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 2026-09-14.

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	13% 9% 4 9% 5% 5% 5% 4 6% 7% 34%	148663
	Night	14% 11% 4 9% 5% 5% 4 4 5% 6% 33%	142691
Transparency	Day	48% 5% 11% 22% 13% 1	152216
	Night	51% 5% 10% 20% 13% 1	147215
Seeing	Day	19% 13% 29% 22% 13% 4	18316
	Night	20% 11% 29% 28% 11% 1	28062

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	13% 18% 10% 21% 6% 4 4 3 4 4 12%	4691
	11th to 20th	Day	11% 13% 9% 16% 9% 8% 6% 4 6% 5% 13%	4710
	21st to end	Day	17% 16% 6% 14% 7% 6% 5% 3 4 5% 17%	5156
February	1st to 10th	Day	15% 15% 5% 13% 7% 5% 5% 4 5% 6% 20%	2020
	11th to 20th	Day	14% 13% 5% 14% 8% 5% 5% 3 5% 6% 22%	2048
	21st to end	Day	16% 13% 7% 12% 5% 5% 4 4 5% 5% 25%	1730
March	1st to 10th	Day	17% 11% 6% 11% 5% 4 3 3 4 5% 31%	4580
	11th to 20th	Day	10% 9% 6% 7% 4 5% 5% 3 6% 8% 36%	4902
	21st to end	Day	17% 15% 7% 10% 6% 5% 4 3 4 5% 21%	5854
April	1st to 10th	Day	18% 14% 4 8% 5% 4 4 3 5% 5% 31%	2520
	11th to 20th	Day	15% 11% 3 7% 4 5% 4 4 5% 6% 37%	2507
	21st to end	Day	18% 9% 3 7% 4 4 4 4 6% 7% 34%	2730
May	1st to 10th	Day	17% 9% 3 7% 4 4 4 4 6% 7% 34%	7616
	11th to 20th	Day	13% 7% 3 7% 4 4 5% 4 6% 8% 38%	7626
	21st to end	Day	14% 6% 2 6% 4 4 4 3 6% 9% 42%	8929
June	1st to 10th	Day	11% 6% 2 5% 4 6% 5% 4 5% 8% 44%	2940
	11th to 20th	Day	10% 5% 2 6% 4 6% 5% 4 6% 8% 43%	2940
	21st to end	Day	10% 5% 2 5% 5% 5% 5% 5% 7% 8% 42%	2940
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 8% 10% 39%	8280
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 4 6% 10% 47%	8280
	21st to end	Day	9% 5% 3 6% 5% 5% 5% 4 7% 9% 43%	8990
August	1st to 10th	Day	13% 5% 2 6% 4 5% 4 4 6% 7% 44%	6018
	11th to 20th	Day	13% 7% 2 7% 5% 5% 5% 4 5% 8% 38%	5859
	21st to end	Day	6% 4 2 6% 5% 5% 4 4 6% 10% 48%	6080
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 6% 8% 47%	2498
	11th to 20th	Day	10% 5% 2 6% 3 4 4 4 5% 8% 49%	2233
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 4 5% 8% 41%	2200
October	1st to 10th	Day	13% 6% 3 6% 5% 4 4 3 5% 7% 44%	2200
	11th to 20th	Day	20% 7% 2 7% 4 3 4 4 6% 7% 36%	1978
	21st to end	Day	20% 11% 4 8% 6% 5% 4 3 6% 7% 27%	1980
November	1st to 10th	Day	13% 10% 2 7% 5% 4 4 4 5% 8% 38%	1852
	11th to 20th	Day	16% 17% 4 8% 6% 4 4 3 5% 6% 28%	1890
	21st to end	Day	20% 17% 6% 8% 4 4 3 3 5% 5% 24%	1890
December	1st to 10th	Day	10% 16% 6% 17% 5% 4 4 3 5% 6% 25%	2838
	11th to 20th	Day	15% 19% 10% 14% 6% 5% 3 3 4 4 16%	3088
	21st to end	Day	27% 19% 7% 14% 6% 3 4 2 3 3 10%	4070
January	1st to 10th	Night	13% 17% 8% 19% 6% 5% 4 3 4 4 16%	8232
	11th to 20th	Night	10% 17% 8% 17% 8% 8% 5% 5% 5% 5% 12%	8280
	21st to end	Night	16% 14% 7% 12% 7% 7% 5% 3 4 5% 20%	9058
February	1st to 10th	Night	17% 14% 5% 11% 6% 5% 5% 3 4 5% 25%	2595
	11th to 20th	Night	14% 13% 5% 10% 7% 5% 4 3 5% 6% 26%	2458
