Convective Indices Help

A measure of instability through the depth of the atmosphere, and is related to updraft strength in thunderstorms.

Represents the “negative” area on a sounding that must be overcome for storm initiation.

The temperature difference between the 500 mb temperature and the temperature of a parcel lifted to 500 mb. Negative values denote unstable conditions.

The higher the value the more unstable. Higher Theta‑e values indicate a greater chance for positive buoyancy. Severe weather and excessive rainfall often occur near or just upstream from a Theta‑e ridge

A measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. Larger values of 3km SRH (greater than 250 m? s-2) and 0-1 km SRH (greater than 100 m s-3) suggest an increased threat of tornadoes with supercells.

Skew-T Diagram Cheat Sheet

1. Axes and Basic Layout

• X-Axis (Bottom): Temperature in °C (skewed right with height)
• Y-Axis (Left): Pressure in hPa (logarithmic scale)
• Isobars: Horizontal constant pressure lines (e.g., 1000, 850, 700, 500, 300 hPa)
• Isotherms: Diagonal lines (skewed right); constant temperature
• Dry Adiabats: Curved, upward-right; dry parcel ascent path
• Moist Adiabats: Curved, upward-left; saturated parcel ascent path
• Mixing Ratio Lines: Dashed, upward-right; constant dew point depression

2. Key Plotted Lines

• Environmental Temperature: Red or black solid line (actual air temperature)
• Dew Point Temperature: Green or blue solid line (moisture profile)
• Parcel Ascent Line: Usually yellow or dashed; follows adiabats for lifted parcel

3. Wind Barbs (Right Margin)

• Full barb: 10 knots
• Half barb: 5 knots
• Pennant (flag): 50 knots
• Direction: Where wind is coming from

4. Important Atmospheric Features

Feature	Interpretation
LCL (Lifting Condensation Level)	Cloud base; where T and Td converge
LFC (Level of Free Convection)	Parcel rises freely; indicates instability
EL (Equilibrium Level)	Parcel cools below environment; near cloud top
CAPE	Positive energy; >1000 J/kg = storms; >2500 = strong storms
CIN	Negative energy; CIN > 50 J/kg may suppress convection
Inversion	Temp increases with height; suppresses convection
Dry Layer	Large T-Td difference; limits clouds/enhances downdrafts
Moist Layer	Small T-Td difference; promotes fog/clouds near surface

5. Rule-of-Thumb Values

Instability – CAPE

CAPE	Interpretation
0–500 J/kg	Stable / weak convection
500–1000	Marginal instability
1000–2500	Moderate instability
2500–3500	Strong instability
>3500	Extreme (severe storm potential)

Wind Shear (0–6 km Bulk Shear)

• <20 kt: Weak shear – pulse storms
• 20–35 kt: Moderate shear – organized storms possible
• >35 kt: Strong shear – supercells/severe potential

Lapse Rate (700–500 hPa)

• >6.5°C/km: Steep lapse rate = more instability
• <6.0°C/km: Stable or neutral layer

6. Specialized Indicators

Indicator	What to Watch For
Veering Winds	Clockwise with height; warm-air advection
Backing Winds	Counterclockwise; cold-air advection
Critical Thickness (~540 dam)	Useful for winter snow prediction
Dry Slot	Mid-level dry air; evaporation/microburst risk

7. Example Scenarios

Thunderstorm Likely

• CAPE > 1000 J/kg
• LCL < 1500 m
• LFC present
• Wind shear > 20 kt
• CIN < 25 J/kg

Fog Formation

• Surface T ≈ Td
• Saturated boundary layer
• Light/calm winds
• Surface inversion

Supercell Environment

• CAPE > 2000
• Shear > 40 kt
• High lapse rates
• Veering winds with height