Salt Lake City Weather Assessment to Support Amateur Astronomy - Dashboard

Instructions Urls Last Verified and Updated 9-20-2025. K. Fisher fisherka@ipns.com

I. Weather Forecasting

II. Satellite Imagery

III. Moonrise-set and Sunrise-set, and Astronomical Twilight

IV. Local Sidereal Time (LST) and RA at North Horizon

V. Deep and Shallow Sky

VI. Air Quality - Pollution and Wildfire Affected Transparency

VII. Purpose and Background

A central nightly question for northern Utah amateur astronomers is "What is the weather forecast tonight? How cloudy will it be?" The dashboard supports links primarily to authoritative government sources and reliable commercial sources to answer that question for northern Utah. Resources provided can be used to rapidly make screen clips to support nightly posts to northern Utah astronomy social media groups. This dashboard is duplicative of the many smart phone weather apps like Accuweather, The Weather Channel or Clear Outside. That is by design. I prefer to work from data closest to the original government source instead of information mediated by commercial providers. Additionally, general internet links are easier to gather weather data into a format suitable for social media posting.

VIII. How to Use this SLC Weather Dashboard:

1. Under Section I: Weather Forecasting, click from left-to-right the first five buttons. Make screen captures as needed.
2. Under Section II: Satellite Imagery, click from left-to-right the first two buttons. Make screen captures as needed.
3. Section I, the last item, and Section III, Moonrise, Local Sidereal Time, report the current UTC time and local Salt Lake City LST. This information is useful for annotating social media posts with date and times.
4. Sometimes for uncertain weather conditions, the National Short-term Forecast Maps under Section I, are useful to discuss the passage of frontal systems over multiple days.
5. The NWS CONUS Cloud Cover Animated Chart is useful for discussing the timing of cloud fronts.
6. The remaining sections and links provide additional useful information, e.g., during the summerime, wildfire smoke can compromise visibility but does not show up on traditional weather charts. Wildfire smoke is graphically forecast in Section V, Air Quality, resources.

IX. Website Specific Notes:

1. Users should feel free to download local copies of this HTML file and to modify the same without this author's consent. To promote this process, this document is covered by a CC BY-SA 4.0 license.
2. GOES Satellite Animations: At night, use the sandwich filter.
3. Moon Phases: The Moon Phases display at TimeandDate dot com is used to identify those times during the calendar month in which the night sky is not washed out by moonlight - the "current dark sky window" - and those times of the month in which the night is compromised by bright moonlight - the "current bright sky window." Amateur astronomers typically do observing and imaging during a dark sky window. Planetary and lunar observing can be done during the bright sky window, although the advent of modern ultra-narrowband filters has enabled skilled amateurs to pursue DSO imaging without regard to the brightness of their local sky.
4. Utah Dept. of Transportation Webcamera System: This link goes to the UDOT Webcam portal. Use requires creating a login account and list of webcams. The use process is too complicated and time-consuming, and is beyond scope of this dashboard. The UDOT webcam system can infrequently be used to verify the current cloud conditions in Utah's remote West Desert.
5. Heaven's Above: This satellite tracking website is best used as a mobile phone application. The website requires a login to particularize charts for a specific observing point.
6. NOAA HRRR Smoke Forecast Map: The HRRR Smoke Map has a maximum 48 hour look-ahead. On the Vertically Integrated Smoke row, choose a look-ahead hour.
7. Skew-T Log P Plot - NWS SLC Upper Air Sounding: Skew-T Log P charts are a technical meteorological topic. On the reference map, click on Salt Lake City to display the Skew-T plot. If the graphical index chart of stations does not display, scroll down to the station list and to the Salt Lake Station "KSLC" row. Click and launch the current Salt Lake City Skew-T chart manually. Online search for "Skew-T Log P tutorial" for educational materials discussing this key type of meteorological chart. Sounding balloons (Radiosondes) are launched twice daily by the NWS at the Salt Lake City International Airport at 6 a.m. MT and 6 p.m. MT. An easy way to study these complex weather balloon soundings is to download the Skew-T plot to your local disk. Then reupload the Skew-T sounding image to a GPT chat Large Large Model. Ask the chatGPT LLM to prepare a narrative evaluation, e.g., using a prompt such as, "From the uploaded Skew-T plot, please prepare a weather report narrative discussing the key features, readings, and notations in the figure." Three handy tables for interpreting Skew-T charts are in the Appendix. The first translates the millibars of pressure on the left-hand scale of the Skew-T plot into approximate feet above ground level. The second is a Celsius to Fahrenheit conversion table. The x-axis of the Skew-T chart is expressed in degrees Celsius. The third CAPE table defines Low, Medium, High and Very High levels of CAPE readings. CAPE measures the instability of the atmosphere. Higher CAPE values mean the atmosphere is more unstable and capable of producing stronger convection (e.g., thunderstorms, severe weather). Lower CAPE values indicate a relatively stable atmosphere with little potential for rapid convection of moist low altitude air to higher altitudes where they turn into thunderstorm producing cumulus clouds.
8. Skew-T Log P Plot - Nationwide Interpolated by Tropical Tidbits: Click anywhere on the nationwide CONUS map. Tropical Tidbits will generate a Skew-T Log P chart for that location from the surrounding NWS Radiosonde stations and the current GFS model run. There are a total of 92 U.S. Radiosonde stations in the United States.
9. Utah Division of Air Quality Pollutants: While the PurpleAir chart is a good summary for real-time particulate pollution measurements in the Salt Lake Valley, PurpleAir does not cover gaseous key summertime pollutants like NOx and Ozone. Because of this omission, the PurpleAIr chart can indicate good air particulate quality that is not healthy when gaseous pollutants are considered. The UDAQ air quality site provides this important additional pollutant information.
10. Federal Inciweb Wildfire Incident Reports: For large Utah wildfires managed by Federal wildfire response teams, daily incident reports are filed. This is the best real-time data for large Utah wildfires on federal lands. Search by fire name.

