 6% 4 4 3 3 4 6% 35%	1777
	11th to 20th	Day	20% 7% 3 6% 4 3 4 2 7% 6% 38%	1680
	21st to end	Day	23% 10% 2 5% 5% 4 4 3 5% 7% 32%	1680
December	1st to 10th	Day	22% 10% 2 6% 3 3 3 3 5% 6% 36%	2690
	11th to 20th	Day	25% 11% 3 7% 4 3 3 2 3 4 37%	2795
	21st to end	Day	22% 10% 3 4 3 3 4 3 6% 6% 37%	3553
January	1st to 10th	Night	13% 12% 4 8% 6% 5% 5% 4 5% 7% 32%	6958
	11th to 20th	Night	17% 11% 3 9% 5% 4 4 4 5% 6% 33%	6465
	21st to end	Night	12% 11% 3 7% 4 4 4 4 6% 7% 39%	6490
February	1st to 10th	Night	18% 12% 3 6% 5% 3 4 4 6% 7% 32%	2600
	11th to 20th	Night	16% 12% 3 8% 4 5% 6% 4 5% 6% 31%	2533
	21st to end	Night	16% 10% 3 8% 6% 4 5% 4 5% 5% 35%	2035
March	1st to 10th	Night	26% 11% 4 7% 4 5% 4 3 4 4 28%	5056
	11th to 20th	Night	21% 13% 4 7% 5% 4 4 4 5% 6% 28%	4979
	21st to end	Night	31% 10% 3 6% 3 3 4 3 3 5% 29%	5466
April	1st to 10th	Night	23% 11% 3 8% 5% 5% 4 3 5% 4 30%	2076
	11th to 20th	Night	25% 7% 3 6% 4 4 4 3 5% 6% 34%	1778
	21st to end	Night	25% 9% 2 7% 4 4 5% 3 5% 5% 31%	1800
May	1st to 10th	Night	28% 11% 3 7% 4 4 4 4 5% 6% 24%	4054
	11th to 20th	Night	23% 8% 2 6% 5% 4 4 4 5% 6% 32%	3711
	21st to end	Night	20% 13% 4 8% 4 4 4 3 4 6% 30%	3948
June	1st to 10th	Night	26% 11% 3 7% 4 3 5% 3 6% 7% 25%	1402
	11th to 20th	Night	22% 8% 3 8% 5% 3 3 4 4 7% 34%	1400
	21st to end	Night	30% 10% 4 6% 4 4 4 2 4 5% 26%	1400
July	1st to 10th	Night	23% 13% 4 9% 5% 5% 4 4 6% 6% 21%	3160
	11th to 20th	Night	23% 18% 3 6% 3 4 4 3 4 6% 25%	3173
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 7% 35%	4354
August	1st to 10th	Night	25% 12% 3 7% 4 4 3 3 5% 6% 27%	2915
	11th to 20th	Night	18% 11% 2 7% 5% 4 4 4 5% 7% 33%	2872
	21st to end	Night	13% 9% 2 5% 4 4 4 3 6% 6% 44%	3245
September	1st to 10th	Night	19% 7% 2 5% 4 4 3 3 4 7% 41%	2010
	11th to 20th	Night	18% 8% 2 5% 3 3 3 3 5% 7% 44%	2200
	21st to end	Night	21% 7% 2 5% 3 3 4 3 5% 6% 42%	2200
October	1st to 10th	Night	20% 6% 2 5% 3 3 4 3 5% 6% 42%	2375
	11th to 20th	Night	19% 8% 2 6% 4 3 3 3 5% 6% 40%	2400
	21st to end	Night	22% 10% 3 6% 4 4 4 3 4 5% 34%	2730
November	1st to 10th	Night	22% 7% 2 6% 4 3 4 4 5% 6% 38%	2740
	11th to 20th	Night	21% 8% 3 6% 4 4 4 4 5% 6% 35%	2940
	21st to end	Night	21% 9% 3 7% 4 4 4 4 5% 7% 34%	2940
December	1st to 10th	Night	18% 11% 4 6% 3 3 4 3 4 6% 37%	4720
	11th to 20th	Night	25% 10% 2 5% 3 3 3 2 4 5% 37%	5629
	21st to end	Night	22% 12% 3 7% 4 4 4 3 4 5% 34%	7667

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	43% 28% 10% 14% 5%	3228
	11th to 20th	Day	38% 28% 8% 19% 7% 1	3582
	21st to end	Day	34% 30% 7% 10% 16% 4	4462
February	1st to 10th	Day	22% 62% 3 7% 4 2	1800
	11th to 20th	Day	21% 61% 4 8% 5%	1761
	21st to end	Day	18% 61% 4 10% 5% 2	1592
March	1st to 10th	Day	45% 29% 6% 9% 8% 3	4773
	11th to 20th	Day	40% 32% 11% 11% 5% 1	4980
	21st to end	Day	49% 29% 4 10% 8% 1	5466
April	1st to 10th	Day	22% 62% 3 7% 5% 2	2319
	11th to 20th	Day	21% 62% 4 9% 4	2572
	21st to end	Day	20% 62% 4 8% 5% 1	2600
May	1st to 10th	Day	46% 34% 6% 11% 3	5897
