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 2025-04-26.

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 9% 5% 4 4 4 5% 7% 34%	113328
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 32%	115445
Transparency	Day	49% 6% 11% 21% 12% 1	116928
	Night	51% 5% 11% 20% 12% 1	119988
Seeing	Day	19% 14% 29% 22% 12% 4	16303
	Night	20% 11% 29% 28% 11% 1	26288

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	12% 16% 10% 20% 7% 4 4 3 4 4 15%	3617
	11th to 20th	Day	13% 14% 9% 16% 10% 6% 6% 4 6% 6% 11%	3620
	21st to end	Day	21% 18% 7% 13% 4 3 3 3 4 5% 19%	3982
February	1st to 10th	Day	15% 15% 6% 12% 8% 6% 5% 4 5% 6% 20%	1923
	11th to 20th	Day	14% 13% 5% 14% 8% 5% 4 3 6% 6% 22%	1948
	21st to end	Day	16% 13% 6% 12% 4 5% 4 4 5% 5% 25%	1650
March	1st to 10th	Day	17% 11% 6% 11% 5% 4 3 3 4 5% 30%	4480
	11th to 20th	Day	10% 9% 6% 7% 4 5% 5% 4 6% 8% 37%	4796
	21st to end	Day	18% 16% 6% 10% 6% 5% 4 3 4 5% 21%	5727
April	1st to 10th	Day	18% 13% 4 8% 5% 4 4 3 5% 5% 31%	2400
	11th to 20th	Day	16% 11% 3 7% 4 5% 4 3 5% 6% 37%	2386
	21st to end	Day	19% 9% 3 7% 4 4 4 4 6% 7% 33%	2535
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	21% 11% 4 8% 6% 5% 4 4 6% 7% 25%	1881
November	1st to 10th	Day	13% 10% 2 7% 5% 4 4 4 5% 8% 38%	1762
	11th to 20th	Day	16% 17% 4 8% 6% 4 4 3 5% 6% 27%	1800
	21st to end	Day	21% 17% 6% 8% 4 4 4 3 5% 5% 24%	1800
December	1st to 10th	Day	10% 16% 6% 17% 5% 4 4 3 5% 6% 26%	2753
	11th to 20th	Day	15% 19% 10% 14% 6% 5% 3 3 4 4 16%	3008
	21st to end	Day	27% 19% 7% 14% 6% 3 4 2 3 3 10%	3982
January	1st to 10th	Night	15% 18% 9% 18% 5% 4 4 3 4 5% 16%	6348
	11th to 20th	Night	13% 19% 8% 16% 7% 7% 5% 5% 5% 5% 11%	6360
	21st to end	Night	19% 15% 7% 11% 5% 4 4 3 4 5% 22%	6996
February	1st to 10th	Night	18% 14% 5% 11% 6% 5% 4 3 4 5% 24%	2472
	11th to 20th	Night	15% 14% 5% 11% 7% 5% 4 3 5% 6% 25%	2338
	21st to end	Night	19% 13% 5% 11% 5% 4 5% 3 4 4 27%	1980
March	1st to 10th	Night	15% 11% 5% 9% 4 5% 4 3 4 6% 34%	5373
	11th to 20th	Night	12% 8% 4 7% 4 4 4 4 5% 7% 42%	5164
	21st to end	Night	18% 11% 4 9% 5% 4 4 4 5% 5% 31%	5243
April	1st to 10th	Night	16% 10% 3 6% 5% 4 5% 4 6% 7% 35%	2000
	11th to 20th	Night	18% 9% 3 8% 5% 5% 3 3 4 5% 38%	1961
	21st to end	Night	17% 9% 3 7% 4 5% 5% 4 5% 6% 36%	1761
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 7% 4 3 3 3 4 6% 34%	2717
November	1st to 10th	Night	13% 12% 3 8% 4 4 4 3 5% 6% 39%	2536
	11th to 20th	Night	14% 15% 4 9% 5% 4 5% 4 6% 6% 28%	2600
	21st to end	Night	18% 17% 5% 9% 5% 4 3 3 4 6% 25%	2600
December	1st to 10th	Night	12% 12% 6% 15% 6% 4 5% 4 6% 7% 24%	4415
	11th to 20th	Night	17% 19% 8% 13% 8% 5% 5% 3 4 5% 14%	5199
	21st to end	Night	26% 19% 6% 11% 6% 4 4 3 4 4 11%	6996