X. Acknowledgements:

1. College DuPage Meterology Department.
2. Danko, Attillia. Clear Sky Chart
3. Dept. of Geology, University of Utah (William Browning Building Webcams)
4. Heaven's Above
5. Jet Propulsion Laboratory
6. Jiménez, Victor and Salvador Rubio, Univ. of Barcelona (Local Sidereal Time code).
7. KSL News (Vortex Doplar Radar Forecast)
8. MesoWest
9. Netweather (Jet Stream Position)
10. NASA
11. NOAA-National Weather Service
12. Observatory Črni Vrh. (Comet Observation Database)
13. PurpleAir
14. Sky and Telescope
15. Time and Date (Weather Forecast, Sunrise Sunset, Moonrise Moonset, Extended Forecast, Planet Positions).
16. TropicalTidbits.com
17. United States Naval Observatory
18. University of Utah Geology Department (WBB Webcams)
19. Utah Dept. of Transportation
20. Utah Division of Air Quality.
21. Utah Wildfire.com (A consortium of Utah State and Federal Fire Agencies).
22. WeatherSpark
23. Wojczyński, Bartosz and PTMA (Transit Finder)

XI. Revision History:

1. 2025-09-20. Added Local Sidereal Time clock with updatable observer point (latitude, longitude).
2. 2025-09-18. Added appendix for understanding and interpreting red, green, and brown-black lines NOAA and Tropical Tidbits Skew-T diagrams. Addded links to separate example Skew-T diagrams.
3. 2025-09-18. Added decoder appendix for abbreviations in NOAA and Tropical Tidbits Skew-T diagrams.
4. 2025-08-21. Added TropicalTibits nationwide interpolated Skew-T Log P charts. Added suggestion for using chatGPT to evaluate Skew-T plots. Added handy tables (millibars-altitude(feet) and C degs-F degs). Added Utah Div. of Air Quality Real Time Chart.
5. 2025-06-24. Added HRRR Smoke map link, Federal Inciweb Wildfire Incident Reports, and Skew-T Log P chart links.
6. 2025-06-02. Updated NOAA Satellite wildfire smoke map links.
7. 2025-05-20. Corrected link to South Rockies GEOS satellite image.
8. 2024-04-23. Updated url change for William Browning Building cameras.

