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 2024-11-25.

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 6% 5% 4 4 3 5% 7% 34%	101692
	Night	21% 10% 3 7% 4 4 4 3 5% 6% 32%	97849
Transparency	Day	28% 47% 7% 11% 6% 1	105239
	Night	29% 47% 6% 11% 6% 1	102391
Seeing	Day	27% 18% 30% 18% 6% 1	16557
	Night	28% 15% 29% 22% 6%	24393

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% 11% 3 7% 5% 5% 4 3 4 5% 37%	2207
	11th to 20th	Day	17% 12% 3 6% 4 3 3 2 3 4 43%	2468
	21st to end	Day	15% 9% 2 7% 6% 5% 5% 4 6% 7% 35%	3120
February	1st to 10th	Day	16% 13% 2 6% 5% 4 5% 4 7% 6% 31%	1710
	11th to 20th	Day	15% 12% 3 8% 6% 4 4 4 6% 7% 32%	1663
	21st to end	Day	12% 11% 4 7% 5% 4 4 4 4 7% 37%	1514
March	1st to 10th	Day	22% 12% 4 7% 5% 4 3 3 4 5% 30%	3325
	11th to 20th	Day	17% 14% 4 9% 6% 6% 5% 4 6% 7% 25%	3480
	21st to end	Day	30% 10% 3 7% 4 4 4 4 4 4 25%	3816
April	1st to 10th	Day	21% 13% 4 6% 6% 4 4 3 5% 6% 29%	2203
	11th to 20th	Day	19% 9% 3 6% 5% 4 3 3 4 7% 36%	2442
	21st to end	Day	23% 7% 3 7% 5% 3 3 4 4 7% 32%	2470
May	1st to 10th	Day	20% 10% 3 7% 6% 5% 5% 4 5% 8% 27%	4106
	11th to 20th	Day	18% 7% 3 7% 5% 4 4 3 6% 7% 36%	4317
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 6% 8% 29%	4862
June	1st to 10th	Day	23% 9% 2 7% 5% 5% 4 4 5% 6% 28%	2850
	11th to 20th	Day	21% 9% 3 6% 5% 3 4 3 5% 6% 35%	2850
	21st to end	Day	29% 8% 2 6% 5% 4 4 3 6% 6% 28%	2850
July	1st to 10th	Day	19% 9% 3 7% 4 5% 4 4 6% 8% 31%	4760
	11th to 20th	Day	23% 11% 2 5% 4 3 4 3 5% 7% 32%	4742
	21st to end	Day	22% 9% 2 6% 4 4 4 3 6% 6% 33%	4480
August	1st to 10th	Day	24% 10% 3 7% 4 4 4 4 5% 7% 29%	4105
	11th to 20th	Day	19% 7% 2 5% 5% 5% 4 4 7% 7% 36%	4045
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 8% 45%	4510
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 8% 44%	2266
	11th to 20th	Day	19% 6% 1 4 3 3 3 3 5% 6% 47%	2090
	21st to end	Day	21% 5% 2 5% 4 3 3 3 4 6% 44%	2090
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 40%	1923
	11th to 20th	Day	20% 6% 2 6% 4 4 4 4 5% 5% 40%	1900
	21st to end	Day	20% 9% 3 6% 4 4 4 3 5% 6% 35%	2004
November	1st to 10th	Day	22% 8% 3 6% 4 4 3 3 4 6% 35%	1691
	11th to 20th	Day	20% 7% 2 6% 4 3 3 3 7% 7% 38%	1600
	21st to end	Day	24% 10% 2 5% 4 4 4 3 5% 7% 32%	1552
December	1st to 10th	Day	26% 10% 2 5% 3 4 4 3 6% 5% 30%	1520
	11th to 20th	Day	25% 13% 3 8% 4 3 3 3 4 5% 29%	1730
	21st to end	Day	23% 9% 3 5% 4 3 4 3 6% 6% 35%	2431
January	1st to 10th	Night	13% 11% 3 7% 4 4 5% 4 5% 7% 37%	4748
	11th to 20th	Night	17% 11% 3 8% 5% 4 4 4 5% 6% 33%	4490
	21st to end	Night	16% 12% 3 8% 5% 4 5% 4 6% 7% 31%	4532
February	1st to 10th	Night	19% 12% 3 6% 5% 4 4 4 6% 7% 32%	2470
	11th to 20th	Night	16% 12% 3 8% 4 5% 6% 4 5% 6% 31%	2403
	21st to end	Night	16% 10% 3 8% 6% 4 5% 4 5% 6% 34%	1937
