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 2024-01-14.

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% 8% 3 5% 5% 3 4 4 5% 7% 35%	74674
	Night	21% 9% 3 6% 5% 3 4 4 5% 6% 33%	75072
Transparency	Day	51% 6% 11% 20% 11% 1	74379
	Night	53% 6% 10% 19% 11% 1	74902
Seeing	Day	27% 18% 30% 18% 6% 1	15327
	Night	28% 15% 29% 22% 6%	23320

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% 12% 5% 7% 5% 3 5% 4 5% 6% 33%	1327
	11th to 20th	Day	22% 13% 4 6% 6% 3 3 3 3 4 32%	1436
	21st to end	Day	14% 9% 3 5% 6% 2 5% 4 6% 7% 39%	1778
February	1st to 10th	Day	16% 13% 3 5% 6% 3 6% 4 6% 6% 32%	1620
	11th to 20th	Day	16% 11% 3 7% 6% 4 4 5% 6% 7% 31%	1573
	21st to end	Day	12% 10% 4 7% 6% 3 4 5% 5% 7% 37%	1427
March	1st to 10th	Day	15% 10% 5% 8% 6% 4 6% 5% 5% 5% 32%	1908
	11th to 20th	Day	17% 8% 3 6% 5% 3 5% 5% 6% 7% 34%	1980
	21st to end	Day	17% 11% 4 7% 5% 3 4 4 4 6% 36%	2178
April	1st to 10th	Day	21% 12% 5% 6% 6% 3 5% 4 5% 6% 28%	2086
	11th to 20th	Day	20% 8% 3 6% 5% 3 4 3 4 7% 37%	2312
	21st to end	Day	25% 7% 3 6% 6% 3 3 4 4 7% 31%	2340
May	1st to 10th	Day	23% 10% 4 6% 6% 3 5% 5% 4 7% 28%	2340
	11th to 20th	Day	24% 7% 3 4 4 2 4 4 5% 5% 38%	2425
	21st to end	Day	22% 8% 3 6% 5% 3 4 5% 6% 8% 31%	2736
June	1st to 10th	Day	24% 8% 3 6% 6% 4 5% 5% 5% 6% 28%	2700
	11th to 20th	Day	22% 8% 4 6% 5% 2 4 4 4 7% 35%	2700
	21st to end	Day	29% 7% 2 5% 5% 3 4 4 5% 7% 28%	2700
July	1st to 10th	Day	19% 7% 2 5% 5% 3 5% 6% 6% 9% 33%	2700
	11th to 20th	Day	19% 9% 3 5% 5% 3 5% 4 5% 8% 34%	2696
	21st to end	Day	20% 8% 3 5% 5% 3 5% 5% 7% 8% 31%	2561
August	1st to 10th	Day	17% 7% 3 6% 6% 3 4 5% 6% 8% 35%	2327
	11th to 20th	Day	19% 7% 2 5% 5% 3 5% 5% 7% 7% 33%	2304
	21st to end	Day	17% 5% 3 5% 4 3 4 4 6% 7% 42%	2566
September	1st to 10th	Day	15% 5% 2 4 5% 3 4 5% 5% 8% 43%	2147
	11th to 20th	Day	19% 6% 2 4 3 2 3 4 5% 7% 45%	1980
	21st to end	Day	21% 5% 1 4 4 2 3 3 4 6% 45%	1980
October	1st to 10th	Day	18% 7% 2 5% 5% 3 4 4 5% 7% 41%	1822
	11th to 20th	Day	20% 6% 2 6% 5% 3 4 5% 5% 5% 39%	1800
	21st to end	Day	21% 8% 3 6% 6% 3 4 4 5% 6% 34%	1899
November	1st to 10th	Day	23% 7% 3 5% 6% 3 4 3 4 6% 36%	1605
	11th to 20th	Day	20% 6% 3 5% 5% 3 4 3 7% 7% 39%	1520
	21st to end	Day	23% 9% 3 4 5% 3 4 4 4 7% 33%	1520
December	1st to 10th	Day	26% 9% 3 5% 4 3 4 5% 5% 6% 30%	1520
	11th to 20th	Day	25% 12% 4 7% 5% 2 3 3 4 5% 29%	1730
	21st to end	Day	24% 9% 3 5% 4 3 4 3 5% 6% 35%	2431
January	1st to 10th	Night	17% 11% 4 6% 6% 3 5% 5% 5% 7% 31%	2838
	11th to 20th	Night	21% 12% 3 6% 6% 3 5% 5% 4 7% 29%	2593
	21st to end	Night	14% 10% 3 7% 6% 3 4 4 6% 9% 34%	2574
February	1st to 10th	Night	19% 11% 3 5% 6% 2 4 5% 6% 8% 32%	2340
	11th to 20th	Night	17% 12% 4 6% 5% 4 6% 5% 5% 6% 30%	2273
