PHM Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PHM Planetarium clear sky chart from 2005-11-06 to 2024-02-25.

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	15% 10% 4 7% 6% 3 5% 5% 5% 7% 33%	78363
	Night	16% 11% 5% 7% 6% 3 4 4 5% 6% 32%	80982
Transparency	Day	50% 6% 11% 19% 11% 2	78065
	Night	52% 6% 11% 18% 11% 1	80801
Seeing	Day	19% 13% 28% 22% 12% 4	14699
	Night	20% 11% 29% 28% 11% 1	24726

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	15% 18% 9% 10% 8% 4 5% 5% 5% 5% 16%	2527
	11th to 20th	Day	15% 16% 9% 12% 9% 5% 6% 6% 7% 6% 10%	2530
	21st to end	Day	27% 21% 8% 10% 5% 2 4 3 3 5% 12%	2783
February	1st to 10th	Day	15% 13% 7% 10% 9% 4 6% 4 5% 6% 20%	1826
	11th to 20th	Day	15% 11% 5% 12% 9% 3 5% 5% 5% 7% 23%	1848
	21st to end	Day	18% 11% 8% 11% 5% 3 5% 5% 5% 5% 24%	1520
March	1st to 10th	Day	16% 11% 5% 9% 8% 4 4 4 5% 7% 27%	1800
	11th to 20th	Day	15% 10% 5% 7% 6% 3 5% 4 6% 7% 31%	1908
	21st to end	Day	20% 13% 5% 7% 6% 4 5% 4 4 6% 26%	2276
April	1st to 10th	Day	18% 12% 5% 6% 6% 3 5% 4 5% 5% 32%	2160
	11th to 20th	Day	17% 10% 3 6% 5% 3 4 4 5% 7% 37%	2144
	21st to end	Day	19% 9% 3 6% 5% 3 4 4 6% 7% 34%	2340
May	1st to 10th	Day	17% 10% 4 6% 5% 3 5% 4 6% 7% 33%	2340
	11th to 20th	Day	19% 9% 3 6% 4 2 5% 4 4 7% 36%	2301
	21st to end	Day	11% 5% 3 6% 6% 4 5% 5% 6% 9% 40%	2691
June	1st to 10th	Day	10% 5% 2 5% 5% 4 6% 5% 5% 8% 45%	2520
	11th to 20th	Day	10% 4 2 5% 5% 4 6% 5% 6% 8% 43%	2520
	21st to end	Day	11% 5% 2 5% 5% 4 6% 6% 7% 9% 41%	2520
July	1st to 10th	Day	7% 6% 2 5% 6% 3 5% 5% 7% 9% 46%	2520
	11th to 20th	Day	10% 4 2 4 6% 3 5% 5% 7% 9% 46%	2520
	21st to end	Day	7% 4 2 4 6% 4 6% 5% 7% 10% 44%	2743
August	1st to 10th	Day	7% 4 2 5% 6% 4 6% 6% 6% 9% 45%	2331
	11th to 20th	Day	9% 4 3 6% 6% 4 5% 5% 6% 10% 43%	2269
	21st to end	Day	9% 4 1 4 6% 3 5% 5% 6% 9% 47%	2376
September	1st to 10th	Day	11% 6% 2 5% 5% 4 4 5% 6% 8% 45%	2142
	11th to 20th	Day	11% 5% 2 5% 5% 3 4 5% 5% 8% 47%	1980
	21st to end	Day	12% 6% 3 6% 5% 4 5% 5% 4 8% 42%	1980
October	1st to 10th	Day	14% 6% 3 5% 6% 3 4 4 5% 7% 42%	1980
	11th to 20th	Day	21% 7% 3 5% 6% 3 4 5% 6% 7% 34%	1782
	21st to end	Day	22% 10% 4 8% 7% 4 4 4 6% 7% 25%	1782
November	1st to 10th	Day	13% 10% 3 6% 6% 3 4 5% 5% 8% 39%	1672
	11th to 20th	Day	16% 15% 5% 7% 7% 3 3 5% 5% 6% 28%	1710
	21st to end	Day	21% 15% 6% 7% 5% 3 4 4 5% 5% 25%	1710
December	1st to 10th	Day	16% 16% 7% 12% 6% 3 4 4 4 5% 23%	1611
	11th to 20th	Day	13% 18% 9% 12% 8% 3 4 4 4 5% 20%	1918
	21st to end	Day	23% 20% 6% 13% 9% 2 5% 3 3 4 10%	2783
January	1st to 10th	Night	17% 17% 9% 10% 7% 3 4 4 5% 5% 18%	4428
	11th to 20th	Night	16% 18% 9% 13% 7% 4 5% 6% 5% 5% 12%	4440
	21st to end	Night	25% 17% 10% 10% 7% 3 3 3 4 4 13%	4884
February	1st to 10th	Night	19% 13% 6% 9% 7% 3 5% 4 5% 5% 24%	2349
	11th to 20th	Night	14% 12% 6% 9% 7% 3 6% 4 5% 7% 26%	2218
