Hydrology I

Jozsef Szilagyi, Professor of Hydrology Department of Hydraulic and Water

Resources Engineering Budapest University of Technology and

Economics

Lecture #4: Formation of precipitation

• Steps necessary to produce rain:

1) Moist air must cool down to the dewpoint2) Condensation (transition from a gas

phase to a liquid one) and dry deposition (transition from a gas phase to a solid one in the form of ice)

3) Dropleth growth4) Continual import of water vapor into the

rain producing clouds

Step (1) is achieved by vertical uplift of the air parcel

Temperature

Elevation

Dry adiabatic lapse rate (~ -1°C)

Moist adiabatic lapse rate (~ -0.5°C)

Condensation level

Q1: Why is the moist lapse rate smaller than the dry one?

Step (2) is achieved by the help of cloud condensation nuclei (CCNs): salt particles,dust, fine sand, volcanic ash, soot, clay minerals, etc.

Step (3) is achieved by:

a) Droplet collision: heavier, bigger droplets have larger fall velocities, thus hit smaller ones and (by grazing) grow in size. When become large enough, uplift of air cannot keep them in the air, therefore they fall out of the sky as rain or snow.

b) Ice-crystal growth: CCNs made of clay cause dry deposition, forming ice crystals. Saturation vapor pressure of air is lower over ice surfaces than over liquid water ones, thus ice crystals continue to grow at the expense of liquid droplets.

• Step (4) is important to produce significant amount of precipitation.

Q2: Why cloud seeding to induce precipitation did not live up to its original expectd potential?

How does uplift of air happen?

Types of convergence:

a) Cyclonic or frontal convergence (characteristic of mid-latitudes)

b) Nonfrontal convergence at

i) ITCZ (Intertropical Convergence Zone): ‘afternoon rain’ region within the tropics

ii) Tropical cyclones (hurricanes): very warm ocean temperatures (over 27 °C) a must for high evaporation rates which lead to intense condensation at the cloud level releasing vast amount of heat which supplies energy into the cyclone. They start in the trade-wind zones, travel west-ward until continents block their way when they turn either north or south

c) Uplift due to orography

• Q3: If moist air contains extra stuff in it (i.e., water vapor) compared to dry air, then how can it be lighter than dry air?

• Q4: Even in the Sahara cloud formation takes place from time to time with occassional thundering and bolting, yet without rain. Why?