Weather Factors

2.1- Energy in the Atmosphere Energy from the Sun – The sun provides most of Earth’s energy. – This energy travels to Earth in the form of electromagnetic waves. – Direct transfer of electromagnetic waves is known as radiation.

Most of the energy that reaches Earth is in the form of visible light and infrared radiation, and a small amount of ultraviolet.

Visible light is a mixture of all the colors seen in a rainbow. Infrared energy has wavelengths longer than visible light. – Not visible, but can be felt as heat. Ultraviolet has wavelengths shorter than visible light and can cause sunburn, skin cancer, and eye damage.

Some energy is reflected back into space by clouds, particles, and Earth's surface. Some is absorbed by the atmosphere or by land and water on Earth's surface.

Scattering is the reflection of light in all directions. The sky appears blue because blue light is scattered more than other colors. Why does the sky change color at sunrise and sunset?

When the Earth’s surface is heated, it radiates some of the energy back into the atmosphere as infrared radiation. Much of this is absorbed by water vapor, carbon dioxide, methane, and other gases in the air. The process by which these gases hold heat in the atmosphere is known as the greenhouse effect.

2.2- Heat Transfer Energy and Temperature – The thermal energy of a substance is the energy of motion of each molecule of that substance. – Air temperature is usually measured with a thermometer. The energy transferred from a hotter object to a cooler one is known as heat.

Energy from the Sun reaches Earth in the form of radiant energy, or radiation. Radiation is energy that is transferred in the form of rays or waves. Earth radiates some of the energy it absorbs from the Sun back toward space.

Conduction is the transfer of energy that occurs when molecules bump into one another. Molecules in warmer objects move faster than molecules in cooler objects. When objects are in contact, energy is transferred from warmer objects to cooler objects.

Earth ’ s surface conducts energy directly to the atmosphere. As air moves over warm land or water, molecules in air are heated by direct contact.

Convection is the transfer of heat by the flow of material. Convection circulates heat throughout the atmosphere.

When air is warmed, the molecules in it move apart and the air becomes less dense. Air pressure decreases because fewer molecules are in the same space. In cold air, molecules move closer together. The air becomes more dense and air pressure increases. Cooler, denser air sinks while warmer, less dense air rises, forming a convection current.

2.3- Winds What Causes Winds? – A wind is the horizontal movement of air from an area of high pressure to an area of low pressure. – All winds are caused by differences in air pressure. – Most differences in air pressure are caused by differences in air temperatures. Temperature differences happen because some parts of the Earth get more energy from the sun than others. Since the sun shines more directly on the equator than on the poles, the air is warmer near the equator.

Since the air near the equator is less dense, it rises forming areas of low pressure. The cold air near the poles sinks because it is more dense, forming areas of high pressure. The air moves in large circular patterns called convection cells.

Wind speed is measured with an anemometer. The force of the wind causes the cups to spin and turns the axle. A speedometer is attached to the axle that shows the wind speed. The wind-chill factor is a measure of how much a wind blowing over your skin removes body heat.

Wind Speed and Isobars Isobars- Isolines of equal pressure The larger the pressure gradient the faster the wind. The closer the isobars the faster the wind

How do pressure differences affect wind speed?

Weather Station Symbols

Draw the tail feather on a station model A. A 5 knot wind blows from the N B. A 10 knot wind blows from the N C. A 15 wind blows from the N D. A 20 knot wind blows from the N E. A 30 knot wind blows from the N F. A 35 Knot wind blows from the N G. Repeat A-F for a Southerly Wind, a North Easterly Wind and a South Westerly Wind

Local Winds Local winds generally move short distances and can blow from any direction. Caused by unequal heating of Earth’s surface within a small area. Unequal heating often occurs on land that is next to a large body of water. It takes more energy to warm water than an equal area of land.

Monsoons are sea and land breezes that change directions seasonally.

Southeast Asian Monsoon

Global Winds Winds that blow steadily from specific directions over long distances. Global wind patterns are caused by: – Coriolis Effect – The land and sea makeup of Earth The movement of air between the equator and the poles produces global winds. – Low air pressure near the equator, high pressure near the poles.

Coriolis Effect Because the Earth rotates, and consists of a mixture of land and sea, the Coriolis Effect exists The Coriolis Effect is the movement of particles (such as air) to the right in the Northern Hemisphere How about the South? – They move to the left

Coriolis Effect

Warm Moist Air Rises

Cool Dry Air Sinks

Warm Moist Air Rises Cool Dry Air Sinks Cold Moist Air Rises

Warm Moist Air Rises Cool Dry Air Sinks Cold Moist Air Rises Very Cold Air Sinks

Warm Moist Air Rises Cool Dry Air Sinks Cold Moist Air Rises Very Cold Air Sinks H L H H H L L

H L H H H L L Wind Moves from HIGH to LOW

Objects are deflected to the RIGHT in the Northern Hemisphere

Remember! Wind gets its name from the direction it is blowing FROM!