XII. Appendix:

A(1) Millibars to Altitude Conversion Table

A(2) Celsius to Fahrenheit Conversion Table

A(3) CAPE Zone Table

A(4) Decoding Skew-T Measurement Abbreviations for NOAA Skew-T Diagrams, including KSLC

Glossary

• Adiabatic - In meteorology, "adiabatic" describes temperature changes in an air parcel that occur without any heat exchange with its surroundings. As air rises and expands due to lower atmospheric pressure at higher altitudes, it cools (adiabatic cooling), while sinking and compressed air warms up (adiabatic warming).

On the body of the Skew-T diagram:

• CCL (Convective Condensation Level): The height at which surface air parcels, if heated enough, become saturated and form cumulus clouds (based on surface temperature and dewpoint).
• LCL (Lifted Condensation Level): The height at which a parcel lifted adiabatically becomes saturated. Usually lower than the CCL. Important for cloud base and storm structure.
• LFC (Level of Free Convection): The LFC is the level where a parcel of air, after being lifted (either by forced ascent, like a front, or by convection reaching the CCL), becomes warmer than the surrounding air. At this point, the parcel becomes positively buoyant and will continue to rise freely without further external forcing, forming towering cumulonimbus clouds (thunderstorms) if sufficient moisture is present.
• M - Most Unstable Parcel (MUP): This is the letter "M" near the center of the graph near the 600 mb horizontal black line. The Most Unstable parcel (MU, or “M”) – the parcel in the lowest ~300 mb that has the highest equivalent potential temperature, i.e., the most buoyant one if lifted. The M parcel can reveal instability that surface parcels miss, especially if there’s a shallow stable layer near the ground but richer moisture a little above it.

Along the top horizontal axis:

• T (F) – Temperature (°F): The ambient air temperature at the surface or at a specified pressure level.
• Td – Dewpoint Temperature (°C or °F): The temperature to which air must cool for saturation (RH = 100%). Indicates atmospheric moisture.
• LI (Lifted Index): A measure of instability. It compares the temperature of an air parcel lifted to 500 mb with the environment at that level. Negative values = unstable (e.g., LI ≤ –4 is strong instability). Positive values = stable.
• SWT (Severe Weather Threat index): A composite index combining instability, shear, and moisture factors to estimate severe storm potential.
• K (K Index): A thunderstorm potential index using temperature and dewpoint at 850, 700, and 500 mb. Higher values indicate greater thunderstorm probability, but not necessarily severity.
• TT (Total Totals index): Another thunderstorm index based on lapse rate and moisture. Values > 50 suggest potential for severe storms.
• Pw (cm) – Precipitable Water (cm): The total depth of water vapor in a vertical column of the atmosphere, expressed as centimeters of liquid water. A measure of available moisture for rainfall.
• CAPE (Convective Available Potential Energy): The amount of buoyant energy available to rising air parcels (J/kg). Higher values = stronger potential updrafts and severe weather.
• CIN (Convective Inhibition): The energy “cap” that suppresses convection (J/kg). Large CIN prevents storms from forming without strong forcing.
• Bmin (Minimum Buoyancy or Buoyancy Minimum): The lowest point of negative buoyancy a lifted parcel encounters before reaching its Level of Free Convection (LFC). Represents the “cap” strength in another form.
• Tc (Convective Temperature): The surface temperature that must be reached to eliminate CIN and trigger convection.
• Cell: Often shorthand for a computed “storm cell” model in sounding software; can provide estimated storm type or parameters.
• SREH (Storm-Relative Environmental Helicity): SREH is a measure of the corkscrew-like motion available to storms due to low-level wind shear (m²/s²). Values > 150 (m²/s²) can support supercells if instability is present.
• VGP (Vorticity Generation Parameter): A composite index estimating potential for rotating updrafts (tornadic supercells).
• Trop (Tropopause height): The altitude of the tropopause, usually given in pressure (mb) or height (km). Important for storm depth and overshooting top potential.

