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 2024-10-25.

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	14% 10% 4 8% 5% 5% 4 4 6% 7% 35%	101072
	Night	15% 11% 4 8% 5% 4 4 4 5% 6% 33%	97777
Transparency	Day	48% 6% 12% 21% 12% 1	100864
	Night	50% 6% 11% 19% 12% 1	97657
Seeing	Day	19% 13% 28% 22% 13% 4	15714
	Night	20% 11% 29% 28% 11% 1	25531

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% 19% 9% 12% 6% 5% 4 4 5% 5% 16%	2527
	11th to 20th	Day	15% 17% 8% 15% 7% 6% 5% 4 7% 6% 9%	2530
	21st to end	Day	27% 23% 7% 11% 4 3 3 2 3 4 12%	2783
February	1st to 10th	Day	15% 15% 5% 12% 8% 6% 5% 4 5% 5% 20%	1826
	11th to 20th	Day	14% 13% 5% 13% 8% 5% 4 4 6% 6% 23%	1848
	21st to end	Day	17% 13% 7% 12% 5% 5% 4 4 4 5% 25%	1570
March	1st to 10th	Day	18% 13% 6% 8% 5% 4 4 3 5% 6% 28%	3120
	11th to 20th	Day	12% 8% 6% 8% 4 5% 5% 4 7% 9% 32%	3352
	21st to end	Day	19% 12% 7% 10% 6% 5% 4 3 4 5% 23%	3995
April	1st to 10th	Day	17% 13% 4 8% 5% 5% 5% 3 5% 5% 31%	2280
	11th to 20th	Day	17% 11% 3 6% 4 5% 4 3 5% 6% 37%	2265
	21st to end	Day	19% 9% 3 7% 4 4 4 4 6% 7% 33%	2470
May	1st to 10th	Day	18% 8% 3 7% 4 4 4 4 6% 8% 34%	4082
	11th to 20th	Day	15% 8% 2 7% 3 4 5% 4 6% 8% 39%	4081
	21st to end	Day	9% 5% 2 6% 5% 4 4 3 6% 10% 45%	4762
June	1st to 10th	Day	10% 6% 2 5% 4 6% 5% 4 5% 8% 44%	2660
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 8% 43%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 41%	2660
July	1st to 10th	Day	12% 7% 2 7% 5% 5% 5% 4 7% 9% 37%	4440
	11th to 20th	Day	8% 4 1 4 4 4 4 4 6% 11% 49%	4440
	21st to end	Day	7% 5% 3 6% 6% 6% 6% 5% 7% 8% 42%	4813
August	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 6% 8% 48%	4108
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	3982
	21st to end	Day	7% 5% 2 6% 5% 5% 4 4 6% 9% 47%	4180
September	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 47%	2260
	11th to 20th	Day	10% 5% 2 6% 3 4 4 4 5% 8% 48%	2090
	21st to end	Day	13% 7% 3 6% 5% 4 5% 4 5% 8% 41%	2090
October	1st to 10th	Day	14% 7% 3 6% 5% 4 4 3 5% 7% 43%	2090
	11th to 20th	Day	20% 7% 2 7% 5% 3 4 4 6% 7% 35%	1880
	21st to end	Day	22% 11% 4 8% 6% 5% 4 3 6% 7% 25%	1824
November	1st to 10th	Day	13% 10% 2 7% 5% 4 4 4 5% 7% 39%	1672
	11th to 20th	Day	16% 17% 4 8% 6% 4 4 3 5% 6% 28%	1710
	21st to end	Day	21% 16% 6% 8% 4 4 4 3 5% 5% 25%	1710
December	1st to 10th	Day	16% 17% 7% 12% 5% 3 4 3 5% 5% 23%	1611
	11th to 20th	Day	13% 20% 8% 13% 7% 4 3 4 5% 4 20%	1918
	21st to end	Day	23% 21% 6% 15% 7% 3 5% 2 3 4 10%	2783
January	1st to 10th	Night	17% 18% 8% 12% 6% 5% 4 3 5% 5% 18%	4428
	11th to 20th	Night	16% 19% 8% 15% 6% 5% 4 5% 5% 5% 12%	4440
	21st to end	Night	25% 18% 9% 11% 6% 4 3 2 4 4 13%	4884
February	1st to 10th	Night	18% 15% 5% 10% 7% 5% 5% 3 4 5% 24%	2349
	11th to 20th	Night	14% 14% 5% 10% 7% 5% 5% 4 5% 6% 26%	2218
	21st to end	Night	20% 13% 5% 11% 5% 4 5% 3 4 4 26%	1884
March	1st to 10th	Night	16% 12% 4 8% 4 5% 4 3 4 7% 34%	3733
	11th to 20th	Night	11% 7% 4 8% 5% 5% 4 4 5% 6% 42%	3608
	21st to end	Night	19% 10% 5% 9% 5% 4 4 4 5% 6% 30%	3663
April	1st to 10th	Night	16% 10% 3 6% 5% 4 5% 4 5% 7% 35%	1900
	11th to 20th	Night	18% 9% 3 8% 5% 5% 3 3 4 5% 37%	1862