	11th to 20th	Day	39% 32% 10% 14% 4 1	6199
	21st to end	Day	41% 32% 8% 11% 8%	6988
June	1st to 10th	Day	19% 63% 5% 9% 4	3000
	11th to 20th	Day	19% 62% 4 10% 5%	3000
	21st to end	Day	20% 60% 4 9% 7% 1	3000
July	1st to 10th	Day	45% 27% 9% 12% 7%	6745
	11th to 20th	Day	40% 27% 10% 18% 5% 1	6721
	21st to end	Day	34% 28% 11% 19% 8%	6268
August	1st to 10th	Day	37% 38% 7% 14% 5%	4222
	11th to 20th	Day	29% 39% 11% 14% 6%	4214
	21st to end	Day	23% 38% 9% 17% 12% 1	4652
September	1st to 10th	Day	16% 58% 6% 12% 7% 1	2385
	11th to 20th	Day	19% 59% 5% 11% 6% 1	2200
	21st to end	Day	14% 57% 5% 14% 9% 1	2200
October	1st to 10th	Day	28% 39% 7% 20% 6%	3297
	11th to 20th	Day	28% 37% 5% 23% 7%	3240
	21st to end	Day	25% 37% 12% 13% 12%	3385
November	1st to 10th	Day	26% 54% 6% 10% 3	1777
	11th to 20th	Day	24% 54% 6% 8% 6% 1	1680
	21st to end	Day	27% 55% 6% 9% 4	1680
December	1st to 10th	Day	32% 36% 11% 16% 5%	2690
	11th to 20th	Day	40% 30% 6% 10% 10% 3	2795
	21st to end	Day	35% 27% 8% 17% 11% 2	3553
January	1st to 10th	Night	38% 28% 8% 21% 4 1	6958
	11th to 20th	Night	39% 28% 9% 19% 6%	6510
	21st to end	Night	32% 30% 5% 17% 13% 3	6490
February	1st to 10th	Night	21% 63% 3 6% 7% 1	2600
	11th to 20th	Night	21% 61% 3 9% 6% 1	2536
	21st to end	Night	20% 62% 2 7% 7% 2	2035
March	1st to 10th	Night	47% 29% 3 9% 9% 3	5065
	11th to 20th	Night	42% 32% 10% 10% 6% 1	4980
	21st to end	Night	49% 28% 3 12% 8% 1	5466
April	1st to 10th	Night	22% 62% 3 6% 6% 3	2076
	11th to 20th	Night	22% 62% 4 8% 5%	1778
	21st to end	Night	22% 62% 4 6% 6% 1	1800
May	1st to 10th	Night	47% 32% 7% 9% 5%	4060
	11th to 20th	Night	39% 31% 9% 16% 4 2	3712
	21st to end	Night	41% 31% 10% 13% 5% 1	3949
June	1st to 10th	Night	20% 63% 4 9% 4	1402
	11th to 20th	Night	19% 61% 6% 10% 4	1400
	21st to end	Night	23% 60% 3 8% 6% 1	1400
July	1st to 10th	Night	53% 26% 7% 8% 7%	3125
	11th to 20th	Night	49% 26% 7% 12% 5%	3131
	21st to end	Night	37% 27% 10% 16% 10% 1	4224
August	1st to 10th	Night	42% 37% 5% 12% 4	2903
	11th to 20th	Night	33% 38% 11% 13% 5%	2899
	21st to end	Night	27% 37% 7% 16% 13% 1	3244
September	1st to 10th	Night	17% 57% 5% 13% 7% 1	2010
	11th to 20th	Night	19% 57% 6% 11% 7%	2200
	21st to end	Night	16% 58% 4 15% 7% 1	2200
October	1st to 10th	Night	30% 37% 9% 16% 8% 1	3882
	11th to 20th	Night	23% 37% 7% 20% 13%	3899
	21st to end	Night	29% 36% 8% 16% 10%	4404
November	1st to 10th	Night	25% 54% 6% 10% 5%	2740
	11th to 20th	Night	24% 55% 5% 10% 6% 1	2940
	21st to end	Night	25% 55% 5% 7% 7% 1	2940
December	1st to 10th	Night	30% 36% 6% 20% 7%	4720
	11th to 20th	Night	37% 31% 5% 14% 11% 2	5630
	21st to end	Night	40% 27% 7% 14% 10% 3	7667

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	30% 28% 28% 13% 1	220
	11th to 20th	Day	38% 19% 30% 8% 4	212
	21st to end	Day	21% 24% 37% 15% 3	436
February	1st to 10th	Day	28% 25% 30% 14% 3	400
	11th to 20th	Day	27% 29% 30% 13% 2 1	396
	21st to end	Day	22% 28% 34% 15% 2	386
March	1st to 10th	Day	27% 24% 34% 12% 3	500
