ZIMSEC O Level Geography Notes: People and Weather: Deliberate Changes
- These are used to grow plants that are susceptible to frost during frosty conditions.
- They create a micro-climate.
- They are made up of glass or a thin layer of polythene usually colored green hence their name.
- Short UV waves from the sun can pass through the glass/plastic.
- When these heat the ground they are converted into longer infra-red waves (heat) which cannot pass through the plastic.
- The result is that greenhouses are warmer than the surrounding areas.
Other methods used to protect crops from frost
- Heating the lower layers of the soil using either primitive oil fired “smudge pots” or modern gas or electrically powered frost prevention heaters.
- Covering the crops using a thin layer of polythene for example growing tunnels also known as polythene tunnels.
- Spraying the crops with water which acts as an insulator although the efficacy of this method is doubtful at best and it is useless against extreme frost/cold.
- Blowing air around the fields in order to mix the cold layers of air below with the warmer lays above.
- Not all condensation leads to precipitation sometimes due to the lack of condensation nuclei in the atmosphere.
- Cloud seeding less well known as rain making involves the injection of silver iodide or dry ice into clouds.
- In a natural process the clouds would have been provided with materials like soot, dust, sea salt, volcanic ash and ice to use as nuclei.
- Silver iodide or dried is sprayed into likely looking clouds in a bid to spur precipitation.
- The clouds’ droplets grow bigger around the introduced nuclei and eventually fall as rain.
- The success of cloud seeding depends on the clouds being seeded.
- Arguments have been made that in most cases rain would have eventually fallen from the seeded clouds.
- There have also been allegations of “rain stealing” where people contend that a seeded cloud would have taken its rain elsewhere (usually neighboring areas) where rain would have fallen were it not for the interference of cloud seeding.
- Dams can change weather conditions in the vicinity by creating a humidity and precipitation.
- The areas surrounding Lake Kariba receive more precipitation when compared to the more further areas ever since the dam was constructed.
- Lake shore breezes not unlike Sea and Land breezes also occur around the Lake.
- Areas around the Lake are generally cooler and the diurnal temperature range smaller than that of areas further from the Lake.
- Research has also shown that Lakes can cause freak rain storms for exam water spouts/mini tornadoes sometimes occur on and around the Lake.
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