18 Weather and Climate

Yogalakshmi K N

1. Learning outcomes
2. Introduction
3. Elements of weather and climate
3.1. Temperature
3.2. Humidity
3.3. Precipitation
3.4. Visibility
3.5. Atmospheric pressure
3.6. Wind
4. Climatic controls
4.1. Latitudinal variation in the solar radiation
4.2. Elevation or altitude
4.3. Distance from the sea
4.4. Pressure and wind systems
4.5. Ocean currents
4.6. Local features
4.7. Human activities
5. Weather and climate predictions
6. Summary

1. Learning outcomes

After studying this module, you shall be able to:

• Define weather and climate
• Differentiate between weather and climate
• Know about various elements of weather and climate
• Know about various factors that determine the weather and climate of a place or region
• Know about how weather and climate predictions are made and their major differences

2. Introduction

Earth’s environment comprises of five important components, viz., atmosphere, ocean, land surface, ice and snow surfaces (both land and ocean areas), and biosphere (both terrestrial and marine). Continuous interactions among these components determine the state and dynamics of our climate system and produces weather at any given place and time. Our forefathers since ages before have been fully aware of roles that weather or climate plays in our lives. One can find numerous direct or indirect references of weather and climate in the historical texts and scriptures; the need and desire to control weather, for the best advantage of the society, led to worshipping and rituals involving natural elements like sun, moon or wind in almost all religions.

Weather and climate determine the way people live at a place; type of clothes they wear, kind of food they eat or type of houses they build in to live. In fact, it is the climate that determines natural vegetation, ecosystem functioning, water supply, agricultural practices, comfort level etc. factors which leads to development of general human or animal habits and habitats in that area; however, instantaneous choices have more to do with the current weather situation.

Although the evolution of weather and climate is largely a natural process, recent scientific evidences suggest that anthropogenic influences on the nature earth have had something too in shaping this evolution around the world and would continue to do so in coming days. It would be interesting and imperative as well, to know how things unfold in future. So, it is necessary to understand various aspects of weather and climate.

Weather and climate: definition and differences

Weather is the day-to-day physical condition or state of the atmosphere (i.e., hot/cold, wet/dry, calm/windy, sunny/cloudy) at a given time and place and its short-term variation in minutes to weeks. Fig. 1 shows one aspect of weather, the surface air temperature measured at 0830 Indian Standard Time (IST) on 10th October, 2016 over Indian region and indicates that warmest temperatures of 30oC and above were experienced by few stations from Tamil Nadu and West Bengal. On the other hand, lower temperatures were experienced by stations from northwestern and northern most parts of the country.

Climate is the average weather pattern of a place over a long time period, often 30 years or more. In a more scientifically accurate way, climate is the statistical description of weather pattern over a place in terms of the mean and its variability over a long time period. Fig. 2 shows the annual climatology of rainfall over India, computed using data for the period 1951-2000. As seen in it, northeast India, and west coast & adjoining Ghats experience highest rainfall. On the other hand, western parts of Rajasthan and southeast Tamil Nadu experience lowest rainfall.

The comparison is further elaborated in the following bullets:

2.2 Difference between Weather and Climate

• Weather and Climate are two different meteorological terms that are related but not interchangeable. For example, it would be wrong to say that the `climate has become so hot` or `Siberia has a cold weather`. In a day to day conversations, it is even common to see, when the sudden changes occur in the weather conditions like rain, temperature etc., some of us wrongly mention that climate has changed
• Climate is what you expect, weather is what you get. In our day to day life, we often discuss a cold morning, a cloudy sky, a sultry afternoon or a warm evening. All these refer to the weather conditions of a given place at a given time. In most places weather may change from minute-to-minute, hour-to-hour, day-to-day, and season-to-season

Climate is the average of weather over time and space. For example, In India we can expect snow in the northern most parts and Himalayan ranges during December to February, dry and hot during April to June over northwest and central parts of India, rains in most parts of the country during the southwest monsoon season (June to September). This is climate. Generally, the climate information of a place includes extreme values such as record high/low temperatures or record amounts of rainfall. When you hear from your local weather agency saying today’s maximum temperature hit a record high, the agency is mentioning it based on the historical climate records of that place.