	21st to end	Night	20% 12% 5% 10% 6% 3 5% 4 3 4 27%	1831
March	1st to 10th	Night	16% 10% 4 8% 6% 4 5% 4 4 6% 33%	2160
	11th to 20th	Night	14% 8% 3 7% 5% 3 4 5% 5% 7% 39%	2052
	21st to end	Night	20% 9% 4 6% 5% 3 4 4 5% 6% 33%	2093
April	1st to 10th	Night	15% 8% 3 6% 5% 3 5% 6% 5% 8% 35%	1800
	11th to 20th	Night	18% 8% 3 7% 6% 4 4 4 4 5% 38%	1763
	21st to end	Night	18% 8% 3 6% 5% 3 5% 4 5% 6% 37%	1620
May	1st to 10th	Night	16% 9% 4 6% 6% 3 5% 4 7% 7% 33%	1620
	11th to 20th	Night	19% 7% 3 5% 5% 3 4 4 4 8% 38%	1592
	21st to end	Night	10% 6% 3 5% 6% 4 5% 5% 5% 8% 43%	1581
June	1st to 10th	Night	8% 5% 3 5% 5% 3 4 5% 6% 8% 49%	1440
	11th to 20th	Night	9% 5% 3 5% 5% 3 5% 5% 6% 9% 45%	1440
	21st to end	Night	10% 4 3 5% 4 3 6% 6% 7% 10% 41%	1440
July	1st to 10th	Night	5% 3 2 5% 4 3 5% 5% 6% 9% 51%	1440
	11th to 20th	Night	8% 4 2 4 3 3 4 5% 5% 10% 52%	1440
	21st to end	Night	5% 4 2 4 4 3 5% 6% 6% 10% 50%	1575
August	1st to 10th	Night	7% 4 1 4 5% 3 5% 5% 6% 9% 51%	1606
	11th to 20th	Night	7% 3 2 5% 4 4 5% 5% 6% 8% 50%	1628
	21st to end	Night	9% 4 2 5% 5% 3 4 4 6% 7% 52%	1980
September	1st to 10th	Night	12% 6% 2 4 4 2 3 5% 5% 6% 49%	1800
	11th to 20th	Night	11% 4 2 4 4 2 4 4 4 8% 54%	1980
	21st to end	Night	13% 7% 2 5% 4 2 3 4 5% 7% 46%	1980
October	1st to 10th	Night	15% 6% 2 5% 5% 3 4 4 4 6% 46%	1980
	11th to 20th	Night	18% 9% 4 6% 5% 3 3 4 4 6% 38%	2174
	21st to end	Night	23% 10% 3 7% 5% 3 3 4 4 5% 34%	2574
November	1st to 10th	Night	13% 10% 4 7% 5% 3 4 4 4 6% 39%	2406
	11th to 20th	Night	14% 15% 4 8% 6% 3 5% 5% 5% 6% 28%	2470
	21st to end	Night	19% 15% 6% 8% 6% 3 3 4 4 6% 27%	2470
December	1st to 10th	Night	17% 15% 8% 10% 6% 3 4 4 5% 6% 21%	2565
	11th to 20th	Night	15% 17% 8% 10% 8% 4 5% 5% 5% 5% 19%	3279
	21st to end	Night	27% 18% 6% 8% 7% 3 4 5% 4 5% 12%	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	68% 8% 11% 10% 3	2528
	11th to 20th	Day	72% 6% 8% 9% 4	2530
	21st to end	Day	77% 4 7% 9% 3	2783
February	1st to 10th	Day	65% 5% 10% 12% 6% 1	1826
	11th to 20th	Day	60% 7% 10% 16% 7%	1858
	21st to end	Day	61% 7% 10% 15% 6% 1	1520
March	1st to 10th	Day	58% 5% 11% 18% 8%	1800
	11th to 20th	Day	51% 6% 13% 19% 10% 1	1908
	21st to end	Day	61% 5% 9% 18% 5% 1	2276
April	1st to 10th	Day	55% 4 11% 18% 12%	2160
	11th to 20th	Day	48% 6% 10% 21% 12% 3	2144
	21st to end	Day	49% 6% 12% 18% 12% 3	2340
May	1st to 10th	Day	50% 7% 12% 22% 9% 1	2340
	11th to 20th	Day	49% 5% 10% 20% 13% 2	2305
	21st to end	Day	40% 8% 12% 24% 14% 2	2691
June	1st to 10th	Day	36% 8% 11% 25% 17% 2	2520
	11th to 20th	Day	37% 8% 11% 23% 19% 2	2520
	21st to end	Day	38% 8% 15% 20% 16% 2	2520
July	1st to 10th	Day	34% 8% 12% 24% 19% 4	2451
	11th to 20th	Day	33% 8% 13% 26% 16% 3	2445
	21st to end	Day	34% 8% 13% 25% 17% 3	2591
August	1st to 10th	Day	33% 8% 15% 27% 15% 2	2317