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	72% 5% 9% 11% 3	3618
	11th to 20th	Day	73% 5% 7% 8% 7%	3620
	21st to end	Day	69% 3 9% 13% 6%	3982
February	1st to 10th	Day	66% 5% 10% 12% 6% 1	1923
	11th to 20th	Day	62% 6% 10% 15% 7%	1958
	21st to end	Day	59% 6% 11% 15% 7% 1	1650
March	1st to 10th	Day	57% 4 9% 21% 8%	4480
	11th to 20th	Day	44% 6% 13% 30% 8% 1	4796
	21st to end	Day	65% 5% 9% 14% 6% 1	5729
April	1st to 10th	Day	55% 4 10% 18% 12%	2400
	11th to 20th	Day	48% 5% 10% 22% 12% 2	2386
	21st to end	Day	49% 6% 12% 19% 12% 2	2535
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	30% 4 8% 35% 21% 1	3480
	11th to 20th	Day	40% 4 10% 21% 25%	3096
	21st to end	Day	50% 3 10% 25% 11% 2	3081
November	1st to 10th	Day	45% 6% 11% 23% 14% 1	1762
	11th to 20th	Day	58% 5% 11% 17% 8% 1	1800
	21st to end	Day	63% 5% 10% 17% 5%	1800
December	1st to 10th	Day	60% 4 15% 18% 3	2753
	11th to 20th	Day	72% 5% 10% 12% 2	3005
	21st to end	Day	81% 3 6% 7% 2	3982
January	1st to 10th	Night	71% 5% 8% 13% 3	6348
	11th to 20th	Night	74% 5% 8% 9% 4	6360
	21st to end	Night	65% 4 13% 11% 7%	6996
February	1st to 10th	Night	62% 5% 11% 14% 8% 1	2472
	11th to 20th	Night	59% 5% 9% 18% 8% 1	2339
	21st to end	Night	61% 5% 11% 15% 7% 2	1980
March	1st to 10th	Night	52% 5% 10% 25% 7% 1	5373
	11th to 20th	Night	41% 5% 12% 28% 12% 2	5164
	21st to end	Night	53% 6% 10% 19% 10% 1	5243
April	1st to 10th	Night	47% 6% 12% 20% 14% 1	2000
	11th to 20th	Night	49% 5% 8% 20% 17% 2	1961
	21st to end	Night	49% 6% 10% 20% 13% 2	1761
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	26% 3 9% 33% 28% 1	3480
	11th to 20th	Night	40% 4 8% 23% 24% 1	3819
	21st to end	Night	40% 3 9% 29% 18% 1	4467
November	1st to 10th	Night	46% 5% 11% 22% 15% 1	2536
	11th to 20th	Night	54% 7% 11% 18% 8% 2	2600
	21st to end	Night	61% 5% 11% 17% 6%	2600
December	1st to 10th	Night	59% 4 14% 19% 4	4415
	11th to 20th	Night	73% 4 10% 10% 3	5201
	21st to end	Night	77% 4 9% 8% 2	6996

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	21% 23% 31% 19% 5% 1	300
	11th to 20th	Day	27% 16% 39% 14% 3 1	300
	21st to end	Day	27% 20% 34% 16% 2	330
February	1st to 10th	Day	28% 22% 34% 12% 3	300
	11th to 20th	Day	29% 22% 33% 13% 3	300
	21st to end	Day	31% 26% 31% 10% 2	249
March	1st to 10th	Day	25% 22% 38% 12% 3	300
	11th to 20th	Day	27% 15% 34% 18% 5% 1	420
	21st to end	Day	32% 18% 29% 16% 5% 1	550
April	1st to 10th	Day	27% 17% 31% 18% 5% 1	500
	11th to 20th	Day	27% 22% 26% 15% 9% 2	492
	21st to end	Day	26% 18% 27% 18% 9% 1	480
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	30% 18% 35% 13% 3 1	330
November	1st to 10th	Day	17% 18% 38% 19% 7% 1	300
	11th to 20th	Day	31% 22% 26% 16% 6%	300
	21st to end	Day	30% 20% 35% 13% 2	300
December	1st to 10th	Day	22% 28% 31% 14% 5%	300
	11th to 20th	Day	21% 27% 34% 14% 4	300
	21st to end	Day	31% 20% 35% 12% 2	330
January	1st to 10th	Night	25% 15% 34% 22% 4	1000
	11th to 20th	Night	24% 17% 36% 19% 3 1	1000
	21st to end	Night	23% 15% 36% 23% 3	1100
February	1st to 10th	Night	28% 13% 35% 22% 3	807
	11th to 20th	Night	22% 16% 35% 23% 4	783
	21st to end	Night	28% 16% 35% 18% 3	660
March	1st to 10th	Night	21% 15% 35% 22% 7% 1	800
	11th to 20th	Night	19% 13% 38% 23% 7%	800
	21st to end	Night	24% 13% 32% 26% 5% 1	785
April	1st to 10th	Night	21% 16% 33% 24% 6% 1	600
	11th to 20th	Night	23% 13% 33% 24% 6% 1	593
	21st to end	Night	22% 12% 32% 26% 9% 1	587
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	27% 12% 33% 21% 6%	836
November	1st to 10th	Night	20% 14% 32% 24% 9% 1	780
	11th to 20th	Night	25% 17% 32% 22% 5% 1	800
	21st to end	Night	28% 12% 33% 23% 4	800
December	1st to 10th	Night	27% 16% 34% 19% 4	900
	11th to 20th	Night	25% 16% 36% 19% 4	1000
	21st to end	Night	32% 16% 31% 19% 2	1100

Page generated at 2025-04-23 09:25:07. Version ac07

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