Northern Wind Patterns Polar Easterlies – 90 – 60 latitude from the east Prevailing Westerlies – 60 – 30 latitude from the west Trade winds – 30 – 0 latitude from the East

Latitude Nicknames Horse Latitudes – 30 latitude – Named because the boats stalled and the horses had to go! Doldrums / Intertropical Convergence Zone (ITCZ) – Equator – Trade winds converge from north and south – Named because boats also got stranded here

Jet Streams Jet Streams form at latitudes where wind systems come together High speed westerly winds form high above the surface

2.4- Water in the Atmosphere Humidity The term humidity is used to describe the amount of water vapor in the air. Water vapor in the atmosphere is extremely important. Without it there would be no clouds and no precipitation. If all the water vapor in the atmosphere were to suddenly condense and fall as rain, it would cover the earth ’ s surface with about 1 inch of water.

Circulation of water in the Atmosphere Within the atmosphere there is an unending circulation of water. Oceans occupy 70% of the earth ’ s surface; we can speak of the circulation of water as beginning over the ocean. Evaporation: transformation of liquid water into water vapor Condensation: water vapor changes back into liquid (this forms our clouds) Precipitation: liquid (or solid) cloud particles grow in size and fall to the earth ’ s surface

The Hydrological cycle: “ Water cycle ” Water molecules travel from the ocean to the atmosphere to the land and back to the ocean Transpiration: process that allows plants to give up moisture. Water absorbed by roots, moves through stem system and emerges from the plant through the underside of leaves. 85% evap from here Remaining 15% from land

Evaporation – A closer look Water molecules at the surface of the water are evaporating and condensing. But more are evaporating so net evaporation is occurring Recall temperature of water is a measure of the average speed of the molecules. Molecules near the surface move fast enough to break away and enter the air.

Saturation When the dish is covered eventually the total number of molecules escaping the liquid will be balanced by the number of returning molecules. This condition is know as saturated air. (No net loss of water vapor) Remove the cover and blow across the jar and evaporation will resume – Why?

Condensation Nuclei If we examine the air above the water in the jar of the previous pictures, we would find that the air molecules are mixed with tiny (microscopic) bits of dust, smoke, ocean salt, etc. Since these all serve as surfaces on which water vapor may condense they are called condensation nuclei. Condensation is more likely to occur when air is cooled because the speed of the water vapor decreases.

Which “ holds ” more moisture? Warm or Cold Air Although condensation is more likely to occur when the air cools, no matter how cold it becomes, there are always a few molecules with sufficient speed (energy) to remain as a vapor. Given the same number of water vapor molecules in the air, saturation is more likely to occur in cool vice warm air. Thus, warm air can “ hold ” more water vapor molecules before becoming saturated than can cold air.

Humidity Any one of a number of ways of expressing the amount of water vapor in air. Absolute humidity: or water vapor density. Comparison of the mass of water vapor with the volume of air in the parcel (g/m³) Specific Humidity: mass of water vapor in parcel compared to total mass of air in parcel (g/kg) Mixing Ratio: mass of water vapor in the parcel compared to the mass of the remaining dry air in the parcel (g/kg)

Relative Humidity When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed. Most commonly used way to describe atmospheric moisture. Also the most misunderstood. RH=water vapor content/water vapor capacity Simply put it is the ratio of the air ’ s water vapor content to its capacity Change in RH brought about in two primary ways: – Changing the air ’ s water vapor content – Changing the air temperature Note normal times of high vs low RH.

RH and Dew Point Dew point is the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur.

Average surface dew-point temperatures (°F) for January.

Average surface dew-point temperatures (°F) for July.

RH and Dew Point

Heat Index To calculate the apparent temperature, find the intersection of the air temperature and the RH. 100 o F with RH 60% HI=130F

Sling Psychrometer Instrument used to obtain the dew point and RH Wet bulb (white sock)- wick-covered thermometer dipped in clean water Spin the sling and water evaporates from the wick and thermometer cools. Wet bulb temperature – lowest temperature that can be attained by evaporating water into air. Dry bulb gives current air temperature. Wet bulb depression – difference between the dry bulb and the wet bulb – Large depression indicates a great deal of water can evaporate into air and the RH is low – WB=DB indicates 100% RH (saturated air)

Why Do Clouds Form? Clouds need three things to form: – Cooling – Condensation Nuclei – Saturated Air

What Causes Cooling? There are Four Processes that lead to the cooling of air- #1 Convective Cooling Rising air expands because pressure decreases Expansion causes the air to become cooler – Adiabatic Temperature Changes are changes in temperature from the expansion or compression of air

