Weather and Climate

Weather is a description of the changeable aspects of the atmosphere, the temperature, rainfall, pressure, and so forth, at a particular time. These changes usually affect your daily life one way or another, but some of them seem more inconvenient than others.

Clouds and Precipitation

Introduction Hydrological Cycle – 4 main parts Evaporation of water from the oceans Transport of water vapor in the atmosphere Condensation and precipitation Return of water to ocean by rivers and streams

The main events of the hydrologic cycle are: (1) The evaporation of water from the ocean, (2) the transport of water vapor through the atmosphere, (3) condensation and precipitation of water on the land, and (4) return of water to the ocean by rivers and streams.

Cloud-forming Processes Upward Air Movement Convection resulting from differences in temperature Barriers such as mountain ranges which provide lift to air masses Meeting of moving air masses with different densities.

Adiabatic Cooling Decrease in temperature of an expanding gas Adiabatic Heating Heating of a gas as it undergoes compression Dry Adiabatic Lapse Rate Rate of cooling in the absence of condensation About 10 OC for each increase or decrease of 1 km (5.5OF/1,000 ft)

As a parcel of dry air is moved upward, it expands and cools according to the dry adiabatic lapse rate. This graph compares the temperature of a rising and adiabatically cooling parcel of dry air with the average temperatures of the surrounding atmosphere when the temperature at the surface is 30OC (86O F).

Atmospheric Stability When the atmospheric lapse rate is less that the dry adiabatic lapse rate Instability When the atmospheric lapse rate is greater than the dry adiabatic lapse rate.

If a parcel of air is moved up or down, it will cool or warm according to the dry adiabatic lapse rate. In a state of atmospheric stability, the parcel of air will always be cooler, and therefore more dense, than the surrounding air at any altitude. It will, therefore, return to the original level when the upward force is removed.

In a state of atmospheric instability, a parcel of air will always be warmer, and therefore less dense, than the surrounding air at any altitude. The parcel will, therefore, continue on in the direction pushed when the upward force is removed.

Wet Adiabatic Lapse Rate The lapse rate for a parcel of air at a slow rate and releasing the temperature equal to its latent heat of vaporization Supersaturated When the air contains more water vapor than is the normal amount of water vapor

When the dew point temperature is reached in a rising parcel of air, the latent heat of vaporization is released as water vapor condenses. This release of heat warms the air, decreasing the density and accelerating the ascent. The new lapse rate from the release of latent heat is called the wet adiabatic lapse rate.

Origin of Precipitation Processes of precipitation formation Coalescence When water droplets merge with millions of other water droplets. Growth of ice crystals Ice crystals can capture other water molecules and grow to enormous sizes.

Supercooled If water remains in the liquid state after the temperature is below the freezing point. Ice-forming nuclei Solid particles on which the ice forms

Precipitation is water in the liquid or solid form that returns to the surface of the earth. The precipitation you see here is liquid, and each raindrop is made from billions of the tiny droplets that make up the clouds. The tiny droplets of clouds become precipitation by merging to form larger droplets or by the growth of ice crystals that melt while falling.

Weather Producers

Introduction There is a shift in weather during the changes of the seasons that is related to: Movement of air masses Leading fronts of air masses high and low pressure areas

Air Masses Polar air mass An air mass that moves from a cold region Tropical Air Mass An air mass that moves from a warm region Continental Air Mass Moves in from a land mass Maritime Air Mass Moves in from over an ocean

Major types of air masses Continental Polar Cold Dry Maritime Polar Moist

Continental Tropical Warm Dry Maritime tropical Moist

The general movement of the four main types of air masses that influence the weather over the contiguous United States. The tropical air masses visit most often in the summer, and the polar air masses visit most often during the winter. During other times, the polar and tropical air masses battle back and forth over the land.

This satellite photograph shows the result of a polar air mass moving southeast over the southern United States. Clouds form over the warmer waters of the Gulf of Mexico and the Atlantic Ocean, showing the state of atmospheric instability from the temperature differences.

Air mass weather When the weather in an area is under the influence of an air mass.

Weather Fronts Front A boundary between two different air masses Cold Front When a cold air mass moves into a warmer area, displacing the warm air mass Provides lift to adiabatically cool the warm air, resulting in towering cumulus and thunderclouds.

(A)A cold air mass is similar to a huge, flattened bubble of cold air that moves across the land. The front is the boundary between two air masses, a narrow transition zone of mixing. (B) A front is represented by a line on a weather map, which shows the location of the front at ground level.

An idealized cold front, showing the types of clouds that might occur when an unstable cold air mass moves through unstable warm air. Stable air would result in more stratus clouds rather than cumulus clouds.

Warm Front When a warm air mass moves into an area, displacing the cold air mass A gently sloping front as the Warm air moves over top of the cooler air. Stationary Front When the edge of a front ceases to advance

An idealized warm front, showing a warm air mass overriding and pushing cold air in front of it. Notice that the overriding warm air produces a predictable sequence of clouds far in advance of the moving front.

Waves and Cyclones Occluded Front One that has been lifted completely off the ground Has s low pressure center and cyclonic activity Cyclone a low pressure area with winds moving into the low pressure area and being forced upward. Friction and the Coriolis effect cause the air to move to the right of the direction of movement. Anticyclone A high pressure center

The development of a low-pressure center, or cyclonic storm, along a stationary front as seen from above. (A) A stationary front with cold air on the north side and warm air on the south side.

(B) A wave develops, producing a warm front moving northward on the right side and a cold front moving southward on the left side.

(C) The cold front lifts the warm front off the surface at the apex, forming a low-pressure center.

(D) When the warm front is completely lifted off the surface, an occluded front is formed.