	21st to end	Night	17% 9% 3 7% 4 5% 5% 4 5% 6% 36%	1710
May	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	2813
	11th to 20th	Night	19% 7% 3 5% 4 3 4 3 5% 9% 39%	2812
	21st to end	Night	14% 6% 2 6% 5% 5% 4 3 4 7% 44%	2786
June	1st to 10th	Night	7% 6% 2 5% 4 4 3 4 7% 8% 48%	1520
	11th to 20th	Night	9% 5% 2 5% 4 6% 4 4 7% 9% 45%	1520
	21st to end	Night	9% 6% 2 6% 4 5% 5% 5% 7% 9% 41%	1520
July	1st to 10th	Night	9% 4 2 6% 4 5% 5% 4 7% 10% 44%	2530
	11th to 20th	Night	7% 4 2 5% 3 3 3 4 5% 9% 55%	2530
	21st to end	Night	6% 5% 3 5% 4 5% 5% 5% 8% 9% 45%	2750
August	1st to 10th	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 45%	2820
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 6% 8% 50%	2843
	21st to end	Night	10% 4 2 6% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 4 5% 6% 51%	1900
	11th to 20th	Night	10% 4 2 4 4 3 3 3 4 8% 55%	2090
	21st to end	Night	13% 8% 2 6% 4 3 3 3 5% 7% 45%	2090
October	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 4 6% 48%	2090
	11th to 20th	Night	17% 9% 3 6% 4 3 3 3 4 6% 39%	2296
	21st to end	Night	22% 10% 3 8% 4 3 3 3 4 5% 34%	2638
November	1st to 10th	Night	13% 11% 3 8% 4 4 4 3 5% 6% 39%	2406
	11th to 20th	Night	14% 15% 4 9% 5% 4 5% 4 6% 6% 28%	2470
	21st to end	Night	18% 16% 5% 8% 5% 4 3 3 4 6% 26%	2470
December	1st to 10th	Night	17% 16% 7% 11% 6% 4 4 3 5% 6% 20%	2565
	11th to 20th	Night	15% 18% 7% 11% 7% 5% 5% 3 5% 5% 19%	3279
	21st to end	Night	27% 19% 6% 9% 6% 4 4 4 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	60% 6% 11% 15% 6% 1	1570
March	1st to 10th	Day	58% 6% 10% 19% 7%	3120
	11th to 20th	Day	46% 8% 13% 26% 8% 1	3352
	21st to end	Day	63% 7% 11% 16% 4	3995
April	1st to 10th	Day	55% 5% 11% 19% 12%	2280
	11th to 20th	Day	48% 6% 10% 22% 12% 3	2265
	21st to end	Day	48% 6% 12% 19% 12% 3	2470
May	1st to 10th	Day	46% 8% 13% 23% 10% 1	4120
	11th to 20th	Day	42% 8% 14% 26% 10% 1	4085
	21st to end	Day	33% 8% 13% 22% 23% 1	4775
June	1st to 10th	Day	36% 8% 11% 25% 17% 2	2660
	11th to 20th	Day	36% 8% 11% 23% 19% 2	2660
	21st to end	Day	38% 8% 15% 20% 16% 3	2660
July	1st to 10th	Day	43% 6% 12% 20% 16% 3	4371
	11th to 20th	Day	30% 5% 14% 32% 16% 2	4365
	21st to end	Day	39% 6% 14% 18% 21% 3	4662
August	1st to 10th	Day	34% 6% 14% 28% 17% 1	4094
	11th to 20th	Day	35% 5% 13% 28% 18% 1	4021
	21st to end	Day	33% 5% 13% 26% 19% 3	4179
September	1st to 10th	Day	35% 7% 14% 21% 18% 3	2260
	11th to 20th	Day	34% 5% 12% 27% 20% 2	2090
	21st to end	Day	41% 6% 11% 24% 17% 2	2090
October	1st to 10th	Day	40% 6% 10% 26% 16% 2	2090
	11th to 20th	Day	47% 5% 11% 21% 15%	1880
	21st to end	Day	59% 5% 11% 15% 9% 1	1824
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	62% 5% 10% 14% 7% 2	1884
March	1st to 10th	Night	51% 6% 12% 23% 8% 1	3733
	11th to 20th	Night	40% 7% 12% 25% 14% 2	3608
	21st to end	Night	54% 8% 13% 18% 7% 1	3663
April	1st to 10th	Night	47% 6% 12% 20% 14% 1	1900
	11th to 20th	Night	49% 5% 8% 19% 17% 2	1862
	21st to end	Night	49% 6% 10% 20% 14% 2	1710
May	1st to 10th	Night	46% 7% 12% 23% 11% 1	2840
	11th to 20th	Night	41% 8% 13% 25% 12% 1	2813
	21st to end	Night	38% 8% 12% 19% 22% 2	2794
June	1st to 10th	Night	32% 7% 12% 29% 18% 3	1520