	11th to 20th	Day	36% 8% 14% 25% 16% 2	2269
	21st to end	Day	33% 6% 13% 24% 18% 5%	2376
September	1st to 10th	Day	37% 7% 15% 20% 17% 4	2142
	11th to 20th	Day	35% 6% 12% 28% 18% 2	1980
	21st to end	Day	40% 6% 11% 24% 18% 2	1980
October	1st to 10th	Day	42% 6% 10% 24% 15% 2	1980
	11th to 20th	Day	48% 5% 12% 21% 14%	1782
	21st to end	Day	59% 5% 12% 15% 8% 1	1782
November	1st to 10th	Day	44% 6% 12% 22% 15% 1	1672
	11th to 20th	Day	57% 6% 11% 18% 8% 1	1710
	21st to end	Day	62% 5% 11% 16% 5%	1710
December	1st to 10th	Day	64% 6% 10% 14% 5%	1611
	11th to 20th	Day	66% 7% 11% 14% 2	1915
	21st to end	Day	80% 4 7% 6% 3	2783
January	1st to 10th	Night	67% 7% 10% 12% 4	4428
	11th to 20th	Night	73% 6% 10% 9% 3	4440
	21st to end	Night	75% 5% 9% 9% 3	4884
February	1st to 10th	Night	62% 5% 11% 13% 8% 1	2349
	11th to 20th	Night	58% 5% 10% 19% 8% 1	2219
	21st to end	Night	61% 5% 10% 15% 8% 2	1831
March	1st to 10th	Night	53% 5% 12% 18% 10% 1	2160
	11th to 20th	Night	45% 5% 13% 21% 13% 2	2052
	21st to end	Night	53% 6% 10% 21% 10% 1	2093
April	1st to 10th	Night	46% 7% 13% 20% 13% 1	1800
	11th to 20th	Night	49% 4 8% 20% 17% 2	1763
	21st to end	Night	49% 6% 10% 20% 13% 2	1620
May	1st to 10th	Night	49% 6% 12% 22% 10% 1	1620
	11th to 20th	Night	45% 7% 10% 20% 16% 2	1593
	21st to end	Night	39% 8% 15% 22% 14% 2	1582
June	1st to 10th	Night	31% 7% 12% 29% 19% 3	1440
	11th to 20th	Night	35% 6% 14% 24% 20% 2	1440
	21st to end	Night	35% 9% 15% 24% 15% 3	1440
July	1st to 10th	Night	30% 8% 14% 24% 19% 5%	1400
	11th to 20th	Night	29% 7% 14% 26% 21% 4	1392
	21st to end	Night	28% 9% 15% 23% 20% 5%	1487
August	1st to 10th	Night	28% 7% 13% 31% 19% 2	1596
	11th to 20th	Night	31% 8% 14% 24% 21% 2	1628
	21st to end	Night	31% 7% 10% 25% 22% 5%	1980
September	1st to 10th	Night	34% 8% 13% 22% 19% 5%	1800
	11th to 20th	Night	30% 5% 9% 30% 24% 2	1980
	21st to end	Night	37% 5% 10% 26% 19% 2	1980
October	1st to 10th	Night	38% 5% 9% 26% 19% 1	1980
	11th to 20th	Night	47% 5% 10% 22% 14% 1	2174
	21st to end	Night	53% 5% 11% 20% 10% 1	2574
November	1st to 10th	Night	45% 6% 11% 21% 15% 2	2406
	11th to 20th	Night	54% 7% 12% 18% 8% 1	2470
	21st to end	Night	60% 5% 11% 17% 7%	2470
December	1st to 10th	Night	63% 6% 11% 13% 6%	2565
	11th to 20th	Night	66% 6% 11% 12% 4	3281
	21st to end	Night	74% 6% 10% 7% 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	23% 24% 30% 18% 4 1	270
	11th to 20th	Day	29% 16% 38% 13% 3 1	270
	21st to end	Day	29% 19% 32% 17% 3	297
February	1st to 10th	Day	28% 22% 33% 13% 4	270
	11th to 20th	Day	29% 23% 31% 14% 3	270
	21st to end	Day	31% 27% 31% 9% 1	210
March	1st to 10th	Day	26% 24% 35% 12% 4	240
	11th to 20th	Day	27% 13% 35% 18% 6% 1	348
	21st to end	Day	34% 17% 28% 15% 6% 1	462
April	1st to 10th	Day	27% 15% 32% 20% 5% 1	420
	11th to 20th	Day	27% 20% 26% 15% 9% 2	412
	21st to end	Day	27% 18% 27% 17% 9% 1	420