	21st to end	Night	18% 13% 5% 12% 5% 4 5% 3 4 4 27%	2076
March	1st to 10th	Night	15% 11% 5% 9% 4 5% 4 3 4 6% 34%	5493
	11th to 20th	Night	12% 8% 4 7% 4 4 4 4 5% 7% 41%	5278
	21st to end	Night	18% 11% 4 9% 5% 4 4 4 5% 5% 31%	5359
April	1st to 10th	Night	17% 10% 3 6% 5% 4 5% 4 6% 7% 34%	2100
	11th to 20th	Night	18% 9% 3 7% 5% 5% 4 3 4 5% 37%	2060
	21st to end	Night	16% 9% 3 7% 4 4 4 4 5% 6% 38%	1890
May	1st to 10th	Night	17% 9% 3 7% 5% 4 4 3 5% 7% 36%	5237
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 9% 43%	5234
	21st to end	Night	15% 6% 2 6% 5% 4 4 3 5% 8% 42%	5234
June	1st to 10th	Night	8% 6% 3 5% 4 4 3 4 6% 8% 48%	1680
	11th to 20th	Night	10% 5% 2 5% 4 5% 4 4 7% 9% 44%	1680
	21st to end	Night	9% 5% 2 6% 4 5% 5% 6% 8% 10% 41%	1680
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 9% 46%	4708
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 5% 9% 52%	4707
	21st to end	Night	8% 6% 2 6% 5% 5% 5% 4 7% 8% 45%	5117
August	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 5% 8% 44%	4130
	11th to 20th	Night	13% 7% 3 7% 4 5% 5% 4 6% 7% 40%	4183
	21st to end	Night	8% 4 2 6% 4 4 4 4 5% 8% 50%	5046
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 4 5% 6% 51%	2100
	11th to 20th	Night	9% 4 2 4 4 3 4 3 4 8% 56%	2234
	21st to end	Night	13% 7% 2 6% 4 3 3 3 5% 7% 45%	2200
October	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 4 6% 49%	2200
	11th to 20th	Night	17% 9% 3 6% 5% 4 3 3 5% 6% 39%	2417
	21st to end	Night	21% 10% 3 7% 4 3 3 3 4 6% 35%	2860
November	1st to 10th	Night	13% 12% 3 8% 4 4 4 3 5% 6% 38%	2666
	11th to 20th	Night	14% 15% 4 9% 5% 4 5% 4 6% 6% 29%	2730
	21st to end	Night	18% 17% 6% 9% 5% 4 3 3 4 6% 25%	2730
December	1st to 10th	Night	13% 12% 6% 15% 6% 4 5% 4 6% 6% 24%	4550
	11th to 20th	Night	16% 19% 8% 13% 8% 5% 5% 3 4 5% 14%	5339
	21st to end	Night	26% 19% 6% 11% 6% 4 4 3 4 4 11%	7150

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	75% 5% 9% 9% 2	4692
	11th to 20th	Day	71% 4 6% 11% 7%	4710
	21st to end	Day	71% 3 9% 12% 5%	5156
February	1st to 10th	Day	65% 5% 11% 12% 6% 1	2020
	11th to 20th	Day	62% 6% 10% 15% 7%	2058
	21st to end	Day	60% 6% 11% 15% 6% 1	1730
March	1st to 10th	Day	57% 4 9% 21% 8%	4580
	11th to 20th	Day	44% 5% 13% 29% 7% 1	4902
	21st to end	Day	65% 5% 9% 14% 6% 1	5856
April	1st to 10th	Day	56% 4 11% 18% 11%	2520
	11th to 20th	Day	48% 5% 11% 22% 12% 2	2507
	21st to end	Day	48% 6% 12% 21% 12% 2	2730
May	1st to 10th	Day	48% 5% 11% 22% 14%	7651
	11th to 20th	Day	44% 6% 13% 23% 14% 1	7630
	21st to end	Day	39% 5% 10% 23% 19% 4	8943
June	1st to 10th	Day	37% 8% 11% 25% 18% 2	2940
	11th to 20th	Day	37% 7% 11% 24% 19% 2	2940
	21st to end	Day	38% 8% 15% 21% 16% 3	2940
July	1st to 10th	Day	39% 4 12% 26% 17% 2	8211
	11th to 20th	Day	33% 4 13% 31% 18% 1	8205
	21st to end	Day	38% 4 11% 25% 19% 2	8794
August	1st to 10th	Day	39% 5% 12% 24% 19% 1	6004