March	1st to 10th	Night	24% 11% 4 7% 4 5% 5% 3 4 4 29%	3514
	11th to 20th	Night	19% 13% 4 8% 7% 5% 5% 4 6% 6% 24%	3479
	21st to end	Night	32% 10% 3 6% 4 4 4 3 4 4 27%	3816
April	1st to 10th	Night	23% 11% 3 8% 5% 4 4 3 5% 4 30%	1972
	11th to 20th	Night	24% 7% 3 6% 4 4 4 3 5% 6% 34%	1688
	21st to end	Night	25% 9% 2 7% 4 4 5% 3 5% 5% 31%	1710
May	1st to 10th	Night	21% 8% 3 8% 5% 5% 4 4 6% 8% 28%	2834
	11th to 20th	Night	19% 7% 3 6% 5% 5% 5% 4 6% 6% 34%	2582
	21st to end	Night	19% 11% 4 9% 5% 4 4 3 5% 7% 29%	2749
June	1st to 10th	Night	26% 11% 3 7% 4 3 5% 4 5% 7% 24%	1332
	11th to 20th	Night	22% 8% 3 8% 5% 3 3 4 4 7% 33%	1330
	21st to end	Night	31% 10% 3 6% 4 5% 4 2 5% 5% 26%	1330
July	1st to 10th	Night	24% 14% 4 9% 5% 4 4 3 5% 5% 23%	2210
	11th to 20th	Night	26% 16% 2 6% 3 4 4 3 4 6% 25%	2223
	21st to end	Night	22% 9% 3 6% 4 4 5% 3 5% 7% 31%	3041
August	1st to 10th	Night	25% 13% 3 7% 5% 4 3 3 5% 6% 25%	2825
	11th to 20th	Night	19% 11% 2 7% 5% 4 4 3 5% 7% 32%	2782
	21st to end	Night	13% 9% 2 5% 4 4 4 3 6% 6% 44%	3145
September	1st to 10th	Night	19% 7% 2 5% 4 4 3 3 4 7% 41%	1909
	11th to 20th	Night	19% 8% 2 5% 3 2 3 3 5% 6% 43%	2090
	21st to end	Night	21% 7% 2 6% 3 3 4 3 5% 6% 42%	2090
October	1st to 10th	Night	21% 6% 2 5% 4 3 4 3 5% 6% 40%	2256
	11th to 20th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 40%	2280
	21st to end	Night	22% 10% 3 6% 4 4 4 3 4 5% 35%	2593
November	1st to 10th	Night	22% 6% 2 6% 4 3 4 4 5% 6% 39%	2606
	11th to 20th	Night	21% 7% 3 6% 4 4 4 4 5% 6% 35%	2800
	21st to end	Night	22% 9% 2 6% 5% 4 4 4 5% 7% 33%	2723
December	1st to 10th	Night	27% 12% 3 6% 4 3 5% 3 4 5% 28%	2660
	11th to 20th	Night	28% 10% 2 6% 4 4 4 3 5% 6% 29%	3464
	21st to end	Night	22% 11% 3 7% 4 4 4 3 4 5% 33%	5236

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% 38% 12% 12% 6%	2208
	11th to 20th	Day	27% 38% 5% 20% 10% 1	2468
	21st to end	Day	31% 43% 9% 9% 9%	3120
February	1st to 10th	Day	21% 65% 3 7% 3 1	1710
	11th to 20th	Day	19% 64% 4 7% 5%	1671
	21st to end	Day	16% 65% 4 9% 5% 2	1514
March	1st to 10th	Day	35% 40% 6% 11% 3 4	3329
	11th to 20th	Day	37% 44% 7% 7% 5% 1	3480
	21st to end	Day	44% 39% 3 6% 6% 1	3816
April	1st to 10th	Day	20% 65% 3 7% 4 2	2203
	11th to 20th	Day	18% 65% 4 8% 4	2442
	21st to end	Day	19% 65% 4 6% 5% 1	2470
May	1st to 10th	Day	30% 48% 8% 13% 2	4120
	11th to 20th	Day	29% 43% 12% 13% 4	4321
	21st to end	Day	32% 44% 10% 7% 7%	4862
June	1st to 10th	Day	18% 66% 4 8% 3	2850
	11th to 20th	Day	18% 65% 4 9% 4	2850
	21st to end	Day	18% 64% 4 8% 6%	2850
July	1st to 10th	Day	38% 37% 7% 11% 7%	4685
	11th to 20th	Day	37% 38% 6% 14% 5%	4661
	21st to end	Day	33% 38% 8% 16% 5%	4337
August	1st to 10th	Day	38% 39% 7% 13% 4	4092
	11th to 20th	Day	29% 41% 11% 14% 6%	4084