May	1st to 10th	Day	26% 16% 31% 16% 9% 2	423
	11th to 20th	Day	25% 13% 28% 21% 9% 4	447
	21st to end	Day	14% 11% 30% 23% 17% 6%	645
June	1st to 10th	Day	11% 10% 31% 30% 13% 4	630
	11th to 20th	Day	14% 9% 29% 29% 16% 4	630
	21st to end	Day	13% 11% 31% 26% 15% 5%	630
July	1st to 10th	Day	10% 5% 21% 29% 22% 13%	621
	11th to 20th	Day	11% 5% 31% 25% 18% 9%	618
	21st to end	Day	8% 7% 25% 34% 18% 8%	684
August	1st to 10th	Day	9% 2 21% 29% 23% 15%	450
	11th to 20th	Day	9% 8% 22% 31% 23% 8%	439
	21st to end	Day	11% 4 23% 29% 25% 8%	495
September	1st to 10th	Day	14% 8% 24% 30% 18% 7%	450
	11th to 20th	Day	9% 6% 21% 25% 28% 11%	450
	21st to end	Day	17% 14% 21% 24% 18% 6%	450
October	1st to 10th	Day	16% 11% 26% 26% 17% 3	450
	11th to 20th	Day	26% 21% 25% 23% 5%	354
	21st to end	Day	32% 17% 37% 11% 2 1	297
November	1st to 10th	Day	17% 18% 38% 20% 7% 1	270
	11th to 20th	Day	29% 23% 26% 16% 6%	270
	21st to end	Day	29% 21% 33% 14% 2	270
December	1st to 10th	Day	23% 28% 30% 13% 6%	270
	11th to 20th	Day	21% 28% 31% 15% 4	270
	21st to end	Day	29% 22% 36% 12% 1	297
January	1st to 10th	Night	25% 16% 33% 22% 4	950
	11th to 20th	Night	25% 17% 34% 19% 3 1	950
	21st to end	Night	24% 14% 35% 23% 3	1045
February	1st to 10th	Night	29% 12% 35% 22% 3	767
	11th to 20th	Night	22% 16% 35% 23% 4	743
	21st to end	Night	28% 15% 35% 18% 3	611
March	1st to 10th	Night	21% 15% 34% 23% 7% 1	720
	11th to 20th	Night	20% 12% 37% 24% 8%	720
	21st to end	Night	24% 12% 33% 26% 5% 1	707
April	1st to 10th	Night	20% 15% 33% 24% 7% 1	540
	11th to 20th	Night	23% 12% 32% 26% 6% 1	533
	21st to end	Night	22% 11% 31% 26% 9% 1	540
May	1st to 10th	Night	21% 9% 31% 31% 8%	540
	11th to 20th	Night	21% 7% 28% 31% 12%	531
	21st to end	Night	13% 5% 26% 37% 16% 1	584
June	1st to 10th	Night	9% 4 29% 43% 14% 1	540
	11th to 20th	Night	12% 4 28% 39% 17% 1	540
	21st to end	Night	10% 4 26% 37% 22% 1	540
July	1st to 10th	Night	7% 2 18% 42% 27% 5%	525
	11th to 20th	Night	10% 2 21% 37% 26% 3	522
	21st to end	Night	7% 3 18% 41% 30% 1	591
August	1st to 10th	Night	8% 1 15% 45% 26% 4	537
	11th to 20th	Night	9% 1 16% 42% 27% 6%	534
	21st to end	Night	11% 3 16% 37% 30% 3	594
September	1st to 10th	Night	16% 3 17% 36% 24% 4	540
	11th to 20th	Night	11% 3 20% 36% 26% 4	720
	21st to end	Night	18% 7% 23% 31% 19% 3	720
October	1st to 10th	Night	18% 7% 26% 30% 18% 1	720
	11th to 20th	Night	23% 17% 29% 25% 7% 1	720
	21st to end	Night	29% 12% 33% 20% 6%	792
November	1st to 10th	Night	19% 14% 32% 24% 10% 1	740
	11th to 20th	Night	25% 17% 31% 22% 5% 1	760
	21st to end	Night	28% 11% 33% 24% 4	760
December	1st to 10th	Night	27% 16% 34% 19% 4	855
	11th to 20th	Night	25% 15% 37% 19% 4	950
	21st to end	Night	31% 17% 31% 18% 2	1045

Page generated at 2024-02-22 06:05:45. Version ac07

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