	11th to 20th	Day	44% 5% 13% 24% 13% 1	5898
	21st to end	Day	33% 4 14% 32% 16% 2	6079
September	1st to 10th	Day	34% 7% 14% 24% 18% 3	2498
	11th to 20th	Day	32% 5% 11% 28% 21% 2	2233
	21st to end	Day	41% 5% 10% 24% 18% 2	2200
October	1st to 10th	Day	30% 4 8% 35% 21% 1	3590
	11th to 20th	Day	40% 4 10% 21% 25%	3194
	21st to end	Day	49% 3 10% 24% 11% 2	3180
November	1st to 10th	Day	45% 6% 11% 23% 14% 1	1852
	11th to 20th	Day	57% 5% 10% 18% 8% 1	1890
	21st to end	Day	63% 5% 10% 17% 5%	1890
December	1st to 10th	Day	61% 4 14% 18% 3	2838
	11th to 20th	Day	71% 5% 10% 13% 2	3085
	21st to end	Day	81% 3 6% 7% 2	4070
January	1st to 10th	Night	71% 4 8% 13% 3	8233
	11th to 20th	Night	72% 5% 9% 11% 3	8280
	21st to end	Night	67% 3 12% 12% 6%	9058
February	1st to 10th	Night	62% 4 11% 14% 8% 1	2595
	11th to 20th	Night	58% 5% 9% 19% 8% 1	2459
	21st to end	Night	61% 4 11% 15% 7% 2	2076
March	1st to 10th	Night	52% 5% 10% 25% 7% 1	5493
	11th to 20th	Night	42% 5% 12% 28% 11% 2	5278
	21st to end	Night	53% 6% 10% 20% 10% 1	5359
April	1st to 10th	Night	48% 6% 12% 19% 14% 1	2100
	11th to 20th	Night	49% 5% 8% 19% 17% 2	2060
	21st to end	Night	47% 5% 11% 22% 13% 2	1890
May	1st to 10th	Night	49% 4 10% 21% 14% 2	5264
	11th to 20th	Night	39% 6% 12% 23% 18% 1	5235
	21st to end	Night	41% 5% 10% 19% 21% 4	5244
June	1st to 10th	Night	33% 6% 11% 29% 19% 3	1680
	11th to 20th	Night	36% 6% 14% 23% 20% 2	1680
	21st to end	Night	35% 8% 15% 24% 15% 2	1680
July	1st to 10th	Night	36% 4 10% 30% 17% 4	4670
	11th to 20th	Night	31% 4 13% 26% 26% 2	4662
	21st to end	Night	36% 4 12% 24% 21% 2	5002
August	1st to 10th	Night	39% 5% 10% 26% 18% 1	4120
	11th to 20th	Night	43% 6% 11% 25% 15% 1	4210
	21st to end	Night	33% 4 10% 33% 18% 2	5046
September	1st to 10th	Night	32% 7% 12% 25% 19% 4	2100
	11th to 20th	Night	28% 5% 8% 30% 27% 2	2234
	21st to end	Night	39% 5% 10% 26% 19% 2	2200
October	1st to 10th	Night	26% 3 8% 33% 29% 1	3590
	11th to 20th	Night	40% 4 8% 23% 24% 1	3940
	21st to end	Night	40% 3 8% 29% 18% 1	4610
November	1st to 10th	Night	45% 5% 11% 23% 15% 1	2666
	11th to 20th	Night	53% 6% 11% 19% 8% 2	2730
	21st to end	Night	61% 5% 11% 17% 7%	2730
December	1st to 10th	Night	59% 4 14% 18% 4	4550
	11th to 20th	Night	73% 4 10% 10% 3	5341
	21st to end	Night	77% 4 9% 8% 2	7150

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% 22% 32% 18% 5% 1	330
	11th to 20th	Day	25% 18% 39% 14% 2 1	330
	21st to end	Day	26% 21% 34% 16% 2	363
February	1st to 10th	Day	26% 20% 35% 14% 4 1	330
	11th to 20th	Day	28% 22% 32% 15% 3	330
	21st to end	Day	29% 27% 33% 10% 1	273
March	1st to 10th	Day	25% 22% 39% 12% 3	330
	11th to 20th	Day	28% 16% 33% 17% 5% 1	456
	21st to end	Day	30% 18% 30% 16% 5% 1	594
April	1st to 10th	Day	29% 17% 31% 18% 5% 1	540
	11th to 20th	Day	25% 22% 28% 15% 8% 2	532