	21st to end	Night	15% 9% 4 7% 6% 3 5% 5% 5% 6% 34%	1826
March	1st to 10th	Night	15% 11% 4 7% 6% 3 5% 5% 5% 6% 34%	2038
	11th to 20th	Night	19% 9% 4 6% 6% 4 5% 4 6% 6% 31%	1979
	21st to end	Night	18% 10% 3 5% 5% 3 4 4 5% 6% 37%	2178
April	1st to 10th	Night	24% 10% 3 7% 5% 3 5% 4 5% 4 30%	1869
	11th to 20th	Night	24% 6% 3 6% 5% 3 4 4 5% 6% 34%	1598
	21st to end	Night	26% 9% 2 6% 5% 2 5% 5% 4 6% 30%	1620
May	1st to 10th	Night	23% 8% 3 6% 6% 3 5% 4 5% 8% 29%	1620
	11th to 20th	Night	26% 7% 2 5% 6% 3 4 5% 4 5% 31%	1466
	21st to end	Night	21% 10% 4 7% 5% 3 4 4 4 7% 30%	1550
June	1st to 10th	Night	27% 11% 3 6% 5% 3 5% 5% 5% 7% 24%	1262
	11th to 20th	Night	23% 7% 4 7% 6% 3 3 4 4 6% 33%	1260
	21st to end	Night	31% 9% 4 5% 5% 3 4 3 4 5% 27%	1260
July	1st to 10th	Night	22% 9% 4 7% 5% 2 6% 5% 5% 7% 28%	1260
	11th to 20th	Night	24% 10% 3 6% 5% 3 5% 4 6% 7% 28%	1260
	21st to end	Night	25% 10% 4 5% 6% 3 6% 5% 5% 7% 25%	1738
August	1st to 10th	Night	17% 7% 3 7% 6% 4 4 5% 6% 8% 34%	1612
	11th to 20th	Night	23% 10% 3 5% 5% 3 4 5% 6% 7% 30%	1596
	21st to end	Night	16% 8% 2 4 5% 2 4 5% 6% 7% 40%	1789
September	1st to 10th	Night	19% 7% 2 5% 4 2 4 4 4 7% 41%	1808
	11th to 20th	Night	20% 8% 2 4 4 2 3 4 6% 7% 42%	1980
	21st to end	Night	21% 6% 2 4 4 2 4 4 4 6% 43%	1980
October	1st to 10th	Night	21% 6% 2 4 5% 3 4 4 5% 6% 41%	2137
	11th to 20th	Night	19% 7% 3 5% 5% 3 4 4 5% 6% 39%	2160
	21st to end	Night	23% 10% 3 5% 5% 3 4 4 4 6% 34%	2456
November	1st to 10th	Night	23% 6% 2 5% 5% 3 3 5% 5% 6% 38%	2472
	11th to 20th	Night	21% 6% 3 5% 5% 3 4 4 5% 6% 36%	2660
	21st to end	Night	21% 8% 3 5% 5% 3 4 5% 5% 7% 34%	2660
December	1st to 10th	Night	27% 11% 4 6% 5% 2 4 4 4 5% 28%	2660
	11th to 20th	Night	28% 9% 3 5% 5% 3 4 4 4 6% 29%	3464
	21st to end	Night	23% 10% 3 6% 5% 3 4 4 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	53% 7% 13% 15% 11% 1	1328
	11th to 20th	Day	59% 5% 9% 18% 8% 1	1436
	21st to end	Day	45% 7% 9% 22% 16% 1	1778
February	1st to 10th	Day	53% 6% 12% 17% 10% 3	1620
	11th to 20th	Day	54% 5% 12% 17% 10% 2	1581
	21st to end	Day	48% 5% 12% 22% 11% 3	1427
March	1st to 10th	Day	55% 5% 9% 20% 10% 2	1912
	11th to 20th	Day	49% 6% 11% 17% 15% 2	1980
	21st to end	Day	51% 5% 9% 18% 13% 4	2178
April	1st to 10th	Day	58% 5% 9% 14% 13% 2	2086
	11th to 20th	Day	49% 4 10% 21% 14% 2	2312
	21st to end	Day	54% 5% 11% 16% 12% 3	2340
May	1st to 10th	Day	57% 6% 9% 17% 10% 1	2340
	11th to 20th	Day	49% 5% 9% 24% 11% 2	2429
	21st to end	Day	53% 7% 12% 19% 8% 1	2736
June	1st to 10th	Day	57% 7% 9% 17% 8% 1	2700
	11th to 20th	Day	51% 6% 11% 20% 11% 2	2700
	21st to end	Day	56% 8% 10% 15% 10% 1	2700
July	1st to 10th	Day	49% 8% 16% 17% 10% 1	2625
	11th to 20th	Day	48% 9% 15% 19% 8% 1	2615