	11th to 20th	Day	27% 21% 34% 14% 3 1	500
	21st to end	Day	32% 23% 29% 10% 5% 1	550
April	1st to 10th	Day	37% 27% 25% 9% 3	500
	11th to 20th	Day	28% 26% 28% 13% 4 1	492
	21st to end	Day	28% 21% 32% 13% 5% 1	500
May	1st to 10th	Day	29% 22% 31% 14% 4	503
	11th to 20th	Day	27% 18% 29% 20% 5% 1	666
	21st to end	Day	30% 17% 29% 17% 5% 1	790
June	1st to 10th	Day	30% 16% 28% 20% 6%	730
	11th to 20th	Day	27% 14% 30% 23% 6% 1	730
	21st to end	Day	34% 9% 30% 19% 7% 1	730
July	1st to 10th	Day	27% 11% 33% 22% 8%	720
	11th to 20th	Day	29% 9% 26% 25% 10% 2	713
	21st to end	Day	26% 11% 36% 21% 6% 1	576
August	1st to 10th	Day	22% 10% 31% 31% 6% 1	526
	11th to 20th	Day	26% 8% 29% 28% 8% 2	519
	21st to end	Day	21% 12% 30% 25% 9% 2	583
September	1st to 10th	Day	20% 10% 35% 25% 8% 2	530
	11th to 20th	Day	24% 13% 29% 22% 9% 3	530
	21st to end	Day	21% 13% 35% 21% 9% 1	530
October	1st to 10th	Day	23% 13% 28% 22% 12% 2	433
	11th to 20th	Day	23% 20% 27% 22% 8% 1	420
	21st to end	Day	26% 20% 32% 17% 4 1	462
November	1st to 10th	Day	30% 19% 30% 15% 4 1	425
	11th to 20th	Day	32% 22% 27% 15% 4	430
	21st to end	Day	33% 23% 30% 11% 3	430
December	1st to 10th	Day	31% 20% 30% 16% 3	430
	11th to 20th	Day	36% 28% 24% 10% 1	288
	21st to end	Day	31% 26% 30% 10% 3	242
January	1st to 10th	Night	27% 22% 31% 17% 3	950
	11th to 20th	Night	30% 20% 28% 18% 4	839
	21st to end	Night	20% 25% 35% 16% 4	880
February	1st to 10th	Night	27% 23% 30% 18% 2	800
	11th to 20th	Night	26% 24% 31% 16% 2	783
	21st to end	Night	25% 18% 31% 23% 3	660
March	1st to 10th	Night	26% 19% 36% 18% 2	800
	11th to 20th	Night	25% 17% 32% 21% 4	800
	21st to end	Night	26% 18% 32% 18% 6%	880
April	1st to 10th	Night	31% 17% 33% 17% 2	696
	11th to 20th	Night	29% 14% 34% 19% 4	594
	21st to end	Night	30% 13% 32% 20% 5%	600
May	1st to 10th	Night	28% 11% 31% 24% 5% 1	600
	11th to 20th	Night	30% 11% 30% 23% 5%	592
	21st to end	Night	28% 11% 35% 22% 4 1	649
June	1st to 10th	Night	33% 7% 26% 26% 7%	402
	11th to 20th	Night	26% 7% 27% 34% 7%	400
	21st to end	Night	39% 3 21% 30% 7% 1	400
July	1st to 10th	Night	28% 7% 24% 32% 9% 1	390
	11th to 20th	Night	32% 4 21% 31% 11% 1	388
	21st to end	Night	27% 6% 26% 30% 10%	621
August	1st to 10th	Night	22% 4 25% 36% 12% 1	598
	11th to 20th	Night	28% 5% 23% 31% 12% 1	592
	21st to end	Night	21% 7% 25% 31% 15% 1	660
September	1st to 10th	Night	24% 4 31% 29% 10% 1	610
	11th to 20th	Night	24% 9% 22% 28% 15% 2	800
	21st to end	Night	25% 8% 28% 26% 13% 1	800
October	1st to 10th	Night	24% 9% 26% 29% 11% 2	800
	11th to 20th	Night	23% 15% 29% 22% 10% 1	800
	21st to end	Night	28% 18% 28% 22% 3	880
November	1st to 10th	Night	29% 15% 29% 19% 7% 1	859
	11th to 20th	Night	27% 21% 26% 20% 6% 1	950
	21st to end	Night	30% 21% 25% 19% 4	950
December	1st to 10th	Night	33% 18% 30% 15% 3	950
	11th to 20th	Night	32% 25% 29% 12% 2	950
	21st to end	Night	30% 22% 28% 16% 4	1045

Page generated at 2026-01-14 02:14:26. Version ac07

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