3. Elements of weather and climate

Atmospheric variables that can be used in combination to describe Weather and Climate are called elements. These variables are temperature, humidity, precipitation, visibility, atmospheric pressure and wind. These atmospheric variables vary from once place to another, with the height and  with time. Table 1 shows list of various weather elements along with instruments used for its measurements.

3.1. Temperature

The temperature refers to measure of degree of warmth or coldness of the atmospheric air at a given place and level with respect to some standard value. At the surface the air temperature is measured by a thermometer (in Celsius, oC; Fahrenheit oF; Kelvin, oK). It is governed by many factors, including incoming solar radiation, humidity and altitude. The temperature of the air has an effect on the rate of evaporation in the atmosphere, the amount of humidity and form of precipitation.

3.2. Humidity

The amount of water vapor in the atmosphere is called humidity. The main sources of water vapor in the lower atmosphere are evaporation from the Earth’s surface and transpiration by plants. In the stratosphere, the breakdown of methane by sunlight is another source. The main sink is precipitation. Meteorologists have defined several different measures of humidity. These can be divided into two categories: those that describe the actual amount, or concentration, of water vapor in the air and those that relate the actual amount to the potential amount that the air could hold if it were saturated with respect to water vapor. Air is saturated when it holds the maximum possible amount of water vapor. At that point, the rate at which water molecules enter the air by evaporation exactly balances the rate at which they leave by condensation. A device to measure relative humidity is called a hygrometer. The simplest hygrometer – a sling psychrometer – consists of two thermometers mounted together with a handle attached on a chain. One thermometer is ordinary. The other has a cloth wick over its bulb and is called a wet-bulb thermometer.

3.3. Precipitation

Precipitation is the condensed water vapor in the form of liquid or solid falling from the air onto the ground. Different forms of precipitation include rain, sleet, snow, hail, and drizzle plus a few  less common occurrences such as ice pellets, diamond dust, fog precipitation and freezing rain. The total amount of precipitation that reaches the ground in a stated period is expressed in terms of the vertical depth of water (or water equivalent in the case of solid forms) to which it would cover a horizontal projection of the Earth’s surface. Snowfall is also expressed by the depth of fresh, newly fallen snow covering an even horizontal surface.

Precipitation gauges (or rain gauges if only liquid precipitation can be measured) are the most common instruments used to measure precipitation. Generally, an open receptacle with vertical sides is used, usually in the form of a right cylinder, with a funnel if its main purpose is to measure rain. The volume or weight of the catch is measured, the latter in particular for solid precipitation. The measurement of precipitation is very sensitive to exposure, and in particular to wind.

3.4. Visibility

Visibility is a measure of the horizontal opacity of the atmosphere at the point of observation and is expressed in terms of the horizontal distance at which a person should be able to see and identify: in the daytime, a prominent dark object against the sky at the horizon; at night, a known, preferably unfocused, moderately intense light source. Visibility affects all forms of traffic: roads, sailing and aviation. Visibility can be reduced by fog, pollution, snow or even sand blown up by the wind. Visibility degradation is caused by the absorption and scattering of light by particles and gases in the atmosphere. For last many years, meteorological visibility was used to be estimated by the human observer judging the appearance of distant objects against a contrasting background, usually the sky. Recently, many synoptic observing stations have sensors which provide a measurement of visibility. Visibility sensors measure the meteorological optical range which is defined as the length of atmosphere over which a beam of light travels before its luminous flux is reduced to 5% of its original value. In most instances this is approximately equivalent to, but not the same as, visibility measured by e contrast of a distant object against its background.