A(5) Line Decoding for NOAA Skew-T Diagrams, including KSLC

Explanation of Each Line Type

• Thick Green Line – Dewpoint Profile
  • Vertical profile of dewpoint temperature from radiosonde data.
  • Shows moisture availability, helps determine cloud bases (LCL/CCL), and signals moist vs dry layers.
• Thick Red Line – Temperature Profile
  • Vertical profile of actual air temperature.
  • Compared with parcel ascent paths to diagnose stability, CAPE, CIN, and storm potential.
• Dashed Red Lines – Dry Adiabats
  • Show how an unsaturated air parcel warms or cools as it rises or sinks (about 9.8°C/km).
  • Used to track parcel paths below the LCL and diagnose instability.
• Dashed Green Lines – Mixing Ratio Lines
  • Lines of constant water vapor mixing ratio (g/kg).
  • Used with dewpoint to determine moisture content and find LCL (where T and Td meet along these lines).
• Horizontal Black Lines – Isobars
  • Constant pressure levels (mb or hPa).
  • Help forecasters determine altitude, locate tropopause, and diagnose pressure-related features.
• Light Brown Lines – Saturated Adiabats (Moist Adiabats) rising from the horizontal axis to the 600mb horizontal line.
  • Show how a saturated air parcel cools as it rises (≈6°C/km).
  • Used to estimate parcel ascent above LCL and cloud/precipitation processes.

A(6) Example interpretation of a Skew-T Graph from NOAA for KSLC

Click thumbnail to expand.

Perplexity AI chatbot generated intrepretation:

Based on the uploaded Skew-T diagram for KSLC (Salt Lake City) at 0000 UTC September 18, 2025, the atmosphere is generally stable with limited convective or thunderstorm potential during this period.

Significant Skew-T Elements

• Temperature/Dewpoint Profile (Red & Green Lines):
  • The temperature profile (red) and dewpoint profile (green) are fairly separated, especially above the surface, which indicates a relatively dry mid-level atmosphere and limited deep moisture for cloud development.

• CCL and LCL (Marked on the Chart):
  • The LCL (Lifted Condensation Level, usually marked lower than CCL) is close to 700 mb, suggesting any convection would yield relatively high cloud bases.
  • The CCL (Convective Condensation Level) is slightly higher than the LCL, indicating both the forced lifting and surface heating threshold for cloud formation are above typical mountain/valley elevations, so cumulus clouds are not easily triggered.

• CAPE, CIN, and Indices:
  • CAPE (Convective Available Potential Energy) is 0.0, revealing no buoyant energy for storms; CIN (Convective Inhibition) is also 0.0, confirming no significant energy cap, but since CAPE is zero, convection is suppressed anyway.
  • TT (Total Totals): 44, and K Index: 19, both below thunderstorm thresholds—these indicate minimal thunderstorm potential.
  • LI (Lifted Index): 4.1 (positive), also suggesting stable air as negative LI is required for true instability.
  • Severe Weather Threat, Cell, SREH, VGP: All low, with SREH at 11 (below supercell threshold), and VGP at 0.

• Moisture and Tropopause:
  • Precipitable Water (Pw): 1.20 cm, showing only modest available moisture but not enough for widespread heavy rain.
  • The tropopause is near 11,390 meters, marking the boundary for deep convection—which is irrelevant here due to lack of instability.

• Most Unstable Parcel (“M” near 600 mb):
  • The “M” marks the most unstable parcel, but since overall instability is negligible, no enhanced localized risk exists at that layer.

• Surface:
  • Surface temperature is 62°F with a surface dewpoint of 30°F, confirming overall dryness at the ground and limited convective potential.

• Wind & Hodograph:
  • The hodograph and wind barbs show weak to moderate winds aloft and little directional shear, indicating little to no threat for organized severe storms or rotating updrafts.

Summary Forecast

• Expect mainly clear skies or only scattered high-based clouds.
• No convective storms or precipitation are expected.
• Winds will be moderate and generally non-gusty, with stable air throughout most of the troposphere. li>Overall, a quiet weather period with no severe weather or significant atmospheric instability is indicated by this Skew-T.