	21st to end	Day	26% 16% 28% 20% 9% 1	540
May	1st to 10th	Day	26% 15% 31% 17% 9% 2	543
	11th to 20th	Day	22% 14% 30% 21% 10% 4	567
	21st to end	Day	14% 10% 31% 23% 16% 5%	804
June	1st to 10th	Day	12% 10% 31% 29% 13% 4	780
	11th to 20th	Day	14% 9% 31% 28% 15% 3	780
	21st to end	Day	12% 9% 31% 27% 15% 5%	780
July	1st to 10th	Day	10% 4 22% 31% 21% 12%	771
	11th to 20th	Day	11% 5% 28% 26% 20% 9%	768
	21st to end	Day	8% 6% 24% 34% 19% 8%	849
August	1st to 10th	Day	10% 2 20% 29% 25% 14%	570
	11th to 20th	Day	10% 8% 22% 31% 21% 8%	559
	21st to end	Day	10% 4 21% 30% 26% 9%	627
September	1st to 10th	Day	12% 8% 25% 28% 18% 9%	570
	11th to 20th	Day	8% 5% 18% 24% 30% 14%	542
	21st to end	Day	19% 12% 20% 23% 19% 6%	530
October	1st to 10th	Day	15% 10% 25% 27% 19% 4	530
	11th to 20th	Day	24% 18% 25% 24% 8% 1	422
	21st to end	Day	28% 17% 36% 15% 3 1	363
November	1st to 10th	Day	18% 18% 39% 19% 6% 1	330
	11th to 20th	Day	30% 23% 25% 16% 5%	330
	21st to end	Day	31% 19% 35% 12% 2	330
December	1st to 10th	Day	22% 26% 31% 15% 5%	330
	11th to 20th	Day	21% 28% 33% 15% 3	330
	21st to end	Day	31% 19% 36% 12% 2	363
January	1st to 10th	Night	24% 15% 34% 22% 4	1050
	11th to 20th	Night	23% 18% 37% 18% 3 1	1050
	21st to end	Night	23% 15% 36% 23% 3	1155
February	1st to 10th	Night	27% 13% 35% 23% 3	847
	11th to 20th	Night	22% 15% 35% 24% 4	823
	21st to end	Night	27% 16% 36% 18% 3	692
March	1st to 10th	Night	21% 15% 35% 22% 6%	840
	11th to 20th	Night	20% 13% 37% 23% 7%	840
	21st to end	Night	24% 13% 33% 25% 4 1	824
April	1st to 10th	Night	22% 16% 33% 23% 6% 1	630
	11th to 20th	Night	23% 13% 33% 24% 6% 1	623
	21st to end	Night	20% 12% 31% 27% 9% 1	630
May	1st to 10th	Night	20% 9% 33% 31% 7%	630
	11th to 20th	Night	20% 7% 29% 32% 13%	621
	21st to end	Night	14% 6% 26% 38% 15% 1	683
June	1st to 10th	Night	10% 5% 28% 42% 14%	630
	11th to 20th	Night	13% 4 28% 38% 16% 1	630
	21st to end	Night	11% 4 24% 39% 21% 1	630
July	1st to 10th	Night	8% 2 17% 43% 26% 4	615
	11th to 20th	Night	9% 3 20% 38% 27% 3	612
	21st to end	Night	7% 2 17% 42% 31% 2	690
August	1st to 10th	Night	9% 1 14% 46% 26% 4	627
	11th to 20th	Night	11% 1 15% 42% 26% 5%	624
	21st to end	Night	10% 2 15% 39% 31% 3	693
September	1st to 10th	Night	14% 3 18% 36% 26% 3	630
	11th to 20th	Night	10% 3 18% 36% 29% 4	813
	21st to end	Night	19% 6% 23% 31% 19% 3	800
October	1st to 10th	Night	16% 6% 26% 32% 19% 1	800
	11th to 20th	Night	22% 16% 28% 26% 8% 1	800
	21st to end	Night	27% 12% 32% 22% 6%	880
November	1st to 10th	Night	20% 14% 33% 24% 9%	820
	11th to 20th	Night	24% 17% 32% 22% 5%	840
	21st to end	Night	29% 13% 32% 22% 4	840
December	1st to 10th	Night	26% 15% 35% 20% 4	945
	11th to 20th	Night	25% 16% 36% 19% 4	1050
	21st to end	Night	32% 16% 31% 18% 2	1155

Page generated at 2026-09-11 04:11:54. Version ac07

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