3.5.  Atmospheric pressure

The force per unit area exerted against a surface by the weight of the air above that surface is called atmospheric pressure. Atmospheric pressure is measured using a metric unit called a millibar  (mb) or hecta Pascal (hPa) and the average pressure at sea level is 1013.25 millibars. Pressure decreases exponentially with altitude. If P(0) is the pressure at the surface and P(z) is the pressure at altitude ‘Z’, the fraction of total atmospheric weight located above altitude z is P(z)/P(0). The atmospheric pressure at about 80 km altitude is about 0.01 hPa, i.e., 99.99% of the atmosphere is below this altitude. The instrument used for measuring the atmospheric pressure is barometer. The most commonly used barometer is mercury barometer. A mercury barometer consists of a thick-walled glass tube, filled with mercury after removing air bubbles is closed at one end and its open end is inverted into a container of mercury. The column of mercury in the tube is supported by the atmospheric pressure and its height depends on the magnitude of the atmospheric pressure. The mean sea level pressure or atmospheric pressure at sea level is the atmospheric pressure normally given in weather reports. The altimeter setting in aviation is an atmospheric pressure adjustment. Average sea-level pressure is 1013.25 hPa or mbar or 76 centimetres of mercury (cm Hg).

3.6. Wind

Movement of air caused by differences in air pressure within the atmosphere is called wind. Air moves from high pressure areas to low pressure areas and the speed of the winds is directly proportional to the difference in pressure. Wind is described with speed and direction. Wind direction is reported by the direction from which it originates. For example, a westerly wind blows from the west to the east. Wind direction is usually reported in cardinal directions or in azimuth degrees. For example, a wind coming from the south is given as 180 degrees; one from the east is 90 degrees. Meters per second (m s-1) is the standard SI unit commonly used for the reporting and forecasting of wind speed. Another commonly used unit of wind speed in meteorology is ‘knot’, which is a non-SI unit. 1 knot is equal to 0.514444 m s-1.

Anemometer is used to measure wind speed and wind vane is used to measure wind direction. A typical wind vane has a pointer in front and fins in back. When the wind is blowing, the wind vane points into the wind. For example, in a north wind, the wind vane points northward. A cup anemometer is a common tool to measure wind speed. The cups catch the wind and produce pressure difference inside and outside the cup. The pressure difference, along with the force of the wind, causes  the cups to rotate. Electric switches measure the speed of the rotation, which is proportional to the wind speed.

4. Climate controls

There are several natural factors that determine the climate of a place. These are called climatic controls. The varying influence of these factors leads to different parts of the earth experiencing differing climates. It is now well accepted that human activities also influences the climate of a place, but its impact is not the same everywhere. For example, changes appear to be happening faster near the poles than in many other places. Some of the most important natural factors are listed and discussed below. The influence of the human activities on the climate is also discussed.

4.1. Latitudinal variation in the solar radiation

Solar illumination at the earth varies in space and time. The annual amount of incoming solar energy varies considerably from tropical to polar latitudes. There is also considerable seasonal variation at the middle and high latitudes. Polar region receive solar radiation at lower angles and after passing through a thicker layer of atmosphere than at the equator. As a result the climate is cooler in the poles than equator. The poles also experience the greatest difference between summer and winter day lengths: in the summer there is a period when the sun does not set at all at the poles; conversely the poles also experience a period of total darkness during winter. In contrast, day length varies little at the equator.

4.2.       Elevation or altitude

In the troposphere, the atmospheric air temperature normally decreases with altitude. As a result, high altitude regions experience cooler climate than adjacent low lands. In regions, where high mountain chains lie in the path of prevailing winds, the transfer of warm or cold air masses get blocked. In addition, the upward movement of air in the wind ward side and down ward movement of air in the leeward side may cause increased (decreased) precipitation in the windward (leeward) side. For example, Western Ghats that runs parallel to the western coast of the Indian peninsula block southwest monsoon winds from reaching the Deccan Plateau resulting large amount of rainfall (wet climate) in the windward side and relatively drier climate in the deccan plateau situated in the leeward side of the mountain ranges.

4.3. Distance from the sea

As oceans store heat, it can moderate climates of coastal areas. Coastal areas may enjoy refreshing breezes in summer, when cooler ocean air moves ashore. Coastal areas are cooler and wetter than inland areas. Clouds form when warm air from inland areas meets cool air from the sea. The interior parts of the continents experience large range of temperatures. For example, in the summer, the northwest India and central India experience very hot and dry climate as moisture from the sea evaporates before it reaches the interior parts.