(E) The cyclonic storm is now a fully developed low-pressure center.

Cyclonic storms usually follow principal storm tracks across the continental United States in a generally easterly direction. This makes it possible to predict where the low-pressure storm might move next.

Air sinks over a high-pressure center that moves away from the center on the surface, veering to the right in the Northern Hemisphere to create a clockwise circulation pattern. Air moves toward a low-pressure center on the surface, rising over the center. As air moves toward the low-pressure center on the surface, it veers to the left in the Northern Hemisphere to create a counterclockwise circulation pattern.

Major Storms Thunderstorms Conditions Uplift of air Cumulus clouds Frontal Thunderstorms Move with the front that produced them

Stages Cumulus Air is lifted and cools adiabatically to the dew point and a cumulus cloud develops. Heat of vaporization is released and accelerates the uplift. Mature When the moisture that is produced reaches the surface of the Earth This series of updrafts, downdrafts, and precipitation release electrical charges which become the lightening associated with the storm Final All of the updrafts are cut off and only downdrafts exist. Lightening Thunder hail

Three stages in the life of a thunderstorm cell Three stages in the life of a thunderstorm cell. (A) The cumulus stage begins as warm, moist air is lifted in an unstable atmosphere. All the air movement is upward in this stage.

(B) The mature stage begins when precipitation reaches the ground (B) The mature stage begins when precipitation reaches the ground. This stage has updrafts and downdrafts side by side, which create violent turbulence.

(C) The final stage begins when all the updrafts have been cut off, and only downdrafts exist. This cuts off the supply of moisture, and the rain decreases as the thunderstorm dissipates. The anvil-shaped top is a characteristic sign of this stage.

Different parts of a thunderstorm cloud develop centers of electric charge. Lightning is a giant electric spark that discharges the accumulated charges.

These hailstones fell from a thunderstorm in Iowa, damaging automobiles, structures, and crops.

Tornados Most violent, focused storm on Earth Speeds in excess of 300 km/hr (200 mi/hr) Hurricanes A hurricane is a violent storm that is produced over the warm tropical ocean near the equator. Tropical Cyclone Typhoon

A tornado might be small, but it is the most violent storm that occurs on the Earth. This tornado, moving across an open road, eventually struck Dallas, Texas.

This is a satellite photo of hurricane John, showing the eye and counterclockwise motion.

Cross section of a hurricane.

Weather Forecasting

Weather predictions are based on information about air masses, fronts, and associated pressure systems in an area. This information is used to produce a model of behavior for weather using a computer. Many models are used and then summarized when the different models agree fairly closely to a model of the weather.

Supercomputers make routine weather forecasts possible by solving mathematical equations that describe changes in a mathematical model of the atmosphere. This "fish-eye" view was necessary to show all of this Cray supercomputer at CERN, the European Center of Particle Physics.

This weather map of the United States shows a cold front running from Houston, Texas, to near Raleigh, North Carolina, where it becomes a stationary front that runs in a northeasterly direction. Note the areas of showers and the temperature predictions.

Climate

Introduction Climate is a generalized pattern for weather over a period of time Weather describes the changes that occur to the atmospheric conditions over short periods of time.

The climate determines what types of plants and animals live in a location, the types of houses that people build, and the life-styles of people. This orange tree, for example, requires a climate that is relatively frost-free, yet it requires some cool winter nights to produce a sweet fruit.

Major Climate Groups Factors Determining Weather Temperature Moisture Movement of Air

Low Latitudes Have vertical solar radiation at noon some times of the year High Latitudes Have no vertical solar radiation at noon some times of the year. Middle Latitudes Between high and low latitudes.

Latitude groups based on incoming solar radiation Latitude groups based on incoming solar radiation. The low latitudes receive vertical solar radiation at noon some time of the year, the high latitudes receive no solar radiation at noon during some time of the year, and the middle latitudes are in between.

Tropical Climate Zone Near the equator and receives the greatest amount of sunlight throughout the year. Polar Climate Zone The Sun never sets during the summer and never rises during the winter. Temperate Climate Zone Average temperatures that are between the two extremes

The principal climate zones are defined in terms of yearly temperature averages, which are determined by the amount of solar radiation received at the different latitude groups.

A wide variety of plant life can grow in a tropical climate, as you can see here.

Polar climates occur at high elevations as well as high latitudes Polar climates occur at high elevations as well as high latitudes. This mountain location has a highland polar climate and tundra vegetation, but little else.

This temperature-climate deciduous forest responds to seasonable changes in autumn with a show of color.

Regional Climate Influence Altitude Higher altitudes radiate more energy back into space. Temperature decreases with altitude Mountains Decreasing temperature with altitude Uplifting effect on air masses

Large Bodies of Water high specific heat and loses energy by evaporation. This has the effect of keeping the temperatures more constant from night to day on a land mass near a large body of water Ocean Currents Currents move large amounts of water from different parts of the world. Can warm or cool land depending upon the origin of the current

Ocean currents can move large quantities of warm or cool water to influence the air temperature of nearby landmasses.

Describing Climates Major Climate Zones Maritime Climate Influenced by air masses from the ocean Continental Climate Influenced by air masses from land Arid dry Less than 25 cm (10 in) of rain per year

Humid Moist More than 50 cm (20 in) of rain per year Semiarid Between arid and humid 25 – 50 cm (10 – 20 in) of rain per year

The idealized general rainfall patterns over the earth shift with seasonal shifts in the wind and pressure areas of earth's general atmospheric circulation patterns.

Local Climates Cities Materials used to build cities have a higher heat holding capacity that natural materials. Also change wind direction and speeds Microclimates A local pattern of climate influenced greatly by the local conditions.

This map highlights the approximate location of the major types of climates in North America.