	21st to end	Day	23% 39% 9% 16% 13% 1	4510
September	1st to 10th	Day	15% 61% 6% 11% 7% 1	2266
	11th to 20th	Day	18% 61% 5% 10% 5% 1	2090
	21st to end	Day	13% 60% 5% 13% 8% 1	2090
October	1st to 10th	Day	29% 40% 7% 20% 5%	3196
	11th to 20th	Day	27% 38% 5% 23% 7%	3140
	21st to end	Day	24% 38% 12% 13% 12%	3280
November	1st to 10th	Day	25% 56% 6% 10% 3	1691
	11th to 20th	Day	23% 57% 6% 7% 6% 2	1600
	21st to end	Day	24% 59% 5% 8% 3	1552
December	1st to 10th	Day	22% 60% 4 9% 4 1	1520
	11th to 20th	Day	35% 48% 4 6% 7%	1730
	21st to end	Day	31% 38% 8% 12% 8% 2	2431
January	1st to 10th	Night	27% 38% 10% 20% 5% 1	4748
	11th to 20th	Night	33% 38% 6% 15% 7% 1	4490
	21st to end	Night	34% 41% 5% 14% 6%	4532
February	1st to 10th	Night	20% 66% 3 6% 6%	2470
	11th to 20th	Night	19% 64% 3 8% 6%	2406
	21st to end	Night	19% 65% 2 6% 6% 2	1937
March	1st to 10th	Night	37% 41% 3 11% 6% 1	3523
	11th to 20th	Night	38% 44% 6% 7% 5% 1	3480
	21st to end	Night	44% 39% 3 7% 5% 1	3816
April	1st to 10th	Night	20% 65% 3 5% 5% 3	1972
	11th to 20th	Night	20% 65% 4 7% 5%	1688
	21st to end	Night	20% 65% 4 5% 6% 1	1710
May	1st to 10th	Night	33% 45% 10% 10% 2 1	2840
	11th to 20th	Night	28% 44% 11% 14% 3	2583
	21st to end	Night	34% 43% 10% 9% 3	2750
June	1st to 10th	Night	19% 66% 4 7% 4	1332
	11th to 20th	Night	18% 64% 5% 9% 3	1330
	21st to end	Night	21% 63% 3 7% 5% 1	1330
July	1st to 10th	Night	46% 36% 4 6% 9%	2175
	11th to 20th	Night	44% 36% 4 11% 4	2181
	21st to end	Night	33% 37% 8% 13% 8% 1	2944
August	1st to 10th	Night	43% 38% 5% 10% 3	2813
	11th to 20th	Night	33% 39% 10% 12% 4	2809
	21st to end	Night	27% 38% 6% 15% 13% 1	3144
September	1st to 10th	Night	16% 60% 5% 12% 6% 1	1909
	11th to 20th	Night	19% 59% 6% 10% 6%	2090
	21st to end	Night	16% 60% 3 14% 6% 1	2090
October	1st to 10th	Night	30% 38% 9% 15% 7% 1	3763
	11th to 20th	Night	22% 38% 7% 20% 13%	3779
	21st to end	Night	28% 37% 8% 16% 10%	4267
November	1st to 10th	Night	23% 56% 6% 10% 5%	2606
	11th to 20th	Night	22% 57% 5% 9% 7% 1	2800
	21st to end	Night	24% 59% 4 6% 7% 1	2723
December	1st to 10th	Night	24% 60% 3 7% 5%	2660
	11th to 20th	Night	33% 48% 5% 9% 5%	3465
	21st to end	Night	35% 38% 7% 12% 4 3	5236

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	29% 27% 28% 14% 1	180
	11th to 20th	Day	37% 21% 32% 8% 2	180
	21st to end	Day	22% 23% 37% 15% 3	403
February	1st to 10th	Day	29% 24% 30% 14% 3	370
	11th to 20th	Day	26% 30% 29% 13% 2 1	366
	21st to end	Day	21% 28% 34% 15% 2	356
March	1st to 10th	Day	26% 25% 35% 12% 3	460
	11th to 20th	Day	26% 22% 33% 14% 3 1	460
	21st to end	Day	32% 23% 29% 10% 5% 1	506
April	1st to 10th	Day	36% 27% 25% 10% 3	460
	11th to 20th	Day	26% 27% 29% 14% 4	452
	21st to end	Day	27% 21% 31% 14% 6% 1	460
May	1st to 10th	Day	28% 23% 32% 13% 4	463