4.4.Pressure and wind systems

The differential heating between high and low latitudes, land and ocean, snow-covered and bare land leads to difference in the atmospheric pressure. The pressure patterns drive wind patterns  which in turn drive the oceanic circulation patterns. The pressure and circulation patterns have significant impact on the precipitation and atmospheric temperature patterns. For example, the southwest monsoon circulation over India is caused by the differential heating between large Asiatic land mass and Indian ocean south of it. Flow of moist air mass from the ocean to the warm land region results in rising of moist air, formation of clouds and precipitation. During the southwest monsoon season, Indian subcontinent experiences a wet climate.

4.5. Ocean currents

Ocean currents are caused by earth’s rotation, surface winds and the Coriolis force. Trade winds force warm water near the equator flow from east to west. The Coriolis effect causes water to be deflected northward away from the equator and sets up a rotational cycle in the oceans, making currents flow clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. When the currents reach the poles, the water cools and sinks. The temperatures (warm or cold) of these currents affect the climate of surrounding areas. Ocean currents induced by the global scale wind systems can also increase or reduce temperatures of a place.

4.6. Local features

Various local features such as the slope of the land, exposure conditions and the characteristics of vegetation and soil impact the climate of the place. In the northern Hemisphere, south facing slopes receive more direct sunlight and have warmer climate than those with a northern exposure, which not only face away from the sun, but are also more open to cold northerly winds. Areas with sandy, loosely packed soil, because of their low heat conductivity are inclined to experience more frosts than do areas with hard packed soils; Valleys normally have more frequent and severe frosts than the adjacent slopes. Cities are usually warmer than the adjacent country sides. The amount of sunlight that is absorbed or reflected by the surface determines how much atmospheric heating occurs. Darker areas, such as heavily vegetated regions, tend to be good absorbers; lighter areas, such as snow and ice-covered regions, tend to be good reflectors. The ocean absorbs and loses heat more slowly thanland as the specific heat capacity of the ocean is higher than that of the land. Ocean water gradually release heat into the atmosphere, which then distributes heat around the globe.

4.7.  Human activities

The factors described above affect the climate naturally. Earlier, impact of human activities on the climate was negligible. However, recently, this impact has become significant due to the increased populations and industrial activities. In the recent several decades, temperatures have been rising steadily throughout the world. However, it is not yet clear how much of this global warming is due to natural causes and how much is due to human activities, such as the burning of fossil fuels and the clearing of forests. Due to global warming, increase in the extreme weather and climate events like heat and cold waves, heavy rainfall, droughts and floods, cyclonic systems etc. have been reported from many parts of the globe.

4.  Weather and climate predictions

Answers to the questions like will it rain today evening and how much will be the rainfall?,

what will be the minimum/ maximum temperature tomorrow, will it be cooler tomorrow than today are provided by weather prediction. At present, weather predictions are prepared using complex mathematical models. Fig. 3 shows a 24 hr rainfall forecast over Indian region using India Meteorological Department’s (IMD) global forecast system model prepared on 10th October, 2016. These models use initial atmospheric conditions such as air pressure, temperature, humidity, winds, precipitation rates etc. to produce the best estimate of the future condition in the atmosphere. The initial atmospheric conditions are obtained from real time observations taken and transmitted using the global observational network set up by the countries around the world through an international arrangement. A schematic diagram of typical global observational network is shown in Fig. 4. This observation network includes surface meteorological stations, balloon measurements, shipboard measurements, and satellite and radar measurements. Once the model outputs are generated, weather forecasting involves interpreting the model outputs to figure out the most likely scenario. The accuracy of weather forecasts depends on the accuracy of the observed data that are used for running the models, capability of the model to simulate the weather phenomena accurately, and skill and experience of the forecaster.