	21st to end	Day	50% 8% 17% 19% 6%	2418
August	1st to 10th	Day	47% 9% 15% 20% 9%	2314
	11th to 20th	Day	49% 9% 13% 21% 8%	2304
	21st to end	Day	42% 7% 14% 24% 12% 1	2566
September	1st to 10th	Day	40% 8% 13% 24% 14% 1	2147
	11th to 20th	Day	41% 6% 13% 23% 15% 3	1980
	21st to end	Day	42% 5% 8% 27% 17% 1	1980
October	1st to 10th	Day	45% 6% 10% 26% 13%	1822
	11th to 20th	Day	47% 6% 12% 26% 9%	1800
	21st to end	Day	51% 5% 11% 20% 13%	1899
November	1st to 10th	Day	52% 5% 10% 21% 11% 1	1605
	11th to 20th	Day	46% 6% 14% 18% 14% 2	1520
	21st to end	Day	54% 6% 12% 20% 7%	1520
December	1st to 10th	Day	57% 7% 10% 19% 7% 1	1520
	11th to 20th	Day	60% 5% 9% 15% 10% 1	1730
	21st to end	Day	52% 5% 13% 18% 10% 2	2431
January	1st to 10th	Night	55% 7% 11% 18% 9% 1	2838
	11th to 20th	Night	58% 7% 10% 16% 8% 1	2593
	21st to end	Night	48% 6% 10% 22% 12% 1	2574
February	1st to 10th	Night	51% 6% 12% 17% 12% 1	2340
	11th to 20th	Night	55% 6% 11% 17% 10% 2	2276
	21st to end	Night	51% 6% 9% 19% 13% 3	1826
March	1st to 10th	Night	52% 5% 8% 18% 14% 2	2047
	11th to 20th	Night	53% 6% 11% 14% 13% 3	1980
	21st to end	Night	50% 6% 8% 19% 15% 3	2178
April	1st to 10th	Night	58% 6% 8% 12% 11% 4	1869
	11th to 20th	Night	50% 5% 11% 18% 14% 1	1598
	21st to end	Night	56% 5% 10% 16% 12% 1	1620
May	1st to 10th	Night	56% 6% 9% 17% 10% 2	1620
	11th to 20th	Night	55% 6% 9% 19% 10% 1	1467
	21st to end	Night	55% 6% 10% 17% 11% 1	1551
June	1st to 10th	Night	60% 6% 9% 17% 7% 1	1262
	11th to 20th	Night	54% 6% 10% 20% 8% 2	1260
	21st to end	Night	62% 5% 7% 16% 9% 2	1260
July	1st to 10th	Night	58% 6% 13% 15% 8%	1225
	11th to 20th	Night	58% 6% 8% 18% 9%	1218
	21st to end	Night	58% 8% 12% 15% 7% 1	1641
August	1st to 10th	Night	49% 8% 13% 22% 8%	1600
	11th to 20th	Night	57% 6% 11% 20% 6%	1596
	21st to end	Night	44% 6% 13% 25% 12% 1	1789
September	1st to 10th	Night	44% 5% 11% 25% 13% 1	1808
	11th to 20th	Night	44% 4 11% 23% 17% 1	1980
	21st to end	Night	44% 5% 9% 26% 15% 2	1980
October	1st to 10th	Night	44% 5% 10% 25% 14% 1	2137
	11th to 20th	Night	46% 7% 11% 25% 10% 1	2160
	21st to end	Night	53% 5% 10% 19% 12% 1	2456
November	1st to 10th	Night	49% 5% 13% 20% 12% 1	2472
	11th to 20th	Night	50% 6% 12% 19% 13% 1	2660
	21st to end	Night	51% 7% 13% 17% 10% 1	2660
December	1st to 10th	Night	59% 5% 10% 18% 8%	2660
	11th to 20th	Night	60% 5% 9% 18% 9%	3465
	21st to end	Night	56% 5% 11% 18% 7% 4	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	35% 21% 34% 8% 2	166
	21st to end	Day	21% 24% 38% 14% 3	370
February	1st to 10th	Day	29% 24% 32% 14% 2	340
	11th to 20th	Day	27% 29% 29% 13% 2 1	336
	21st to end	Day	19% 30% 35% 14% 2	323
March	1st to 10th	Day	24% 25% 36% 12% 3	420
	11th to 20th	Day	24% 22% 36% 15% 3 1	420