	11th to 20th	Day	26% 18% 29% 21% 5% 1	619
	21st to end	Day	29% 18% 29% 18% 5% 1	735
June	1st to 10th	Day	30% 16% 29% 19% 7%	680
	11th to 20th	Day	27% 13% 30% 24% 6% 1	680
	21st to end	Day	34% 10% 28% 19% 7% 1	680
July	1st to 10th	Day	27% 11% 32% 22% 8%	670
	11th to 20th	Day	29% 8% 26% 25% 10% 2	663
	21st to end	Day	27% 10% 35% 22% 6% 1	532
August	1st to 10th	Day	23% 10% 30% 30% 6% 1	486
	11th to 20th	Day	27% 8% 29% 27% 7% 1	479
	21st to end	Day	22% 12% 29% 26% 10% 2	539
September	1st to 10th	Day	19% 11% 36% 25% 8% 1	490
	11th to 20th	Day	25% 12% 29% 21% 9% 3	490
	21st to end	Day	22% 12% 33% 21% 9% 1	490
October	1st to 10th	Day	24% 13% 28% 21% 12% 2	402
	11th to 20th	Day	24% 19% 28% 21% 8% 1	390
	21st to end	Day	25% 19% 33% 17% 4 1	429
November	1st to 10th	Day	29% 19% 30% 15% 5% 2	395
	11th to 20th	Day	31% 23% 27% 15% 4	400
	21st to end	Day	34% 24% 29% 10% 2 1	382
December	1st to 10th	Day	32% 20% 29% 15% 3	370
	11th to 20th	Day	37% 29% 23% 10% 1	242
	21st to end	Day	31% 28% 29% 10% 2 1	198
January	1st to 10th	Night	27% 22% 30% 18% 3	850
	11th to 20th	Night	30% 21% 28% 17% 4	772
	21st to end	Night	21% 24% 35% 16% 4	836
February	1st to 10th	Night	27% 23% 30% 17% 2	760
	11th to 20th	Night	26% 24% 31% 17% 2	743
	21st to end	Night	25% 19% 31% 22% 4	628
March	1st to 10th	Night	25% 19% 37% 18% 2	760
	11th to 20th	Night	25% 18% 31% 21% 4	760
	21st to end	Night	25% 18% 32% 19% 6%	836
April	1st to 10th	Night	31% 16% 33% 17% 3	661
	11th to 20th	Night	28% 14% 34% 19% 4	564
	21st to end	Night	29% 14% 32% 20% 5%	570
May	1st to 10th	Night	27% 11% 32% 24% 5% 1	570
	11th to 20th	Night	29% 11% 30% 24% 6%	562
	21st to end	Night	28% 11% 34% 22% 4 1	616
June	1st to 10th	Night	34% 7% 26% 26% 7%	382
	11th to 20th	Night	26% 8% 25% 34% 7%	380
	21st to end	Night	40% 2 21% 29% 8% 1	380
July	1st to 10th	Night	29% 7% 24% 32% 9% 1	370
	11th to 20th	Night	33% 4 21% 30% 11% 1	368
	21st to end	Night	28% 6% 25% 31% 10%	590
August	1st to 10th	Night	23% 4 26% 35% 12% 1	568
	11th to 20th	Night	29% 5% 23% 31% 12% 1	562
	21st to end	Night	22% 7% 25% 30% 15% 1	627
September	1st to 10th	Night	24% 4 32% 28% 11% 1	579
	11th to 20th	Night	25% 9% 22% 27% 15% 1	760
	21st to end	Night	25% 9% 27% 25% 13% 1	760
October	1st to 10th	Night	25% 9% 26% 29% 10% 2	760
	11th to 20th	Night	23% 15% 30% 21% 10% 1	760
	21st to end	Night	29% 18% 29% 22% 3	836
November	1st to 10th	Night	28% 15% 29% 20% 8% 1	815
	11th to 20th	Night	27% 21% 25% 20% 6% 1	900
	21st to end	Night	31% 22% 24% 19% 5%	873
December	1st to 10th	Night	34% 18% 31% 14% 3	850
	11th to 20th	Night	32% 24% 30% 12% 2	850
	21st to end	Night	31% 22% 27% 15% 4	935

Page generated at 2024-11-22 09:02:26. Version ac07

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