The accuracy in the estimate of the initial conditions specified to the computer models decides the accuracy of the weather prediction. In spite of the large size of the current observational network, the observed data have several gaps as observations are made only from limited locations. Further these measurements are not always perfectly accurate. This causes imperfection in the initial conditions, which contributes some amount of uncertainty in the weather predictions even if understanding of the physics of the weather is perfect. Due to imperfect initial conditions, the errors in the model simulations of weather tend to grow. Thus longer the weather model is run into the future, the less accurate will be the prediction. Predictions of the weather just a week or two in advance, let alone decades, become highly problematic. As per the present capability, short-term weather forecasts are accurate only for around 5-10 days.

Climate predictions provide a much longer-term view and less concerned with exact time and place. It focuses on spatially and temporally averaged conditions. It provide answers to questions like how much warmer will the earth be 50 to 100 years from now?, which part of the globe (ocean or land, southern or northern hemisphere, low or high latitudes) will warm faster?, how much more precipitation will be there?, how much will sea level rise? what will be impact on the extreme weather events like cyclones, heavy rainfall, heat and cold waves etc. Climate predictions are made using global climate models. The climate prediction of annual mean surface temperature change & average percent change in annual mean precipitation during 2081-2100 with respect to 1986-2005 provided by InterGovernmental Panel on Climate Change (IPCC) in the 5th assessment report published in 2013 is given as Fig. 5. As can be seen, the warming is expected to be greatest over land; most increase could be at high northern latitudes and the least over the Southern Ocean and parts of the North Atlantic Ocean. Increase in precipitation is very likely in high latitudes and decrease is likely in most subtropical land regions.

Unlike in weather prediction where the accuracy of the prediction depends on the initial conditions of the atmosphere, the accuracy of the climate prediction depends on a host of boundary conditions, many of which are linked to the atmosphere’s energy. Boundary conditions are both natural and that influenced by human activities. Solar radiation and volcanic aerosols are natural boundary conditions. During the last about 1150 years, total solar insolation, observed at the top of the atmosphere, has varied by about 2 Wm-2 around an average of about 1361 Wm-2. Similarly, large changes in the concentration and distribution of aerosols in the atmosphere associated with large volcanic eruptions produce changes in the reflectivity of the incoming solar radiation.

Human influenced boundary conditions include changes at the surface and changes in the atmosphere. At the surface, changes in the land use like cutting of forest for pasture and crops modifies the surface reflectivity and moisture, heat, and momentum exchanges between land and atmosphere. In the atmosphere, the most important human influenced changes are those that affect greenhouse gases. Greenhouse gases principally water vapor and carbon dioxide keep earth habitable by absorbing enough long-wave radiation to keep surface temperatures tens of degrees Celsius warmer than they would be otherwise. The warming of earth’s surface due to the presence of greenhouses gases in the atmosphere is known as greenhouse effect. However, due to the rapid increase in the emission of different greenhouse gases into the atmosphere over the past two centuries, primarily due to burning of fossil fuels has caused significant warming of the global surface temperatures in recent decades. This is known as global warming.

Warming of air inside a car parked in the sunlight can be used an analogues example to understand the trapping of heat in the atmosphere by greenhouse gases. Sun’s energy entering into the car through the windshield warms the air inside and gets trapped as the warm air cannot pass outside the car through the windshield, causing inside of the car to warm up.

Earth’s energy balance is also altered by the human emissions of atmospheric aerosols. The composition of the aerosols and their distribution decide how they contribute to both warming and cooling of the climate. However, aerosols are thought to contribute an overall cooling effect equal to about half of the warming caused by greenhouse gases when averaged over the globe.

6. Summary

• Weather is the instantaneous physical condition or state of the atmosphere at a given time and place and its short-term variation in minutes to weeks.
• Climate is the statistical description of weather pattern over a region in terms of its mean and variability over a period of 30 yrs or more.
• Climate is what you expect, like a very hot summer, and weather is what you get, like a hot day with pop-up thunderstorms or a cool evening due to sudden summer rains.
• Atmospheric state variables or elements generally used to describe weather and climate over a place are temperature, humidity, precipitation, visibility, atmospheric pressure and wind. These atmospheric variables vary from once place to another, with the height and with time.
• The climate of a place is determined by influence of several natural factors called climate controls.