Adiabatic Temperature Change (Convective Cooling Continued) – For dry air, temperature changes 1ºC every 100 meters in elevation change – 5.4ºF for every 1000 feet – For moist air, the cooling rate varies from 0.5ºC per 100 m for air with a high moisture content to 0.9ºC per 100 m for air with a very low moisture content – Average is 0.7ºC per 100 m

Convective Cooling (Continued) When the rising air reaches the dew point temperature, water vapor begins to condense The condensation level is the height above the ground at which condensation takes place

#2 Forceful Lifting – An event occurs that forces the air to rise – Air moving up and over a mountain

#3Temperature Changes – Two air masses with different temperatures mix – The temperature change of the combined air masses may be cooler than the dew point

#4 Advective Cooling – Warm, moist air moving over cool land or water causes the air temperature to drop – Forms low clouds or fog

Cloud Condensation Level

Condensation Nuclei The water vapor needs a surface on which to condense. Condensation nuclei are tiny, less than mm in diameter. There MUST be solid particles in the air for the water vapor to condense onto. Examples include: Suspended particles of atmospheric dust mineral particles ash from fires volcanic dust microscopic organisms vaporized meteors – salt from sea spray

Saturated Air Air temperature must be equal to the dew point. Thus, 100% humidity... Clouds form!

Let ’ s Make a Cloud! Magic?!... No, science.

Cloud Types 1 Stratus Clouds – sheet-like, or layered. Cumulus Clouds – puffy, like cotton balls. Cirrus Clouds – thin and wispy.

Cloud Types 2 High Clouds (above 6,000 m) – Cirrus – Cirrostratus Mid-level Clouds – Altostratus – Altocumulus Low-level Clouds – Stratus – Cumulus, Cumulonimbus

Cirrus Clouds (thin and wispy)

Cirrostratus Clouds

Altocumulus Clouds

Fair-Weather Cumulus Clouds

Cumulonimbus 1

Cumulonimbus 2

Finally, what do we call a cloud on the ground? FOG!

Precipitation Formation For precipitation to form, millions of cloud droplets must somehow coalesce into drops large enough to sustain themselves during their descent. The two mechanisms that have been proposed to explain this phenomenon are: Bergeron process  the Bergeron process, which produces precipitation from cold clouds (or cold cloud tops) primarily in the middle latitudes, and collision-coalescence process  the warm cloud process most associated with the tropics called the collision-coalescence process.

Collision Coalescence Process

Precipitation Types The two most common and familiar forms of precipitation are:  rain  rain (drops of water that fall from a cloud and have a diameter of at least 0.5 millimeter) and  snow  snow (precipitation in the form of ice crystals or, more often, aggregates of ice crystals).

Precipitation Types (cont.) Other forms include:  sleet  sleet (falling small particles of ice that are clear to translucent),  glaze  glaze (formed when supercooled raindrops turn to ice on colliding with solid objects),  hail  hail (hard, rounded pellets or irregular lumps of ice produced in large cumulonimbus clouds), and  rime  rime (a deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects whose surface temperature is below freezing). drizzledrizzle (smaller droplets of rain, yet larger than mist) mistmist (smallest water droplets visible) gaupelgaupel (watery hail)

Rime Ice Hail Sleet formation

Rain Measurement Rain, the most common form of precipitation, is probably the easiest to measure. The most common instruments used to measure rain are: standard rain gauge  the standard rain gauge, which is read directly, and tipping bucket gaugeweighing gauge  the tipping bucket gauge and weighing gauge, both of which record the amount of rain.

A standard rain gauge

Snow Measurement The two most common measurements of snow are depth and water equivalent. Although the quantity of water in a given volume of snow is not constant, a general ratio of 10 units of snow to 1 unit of water is often used when exact information is not available.

Weather Modification Weather modificationWeather modification is deliberate human intervention to influence atmospheric processes that constitute the weather. Weather modification falls into three categories: energy  (1) the use of energy to forcefully alter the weather, modifying land and water surfaces  (2) modifying land and water surfaces to change their natural interaction with the lower atmosphere, and atmospheric  (3) triggering, intensifying, or redirecting atmospheric processes.

Intentional Weather Modification Techniques The focus of intentional weather modification using modern weather technology is on:  cloud seeding  cloud seeding,  fog and cloud dispersal  fog and cloud dispersal,  hail suppression  hail suppression, and  frost prevention

Cloud seeding

Key Terminology CondensationCirrusCumulus Weather modificationRadiation fogAdvection fog Upslope fogSteam fogFrontal fog Bergeron ProcessWhite frostDew Collision-coalescence processRain gaugeStratus