A(7) Decoding Skew-T Measurement Abbreviations for Tropical Tidbits Diagrams, including the western Unita Mountains

• SRH 0–1 km - Storm-Relative Helicity (0–1 km layer): A measure of the corkscrew-like motion in the lowest 1 km of the atmosphere, important for rotating storms. Units: m²/s². Values > 100 m²/s² = supportive of mesocyclones if instability is present.
• SRH 0–3 km - Storm-Relative Helicity (0–3 km layer): Same as above, but integrated through the lowest 3 km. Higher values support stronger, more organized rotating updrafts.
• SBCAPE - Surface-Based Convective Available Potential Energy: The buoyant energy (J/kg) available to a parcel lifted directly from the surface. Reflects instability for the near-surface air.
• MLCAPE - Mixed-Layer CAPE: CAPE calculated from a parcel averaged over the lowest ~100 mb of the atmosphere. This is a more realistic measure of instability since storms usually ingest mixed air, not a single surface parcel.
• SBCIN – Surface-Based Convective Inhibition: The amount of “negative energy” preventing surface parcels from rising (J/kg). This represents the “cap” holding convection down.
• MLCIN – Mixed-Layer Convective Inhibition: CIN calculated for the lowest mixed layer, rather than just the surface.
• DCAPE – Downdraft CAPE: The potential energy available to downdrafts, representing the strength of cold pools and outflow winds. Higher values = stronger potential downbursts.
• SHR 200–850 mb – Deep-Layer Shear (200–850 mb wind shear): The vector difference in wind between ~200 mb (upper level) and 850 mb (low level). Stronger values (>30–40 kt) favor organized convection.
• RH 300–850 mb – Relative Humidity (average between 300–850 mb): A measure of mid-level moisture. Low values suggest dry air entrainment, which can enhance downdrafts but limit storm longevity.
• PWAT – Precipitable Water (inches): The total atmospheric column water vapor, condensed as liquid. Higher PWAT means more moisture available for rainfall.
• Omega (Pa/s): Vertical velocity in pressure coordinates. Negative values = rising motion (conducive to storms), positive = sinking motion.
• Equivalent Potential Temperature (θe, K): The temperature a parcel would have if lifted to saturation and then compressed back to 1000 mb. A measure of moist static energy. Large differences in θe between layers indicate instability.
• DGZ (Dendritic Growth Zone): The layer (typically –10°C to –20°C) where ice crystal growth by deposition is most efficient. Important for snowfall microphysics and identifying layers favorable for hail growth in convection.

A(8) Line Decoding for Tropical Tidbits Skew-T Diagram, including the western Unita Mountains

Explanation of Each Line Type

• Light solid red lines trending from upper left to lower right (Dry Adiabats)
  These represent the paths an unsaturated air parcel would follow if it rises or sinks without exchanging heat (dry adiabatic process). They slope diagonally upward to the left. Forecasters use them to determine how rising unsaturated air will cool with height (~9.8 °C per km).
• Light solid grey lines trending from upper left to lower right (Moist Adiabats / Saturated Adiabats)
  These represent the cooling rate of a saturated air parcel rising with condensation. Because latent heat is released, the lapse rate is slower than dry adiabats. Forecasters use these to estimate cloud formation, storm development, and stability once an air mass reaches saturation.
• Horizontal grey lines (Isobars of pressure in hPa)
  Each horizontal line marks a level of constant pressure (1000 hPa near the surface to 100 hPa at high altitudes). Forecasters use these as vertical reference levels instead of height, making it possible to measure cloud depth, freezing levels, and storm heights.
• Light dashed black lines sloping from lower right to upper left (Isotherms, °C)
  These straight diagonal reference lines indicate constant temperature. They tilt because of the skewed temperature axis (hence “Skew-T”). Meteorologists compare actual temperature and dewpoint traces to the isotherms to find freezing levels, inversion layers, and gradients.
• Light dotted lines trending upward to the right, ending near 500 mb, labeled 1 to 36 (Mixing Ratio Lines, g/kg)
  These represent constant values of water vapor mixing ratio (grams of water vapor per kilogram of dry air). They are used to estimate dewpoint depressions, parcel moisture content, and precipitable water potential. They slope upward to the right because warmer air can hold more moisture.

