LAB 6 10/16

Stability – Lapse Rate The rate at which a parcel cools as it rises. A dry* parcel cools at 10 degrees Celsius per kilometer***. A moist** parcel cools at 6 degrees Celsius per kilometer. Lapse Rate = Bottom – Top) / Depth Example: The surface temperature is measured at 280K and the 850mb temperature is measured at 270K. What is the lapse rate of this layer? Assume the 850mb surface is around 1500m. Lapse Rate = (280K – 270K) / 1.5km = 6.7 degrees C / km * Dry = unsaturated ** Moist = saturated*** 1km = 1000m

Stability - Types UNSTABLE: Parcel’s lapse rate is less than environmental lapse rate. Positively buoyant – if nudged upward, parcel will be less dense than its surroundings and will continue to rise. STABLE: Parcel’s lapse rate is greater than environmental lapse rate. Negatively buoyant – if nudged upward, parcel will be more dense than its surroundings and will sink back down.

Stability - Types

CONDITIONALLY UNSTABLE: Moist adiabatic lapse rate is less than the environmental lapse rate. Dry adiabatic lapse rate is greater than the environmental lapse rate. If nudged upward, moist parcels will be positively buoyant, dry parcels will be negatively buoyant.

Lifting Condensation Level The height at which cloud bases form. Dew point remains constant as an unsaturated parcel rises, how does the temperature change? Cloud forms when temperature cools to the dew point. As a saturated parcel rises, both temperature and dew point decrease (they remain equal to each other)

Lifting Condensation Level - Example The surface temperature is 24 degrees Celsius with a dew point of 14 degrees Celsius. At what height will a cloud form if uneven heating causes parcels to become positively buoyant at the ground? How many degrees does the parcel have to cool before reaching the dew point? 10 degrees Celsius. What is the parcel’s lapse rate? Is it dry or moist? Dry adiabatic lapse rate is 10 degrees Celsius per km. So, what is the final answer? Clouds will form at 1km above the surface!

Stability and Thunderstorms Often on sunny summer days, the surface receives uneven heating. Warmer parcels of air are forced to rise. If the environmental lapse rate supports unstable conditions, air will continue to rise and form tall cumulonimbus clouds, or thunderstorms!

High-Level Heat Sources Mountains can initiate convection because when the sun shines, the air just above them gets warmer than the surrounding air (which is high above the ground below) Warm air surrounded by cold, dense air will rise If there is enough moisture, thunderstorms will form over mountains as the warm air continues rising

Lab Assignment 8.1 (a, b, c) 8.2 (a, b, c, d) 8.7 (a, b) 8.9 (a) 8.11 (a, b) (a, b) 9.14 Paul will be back for his office hours Thursday from 6- 7pm, pay him a visit if you need help!