	21st to end	Day	30% 23% 30% 11% 5% 1	462
April	1st to 10th	Day	37% 26% 23% 10% 3	420
	11th to 20th	Day	26% 25% 30% 14% 4	412
	21st to end	Day	29% 20% 30% 15% 6% 1	420
May	1st to 10th	Day	28% 23% 32% 13% 4	423
	11th to 20th	Day	28% 18% 30% 18% 5% 1	571
	21st to end	Day	30% 17% 29% 18% 6% 1	680
June	1st to 10th	Day	30% 16% 28% 19% 7%	630
	11th to 20th	Day	27% 13% 29% 23% 7% 1	630
	21st to end	Day	33% 9% 28% 20% 8% 1	630
July	1st to 10th	Day	27% 11% 31% 22% 8%	620
	11th to 20th	Day	28% 8% 26% 25% 11% 2	614
	21st to end	Day	26% 10% 35% 22% 6% 1	488
August	1st to 10th	Day	22% 10% 31% 31% 6% 1	446
	11th to 20th	Day	28% 8% 28% 28% 7% 1	439
	21st to end	Day	23% 12% 27% 27% 10% 1	495
September	1st to 10th	Day	19% 10% 36% 26% 8% 2	450
	11th to 20th	Day	26% 13% 28% 21% 9% 3	450
	21st to end	Day	23% 13% 34% 21% 8% 2	450
October	1st to 10th	Day	25% 14% 28% 20% 11% 2	371
	11th to 20th	Day	24% 19% 28% 21% 8% 1	360
	21st to end	Day	27% 19% 33% 17% 4 1	396
November	1st to 10th	Day	28% 20% 29% 16% 5% 2	365
	11th to 20th	Day	28% 23% 28% 16% 5%	370
	21st to end	Day	32% 25% 30% 11% 2 1	370
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	29% 21% 29% 17% 4	747
	21st to end	Night	21% 24% 35% 16% 5%	792
February	1st to 10th	Night	27% 23% 31% 17% 2	720
	11th to 20th	Night	26% 24% 31% 16% 3	703
	21st to end	Night	23% 20% 32% 21% 4	592
March	1st to 10th	Night	24% 19% 38% 18% 2	720
	11th to 20th	Night	25% 17% 31% 21% 5%	720
	21st to end	Night	24% 18% 33% 20% 6% 1	792
April	1st to 10th	Night	32% 16% 33% 17% 3	627
	11th to 20th	Night	28% 13% 34% 19% 4	534
	21st to end	Night	29% 14% 32% 19% 6%	540
May	1st to 10th	Night	27% 10% 32% 25% 5% 1	540
	11th to 20th	Night	31% 11% 30% 23% 5%	532
	21st to end	Night	29% 10% 34% 22% 4 1	583
June	1st to 10th	Night	35% 7% 26% 25% 7%	362
	11th to 20th	Night	26% 8% 24% 34% 8%	360
	21st to end	Night	39% 2 21% 29% 8% 1	360
July	1st to 10th	Night	28% 7% 23% 32% 9% 1	350
	11th to 20th	Night	32% 4 22% 31% 11% 1	348
	21st to end	Night	29% 6% 25% 30% 10%	558
August	1st to 10th	Night	22% 4 26% 36% 12% 1	538
	11th to 20th	Night	30% 5% 23% 30% 11% 1	532
	21st to end	Night	22% 7% 25% 30% 15% 1	594
September	1st to 10th	Night	24% 4 32% 27% 11% 1	548
	11th to 20th	Night	26% 9% 22% 27% 14% 2	720
	21st to end	Night	26% 9% 28% 25% 12% 1	720
October	1st to 10th	Night	25% 9% 26% 28% 10% 2	720
	11th to 20th	Night	23% 15% 30% 21% 10% 1	720
	21st to end	Night	29% 18% 29% 21% 3	792
November	1st to 10th	Night	28% 15% 28% 20% 8% 1	771
	11th to 20th	Night	27% 21% 26% 20% 6% 1	850
	21st to end	Night	29% 22% 25% 19% 5%	850
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 2024-01-11 05:22:37. Version ac07

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