	11th to 20th	Night	35% 6% 13% 23% 20% 2	1520
	21st to end	Night	35% 8% 15% 23% 15% 3	1520
July	1st to 10th	Night	36% 5% 14% 25% 14% 7%	2490
	11th to 20th	Night	27% 5% 14% 25% 26% 3	2482
	21st to end	Night	35% 7% 15% 17% 24% 3	2662
August	1st to 10th	Night	36% 7% 12% 30% 15% 1	2810
	11th to 20th	Night	33% 6% 11% 30% 19% 1	2870
	21st to end	Night	34% 5% 9% 30% 19% 3	3476
September	1st to 10th	Night	32% 7% 13% 22% 20% 5%	1900
	11th to 20th	Night	29% 5% 8% 29% 26% 2	2090
	21st to end	Night	39% 5% 10% 26% 18% 2	2090
October	1st to 10th	Night	37% 5% 9% 27% 20% 1	2090
	11th to 20th	Night	47% 5% 10% 22% 15% 1	2296
	21st to end	Night	52% 4 10% 21% 11% 1	2638
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	32% 26% 32% 9% 2	225
March	1st to 10th	Day	25% 23% 36% 12% 3	270
	11th to 20th	Day	28% 14% 34% 18% 6% 1	384
	21st to end	Day	33% 17% 27% 16% 6% 1	506
April	1st to 10th	Day	27% 17% 31% 19% 5% 1	460
	11th to 20th	Day	27% 21% 27% 14% 9% 2	452
	21st to end	Day	27% 18% 28% 18% 8% 1	460
May	1st to 10th	Day	26% 16% 32% 16% 8% 2	463
	11th to 20th	Day	23% 13% 28% 22% 10% 4	487
	21st to end	Day	14% 11% 31% 23% 16% 6%	698
June	1st to 10th	Day	12% 10% 31% 30% 13% 4	680
	11th to 20th	Day	13% 8% 31% 29% 15% 4	680
	21st to end	Day	13% 10% 32% 25% 15% 5%	680
July	1st to 10th	Day	11% 4 23% 29% 20% 12%	671
	11th to 20th	Day	11% 5% 30% 26% 18% 9%	668
	21st to end	Day	8% 7% 24% 34% 19% 8%	739
August	1st to 10th	Day	9% 3 22% 29% 23% 14%	490
	11th to 20th	Day	9% 7% 23% 31% 22% 7%	479
	21st to end	Day	10% 4 22% 30% 26% 8%	539
September	1st to 10th	Day	12% 8% 25% 29% 18% 7%	490
	11th to 20th	Day	8% 6% 20% 25% 29% 12%	490
	21st to end	Day	20% 13% 20% 24% 17% 6%	490
October	1st to 10th	Day	15% 11% 26% 27% 18% 3	490
	11th to 20th	Day	24% 20% 26% 22% 7% 1	388
	21st to end	Day	31% 17% 36% 13% 3 1	311
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	29% 16% 34% 18% 3	628
March	1st to 10th	Night	21% 15% 34% 22% 7% 1	760
	11th to 20th	Night	19% 13% 38% 23% 7%	760
	21st to end	Night	25% 12% 32% 26% 5% 1	746
April	1st to 10th	Night	21% 16% 33% 23% 6% 1	570
	11th to 20th	Night	23% 13% 33% 25% 6% 1	563
	21st to end	Night	22% 12% 32% 25% 9% 1	570
May	1st to 10th	Night	20% 9% 32% 31% 8%	570
	11th to 20th	Night	21% 7% 28% 32% 13%	561
	21st to end	Night	13% 6% 26% 37% 16% 1	617
June	1st to 10th	Night	9% 5% 29% 42% 14% 1	570
	11th to 20th	Night	12% 4 28% 39% 16% 1	570
	21st to end	Night	11% 4 25% 37% 22% 1	570
July	1st to 10th	Night	7% 2 18% 43% 26% 5%	555
	11th to 20th	Night	9% 3 21% 36% 26% 4	552
	21st to end	Night	6% 2 17% 42% 31% 1	624
August	1st to 10th	Night	9% 1 15% 46% 25% 4	567
	11th to 20th	Night	9% 1 16% 42% 26% 5%	564
	21st to end	Night	11% 3 15% 37% 31% 3	627
September	1st to 10th	Night	15% 3 18% 35% 26% 4	570
	11th to 20th	Night	10% 3 19% 36% 28% 4	760
	21st to end	Night	19% 6% 23% 31% 18% 3	760
October	1st to 10th	Night	17% 7% 27% 31% 18% 1	760
	11th to 20th	Night	22% 16% 28% 25% 8% 1	760
	21st to end	Night	28% 12% 32% 21% 6%	812
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 2024-10-22 20:21:01. Version ac07

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