Plotted Data Lines

• Thick wandering green line (Dew Point Temperature Profile)
  This shows the observed or modeled dewpoint at each pressure level. The proximity of the green (dewpoint) and red (temperature) lines indicates atmospheric moisture: when close together, the air is near saturation (clouds likely); when separated, the air is dry.
• Thick wavy red line (Environmental Temperature Profile)
  This is the actual ambient temperature of the atmosphere at different levels. It is compared against dry/moist adiabats to assess stability. For instance, a steep lapse rate (temperature dropping quickly with height) indicates instability conducive to convection.
• Thick wavy blue line (Wet-Bulb Temperature Profile / Pseudoadiabat)
  This line shows the wet-bulb temperature of the air column and closely follows the adiabat a parcel would take if lifted while allowing moisture to condense. It is used to approximate cooling if the atmosphere is forced to saturation.

Use of Main Lines in Forecasting

• By comparing the red (temperature) and green (dewpoint) lines, forecasters assess cloud bases, humidity layers, and potential precipitation.
• Following dry vs moist adiabats, they test parcel ascent: if a rising air parcel (following an adiabat) becomes warmer than the environment (red line), convection and storms are possible.
• The mixing ratio lines help determine condensation levels and water vapor availability.
• Forecasters also check for inversions (red line bending right with height), freezing level (intersection with 0 °C isotherm), and stability indices derived from these lines.

A(9) Example interpretation of a Skew-T Graph from Tropical Tidbits for the western Unita Mountains.

Click thumbnail to expand.

Perplexity AI chatbot generated intrepretation:

For this sounding near the Uintas (40.8°N, 110.6°W), the Skew-T diagram reveals the following forecast implications by referencing the graph’s labeled lines:

Temperature and Dewpoint Profiles

• The thick wavy red line shows the actual environmental temperature decreasing steadily with height, starting around 13 °C near the surface (about 1000 hPa).
• The thick wavy green line shows the dewpoint temperature profile, starting close to 7 °C near the surface but dropping more quickly with height than temperature, indicating relatively dry air aloft.

Moisture and Cloud Potential

• The separation between the green (dewpoint) and red (temperature) lines at most levels is fairly wide, indicating generally unsaturated air and low likelihood of widespread cloud formation.
• Near the surface (≈1000 hPa), the dewpoint and temperature come closest, suggesting higher humidity near the ground but still below saturation.

Stability and Atmospheric Layers

• The environmental temperature (red line) is steeply declining with height near the surface, which suggests a moderately unstable layer conducive to some surface heating and mixing.
• Comparing the red line to the dry adiabats (solid light red lines slanting upward left), parcels lifted dry adiabatically would cool quickly, and since the environmental temperature profile is somewhat steep, there is potential for buoyant lifting near the surface.
• The presence of the moist adiabats (solid light grey lines) deeper up shows that even if parcels become saturated, their cooling rate would slow, which affects cloud development.

Moisture Content and Precipitation

• The mixing ratio lines (light dotted lines trending up to right, labeled 1-36) show relatively low moisture contents, consistent with relatively dry air—this reduces precipitation potential.
• The wet-bulb temperature (thick blue line) lies near the dewpoint but remains well below the environmental temperature, reinforcing dry conditions aloft.

Overall Forecast Implications

• Surface air is moderately warm and moderately humid, but drier air above and significant temperature-dewpoint separation imply limited cloud cover and little chance of precipitation.
• The steep environmental lapse rate near the bottom indicates some surface instability, which may promote afternoon mixing or weak convection but not strong storms.
• The freezing level (where the red line crosses the 0 °C isotherm) appears around the 750-800 hPa level (≈2 to 2.5 km altitude), marking where snow could begin if precipitation occurred.

Summary

• Expect a generally dry atmosphere with limited cloudiness.
• Some surface heating and modest instability might yield light convective mixing but little chance of precipitation.
• The freezing level is relatively high, so any precipitation falling would be rain at lower altitudes, with snow only possible at higher elevations above ≈2 km.
• This analysis, referencing the labeled lines on the Skew-T diagram, supports a stable or mostly fair weather forecast for this time and location near the Uintas mountains.

End