Aridity is the Thornthwaite’s concept to describe water deficiency experienced by plants. Thornthwaite gave the following formula for computing aridity index (AI):

 
    	 PE – AE
 AI = ------------- X 100
           PE               

Where, PE - Potential Evapotranspiration, AE - Actual Evapotranspiration

The Standardized Precipitation Index (SPI) is a tool which was developed primarily for defining and monitoring drought (McKee et al 1993). It allows an analyst to determine the rarity of a drought at a given time scale (temporal resolution) of interest for any region with historical data. It can also be used to determine periods of anomalously wet events. The SPI is based only on precipitation. This index is negative for drought and positive for wet conditions. As the dry and wet conditions become more severe the index become more positive or negative. The World Meteorological Organization (WMO) recommends, that all national meteorological and hydrological services should use the SPI for monitoring of dry spells (Press report December 2009, WMO No. 872). “Standardized Precipitation Index User Guide”, WMO-No. 1090 can be referred to for methodology and other key points.

The Standardized Precipitation Evapotranspiration Index (SPEI) is an extension of the widely used Standardized Precipitation Index (SPI). The SPEI is designed to consider both precipitation and potential evapotranspiration (PET) in determining drought. Thus, unlike the SPI, the SPEI captures the main impact of increased temperatures on water demand. Like the SPI, the SPEI can be calculated on a range of timescales from 1-48 months. It can be used for determining the onset, duration and magnitude of drought conditions with respect to normal conditions in a variety of natural and managed systems such as crops, ecosystems, rivers, water resources,etc.

River basins have an important environmental and social role, supplying freshwater, regulating water flow and quality, protecting from natural hazards such as floods and landslides, and enabling the conservation of biodiversity.

Forecasting river levels depends on many factors, such as precipitation, soil moisture, snow and ice cover, and the characteristics of the river basin. Even with accurate estimates of how much precipitation will occur during a storm event, determining how high a river may rise is complicated. How quickly the precipitation falls, where in the basin the precipitation falls, the type of precipitation, and the amount of moisture in the soil all can greatly affect the eventual river crest. Based on the IMD extended range forecast IMD is giving actual rainfall and water volume forecast for the all river basin of India.

EHF stand for Excess Heat factor. EHF is heatwave index, measure to define the heatwave. Positive EHF expresses the sign of heatwave. EHF is computed using two heatwave indices namely Excess Heat Index and Heat stress. Excess heat factor is combined effect of above two indices, which provide a comparative measure of frequency, duration and spatial distribution. More details are available at https://imdpune.gov.in/caui.php

EHF is computed using Maximum and minimum temperature(C) of day, hence it considers the moisture effect also. For more details: https://doi.org/10.1007/s40808-022-01450-2, 2022.

The life cycle of vector borne diseases like malaria and dengue viruses involves two hosts: humans and mosquitoes. Infected persons are bitten by female Anopheles and Aedes mosquitoes, respectively, to transmit the viruses. The viruses undergo multiple developmental stages within the mosquito before being transmitted to another human. Both the virus is transmitted by mosquitoes and their life cycle is closely linked to climatic factors. Temperature and humidity heavily affect the development of malaria and dengue viruses in mosquitoes. Warmer temperatures accelerate the development of the virus, while cooler temperatures slow it down. High humidity levels also favour the development. Climate and weather impact transmission cycles of vector-borne pathogens directly by effects of temperature on the duration of the extrinsic incubation period (EIP) of pathogens in insect vectors, which is a crucial factor determining whether or not insect-borne diseases can persist or not.