1. Explain the characteristics of each weather element; 2. Know the instruments used for measuring the weather elements; 3. Describe measuring of weather

1. Explain the characteristics of each weather element;

2. Know the instruments used for measuring the weather elements;

3. Describe measuring of weather elements.

After successful completion of 7 Lessons in this Module, you have learned to:

Virtual Academy for the Semi Arid Tropics

Course on Crop-Weather Relationships

Module III: Weather Elements

Lesson 1: Rainfall Lesson 2: Relative HumidityLesson 3: SunshineLesson 4: Air-TemperatureLesson 5: Air-PressureLesson 6: Wind velocityLesson 7: Cloud cover

The 7 Lessons in this Module are:




After completing this Lesson, you will be able

to answer:

1. How rainfall occurs? 2. Why study about rainfall? 3. What are the rainfall characteristics? 4. What are monsoon rains? 5. How drought results? 6. How rainfall is measured?


Lesson 1: What are the basic features of rainfall?







• Rainfall occurs when moisture in the atmosphere condenses into drops.

• When water droplets in clouds become too heavy to stay aloft, they fall out of the cloud,

towards the ground as the most common form of rainfall.





• Rainfall control our water supplies.

• That water supply is the basis of crop production.

• Rainfall is especially important in rainfed or dry land agriculture.

Sufficient Rainfall – Good Crop Low Rainfall – Poor Crop





Knowing the nature and characteristics of rainfall, we can predict the cropping period and understand its effect on the crop production.

Location Annual RF Cropping period

Jodhpur 379 mm 11 weeks

Anantapur 561 mm 13 weeks

Hyderabad 812 mm 22 weeks

Indore 1008 mm 36 weeks





• Intensity of rainfall refers to the amount of rain received during unit time.

10 mm received in 1 hour

• High intensity rains are usually received during short durations.

60 mm received in 30 minutes – High Intensity rainfall;

• Low intensity rains can be of long duration. 10 mm received in 1 hour – Low Intensity Rainfall;

• We can expect a less intense rainfall (amount also) over a large area than over a small area.

25 mm received in 3 adjoining villages;

100 mm received in few fields in a village.





• High intensity storms have a larger drop size than low intensity storms.

• Rains with large drop sizes will have high soil erosion potential.

soil erosion





• Rainfall duration within a given day/week/month/ crop season is highly variable.

Year Std. Week Rain (mm)

2004 10 0

2004 11 0

2004 12 0

2004 13 4

2004 14 24.8

2004 15 0

2004 16 0

2004 17 8.19

2004 18 0

2004 19 19.8

Date Rainfall (mm)

5/11/04 05/12/04 05/13/04 195/14/04 05/15/04 05/16/04 6.25/17/04 0.45/18/04 33.45/19/04 05/20/04 0

Year Month Rain (mm)2004 1 55.62004 2 02004 3 42004 4 332004 5 80.42004 6 29.692004 7 72.89

Between days Between weeks Between months





• In India most of the rains are monsoon rains.

• The word "monsoon" appears to have originated from the Arabic word mausim which means season.

• The two monsoon seasons are South-west monsoon and North-east monsoon.

• South-west monsoon is from June to September

• North-east monsoon is from October to December.





• The average rainfall in India is 110 cm. Out of this, 88 cm rainfall is attributed to the South-west monsoon.

• South-west monsoon rains are received in North, North-east, Central and Western India.

• North-east Monsoon season is the major period of rainfall activity over south peninsular India - Coastal and Rayalaseema regions in Andhra Pradesh, Tamil Nadu, and Pondicherry.





Monsoon RainsFor example:

• The rainfall of Andhra Pradesh is influenced by both the South-west and North-east monsoons.

• The normal annual rainfall of the state is 925 mm. Major portion (68.5%) of the rainfall is contributed by South-west monsoon (June-Sept) followed by North-east monsoon (Oct.-Dec. 22.3%).

• The rest (9.2%) of the rainfall is received during the Winter and Summer months.





Monsoon Rains

• The onset, distribution of rainfall, and end of monsoon season are important in crop production.

• The rainfall during monsoon varies from year to year, season to season, and even on a daily basis.





Onset of South-West Monsoon Retreat of Monsoon





Monsoon Rains

• Rainfall during monsoon is not uniform over time and space during the whole season.

• There may be 'breaks' in the monsoon rains and may occur in any of the monsoon months.

• Such prolonged “breaks” during the monsoon results in drought conditions.





Early season “Break” Mid season “Break”

Late season “Break”





Drought

• According Indian Meteorological Department, drought occurs if the annual rainfall is less than 75% of the normal in an area.

• If the annual rainfall is less than 50% of the normal in an area, the situation is referred as severe drought.

To learn more about drought, please review the

course on Coping with Drought http://www.vasat.org/learning/agri/drought.htm





Monsoon Rains

So far you have learnt about characteristics of monsoon rains.

From the next two screens, you will learn briefly about how monsoons occur.





How monsoons result?

• In the summer, the difference in air temperatures between the land (in India) and ocean (Indian Ocean) is highest.

• This causes a pressure gradient between land and ocean.

• Due to this pressure gradient, the winds laden with moisture will travel towards the land from the ocean surface.





How monsoons result?

• These winds will enter the land and are raised because of Westernghats, and shed the moisture resulting in rainfall.

• Also, the summer temperatures cause the air over the land to heat up and expand, and become less dense and rise. This rising air is then replaced with neighboring moisture-rich air from over the ocean surface resulting in monsoon rains.





Differential heating between land and ocean and the resultant movement of winds.

Ref.:http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/sea/htg.rxml





Cyclones

• Cyclones result due to intense low-pressure systems that form over oceans with sustained winds exceeding 63 km/h.

• Severe tropical cyclones are called typhoons in Asia whilst in the USA and Caribbean they are known as hurricanes.

• Cyclones cause enormous damage and loss of life in coastal areas as they don't reach very far inland because the ocean is their source of energy.

• Cyclones occur in the Bay of Bengal in chiefly two seasons: April-May and October-November.





Thunderstorms

• Thunderstorms form when warm, moist air near the ground rises to great heights.

• The rising air cools and, eventually, the moisture in the air (water vapor) condenses, forming a towering rain cloud.

• You'll recognize a thunderstorm

by the sound of its thunder, wind,

lightning, the rain showers and,

sometimes, hail.





How rainfall is measured?

• Rainfall is usually measured by first collecting it in a rain gauge.

• Rain gauges (Fig.) are usually about 50 cm tall and are placed on the ground just high enough to avoid splashes.

• Rain water that is caught in a funnel on top, runs down into a measuring cylinder below – where it can be recorded in mm.






You can also make a simple rain gauge and measure rainfall near your house or in the field.






• A recording rain gauge, gives a record of rainfall events at different times.

• Hence, intensities can be determined from recording gauges.

This concludes Lesson 1- What are the basic features of rainfall? - in this Module.

In the next Lesson, information on relative humidity and its measurement are briefly explained.

Select Lesson 2 from Module III contents




Documents

1. Explain the characteristics of each weather element; 2. Know the instruments used for measuring the weather elements; 3. Describe measuring of weather